Universiteit van
Amsterdam
Faculty: Anton
Pannekoek Instituut
On the carriers of Diffuse
Interstellar Bands
A search for their identity with recently obtained IR spectra
Bachelor project (Chemistry)
07/01/2013 – 29/03/2013
By Abel Schootemeijer (5669146)
Supervisors: prof. dr. Lex Kaper (UvA), prof. dr. Harold Linnzartz
Second reviewer: dr. Steen Ingemann Jorgensen
Table of Contents
Populairwetenschappelijke samenvatting (dutch)…………………………………………………………………..
Abstract…………………………………………………………………………………………………………………………………...
Introduction……………………………………………………………………………………………………………………………..
 Diffuse interstellar bands (DIBs)………………………………………………………………………………….
 The interstellar medium.……………………………………………………………………………………………..
 Reddening ……………………………………………………………………………………………………………………
 Potential DIB-carriers……………………………………………………………………………………………………
 How are DIBs identified?.................................................................................................
 Very Large Telescope/X-shooter…………………….……………………………………………………………
 DIBs in IR region……………………………………………………………………………………………………………
 How to match a DIB from star’s spectrum
and a DIB-candidate’s absorption spectrum……………………………………………………………….
 Research in this project……………………………………………………………………………………………….
Experiment part I….………………………………………………………………………………………………………………….
 IBBCEAS……………………………………………………………………………………………………………………….
 MIS……………………………………………………………………………………………………………………………….
Experiment part II……………………………………………………………………………………………………………………..
 NIR spectra…………………………………………………………………………………………………………………..
 The 485 μm DIB……………………………………………………………………………………………………………
Results, ……………………………………………………………………………………………………………………………………
 NIR spectra…..……………………………………………………………………………………………………………..
 The 485 μm DIB & DIBs in the optical range………………………………………………………………..
Discussion………………………………………………………………………………………………………………………………..
 NIR spectra………………………………………………………………………………………………………………….
 The 485 μm DIB & DIBs in the optical range……………………………………………………………….
Conclusions………………………………………………………………………………………………………………………………
Future research………………………………………………………………………………………………………………………..
Acknowledgements………………………………………………………………………………………………………………….
References……………………………………………………………………………………………………………………………….
Appendix A……………………………………………………………………………………………………………………………….
Appendix B ………………………………………………………………………………………………………………………………
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1
Populairwetenschappelijke samenvatting
Niet iedereen weet dat het heelal niet alleen sterren en planeten bevat; een deel van de materie die
in het heelal aanwezig is, bevindt zich in interstellaire gaswolken. Deze gaswolken zijn heel koud, met
typische temperaturen van 50 K tot 100 K (-220 °C tot -170 °C). Ook hebben deze gaswolken een hele
lage dichtheid. Deze gaswolken zenden zeer weinig licht uit, en zeker als ze ver weg staan zijn voor
ons daardoor zeer moeilijk waarneembaar. We kunnen ze wel op een andere manier zien, namelijk
door naar sterren te kijken die zich achter die gaswolken bevinden. De moleculen in de gaswolken
absorberen licht dat door die sterren wordt uitgezonden: zo weten we dat ze er zijn.
De verschillende moleculen in die gaswolken absorberen licht bij verschillende golflengtes. Dit komt
doordat de moleculen in staat zijn om fotonen met een bepaalde hoeveelheid energie (daarbij hoort
een bepaalde golflengte van het foton) te absorberen en daarbij in een hogere energietoestand
geraken. Aan de hand van de golflengtes van fotonen waarbij bepaalde moleculen absorberen, kan
bepaald worden welke moleculen zich in de gaswolken bevinden. Dit is echter niet altijd even
makkelijk: in het gebied van het zichtbare licht (golflente tussen 400 nanometer en 1000 nanometer)
worden ongeveer 400 absorptiebanden waargenomen van interstellaire gaswolken waarvan het
onbekend is welke moleculen ze veroorzaken. Deze absorptiebanden worden de ‘Diffuse Interstellar
Bands’ (DIBs) genoemd. Met een telescoop die ook nauwkeurig het licht in het nabije
infraroodgebied kan meten (dit was vroeger moeilijk), zijn in het infrarood ook recentelijk DIBs
waargenomen. In dit project is onderzocht of er moleculen geïdentificeerd konden worden die deze
absorptiebanden in het nabije infrarood konden verklaren. Hiervoor zijn de absorptiespectra van
bepaalde polycyclische aromatische koolwaterstoffen (PAKs, dit zijn grote moleculen die uit
meerdere benzeenringen bestaan) en negatief geladen koolstofketens (C12-, C14-, C16-, C18-, C20-)
vergeleken met de absorptiebanden in het nabije infrarood. Het resultaat is dat beide types
moleculen de absorptiebanden in het nabije infrarood niet kunnen verklaren.
Verder is er nog een pas ontdekte absorptieband met een lange golflengte van 485 μm (ver
infrarood) onderzocht. Het vermoeden is dat deze absorptieband veroorzaakt kan worden doordat
een bepaald molecuul in een hogere vibratie-energietoestand raakt door een foton met deze lange
golflengte (=lage energie) te absorberen. Als die aanname klopt, zou het kunnen dat er andere
overgangen te zien zijn in energietoestanden waarbij de vibratietoestand ook verandert. Stel dat die
andere overgangen licht in het zichtbare deel van het spectrum zouden absorberen (pas op,dit is
weer een aanname!). In dat geval zouden er DIBs waargenomen kunnen worden die vlakbij elkaar
liggen, op een bekende afstand uit elkaar(namelijk: het energieverschil tussen de
vibratietoestanden). Door de bekende DIBs te analyseren zijn er drie DIBs gevonden die inderdaad de
juiste posities ten opzichte van elkaar hadden qua golflengte.
Door het grote aantal DIBs in het zichtbare deel van het spectrum kan dit echter ook toeval zijn. In
een volgend onderzoek zou dit onderzocht kunnen worden door langs andere gezichtslijnen naar
sterren te kijken; als die DIBs in een andere verhouding zichtbaar zijn in die andere gezichtslijnen,
dan kunnen deze DIBs niet door hetzelfde molecuul worden veroorzaakt. Als de DIBs wel in dezelfde
verhouding aanwezig zijn, zouden ze inderdaad veroorzaakt kunnen worden door energieovergangen binnen hetzelfde molecuul. De volgende stap zou dan zijn om erachter te komen welk
molecuul dit zou kunnen zijn.
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Abstract
In this research, recently obtained infrared spectra of stars have been used to search for the identity
of Diffuse Interstellar Bands (DIBs). With VLT/X-shooter, DIBs have been identified in the near
infrared region (1000 nm – 2500 nm). Spectra of medium to large sized PAHs and carbon chain
anions have been compared to DIBs in the NIR spectrum of the O-supergiant in the high-mass binary
4U 1907+07. None of the molecules showed an interesting match with one or more DIBs.
Furthermore, a possible far infrared DIB at 485 μm has recently been reported. The assumption has
been tested that this absorption band is caused by a vibrational transition. The DIBs at λair = 769,6
nm, 770,8 nm and 772,0 nm had energy separations that match the energy of a 485 μm photon, and
they also show the absorption line characteristics that are expected if the assumption is true. In
future research, this assumption can be tested further by determining whether the three DIBs are
correlated: if they are not, they cannot have the same carrier.
-------------------------------------------------------------------------------------------------------------------------------------DIBs are best observed in the spectra of hot OIntroduction
type and B-type stars. One reason is that these
stars are much brighter than stars of a colder
Diffuse Interstellar Bands
spectral type, so that a longer sightline can be
Diffuse Interstellar Bands (DIBs) were first
probed. The second reason is that these stars
observed over 90 years ago, in 1922, by Mary
are preferred when researching DIBs, is that
Lea Heger (Heger, 1922). When observing
they have most of their stellar spectral
heavily reddened binary stars, she noticed
absorption lines in the far UV, near the peak
that the positions of some of the absorption
of the energy distribution; therefore they have
lines in the stellar spectra were varying with
relatively few absorption lines in the DIBtime, while other lines remained stationary.
range that would contaminate DIB-spectra.
The fact that some of the lines had their
position changing with time can be explained
by the Doppler effect; wavelengths are
shortened when a source is moving towards
us and stretched when the source is moving
away from us.
Δ𝜆
𝜆
𝑣
=𝑐
(Doppler shift)
Since stars in binaries are moving in elliptical
orbits around each other, their velocity
towards us is varying with time and therefore,
the positions of the centers of the absorption
lines are varying with time. But how could the
non-moving lines be explained? The answer is
that a stationary cloud that is causing the
absorption must be in the Interstellar Medium
(ISM). It could consist of either dust particles
or particles in the gas phase (or both); they
are called the DIB-carriers.
DIBs are mostly observed between λ = 400 nm
and λ = 1000 nm; this includes the near
ultraviolet (near UV), the visible and the near
infrared (NIR) part of the electromagnetic
spectrum (Herbig, 1995). See figure 1. They
are
characterized
by
their
broad,
diffuseprofile, with a FWHM (Full Width at
Half Maximum) ranging from 0,05 nm to 3 nm
(Cox, 2011).
Figure 1: The diffuse interstellar bands in the near UV,
visible and NIR. Image was created by the McCall
group.
Currently, over 400 DIBs are known (Hobbs et
al, 2009). Most of these DIBs are relatively
narrow and shallow, while fewer bands are
very broad and/or deep. DIB-spectra are not
identical for different stars. For example, in an
earlier research (Hobbs, 2008) along another
line of sight, a part of the observed bands
(around 30%) showed that were not found in
the research in 2009 and vice versa.
The ISM
The fact that different clouds do not have
exactly matching DIB-spectra is not surprising:
we do not expect these different clouds in the
ISM to have exactly the same excitation and
ionization conditions since the local conditions
3
are not the same everywhere. There are
variations in cloud densities, cloud
temperatures and exposure to radiation.
Especially UV-photon intensities are an
important factor: UV-photons have energies
high enough to break covalent bonds in
molecules. If a strong UV-radiation field is
present in a cloud in the ISM, hydrogen is
mainly present as unbound H-atoms since the
covalent bond in molecular hydrogen, H2
(bond dissociation energy: 4,52 eV) can be
destroyed by UV -photons with an equal or
larger energy, i. e. λ ≤ 275 nm (Herbig, 1995).
In dense clouds, hydrogen and other atoms or
molecules in the outer layers of the cloud
absorbs the UV-photons, protecting hydrogen
in the core of the cloud and reducing the rate
at which hydrogen molecules are destroyed.
Among sightlines of dense clouds, molecular
hydrogen fractions can be as high as fH2 = 0,6,
while fH2 ≤ 0,1 for diffuse clouds (Vos et al.,
2011). This shielding effect is called the ‘skin
effect’.
Although no DIBs have been assigned to any
molecule with certainty, a large number of
molecules has been found in the ISM by
identifying absorption lines at other
wavelengths than the DIB-region (i.e. 400 nm
– 1000 nm). H2 is by far the most abundant
molecule in the ISM. Other molecules that are
relatively abundant are small molecules like
CO, CO2, H2O and NH3 (Winnewisser & Kramer,
1999, Cheung et al., 1968).
In the mm - μm region, rotational transitions
of diverse small molecules and also of medium
sized carbon chains have been found
(Ehrenfreund & Charnley, 2000). This method
has it flaws: only polar molecules can be
detected, since their dipole moment changes
while they are rotating. Non-polar molecules
without dipole moment are therefore not
observable. (Winnewisser & Kramer, 1999)
By mid-IR spectroscopy, polycyclic aromatic
hydrocarbons have been detected in diffuse
interstellar clouds. In the wavelength region
between 3 μm and 15 μm, vibrational and
vibrational bending transitions were identified
that arose from C-H, C-C and C=C bonds
(Hudgins & Allamandola, 2004). Although this
method did not allow for the identification of
specific PAHs, the authors were able to
conclude that PAHs are present in the ISM.
Other complex molecules that were found in
the ISM are carbon chains: e. g., spectroscopy
in the microwave region resulted in the
detection of low column densities of HC11N
(Bell et al, 1997).
Reddening
The spectral energy distribution of a
blackbody depends only on its effective
temperature. Thus, the intrinsic ratio of the
photon intensities at different wavelengths is
known if the spectral type of a star is
identified (Fitzgerald, 1970). This information
is used to calculate the flux ratio of light
around λ=400 nm (blue) called ‘B’ and around
λ=650 nm (green, or ‘visible’) called ‘V’. This
ratio is called the intrinsic (B-V), or intrinsic
color.
Dust in the ISM scatters blue light with λ = 400
nm more than visible light with λ = 650 nm, so
‘reddening’ is observed in stars behind clouds
that contain dust. The amount of reddening,
E(B-V), is expressed by subtracting the intrinsic
(B-V) from the observed (B-V):
E(B-V) = (B-V)observed - (B-V)intrinsic
Reddening is proportional to the column
density of interstellar dust. Reddening and
column density are also proportional to DIBstrengths, although the correlation is not
perfect (Friedman et al., 2011). This means
that the presence of interstellar dust is
associated with the presence of DIB-carriers,
or that the interstellar dust itself is the DIBcarrier.
Potential DIB carriers
A variety of DIB-carriers has been proposed.
The most important classes of potential DIBcarriers are listed below.
Fullerenes
Fullerenes are molecules that are entirely (if
spherical or ellipsoidal) or almost entirely (if
tubular, with hydrogen atoms at the side
ends) composited of carbon atoms. The best
known
fullerene
is
C60,
or
buckminsterfullerene, which has a footballlike structure (see figure 2).
4
calculations, and a mechanism for the
synthesis of these tubular fullerenes was
proposed (Zhou et al., 2006). However, they
concluded that the error margin of their
calculations was too large to be able to assign
them to known DIBs.
Figure 2: C60. Image was taken from
www.nature.com
Fullerenes are candidates DIB-carriers because
they are large molecules, and the presence of
large molecules in the ISM is a good
explanation for the broad rotational bands.
Especially C60, is a popular candidate because
it is known for its stability (Kroto et al, 1985).
C60 and C70 molecules have been found in
planetary nebula TC1 (Cami et al., 2010).
Unfortunately, no absorption lines in the
optical (DIB) range were detected in that
planetary nebula for C60 (Garcia-Hernandez &
Diaz-Luis, 2013), meaning that the column
density is below the detection limit. A reason
for the column density being under the
detection limit might have been that most of
these molecules are ionized and are present
as C60+. In the article by Garcia-Hernandez &
Diaz-Luis, the authors suggested that buckyonions may be responsible for the DIBs
instead. Bucky-onions are relatively small
spherical fullerenes like C60 contained in larger
spherical fullerenes like C240, hence the name
‘onions’. The absorption spectra of buckyonions are hard to test experimentally; they
are difficult to synthesize due to their bad
solubility.
The ionized C60+ is tentatively assigned to the
DIBs at 957,7 nm and 963,2 nm by matrix
isolation
spectroscopy
(Foing
and
Ehrenfreund, 1994), but assignment has still to
be confirmed by gas phase spectroscopy (Cami
et al., 2010), which is a more accurate method
(see chapter 2: experiment).
Tubular fullerenes are also proposed as DIBcarriers. These molecules have been
investigated
by
electronic
structure
PAHs
Polycyclic Aromatic Hydrocarbons (PAHs) are
molecules consisting of multiple benzene
rings, with 2n+1 double bonds in the molecule
(definition of ‘aromatic’). The simplest PAH is
naphthalene, C10H8, which contains two rings
(see figure 3).
Figure 3: Structures of the small PAHs naphtalene and
pyrene. Image was created with ChemDraw.
PAHs are considered possible DIB carriers
because they are resistant to destruction by
UV-photons, because of the high abundance
of C-atoms in the ISM and because their
transitions are expected to be seen in the
optical part of the spectrum (Cox, 2011). The
absorption spectra of small PAHs like
naphthalene and pyrene (four rings in a
lozenge-like arrangement) were investigated,
but no DIBs could be assigned to their spectra
(Herbig, 1995). Also, their absorption bands
were too narrow to be DIBs. In another
research, Steglich et al., 2011 found no
individual medium sized PAHs that matched
either any DIB in the 400 nm-1000 nm range
or the 217,5 nm UV bump. They suggested
that the UV-bump might, however, be caused
by a variety of medium to large sized PAHs.
In a research by Iglesias-Groth et al, 2008,
absorption lines of the naphthalene cation,
C10H8+ seemed to match two previously
unknown DIBs. These DIBs did not appear
along other lines of sight and corresponded to
only a low column density. Still, this finding
makes the case of the PAH hypothesis
stronger.
Unsaturated carbon chains
Carbon chains are known to have strong
transitions in the visible part of the spectrum,
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and polar C-chains have been found in
interstellar molecular clouds by millimeter
spectroscopy (Schmidt & Sharp, 2005). For
these reasons, C-chains are logical candidates
as DIB-carriers. Remember that non-polar
molecules cannot be found in by mmspectroscopy, as mentioned earlier in the
introduction. Polar carbon chains are for
example molecules with radicals containing
one H-atom, C2nH, or chains containing a Natom.
Linnartz etc al, 1998 and Motylewski et al.,
2000, studied the spectra of carbon chain
radicals. Molecules they took spectra of
included C6H, C8H and C10H, HC4H+, HC6H+, and
also C-chain radicals with one or more
nitrogen atoms (NC4N+, NC6N+, HC5N+). They
were unable to convincingly match any of the
absorption spectra of these molecules to DIBs.
HC5N+, however, showed a correlation with
the weak 581,9 nm DIB and it cannot be
excluded that it causes this DIB. C-chains with
a hydrogen atom at both ends, HC(2n+1)H with
3≤n≤6 have been studied, but did also not
match any DIBs (Ball et al, 2000).
Because of the large variety of DIBs,
coincidental (near) matches occur from time
to time. For example, Güthe et al. (2001)
claimed that l-C3H2- was found in the ISM, but
higher resolution laboratory data proved that
the wavelengths did not match the assigned
DIBs (McCall et al, 2002).
The spectrum of C7- showed striking
similarities with several DIBs (Tulej et al.
1998), but later discrepancies in line shape
and line position (of one of the lines) were
found (McCall et al, 2000), so C7- was
discarded as a DIB-carrier.
Molecular hydrogen
It has been suggested that molecular
hydrogen, H2, is responsible for DIBs in the
768 nm – 788 nm interval (Sorokin & Glownia,
1995). They suggested a mechanism where
the vibrational energy levels of molecular
hydrogen in the vicinity of hot O-type stars (T
= 45000 K) were occupied by electrons excited
by Ly-α photons combined with other photons
(two-photon absorption). In a critique, Snow
(1995) argued that the H2-spectrum did not
completely match their assigned DIBs.
Furthermore, the general belief is that DIBs
are not (only) caused by gas clouds near hot
stars; DIBs have also been observed in the
spectra of much colder (T = 10000 K) stars. In
response, Sorokin & Glownia (1996) adjusted
their theory, but concluded that even though
there seem to be similarities between DIBs
and the H2-spectrum, they were not able to
proof their H2-hypothesis. For that, more
research would be required.
This H2-hypothesis is not the most popular
explanation for DIBs because it assumes the
absorption lines originate from clouds near Ostars. Also, the fact that the theory requires a
lot of tuning is a bad sign. Still, it is not ruled
out and in the research described in this
paper, H2 is one of the investigated DIBcandidates.
There is another theory suggesting H2 as DIBcarrier. It is estimated that H2 is roughly
106 times more abundant in the ISM than the
most abundant large carbon based molecules
(Schmidt & Sharp, 2005).
Due to the high abundance of molecular
hydrogen, H2, in the interstellar gas clouds,
lines in the DIB-spectrum are possibly caused
by indistinct absorption lines of H2 with low
oscillator strength. However, this theory is
also disputable since DIB-strengths seems to
correlate well with neutral atomic hydrogen
column densities and poorly with H2 column
densities (Friedman et al., 2011)
Dust grains
Interstellar dust grains are small condensed
dust particles. Their sizes varies, but most stay
smaller than 400 nm: this is known because
they scatter blue light ( λ ≈ 450 nm) more
efficiently than red light (λ = ≈ 650 nm).
Grains were once a popular candidate for DIBcarriers because, as mentioned, DIB-strengths
correlate with reddening and reddening
correlates with the column density of grains
(therefore, DIB-strength correlates with grain
column density), but now they are out of
fashion because of lack of evidence of grains
as DIB-carriers (Herbig, 1995), and counterevidence has been proposed: grains are
known to align to a magnetic field, which
should cause the DIBs to be polarized if these
DIB-carrying grains are situated in a magnetic
field. This effect, however, has never been
observed (Cox et al, 2011).
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How are DIBs identified?
To be able to identify the DIB-carriers, their
spectra should be compared to the observed
DIBs. Obtaining the spectrum of a DIBcandidate is often challenging, since the
circumstances in the ISM are different from
the circumstances here on Earth; low densities
of particles, low temperatures and usually a
high exposure to UV light. These
circumstances can result in the presence of
molecules that are not stable in circumstances
here on Earth, like radicals. Since radical
molecules are very reactive and consequently
have a short lifetime, it is not possible to
create a sample of say, HC7H and measure its
spectrum by conventional methods like IRspectroscopy.
Instead, several other methods are available
in which candidate DIB-carriers are stable so
their spectra can be measured. Three
methods are discussed here:
-Matrix Identification Spectroscopy
-Cavity Ringdown Spectroscopy
-Computational methods
In Matrix Identification Spectroscopy (MIS),
possible DIB-carrier molecules are caught in a
very cold (temperatures as low as 5K) matrix
of a noble gas, most often neon (Freivogel et
al., 1994). Then, the absorption of the matrix
is measured to obtain the spectra of the
molecules. The advantage of this method is
that high column densities of target molecules
can be achieved. A major downside, however,
is that interaction of the molecules with their
environment (the neon atoms) distorts their
absorption spectra. Therefore, this method is
inaccurate and it has partly been taken over
by gas-phase spectroscopy in the past few
years.
Cavity Ring-Down spectroscopy (CRD) is a
method in which molecules in the gas phase
are released in a low-pressure cavity at high
(usually supersonic) velocities. In the cavity,
light is reflected between two mirrors before
it will reach the detector: this is done to
increase the path length of the photons, so
the absorption can be measured more
accurately.
In computational methods, simulations are
used to calculate spectra of molecules. These
simulations are not perfectly accurate, but are
able to give a good approximation of the real
absorption spectra (Zhou et al., 2006).
VLT/X-shooter
The Very Large Telescope (VLT) and X-shooter
have been built recently by ESO (European
Southern Observatory) . The VLT (figure 4)is a
reflective telescope with a 8,2 m main mirror.
Figure 4: ESO’s Very Large Telescope on the mountain
Cerro Paranal in, Chile. Photo was taken by ESO.
To translate the images taken by this
telescope into spectra, the ‘X-shooter’
instrument was used. X-shooter consists of
three arms; beam splitters split the incident
light into them, separating Near Infrared (NIR),
visible and Ultraviolet (UV) light (Vernet et al.,
2011). After the beams are split, they all pass
through a grating, separating the photons with
different wavelengths. A Charge Coupled
Device (CCD) measures the photon intensities
at different wavelengths.
This design of X-shooter allows it to measure a
very wide range of wavelengths: from the UV
with λ = 300 nm to the NIR part of the
spectrum with λ = 2500 nm. This wide range
allows a reliable study of the whole DIB-range,
and also the possible detection of DIBs in a
region of which no good data was available in
the past, i. e. the λ = 1000 nm to λ = 2500 nm
range. A detailed description of X-shooter is
given in Vernet et al., 2011.
The NIR-spectrum of 4U 1907+07, which is
used in this research, was taken with VLT/Xshooter.
DIBs in the (N)IR region
Until recently, DIBs were only observed in the
region between λ = 400 nm and λ = 1000 nm.
Two DIBs were discovered at λ=1179 nm and
λ=1317 nm (Joblin et al., 1990) and with the
VLT/X-shooter, thirteen DIBs were found
between λ=1500 nm and λ=1800 nm (Geballe
7
et al., 2011). Very recently, a DIB in the far IR
region has been found at 486 μm (Müller et al,
not yet published). These IR DIBs have not yet
been studied intensely and might, if they can
be identified, hint scientists in which direction
they have to search for the identity of the DIBs
in the optical range. In this research, available
spectra from times when no near IR-DIBs were
known are compared to these newly obtained
DIBs.
The wavelength of the far IR 486 μm DIB
corresponds to a wavenumber of 20,599 cm-1,
𝑐
or an energy of 𝐸 = ℎ 𝜆 = 2,56 . 10-3 eV. This
energy value might also be the energy
difference between two rotational states of a
DIB carrier. In that case, two DIBs should be
found spaced 20,599 cm-1 apart more on this
in the ‘experiment’ chapter).
This research project
In this research, a description is given of two
instruments that use the two most important
methods to obtain spectra of candidate DIBcarriers; Cavity Ring-Down Spectroscopy
(CRDS) and Matrix Isolation Spectroscopy
(MIS).
In the remainder of the study, the newly
obtained IR-spectra are investigated. NIR-DIBs
are compared to absorption spectra of
possible DIB-carriers. Finally, a DIB-catalog
was used to search for DIBs that had
difference in wavenumber that matched the
wavenumber of the 485 μm DIB.
To summarize, there are two goals in this
research project: 1) Answer the question:
What are DIBs and how can we identify them?
2) Use newly obtained IR DIB-data to search
for the identity of DIB-carriers.
How to match a DIB from star’s spectrum and
a DIB-candidate’s absorption spectrum
To be able to assign an absorption lines of an
investigated molecule to a DIB, the first
criterion is that the wavelengths of both lines
have to match. Secondly, the shapes of the
lines have to be similar. The third criterion is
that, if a molecule has two or more absorption
lines, all of them have to show in a line of
sight, and not just one. For example: a
suspected DIB-carrier has absorption lines at
500 nm and 550 nm, and a DIB is seen at 500
nm but not at 550 nm, there is no match.
Because of the large number DIBs (over 400)
in the relatively small range between 400 nm
and 1000 nm, a large chance exists that an
absorption line and a DIB will coincidentally
seem to match. Therefore, a lot of care should
be taken to assign a line to a DIB; various false
assignments have been made in the past (see
chapter ‘potential DIB-carriers’). To prevent
wrong DIB identifications (and to identify a
DIB with enough certainty), accurate methods
with small error margins are required.
Experiment part I
Why do we want to identify DIBs?
-To obtain information about the constitution
of the interstellar medium. A problem is that
by rotational spectroscopy, only polar
molecules can be detected. If DIBs can be
identified, more accurate information will be
available.
In the first part of this chapter, two examples
are described of methods that can be used to
obtain spectra of molecules of interest.
IBBCEAS
In Incoherent Broadband Cavity-Enhanced
Absorption Spectroscopy (IBBCEAS), the
absorption is measured of a plasma that is
periodically released in a cavity. In that cavity,
incoherent light (i. e. photons have different
phase and wavelengths) is trapped between
two mirrors. Trapping the light in a cavity
increases the path length of the photons and
as a result, the absorption. The absorption of
the plasma is measured only when the plasma
is present in the cavity; this is achieved using
an optomechanical shutter.
The plasma
As mentioned in the introduction, candidate
DIB-carriers like C-chains with radicals are
unstable under normal terrestrial conditions.
Therefore, they have to be created in situ.
The reactant is acetylene(H-C≡C-H) gas, 0,5%
diluted in 1:1 He/Ar gas. This gas is stored
behind the nozzle at a pressure of 12 bar.
When the gas runs through the nozzle, a
voltage of -1200 V is applied between the jaws
of the nozzle and the grounded metallic plate
in the slit. As a result, a current will flow
8
Figure 5: The IBBCEAS setup displayed schematically. The abbreviations have the following meaning: TS=telescope, M 1 &
M2=high reflectivity mirror, PDN=pulsed discharge nozzle, A1 & A2=aperture, L1 & L2=lens with an f of 7,5 cm,
OS=optimechenical shutter, BS=beam splitter, L3= lens with an f of 5,0 cm, PD=photodiode, F=interference filter, OF=optic
fibre bundle, CCD=charge coupled device. Image was taken from Walsh et al., 2013.
through the gas (depending on its resistivity)
and a variety of carbon chain molecules is
expected to be produced.
After passing the nozzle the gas enters the
cavity, which is kept at a very low pressure:
±10-5 bar. Because of the large difference in
pressure between the gas chamber and the
cavity, the gas will expand at a very high speed
(v > vs, hence the ‘supersonical’ expansion).
This rapid expansion results in a rapid cooling
of the gas to a rovibrational temperature of
roughly 15 K, which is in the range of the
temperatures of diffuse interstellar clouds.
The nozzle releases the plasma in pulses
rather than constantly. This is because with
pulsed release, the pressure in the cavity can
be kept at a lower value than if the release
were constant. This low pressure in the cavity
results in a fast expansion of the plasma when
it is released; the result is a cold plasma with a
temperature that correlates well with the
targeted temperature.
Carbon chain molecules are expected to form
in the plasma, but the structure of the
products is not exactly known. If the produced
carbon molecules do not match DIB
absorption lines, no further research takes
place. If one of the lines of the obtained
spectrum is a possible match, further research
is done to obtain the identity of the absorber.
This can be done with MIS and computational
methods.
The setup
A beam of incoherent light sent by a 300 W
Xe-Arc lamp is directed at a cavity of two
mirrors. A Xe-arc is a lamp with a potential
different which causes an arc of current to
flow in the gas in the lamp, which is
constituted of xenon and other gasses; it is
designed to emit a continuum spectrum. Both
mirrors have a reflectivity of roughly 0,99995
(i. e. a fraction of 5*10-5 of the light passes
through the mirrors). This means that only a
small fraction of the light sent by the lamp
actually enters the cavity: most of it gets
reflected by the first mirror. The light that
passes the first mirror then gets trapped in the
cavity. Half of it will eventually pass through
the first mirror and half of it will pass through
the second mirror, behind which the detector
is situated. The path of the light is directed by
the telescope, the lenses and the apertures.
A pulsed slit discharge nozzle sends plasma in
the cavity at a rate of 4 Hz, with pulses lasting
roughly 1000 μs. Measurements are only done
when plasma is present in the cavity to
achieve a high S/N ratio. Therefore, an
optomechanical shutter is used. This
optomechanical shutter is activated just after
the plasma enters the cavity (using delays)
and it is designed to stay open for 500 μs,
shutting just before the plasma leaves the
cavity. It is open for a shorter time than the
plasma stays in the cavity, to make sure that
the measurements are only done when the
plasma is present.
Behind the cavity, a beam splitter is used to
split the beam of light; it reflects 10% of the
light to a photodiode, to measure whether the
optomechanical shutter is letting light through
the cavity or not (i.e.: is the measured
9
absorption of importance or not?). The other
90% of the light is let through to pass the
interference filter (or not, if the light does not
have a wavelength of interest). Then, the light
is guided by the lens and the fiber bundle to
the spectrograph and the CCD, where its
spectrum is taken.
Measuring the absorption
The absorbance of a sample is defined by the
following formula. I0 is the light’s intensity
without a sample present in the cavity; I is the
light’s intensity with sample present.
A = ln(I0 / I)
Which can be approximated by:
A = I0 / I - 1
Because the light in the cavity is trapped
between two mirrors, its path length is greatly
increased. Therefore, an (1 – R) term has to be
introduced in the equation to prevent the
absorbance from being overestimated by a
large margin. After correcting for this,
equation then becomes:
A = ((I0 / I )– 1)(1 – R)
In this instrument, the emission effects of the
plasma have to be considered. Also, there is
some ‘dark current’, a signal that is always
detected by the detector.
Four scenarios are discerned:
I:
‘Normal’ run, plasma is present and
the Xe/Arc lamp is on
I0:
Plasma is not present, lamp is on
BGplasma: Background measurement: plasma is
running, but the lamp is turned on
BG:
Background measurement with both
the plasma not present and the light turned
off
By subtracting BG (dark current) from I0, the
light’s intensity of the experiment with empty
cavity is obtained; by subtracting BGplasma (dark
current & emission effects) from I, the light’s
intensity in a cavity with plasma is obtained.
Thus, the equation for absorption becomes:
A = ((I0 – BG) / (I - BGplasma) – 1)(1 – R)
Matrix Isolation Spectroscopy: OASIS
OASIS is the name of the Matrix Identification
Spectroscopy (MIS) setup used in this
experiment. Solid phase DIB-candidates are
studied. In a low-pressure chamber, a matrix
and the molecules of interest are deposited
on a glass plate. The pressure in the chamber
is extraordinary low, around 10-10 bar. Also,
the temperature in the chamber is very low: it
can be varied for different purposes, but a
minimum of 12 K is attainable.
Various choices are possible for the matrix
compound: noble gases like neon and argon
can be used to minimize shifts in absorption
lines, which allows the most accurate
measurements for compounds compared to
their gas phase absorption spectrum. OASIS is
still not as accurate as direct gas-phase
measurements like IBBCEAS, but it has its
advantages: 1) it is easier to perform and 2)
large molecules, which are hard or impossible
(depending on their size) to put in the gas
phase, can be studied.
MIS can also be applied for researching solid
phase DIB-candidates, although these solid
phase DIB-carriers are not the most popular
DIB-candidates. Among the most likely
environment for solid phase DIB-carriers are
H2O and NH3, which are relatively abundant
molecules in the ISM.
H2O and NH3, being polar molecules, cause a
large shift of the spectra of matrix-solved
molecules.
The setup
In a cooled low pressure chamber, a glass
plate is situated at which the matrix
Figure 6: Destructive (upper graph) and constructive
interference (lower graph). Image was created with
Mathematica.
compound and the molecules of interest are
deposited. A Xe/Arc lamp directs a beam of
light with a continuum spectrum through the
glass plate into a light detector. This detector
uses a grating to disperse the light and a CCD
10
to measure the light’s intensity at different
wavelengths. To measure the thickness of the
matrix, a laser beam is directed towards it.
The intensity of this beam is measured; the
intensity varies because destructive or
constructive interference occur when the light
is reflected towards the laser detector. In
figure 6, it is shown that if the matrix surface
moves 0,25 λ to the left, the interference
becomes constructive rather than destructive.
So the time interval between two nodes is the
time it takes for the matrix to grow a distance
of 0,5 λ of the incident light beam (see
illustration). Combining the duration and the
speed of the matrix growth, the thickness of
the matrix can be calculated.
Experiment part II
In the second part of this chapter, it is
described how carbon chain anions and PAH
molecules were sought in the ISM by
comparing the NIR spectra of these molecules
with the spectrum of an O-star.
NIR-DIBs
A spectrum of the O 9.5 type star 4U 1907+09,
taken by VLT/X-shooter, was used to compare
near infrared (NIR) DIBs to absorption spectra
of various molecules. The spectrum of this star
was normalized in a previous bachelor project
by Suzanne van Hooff (2012). Eight of the
thirteen NIR-DIBs found by Geballe et al.
(2011) showed up in this spectrum. Also, the
two NIR-DIBs found by Joblin et al. (1990) at
1178 nm and 1317 nm were visible.
NIR DIB-spectra of carbon chain anions
By matrix identification spectroscopy,
absorption lines were measured of various
carbon chains, in a research by J. P. Maier
(1997). The chains in this research that are of
interest for this paper are negatively charged
bare carbon chains with 12, 14, 16, 18 or 20
carbon atoms (C12-, C14-, C16-, C18-, C20-); these
were the only molecules that had absorption
lines in the NIR range.
In this research, interactions with the matrix
compound causedshift of the absorption lines:
the maximal Δλ/λ was 10-2. These shifts could
be in the direction of either longer or shorter
wavelengths. It is assumed that Δλ/λ is
constant.
Thus, the shifts should not be larger than 20
nm for absorption lines at lower wavelength
than 2000 nm; a shift of Δλ≤20 nm was
assumed for the absorption lines at 1269 nm,
1460 nm and 1729 nm. For the absorption
lines at 2069 nm and 2440 nm, a shift of
Δλ≤30 nm was assumed.
NIR DIB-spectra of medium to large sized PAHs
By matrix identification spectroscopy, spectra
of cations and anions of 27 medium to large
sized PAH molecules have been taken in the
near infrared range (Mattioda et al. 2005).
Because the matrix in MIS can cause the
centers of absorption lines to shift, this
method is not perfectly accurate.
A maximum line shift of 30 nm was assumed;
lines further away from DIBs were considered
to be too far away to be a possible match. This
value was chosen because it is known that
matrices can cause a shift of Δλ/λ≈0,01 for
carbon chains (Maier, 1997), which would
correspond to 10-18 nm for the DIBs in the
NIR range; to be on the safe side, and because
the author noted that larger molecules tended
to show larger shifts, 30 nm was chosen for
medium to large sized PAHs.
Furthermore, only absorption lines with
absolute intensities larger than 10*103 km
mol-1 were taken into account; absorption
lines with intensities lower than this value are
relatively small absorption lines in the spectra,
which are not expected to cause DIBs. If they
would cause the DIBs, much larger absorption
lines should be visible from transitions with
higher oscillator strengths. Spectra without
absorption lines matching these two criteria
will not be discussed.
The 485 μm DIB
DIBs are typically located in the UV/VIS and
NIR, energy ranges that are associated with
electronic transitions. In parallel, also
vibrational and/or rotational energy levels can
be involved, and therefore DIBs are likely due
to unresolved rovibronic bands, i.e. transitions
involving composed electronic, vibrational and
rotational bands. The amount of energy
needed to excite vibrational and rotational
modes is substantially smaller than needed for
an electronic excitation.
11
A vibrational transition is a transition between
two vibrational states (indicated by v). A pure
vibrational transition involves a transition
between vibrational states within one
electronic state, a vibronic transition is a
transition between vibrational levels in two
different electronic states.
The goal of this part is to link the UV/VIS
spectra of DIBs to a new FIR feature around
485 m (Müller et al., not yet published) that
has been linked to the diffuse interstellar
medium and that is typical for a purely
vibrational mode.
This recently observed 485 μm DIB-analog
might occur from a purely vibrational
transition (in the electronic ground state or an
excited state) of a DIB-carrier. In that case,
there might be observable transitions in which
the electronic and the vibrational energy
levels change (see figure 7). In the sketch, the
three transitions are sketched that are most
likely to occur. The sketched transition are
from the electronic ground state and the first
excited state:
1) from v’’=0 to v’=1 (blue arrow in figure
5)
2) from v’’=0 to v’=0 (purple arrow)
3) from v’’=1 to v’=0 (red arrow)
(Note: figure 7 describes a simplified
situation, more energy levels exist in a more
realistic scenario).
The energy difference that we are looking for
between transition 1) and transition 2), and
between transition 2) and transition 3) is 𝐸 =
𝑐
ℎ 𝜆 = 0.00256 eV; an energy difference that
corresponds to a difference in
Figure 7: Transitions between different electronic (E=0,
E=1) and vibrational (v=1,2) energy levels.
wavenumbers of 20.599 cm-1 and the energy
of a 485 μm photon.
Thus, if two or more DIBs are spaced each
20.599 cm-1 apart, they might arise from the
same carrier that has mentioned electronic
and vibrational transitions.
The transition from v’’=0 to v’=0 (purple arrow
in figure 7) is expected to occur most often,
since most electrons are expected to be in the
ground state in molecules under interstellar
conditions (i. e. low temperatures). The
absorption line caused by this transition is
often referred to as the ‘origin band’. The
transition represented by the blue arrow from
v’’=0 to v’=1 , also from the ground state, is
expected to cause an absorption line of nearly
the same strength. In some cases, this line can
even be stronger than the origin band.
The v’’=1 to v’=0 transition (red arrow) would
generally have the lowest line intensity; this is
because it cannot occur unless an electron is
in the first excited vibriational state, which is
less populated than the ground state under
those conditions.
In figure 8, an impression of the expected
intensities of the three mentioned transitions
is displayed. The dashed parts of the lines
indicate that there is an uncertainty in the
magnitude of the intensities.
A downside of this method is that a molecule
might be distorted when it’s in the first
excited electronic state, causing the
vibrational energy levels to change a bit. Then,
the energy difference between v’’=0 and v’’=1
is not equal to the energy difference between
v’=0 and v’=1. The result is that that the ‘blue’
transition might have a different wavenumber
separation from the ‘purple’ transition than
the ‘red’ transition has.
The DIB catalogue of Hobbs et al., 2009, was
used to obtain the wavelengths of 488 DIB
Figure 8: Relative expected line intensities for
transitions at different wavelengths
12
lines. 71 of these lines were unconfirmed
DIBs. The wavelengths from this catalogue,
which are measured in air, were converted to
wavelengths in vacuum with a method
described by Morton in 1991 (see references).
Then, the wavelengths were converted to
reciprocal centimeters. An uncertainty of
±1,00 cm-1 was assumed.
Results
NIR DIB-spectra of carbon chain anions
C12-, C14- , C16-; 30 nm for C18-, C20-) of one of the
DIBs observable among the line of sight of 4U
1907+07, which are listed in table 2.
NIR DIB-spectra of medium to large sized PAHs
molecules
Ten molecules with absorption lines close to
NIR DIBs are listed below. In the figures, the
VLT/X-shooter spectrum of 4U 1907+09 is
shown; the location of the absorption lines of
the potential DIB-carriers is indicated with a
black line, the location of the DIBs is indicated
with a dashed line.
The structural formulae of these PAH
molecules can be found in appendix A.
Cations:
Molecule 1: Chrysene, C18H12+
Chrysene has two transitions in the NIR range,
one of them is 11,8 nm away from the 1179,8
DIB.
λ (in nm)
1001
1168
A (in 103 km mol-1)
37
120
Table 3: Wavelengths and absolute intensities of the
NIR absorption lines of chrysene.
Figure 9: The absorption spectra of five carbon chain
anions obtained by MIS. This image was taken from J.
P. Maiers paper (1997), see references.
Molecule
C12C14C16C18C20-
λ (in nm)
1249
1460
1729
2069
2440
Table 1: Centers of the main absorption lines of carbon
chain anions within the NIR-range
DIBS in 4U 1907+07 ( λ in nm)
1179,8
1317,5
1522,2; 1526,7; 1561,1; 1565,8; 1566,7
1656,8; 1658,0
1780,0
Table 2: Observed DIBs along the line of sight of 4U
1907+07.
Of the absorption lines of the carbon chain
anions (see figure 9 and table 1), none is
within the assumed error margin (20 nm for
Figure 10: The second NIR absorption line of chrysene
(solid line) and the 1180 nm DIB (dashed line).
If it is assumed that the absorption line of
chrysene at 1168 nm (λ0) would cause the
1180 nm DIB, it’s expected that its other
absorption line at 1001 nm (λ1) should have a
shift of Δλ1=(Δλ0/λ0)*λ1, towards a longer
wavelength as well (since Δλ/λ is supposed to
to be constant). Thus, an absorption line is
13
expected at 1011,3 nm. This absorption line
should have an absolute intensity with a value
around 0,31 times the absolute intensity of
the 1179,8 nm DIB, and its location should be
at the dotted line in figure 11.
Figure 9: The third NIR absorption line of molecule 2
(solid line) and the 1317 nm DIB (dashed line).
Figure 11: Location of the first NIR absorption line of
chrysene (solid line) and its expected location if
chrysene would be the carrier of the 1180 nm DIB
(dashed line).
The absorption line that would be expected if
chrysene were the carrier of the 1179,8 nm
DIB does not show in figure 11.
Molecule 2: 3,4;5,6;10,11;12,13-tetrabenzoperopyrene, C36H16+
Of the NIR absorption lines of molecule 2, the
1340 nm absorption line is the only one in the
vicinity of a NIR DIB; it is at 22,5 nm from the
1317,5 nm DIB.
λ (in nm)
1108
1210
1340
Figure 10: Location of the first NIR absorption line of
molecule 2 (solid line) and its expected location if
molecule 2 would be the carrier of the 1317 nm DIB
(dashed line).
A (in 103 km mol-1)
13
60
20
Table 4: Wavelengths and absolute intensities of the
NIR absorption lines of molecule 2.
Shifts of Δλ1=(Δλ0/λ0)*λ1 would result in
absorption lines at 1088,6 nm and 1188,9 nm.
The absorption line at 1088,6 nm should have
0,65 times the value of the absolute intensity
of the 1317,5 nm DIB; the absolute intensity of
the 1188,9 nm line should be 3,0 times larger
than the absolute intensity of the 1317,5 nm
DIB.
Figure 11: Location of the second NIR absorption line of
molecule 2 (solid line) and its expected location if
molecule 2 would be the carrier of the 1317 nm DIB
(dashed line).
14
In figure 13, no absorption line is observed at
1088,6 nm with a value of 0,65 times the
absolute intensity of the 1317,5 nm NIR DIB.
The large absorption line that would be
expected at 1188,9 nm does not appear either
in figure 14.
The absorption line at 1423,5 nm (dotted line
in figure 17) would have an intensity of 0.56
times the absolute intensity of the 1179,8 nm
DIB.
Molecule 3: dipyreno-(1’,3’;10,2),(1’’,3’’;5,7)pyrene, C40H18+
This molecules has only one absorption line in
the NIR range at 1301 nm, 16,5 nm away from
the 1317,5 nm DIB. It has an absolute intensity
of 140 . 103 km mol-1.
Comparing the 1317,5 nm DIB (see figure 12)
and the absorption line at 1301 nm from this
molecule from the research by Mattioda et al.
(2005), it can be seen that the 1317 nm DIB is
much less wide.
Molecule 4: difluoranthen-(3’,5’;4,6), (4’’,6’’;
9,11)-coronene, C48H20+
This molecule has, just like molecule 3, only
one significant absorption line in the NIR
range (with absolute intensity of 330 . 103 km
mol-1). This line is at 1291 nm, 26,5 nm away
from the 1317,5 nm DIB. It seems to be too
broad to match the 1317 nm DIB.
Figure 12: The second NIR absorption line of molecule 5
(solid line) and the 1180 nm DIB (dashed line)
Molecule 5: 12-13-o-phenylene-1,2;3,4;5,6;
7,8;9,10-pentabenzoperopyrene, C48H22+
In the spectrum of this molecule, the 1154 nm
absorption line is within the error margin
range of a DIB; it is 25,8 nm away from the
1179,8 nm DIB.
λ (in nm)
1056
1154
1393
A (in 103 km mol-1)
160
25
14
Table 5: Wavelengths and absolute intensities of the
absorption lines of molecule 5.
If the 1056 nm and 1393 nm absorption lines
are shifted with Δλ1=(Δλ0/λ0)*λ1, their centers
should be at 1079,1 nm and 1423,5 nm,
respectively.
The absorption line that would be expected at
1079,1 nm should have an absolute intensity
that is 6,4 times larger than the absolute
intensity of the absorption line that causes the
DIB at 1179,8 nm: a rather large line should be
seen near the dotted line in figure 16.
Figure 13: Location of the first NIR absorption line of
molecule 5 (solid line) and its expected location if
molecule 5 would be the carrier of the 1317 nm DIB
(dashed line).
An absorption line shows within 1 nm of the
dotted line in figure 16. However, it is
comparable in size to the 1179,8 nm DIB,
while it should be over six times larger if it
were caused by molecule 5.
The wavelength interval near the third
expected line of molecule 5 (figure 17)
contains too much noise to identify a line that
would match.
15
Figure 14: Location of the third NIR absorption line of
molecule 5 (solid line) and its expected location if
molecule 5 would be the carrier of the 1317 nm DIB
(dashed line).
Figure 15: The second NIR absorption line of molecule 7
(solid line) and the 1180 nm DIB (dashed line).
Anions
Molecule 6: 3,4;5,6;7,8-tribenzoperopyrene,
C34H16This molecule has, just like molecule 3 and 4,
only one significant absorption line in the NIR
range (with absolute intensity of 130 . 103 km
mol-1). This line is at 1160 nm, 19,8 nm away
from the 1179,8 nm DIB.
Molecule 7: 3,4;5,6;10,11;12,13-tetrabenzoperopyrene, C36H16In the spectrum of this molecule, the 1153 nm
absorption line is close to a DIB: 26,8 nm away
from the 1179,8 nm DIB.
λ (in nm)
1055
1153
A (in 103 km mol-1)
5,8
11
Table 6: Wavelengths and absolute intensities of the
absorption lines of molecule 6.
If the 1055 nm absorption line is shifted with
Δλ1=(Δλ0/λ0)*λ1, its center should be at 1079,1
nm. Also, that 1079,1 nm absorption line
should have 0,53 times the absolute intensity
of the 1179,8 nm DIB.
Figure 19: Location of the first NIR absorption line of
molecule 7 (solid line) and its expected location if
molecule 7 would be the carrier of the 1180 nm DIB
(dashed line).
In figure 19, a narrow, deep line shows within
1 nm left of the location of the expected
absorption line. This line is not smaller than
the 1179,8 nm DIB, while its intensity should
be nearly two times smaller. To the right,
there is also a broad, shallow line. This line’s
intensity is also larger than it should be if it
were a match. It can be concluded that there
is no matching line for the first NIR absorption
line of molecule 7.
16
Molecule 8: 3,4;4,5;10,11;12,13-tetrabenzoperopyrene, C36H16λ (in nm)
1353
1519
A (in 103 km mol-1)
16
21
Table 7: Wavelengths and absolute intensities of the
absorption lines of molecule 8.
In the spectrum of this molecule, the 1519 nm
absorption line is close to two DIBs; 3,2 nm
away from the 1522,2 nm DIB and 7,8 nm
away from the 1526,8 nm DIB.
If the 1353 nm absorption line is shifted with
Δλ1=(Δλ0/λ0)*λ1, its center should be at 1355,9
nm if it corresponds to the first DIB, or 1360,0
nm if it corresponds to the second DIB. The
absolute intensity of the line that would be
expected is a factor 0,76 of the DIB it would
match with.
The spectrum of 4U 1907+07 contains a lot of
noise in the wavelength region of the 1353 nm
absorption line (figure 21).
At 1355,9 nm, an absorption line is visible.
However, this line is narrower and deeper
than the 1522,2 nm DIB. The total intensity of
the absorption line is larger than the total
intensity of the 1522,2 DIB, while it is
expected to have a value of 0,76 times the
value of the 1522,2 nm DIB’s absolute
intensity. Because of the large amount of
absorption lines in this region, the chance of
coincidentally having an absorption line there
is considerable.
At 1360,0 nm, no absorption line with 0,76
times the absolute intensity of the 1526,8 nm
DIB is observed.
Molecule 9: difluoranthen-(3’,5’;4,6), (4’’,6’’;
9,11)-coronene, C48H20λ (in nm)
1313
1468
1797
A (in 103 km mol-1)
160
160
46
Table 8: Wavelengths and absolute intensities of the
absorption lines of molecule 9.
Figure 20: The second NIR absorption line of molecule 8
(solid line) and the 1522 nm and 1526 nm DIBs (dashed
lines).
In the spectrum of this molecule, two
absorption lines are close to a DIB: the 1313
nm absorption line is 4,5 nm away from the
1317,5 nm DIB, and the 1797 nm absorption
line is 16,9 nm away from the 1780,1 nm DIB.
If it is assumed that the 1313 nm absorption
line is causing the DIB, an absorption line with
identical absolute intensity as the 1317,5 nm
DIB would be expected at 1473,0 nm if a shift
of Δλ1=(Δλ0/λ0)*λ1 is assumed. An absorption
line with an absolute intensity a factor 0,29
times of that of the 1317,5 nm DIB would be
expected at 1803,1 nm.
The absorption line that would be expected
does not show in figure 23. It should be noted
that no data are available in the nearby
interval between 1473,7 nm and 1474,9 nm.
In figure 24, an absorption line is visible
around 1.5 nm short of the expected line’s
location. This interval is crowded with
absorption lines, which
Figure 21: Location of the first NIR absorption line of
molecule 8 (solid line) and its expected locations line 1
and line 2 if molecule 8 would be the carrier of the 1522
nm or 1526 nm DIB, respectively (dashed line).
17
Secondly, the case if the 1797 nm absorption
line were the DIB–carrier is examined. If
Δλ1=(Δλ0/λ0)*λ1 is assumed, absorption lines
would be expected at 1300,5 nm and 1454,1
nm. Both absorption lines should have an
absolute intensity a factor 3,5 times larger
than the 1780,1 nm DIB.
The expected line at 1300,5 nm does not show
in figure 26. There is an absorption line that
would have roughly the same size at λ=1297
nm, however.
Figure 22: The first NIR absorption line of molecule 9
(solid line) and the 1317 nm DIBs (dashed lines).
Figure 25: he third NIR absorption line of molecule 9
(solid line) and the 1780 nm DIB (dashed lines).
Figure 23: Location of the second NIR absorption line of
molecule 9 (solid line) and its expected location if
molecule 9 would be the carrier of the 1317 nm DIB
(dashed line).
Figure 26: Location of the first NIR absorption line of
molecule 9 (solid line) and its expected location if
molecule 9 would be the carrier of the 1780 nm DIB
(dashed line).
Figure 24: Location of the third NIR absorption line of
molecule 9 (solid line) and its expected location if
molecule 9 would be the carrier of the 1317 nm DIB
(dashed line).
18
If the 1120 nm absorption line is shifted with
Δλ1=(Δλ0/λ0)*λ1, its center should be at 1106,8
nm. Also, that 1106,8 nm absorption line
should have 0,55 times the absolute intensity
of the 1179,8 nm DIB.
In figure 29, no absorption line is found with
the intensity that would be expected if
molecule 9 were the carrier of the 1317,5 nm
DIB.
Figure 27: Location of the second NIR absorption line of
molecule 9 (solid line) and its expected location if
molecule 9 would be the carrier of the 1780 nm DIB
(dashed line).
In figure 27, no absorption line is found with
the intensity that would be expected if
molecule 9 were the carrier of the 1780.1 nm
DIB.
Molecule 10: 1,14-benzodiphenanthreno(1’’,9’’;2,4),(9’’’,1’’’;11,13)-bisanthene, C50H22λ (in nm)
1120
1333
3
-1
A (in 10 km mol )
40
73
Table 9: Wavelengths and absolute intensities of the
absorption lines of molecule 10.
Figure 28: The first NIR absorption line of molecule 10
(solid line) and the 1317 nm DIB (dashed lines).
In the spectrum of this molecule, the 1333 nm
absorption line is close to a DIB: 15,5 nm away
from the 1317,5 nm DIB.
Figure 29: Location of the first NIR absorption line of
molecule 10 (solid line) and its expected location if
molecule 10 would be the carrier of the 1317 nm DIB
(dashed line).
The 485 μm DIB
Assuming an error margin of Δν=1,00 cm-1, 77
pairs were found that are separated between
19.599 cm-1 and 21.599 cm-1 apart (see tables
11, 12 and 13 in appendix B). Fourteen of
these pairs included an unconfirmed DIB. The
DIB-pairs which had separations closest to
20,599 cm-1 (Δν=0,25 cm-1) are listed in table
11. Nineteen of such pairs were found, with
four including an unconfirmed DIB.
In Table 10, subsequent DIB-pairs are listed.
Eight cases were found where three
subsequent DIBs were all spaced 20,6 ± 1,0
cm-1 apart. In four cases, four DIBs were
subsequently spaced with a separation of 20,6
± 1,0 cm-1.
In the DIB-rich region between DIB-numbers
300 (670,8 nm) and 350 (683,6 nm), 38 of 51
DIBs were part of a of 20,6 ± 1,0 cm-1 DIB pair.
In this interval, five of the twelve subsequent
DIB-pairs are found.
19
Discussion
NIR spectra: carbon chain anions
None of the absorption lines of the main
transition of any carbon chain anion is within
the error margin of a DIB. Therefore, these
molecules are unlikely to cause any of the NIR
DIBs in 4U 1907+07.
NIR spectra: PAHs
Molecules 1, 2 and 5 had one or more other
absorption lines in the NIR, apart from the one
near a DIB. It was assumed that the other
absorption lines shift with Δλ1=(Δλ0/λ0)*λ1.
The spectrum of 4U 1907+07 showed no
absorption lines that would correspond to
these predicted absorption lines. Thus, if our
assumption that the other absorption lines
shift with Δλ1=(Δλ0/λ0)*λ1 is correct, it can be
concluded that none of these molecules is
causing the NIR-DIBs.
Molecules 3 and 4 have only one absorption
line in the NIR range. Therefore, it is hard to
draw any conclusion about whether or not
this molecule is a DIB-carrier; the fact that a
line occurs within 30 nm of a DIB might very
well be coincidence. The fact that the
absorption lines are much broader in the DIBs
than in the spectrum of 4U 1907+07 implies
that these molecules do not cause the NIR
DIBs, but it has to be taken into account that
broadening is caused by matrix interactions
and far higher column densities in MIS. The
absorption lines are with a Δλ of 16,5 nm for
molecule 3 and a Δλ of 26,5 nm for molecule 4
quite far from the DIBs; these separations do
no rule them out as DIB-carriers since this
separation is within the error margin, but it
does make them less likely (especially
molecule 4).
For molecule 6, the same goes as for
molecules 3 and 4. A large Δλ of 20 nm makes
it less likely for molecule be a DIB carrier
The spectrum of 4U 1907+07 has an
interesting looking absorption line close the
wavelength of the expected absorption line
from molecule 7, but a closer look at this line
showed that it cannot be from molecule 7.
Molecule 8 could not be examined due to the
noise in the part of the spectrum that was of
interest. No absorption lines were found in
the spectrum of 4U 1907+07 that were likely
to come from absorption lines of molecules 9
and 10 that were not close to a DIB.
Of the ten medium to large sized PAHs
discussed above, not a single one showed any
evidence that it is likely to cause a NIR DIB.
The same goes for the other 17 molecules
from the paper of Mattioda et al. (2005) that
had no absorption lines in the vicinity of DIBs.
For the three molecules that had only one
absorption line in the NIR range, it is hard to
prove they don’t cause the DIBs, because it is
hard to draw any conclusions from a single
absorption line if there are as many
uncertainties as in this research (e.g. in
absorption line width, and location of the
absorption line center).
If we do not take these molecules with only
one NIR absorption line into consideration, it
can be concluded that every medium to large
sized PAH from Mattioda et al.’s research has
an absorption spectrum that does not match
with the NIR DIBs. Therefore, it is highly
unlikely that these medium to large sized PAH
are the carriers of the NIR DIBs.
The next question to answer is: ‘do these
medium/large PAHs not cause the NIR DIBs, or
can we say that no medium or large PAHs
cause the NIR DIBs? It is of course hard to
prove that the PAHs that are not investigated
do not cause the NIR DIBs, but the PAHs that
were investigated have quite ‘basic’ structures
(see appendix A for the structures with
absorption lines near DIBs, and Mattioda’s
paper for all structural formulae). Of course, it
cannot be proven in this research that in the
ISM, no slightly different PAH structures are
formed that do cause the NIR DIBs.
As a general conclusion: it cannot be proven
by the method used in this research that
medium to large sized PAHs do not cause the
NIR DIBs, but based on the results, it seems
unlikely that they do.
The 485 μm DIB
What do we expect to see? As explained in the
chapter ‘Experiment’, we are looking for pairs
of transitions separated 20,6 cm-1.
It is interesting to look at sequences of DIBpairs spaced 20,6 ± 1,0 cm-1 away rather than
individual pairs, because a lot of pairs can be
expected to be spaced that amount of
wavenumbers apart by coincidence; if three or
20
Subsequent pairs
DIBs
Δν
1st line
EW
FWHM
2nd line
EW
FWHM
Ratio
in
𝑙𝑖𝑛𝑒 1
𝑙𝑖𝑛𝑒 2
𝐸𝑄𝑊
𝐸𝑄𝑊
Ratio
in
𝑙𝑖𝑛𝑒 1
𝑙𝑖𝑛𝑒 2
𝐹𝑊𝐻𝑀
𝐹𝑊𝐻𝑀
189 & 192
192 & 194
cm-1 mÅ
Å
20,2
151,3 2,47
20,7
7,8 0,64
241 & 243
243 & 247
247 & 249
19,8
19,8
19,8
2
10,1
10,1
0,45
0,84
0,92
10,1
10,1
3
0,84
0,92
0,67
0,20
1,00
3,37
0,54
0,91
1,37
268 & 271
271 & 275
21,0
20,0
341,6
17,1
1,08
0,71
17,1
4,6
0,71
0,48
19,98
3,72
1,52
1,48
282 & 286
286 & 290
19,9
19,8
3,9
59,7
0,69
0,67
59,7
9,2
0,67
1,17
0,07
6,49
1,03
0,57
301 & 303
303 & 306
306 & 312
19,6
20,6
21,4
18,7
5,3
7,6
0,96
0,62
0,71
5,3
7,6
10,9
0,62
0,71
0,81
3,53
0,70
0,70
1,55
0,87
0,88
301 & 304
304 & 307
21,5
20,6
18,7
7,7
0,96
0,71
7,7
7,7
0,71
0,7
2,43
1,00
1,35
1,01
305 & 310
310 & 314
314 & 317
20,4
20,3
21,5
2,6
10,7
11,3
0,67
1,24
0,89
10,7
4,3
13,4
1,24
0,89
1,13
0,24
2,49
0,84
0,54
1,39
0,79
323 & 328
328 & 332
332 & 337
21,1
19,9
20,8
5,1
7,9
6,2
0,71
1,2
0,86
7,9
6,2
7,1
1,2
0,86
1,19
0,65
1,27
0,87
0,59
1,40
0,72
326 & 331
331 & 335
19,8
19,8
4,3
2
0,74
0,54
2
16,2
0,54
0,8
2,15
0,12
1,37
0,68
448 & 450
450 & 451
21,5
20,0
7
9,5
0,98
1,12
9,5
19,7
1,12
2,58
0,74
0,48
0,88
0,43
453 & 458
458 & 462
19,9
20,2
21,3
41,3
1,86
5,61
41,3
52,7
5,61
1,5
0,52
0,78
0,33
3,74
464 & 467
467 & 468
20,5
20,5
17,8
11,1
0,86
0,72
11,1
5,9
0,72
0,68
1,60
1,88
1,19
1,06
mÅ
Å
7,8
8,1
0,64
0,88
19,40
0,96
3,86
0,73
Table 10: All subsequent DIB-pairs with a separation in wavenumbers of 19,6 < Δν < 21,6. The DIBs are from Hobbs’ DIBcatalogue (Hobbs et al., 2009).
21
more DIBs have wavenumber separations of
20,6 ± 1,0 cm-1, this is much less likely to be
coincidental.
The transitions we are looking for are
assumed to be from the electronic ground
state to the first excited electronic state, of
which at least one transition should also have
a change in the vibrational energy level (see
figure 7).
In
section ‘Experiment part II’, it was
explained that the most transitions with the
highest intensity (and thus, highest EW) are
expected to be the ones with the shortest
wavelength and the lowest DIB number (see
figure 8). This is because the ground state is
expected to have the highest occupation
under interstellar conditions. Also, we expect
the absorption lines to have similar shapes
and therefore, the FWHMs of the absorption
lines should be comparable.
In short, for the transitions we are looking for,
we expect four features to be observed in
these subsequent DIB-pairs:
1) the EWs of the two absorption lines
with the shortest wavelengths are the
largest, the EW of the smallest energy
transition (and thus, the longest
wavelength) is the smallest
2) comparable FWHMs
3) equal separations in wavenumbers
between absorption lines
4) a value in wavenumber separations
close to 20,6 cm-1
Due to a very high number of DIBs in the
relatively small interval between 670,8 nm
and 683,6 nm (DIBs 300-350), a lot of
coincidental matches can be expected there.
When the five subsequent DIB-pairs in this
interval were examined, they all seemed to
show random values of wavenumbers
between lines, line EWs, and line FWHMs.
Therefore, they were judged to be most likely
coincidental. These are pairs (301 & 303 + 303
& 306 + 306 & 312), (301 & 304 + 304 & 307),
(305 & 310 + 310 & 314 + 314 & 317), (323 &
328 + 328 & 332 + 332 & 337) and (326 & 331
+ 331 & 335). The other subsequent DIBs are
discussed below.
189 & 192 + 192 & 194
These pairs are not very interesting because
the first absorption line has a very high EW,
while this line is not expected to be (much)
larger than the origin band. Furthermore, the
FWHMs are not very similar and the third line
does not have a smaller EW than the second,
contrary to what would be expected. The
wavenumber separations between the lines
are not similar. All in all, no convincing
evidence is found that these lines originate
from different vibrational states.
241 & 243 + 243 & 247 + 247 & 249
The similarity in separations in wavenumbers
between these lines is striking: all three have a
value of 19,8 cm-1. However, the EWs did not
show a pattern of EW decreasing with
wavelength: the DIB with the lowest
wavelength is very small (EW=2mÅ), and the
second and third DIB have the same, larger
EW. Therefore, we cannot assume that these
subsequent DIB-pairs originate from the
transitions of different vibrational states that
we are looking for.
268 & 271 + 271 & 275
The separations in wavenumbers from these
DIBs are, with 21,0 cm-1 and 20,0 cm-1, not
very similar. Also, the EW of the first line is
almost 20 times larger than the EW of the
origin band, which is not what we expect to
see if these DIBs are absorption lines from the
transitions we are looking for.
282 & 286 + 286 & 290
These subsequent DIB-pairs can be discarded,
since the EWs and FWHMs of the absorption
lines show do not match the criteria at all
(although the wavenumber separations are
similar).
448 & 450 450 & 451
These DIB-pairs do not match any of the three
criteria that are set in this research for
subsequent DIB-pairs; there is no reason to
assume these pairs occur from different
vibrational states of the same carrier.
453 & 458 + 458 & 462
The ratio in EW between the first transition:
origin band is 0,52, which is a reasonable
22
value. However, the EW of the last transition
is too large. Also, the FWHMs are not similar,
so it can be concluded that there is not
enough reason to assume that these pairs
occur from different vibrational states of the
same carrier (although the wavenumber
separations match reasonably).
464 & 467 +467 & 468
These two DIB-pairs are very interesting since
they have equal wavenumber separations and
comparable FWHM values. The only downside
of these pairs is that the first transition has a
larger EW than the origin band (ratio=1,60),
while the origin band is expected to be the
strongest transition. This does not rule out
these pairs as being transitions with different
vibrational energy levels, since the origin band
is not always the strongest transition.
It should be noted that DIB 468 is an
unconfirmed DIB.
Of the twelve subsequent DIB-pairs found,
most are most likely coincidental; they
showed no (or not enough) consistency in the
values of their EWs and FWHMs, or in their
separation in wavenumbers. Although
multiple
subsequent
DIB-pairs
look
interesting, the only ones that match all the
criteria that were set are the 464-467-468
DIBs; the last transition had the smaller EW
while the first two were comparable in
magnitude, they had equal wavenumber
separations, and a value of the wavenumber
separation very close to the value 20,6 cm-1
(being 20,5 cm-1). This are the DIBs at λair =
769,6 nm, 770,8 nm and 772,0 nm. Therefore,
the 464-467-468 DIBs are the most likely DIBs
to be associated with the energy of the 485
μm DIB, if the assumption that the 485 μm
DIB occurs from a vibrational transition is
correct.
Of the other subsequent DIB-pairs, the 241243-247-249 DIBs stand out the most because
of their similar separations in wavenumbers of
three DIB-pairs. However, we are not able to
explain the pattern in absorption line EWs,
which are highest for the two lines in the
middle. Also, with their separation of 19,8 cm1
, they are only just within the error margin.
Therefore, these subsequent DIB-pairs do not
seem to be the most interesting topic for
future research in this area.
Conclusions
- The absorption lines of carbon chain anions
C12-, C14-, C16-, C18-, C20- do not match with any
of the NIR DIBs. Anion chains with a different
number of carbon atoms had no NIR
absorption lines. It is concluded that no
relation is found between carbon chain anions
and NIR DIBs.
- Of the 27 medium to large sized PAH spectra
taken by Mattioda et al., 24 did not match
with the NIR DIBs observed in 4U 1907+07.
The other three PAH spectra had only one
absorption line in the NIR range, so it was hard
to draw any conclusions for these PAHs; the
assumed error margin Δλ/λ was relatively
large. In conclusion, there is no reason to
assume that these medium to large sized PAHs
are the carriers of the NIR DIBs, but the
method used in this research is not accurate
enough to completely rule them out.
- Of the sets of subsequent DIB-pairs, there
was one set of three DIBs that matched all the
criteria that were set for absorption lines
associated with transitions from different
vibrational energy levels. This were the DIBs
with indicated in this research with number
464, 467 and 468, with λair = 769,6 nm, 770,8
nm and 772,0 nm. Some other subsequent
pairs also showed interesting features, but did
not match the criteria. Therefore, they were
considered to be most likely coincidental.
Future research
NIR spectra
The results of this research indicated no
relation between the NIR DIBs and both
carbon chain anions or medium to large sized
PAHs. Therefore, I do not recommend further
research projects to identify NIR DIBs to focus
on these classes of molecules.
The 485 μm DIB
It was found in this research that the λair =
769,6 nm, 770,8 nm and 772,0 nm DIBs are
the most likely DIBs to be associated with
different vibrational energy states, with
energy difference corresponding to the energy
23
of a 485 μm photon. The first step to test our
hypothesis, is to investigate if these DIBs are
correlated. Correlated means, in this context,
that the DIBs have the same ratios of EWs
along other lines of sight. If that’s not the
case, the DIBs are very unlikely to have the
same carrier. Also, because the 485 μm DIB is
thought to occur due to a vibrational
transition of the same carrier as the 464-467468 DIBs, the EW of the 485 μm DIB should
correlate with the EW of these DIBs.
If research would prove that the DIBs are
correlated, the next step would be to search
for the identity of the carrier.
Acknowledgements
I would like to thank Lex Kaper for helping me
find a project where I could combine
chemistry and astrophysics, and being helpful
and thinking with me while I was working on it
at the Anton Pannekoek Instituut. I would like
to thank everyone at the API for having me
here for three months. I would like to think
Harold Linnartz and various other people at
the Sterrewacht Leiden for their hospitality
and for how they have helped me with my
project.
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Appendix A
Figure 30: Structures of the ten PAH molecules with absorption lines near NIR DIBs. Molecules 1-5 are cations, molecules
6-10 are anions. Molecule 4 and 9 are actually the same molecule, but with a different charge. Image was created with
ChemDraw.
26
Appendix B
DIBs
1st line
Λ
Ν
EW
cm-1
Å
cm-1
mÅ
20,7
20,7
20,4
20,7
20,7
20,7
20,5
20,7
20,6
20,6
20,4
20,5
20,8
20,4
20,5
20,7
20,7
20,5
20,5
4679,81
5537,36
5813,52
6129,9
6214,72
6318,45
6536,35
6623,54
6720,23
6721,11
6726
6757,85
6788,42
6823,57
6825,48
7356,97
7560,52
7698,2
7710,37
21368,4
18059,2
17201,3
16313,5
16090,8
15826,7
15299,1
15097,7
14880,4
14878,9
14867,7
14797,6
14731
14655,1
14651
13592,6
13226,6
12990,1
12969,5
4,8
139
16,8
11,3
7,8
45,7
329
5,5
5,3
7,7
2,6
7,9
6,2
13,8
6,5
14,5
21,3
17,8
11,1
Possible Δν
DIB?
20,35 ˂ Δν ˂ 20,85
11 & 12
both
64 & 65
102 & 104
169 & 171
192 & 194
210 & 211
256 & 258
270 & 274
303 & 306
304 & 307
305 & 310 305
318 & 322
332 & 337
345 & 349
346 & 350 350
426 & 431
453 & 459
464 & 467
467 & 468 468
FWHM
2nd line
λ
ν
EW
FWHM
Å
Å
cm-1
mÅ
Å
4684,35
5543,71
5820,42
6137,68
6222,74
6326,73
6545,1
6632,65
6729,54
6730,41
6735,24
6767,25
6798,01
6833,1
6835,03
7368,19
7572,37
7710,37
7722,57
21347,7
18038,5
17180,9
16292,8
16070,1
15806
15278,6
15076,9
14859,9
14857,9
14847,3
14777,1
14710,2
14634,7
14630,5
13571,9
13205,9
12969,5
12949,1
3,8
14,1
7,7
6,1
8,1
19,8
13,5
5,3
7,6
7,7
10,7
5,2
7,1
1,8
2,2
25,3
5,8
11,1
5,9
0,74
3,91
1,06
2,18
0,64
2,23
12,7
0,67
0,62
0,71
0,67
1,06
0,86
1,32
0,56
0,96
1,86
0,86
0,72
0,54
1,26
0,65
1,41
0,88
0,84
0,83
0,56
0,71
0,7
1,24
1,13
1,19
0,6
0,83
0,93
0,75
0,72
0,68
Table 11: DIB-pairs with a separation in wavenumbers close to the wavenumber of the 485 μm DIB (20,35 ˂ Δν ˂ 20,85).
27
DIBs
1st line
Λ
Possible Δν
DIB?
19,6 ˂ Δν ˂ 20,35
74 & 75
107 & 110
122 & 127 127
131 & 132 131
141 & 143
161 & 163
166 & 168
189 &192
197 & 198
204 & 205
212 & 213
229 & 232
241 & 243
243 & 247
247 & 249 249
271 & 275
282 & 286 282
286 & 290
301 & 303
310 & 314
316 & 319
326 & 331 Both
328 & 332 328
331 & 335 331
336 & 340
339 & 343
344 & 348
394 & 395
425 & 429
450 & 451
453 & 458
457 & 461
458 & 462
cm
-1
19,7
20,1
19,8
19,8
20,2
20,2
20,3
20,2
20,1
20,2
19,9
20,3
19,8
19,8
19,8
20
19,9
19,8
19,6
20,3
20,3
19,8
19,9
19,8
20,3
20,3
19,6
20,1
19,9
20
19,9
19,8
20,2
ν
EW
-1
Å
cm
5638,94
5839,71
5948,96
5977,4
6022,04
6104,12
6118,53
6206,92
6228,01
6271,66
6331,8
6412,12
6454
6462,28
6470,54
6624,75
6653,74
6662,57
6711,39
6735,24
6749,77
6776,22
6779,25
6785,31
6797,17
6805,25
6820,19
7108,27
7351,88
7534,8
7560,52
7570,3
7571,91
17733,8
17124,1
16809,7
16729,7
16605,7
16382,4
16343,8
16111,1
16056,5
15944,7
15793,3
15595,5
15494,3
15474,4
15454,7
15094,9
15029,2
15009,2
14900
14847,3
14815,3
14757,5
14750,9
14737,7
14712
14694,5
14662,4
14068,1
13602
13271,8
13226,6
13209,5
13206,7
FWHM
2nd line
Λ
ν
-1
mÅ
Å
Å
cm
4,5
1,9
14,6
3,4
11,9
3,5
13,8
151
5,2
256
17,9
11,1
2
10,1
10,1
17,1
3,9
59,7
18,7
10,7
12,3
4,3
7,9
2
10,6
9,1
9,5
63,4
16,5
9,5
21,3
11,7
41,3
1,23
0,46
0,78
0,4
0,76
0,64
0,91
2,47
0,66
1,32
0,73
0,98
0,45
0,84
0,92
0,71
0,69
0,67
0,96
1,24
1,19
0,74
1,2
0,54
0,62
0,6
1,49
3,99
0,81
1,12
1,86
0,87
5,61
5645,22
5846,58
5955,98
5984,47
6029,37
6111,64
6126,13
6214,72
6235,83
6279,09
6339,77
6420,48
6462,28
6470,54
6478,84
6633,54
6662,57
6671,35
6720,23
6744,44
6759,05
6785,31
6788,42
6794,42
6806,56
6814,64
6829,32
7118,44
7362,65
7546,19
7571,91
7581,68
7583,5
11714,1
17104
16789,9
16709,9
16585,5
16362,2
16323,5
16090,8
16036,4
15924,6
15773,4
15575,2
15474,4
15454,7
15434,9
15074,9
15009,2
14989,5
14880,4
14827
14795
14737,7
14731
14718
14691,7
14674,3
14642,7
14048
13582,1
13251,7
13206,7
13189,7
13186,5
EW
FWHM
mÅ
Å
2
11,6
5,9
12,1
57,8
7,5
4,2
7,8
18,9
27,6
2,8
5,8
10,1
10,1
3
4,6
59,7
9,2
5,3
4,3
6,5
2
6,2
16,2
6,8
25,1
29,7
8,3
12,2
19,7
41,3
24,6
52,7
0,65
0,63
0,6
0,85
2,08
0,72
1,08
0,64
0,65
1,35
0,5
0,67
0,84
0,92
0,67
0,48
0,67
1,17
0,62
0,89
0,9
0,54
0,86
0,8
1,38
1,41
1,04
0,71
0,68
2,58
5,61
1,14
1,5
Table 12: DIB-pairs with a separation in wavenumbers smaller than the wavenumber of the 485 μm DIB but still within
the error margin: 19,6 ˂ Δν ˂ 20,35.
28
DIBs
1st line
λ
Possible Δν
DIB?
20,85 ˂ Δν ˂ 21,6
16 & 17
16
25 & 26
54 & 56
134 & 135
147 & 149
148 & 151
154 & 155 155
164 & 167
193 & 195
199 & 200
245 & 248
268 & 271
283 & 288
297 & 300
301 & 304
306 & 312
314 & 317
323 & 328 328
329 & 333
338 & 342
353 & 359
361 & 363
402 & 406
413 & 415
448 & 450
cm
-1
20,9
20,4
21,4
21,5
21,3
21
21,4
21,3
21,3
21,1
20,9
21
21,2
21,3
21,5
21,4
21,5
21,1
21,3
21,1
21,4
21,4
21,3
21
21,5
ν
EW
-1
Å
cm
4759,22
4965,37
5489,22
5989,88
6035,26
6059,29
6072,9
6112,48
6217,15
6238,59
6467,29
6615,53
6656,47
6698,91
6711,39
6729,54
6744,44
6769,56
6780,92
6803,42
6839,64
6854,43
7154,22
7259,44
7722,6
21011,9
20139,5
18217,5
16694,8
16520
16503,6
16466,6
16360
16084,5
16029,3
15462,4
15116
15023
14927,8
14900
14859,9
14827
14772
14747,3
14698,5
14620,7
14589,1
13977,8
13775,2
13293,3
FWHM
2nd line
Λ
ν
-1
mÅ
Å
Å
cm
176
26,4
236
10,9
5,4
5,1
8,8
2,6
10,4
10,1
5,1
342
11,5
5,9
18,7
7,6
11,3
5,1
5,2
17,4
8,4
15,7
15
22,5
7
14,5
0,72
3,38
0,87
0,98
0,68
1,16
0,57
1,62
0,7
0,64
1,08
1,12
0,73
0,96
0,71
0,89
0,71
0,64
0,8
0,72
0,8
2,26
1,36
0,98
4763,95
4970,66
5495,69
5997,59
6061,09
6066,09
6080,79
6120,43
6225,38
6246,82
6476,05
6624,75
6656,89
6708,46
6721,11
6739,26
6754,22
6779,25
6790,71
6813,19
6849,67
6864,5
7165,13
7270,53
7534,8
20991
20118,1
18196,1
16673,4
16498,7
16482,6
16445,2
16338,7
16063,3
16008,2
15441,5
15094,9
15001,8
14906,6
14878,5
14838,4
14805,6
14750,9
14726
14677,4
14599,3
14567,7
13956,5
13754,2
13271,8
EW
FWHM
mÅ
Å
127
7,5
31,2
9,3
10,6
13,9
14,9
4,4
10,3
18,8
12,4
17,1
7,6
6,5
7,7
10,9
13,4
7,9
14,3
27,6
6,5
11,6
15,9
19,2
9,5
2,5
0,89
0,69
0,83
0,91
0,6
3,51
0,65
0,51
1,36
1,25
0,71
1,07
0,96
0,71
0,81
1,13
1,2
1,02
0,93
0,76
0,65
0,84
1,29
1,12
Table 13: DIB-pairs with a separation in wavenumbers larger than the wavenumber of the 485 μm DIB, but still within the
error margin: 20,85 ˂ Δν ˂ 21,6.
29