Diffuse bands, extinction and anomalous
microwave emission in the Interstellar
medium:the role of fullerenes.
Susana Iglesias-Groth
Instituto de Astrofı́sica de Canarias sigroth@iac.es
Summary. Photoabsorption by fullerenes and buckyonions (multishell fullerenes)
can explain the shape, width and peak energy of the most prominent feature of the
interstellar absorption, the UV bump at 2175 Å. The predicted optical and nearinfrared transitions for these molecules also offer a potential explanation for the
long-standing problem of the identity of the diffuse interstellar bands. Comparing
theoretical cross sections and astronomical data, we estimate a density of fullerenes
in the diffuse interstellar medium of 0.1-0.2 parts per million, consistent with the
findings in meteorites. Fullerene-based molecules could be a major carbon reservoir
in the interstellar medium. We also study the rotation rates and electric dipole
emission of hydrogenated icosahedral fullerenes (fulleranes) in various phases of the
interstellar medium. Using the formalism of Draine & Lazarian for the rotational
dynamics of these molecules in various astrophysical environments, we find effective
rotation rates in the range 5-70 GHz with a trend toward lower rotational frequency
as the radius of the molecule increases. Owing to the moderately polar nature of the
CH bond, fulleranes are expected to have a net dipole moment and produce electric
dipole radiation. Adopting the same size distribution proposed for fullerenes in the
study of the UV extinction bump, we predict the dipole electric emission of mixtures
of fulleranes for various levels of hydrogenation. We find that these molecules could
be the carriers of the anomalous microwave emission recently detected by Watson
et al. in the Perseus molecular complex and Cassasus et al in the dark cloud LDN
1622.
1 Introduction
In 1985 it was proposed the existence of a new allotropic form of carbon: the
fullerenes [1]. The research conducted on samples of vaporized graphite using
laser beams, initially conceived to reproduce the chemistry of the atmospheres
of carbon enriched giant stars gave as a result the unexpected discovery of
the C60 (the 60 carbon atoms fullerene). The first experiments by Kroto and
Smalley, and others, showed the existence of carbon aggregates with a larger
number of atoms: C70 , C84 , C240 and established that these aggregates are
also stable albeit less than those formed by 60 and 70 atoms.
2
Susana Iglesias-Groth
The lack of spectroscopic data and the poor knowledge of the photoabsorption spectrum of these complex molecules have prevented us from making
any firm conclusion about their existence in the interstellar medium. The detection of fullerenes C60 -C400 in meteoritic material [2, 3] and the tentative
identification of some diffuse interstellar bands (DIBs) as potentially produced by fullerene ions [4] has renewed interest on this possibility. Fullerenes
can adopt multilayered configurations in which one is encapsulated inside another, like layers of an onion. Diverse laboratory experiments proved that
these molecules, commonly known as buckyonions, can be formed with tens
of layers. The buckyonions have also been synthesized, exposing carbon dust
to thermal treatments [5]. Laboratory experiments have shown that fullerenes
and buckyonions are highly stable molecules that can survive the harsh conditions of interstellar space more easily than other carbon aggregates. Processes
leading to the formation of fullerenes and buckyonions in the laboratory [6]
may also take place in the circumstellar envelopes of evolved stars and other
astrophysical environments.
Very little is known about the electronic structure of fullerenes and buckyonions. Ab initio computations of the electronic structure are available only
for fullerenes with small number of atoms and shells. Given the complexity of
these models, other approximations have to be followed in the study of larger
fullerenes. Some semi-empirical Hückel (tight-binding) and Pariser-Parr-Pople
(PPP) types of models have been successfully used in the study of the electronic structure of C60 [9]. Also it has been used PPP models and a Hückel
approximation to predict the electronic photoabsorption spectra of icosahedral
fullerenes and buckyonions taking into account the screening effects in these
highly correlated electronic systems. [10, 11, 12, 13, 14]. Model parameters are
first fixed fitting the laboratory spectrum [15] of C60 in the energy range 1-40
eV and then properly adjusted for larger fullerenes. All the theoretical spectra
show a prominent band associated to the π-π ∗ plasmon transition at energies
in the range 5.5-6 eV close to that of the UV bump (5.7 eV). Measurements
of laboratory spectra of buckyonions mixture confirm these results [16].
2 The role of fullerenes in interstellar medium.
The most prominent feature in the ISM extinction curve is the UV bump at
2175 Å. The carrier of this band was soon associated with some form of carbonaceous material, but the exact physical nature of the material ultimately
responsible for the band is still unknown. The increase of interstellar extinction at shorter wavelengths and the existence of the mysterious, much weaker
DIBs in the optical and near-infrared are also some of the intriguing observational properties of the interstellar material [7, 8]. The DIBs are believed to
arise from gas-phase organic molecules (rather than from dust grains) in the
ISM, but in spite of more than 80 years of research since the discovery of the
first DIB, the nature of the carriers remains unknown.
Although the high level of symmetry of C60 indicates that this particular fullerene is unlikely as a carrier of the complex spectrum of the diffuse
Interstellar medium:the role of fullerenes
3
0.75
1.75
2.75
3.75
4.75
5.75
Energy(eV)
6.75
7.75
8.75
4000
6000
9632 A
7000
Amstr
9577 A
a8620 A
7429 A
6940 A
5779 A 5780 A
6284 A
6533 A
5000
5535 A
0.5
4760 A
4882 A
4180 A
A(lambda)/A(V)
4429 A
3.5
6177 A
interstellar bands, diverse studies have investigated possible mechanisms for
which this molecule can acquire a complex absorption spectrum in the optical.
Leger et al. [17] , suggested that the spectrum of ionized C60 is much more
complex than the neutral molecule and could produce absorption bands in
the optical and in the infrared. Foing and Ehenfreund [4] found two diffuse
bands at 9577 and 9632 Å coinciding within 0.1 % with the laboratory measurements of the bands of C+
60 observed in a Neon mould. This was considered
as evidence of the existence of the C+
60 in the interstellar medium. There are
also several proposals associating DIBs with the hydrides of the C60 ,C60 Hn
[18, 19, 20, 21]. Although there are also alternative suggestions that the carrier of these bands could be related to PAHs and hydrogenated amorphous
carbon (HACs) compounds.
Iglesias-Groth [13] has shown that photoabsorption by fullerenes and buckyonions (multishell fullerenes) can explain the shape, width and peak energy
of the most prominent feature of the interstellar absorption, the UV bump
at 2175 Å. Comparing theoretical cross sections and astronomical data (see
Fig.1), it is derived a density of fullerenes in the diffuse interstellar medium
of 0.1-0.2 ppm, consistent with the findings in meteorites (see [2]). Fullerenebased molecules could be a major carbon reservoir in the interstellar medium.
8000
9000
10000
Fig. 1. Left:Best fit of the UV bump for a fullerene size distribution consisting
of a power law N(R)∼ R−3.5 when the reddening factor in the diffuse interstellar
medium is RV =3.1 Right: Comparison of predicted theoretical transitions (lines)
with the wavelengths of the 16 stronger DIBs (crosses) [38].
3 Anomalous microwave emission in the Galaxy and
hydrogenated fullerenes.
Recent experiments dedicated to the study of the anisotropy of the Cosmic
Microwave Background have found evidence for galactic microwave emission
4
Susana Iglesias-Groth
in the range 10-90 GHz correlated at high galactic latitudes with thermal
emission (DIRBE 100µm map) from interstellar dust [22, 23, 24, 25, 26, 27, 28].
An explanation for this anomalous dust-correlated microwave emission based
on electric dipole emission from fast rotating carbon-based molecules has been
proposed by Draine & Lazarian [29, 30]. These models appear to reproduce
the major features of the so-called anomalous microwave emission [27] but do
not identify the actual carrier of the emission. This foreground would add to
the classical components of free-free emission (thermal bremsstrahlung) and
synchrotron emission. Tentative evidence for “anomalous” microwave emission
in astronomical objects has been found by Finkbeiner [27] and Casassus [32].
Unambiguous evidence for this new emission mechanism is provided by recent
observations in the Perseus molecular complex [33] and in the dark cloud LDN
1622 [34]. The recent results obtained in the Perseus complex using the Very
Small Array Interferometer at 33 GHz [35] show a good spatial correlation
between the anomalous microwave emission and the thermal dust emission at
12 and 25 µm.
10
5.0e−18
4.0e−18
1.0
F (Jy)
3.0e−18
j v/n H
DC
CNM
15
K
2.0e−18
PDR
T=
1.0e−18
free−free
0.0e+00
0
20
40
60
frequency (GHz)
80
100
0.1
10
ν (GHz)
100
Fig. 2. Left: Observations of Perseus anomalous microvawe emission by Watson et
al. (2005); filled diamond) and predicted rotacional emissivity per H of a mixture
of fulleranes and hydrogenated buckyonions in CNM conditions (dotted curve and
dash-dotted curve) and a combination of CNM and WIM conditions (solid curve
and dashed curve ). See text for details and Iglesias-Groth [36]. Right:Flux emission
curves for fulleranes for a degree of hydrogenation s=1/3 under a combination of
plausible physical conditions in LDN 1622. Dotted curves: Predicted emission curves
for fulleranes in the dark cloud (assuming ionization fraction f=3%), CNM (emission
in a region equivalent to 17% the area of the cloud) and PDR (5% the area). Thick
solid line: Total contribution of DC,CNM and PDR emission including free-free (1.7
emissivity index) and thermal dust emission (15 K). Dyamonds: Observations of
LDN 1622 anomalous microwave emission by Casases et al. [34]. See text for details
and Iglesias-Groth [37].
Interstellar medium:the role of fullerenes
5
Iglesias-Groth [36, 37] has investigated the electric dipole rotational emission of hydrogenated forms of icosahedral fullerenes and its relation to the
anomalous microwave emission. Because of the slightly polar nature of the CH bond, hydrogenated C60 and other fullerene hydrides are expected to have
a net electric dipole moment. Assuming that these dipole moments are proportional to the square root of the number of C-H bonds in each molecule, she
estimates their effective rotational rates and electric dipole emissivity in various astrophysical environments. Rotational damping and excitation processes
are computed following the comprehensive treatment by Draine and Lazarian
[29, 30]. The abundance of fullerenes is estimated by fitting theoretical photoabsorption spectra to the characteristics of the UV bump extinction in the
environment and she finds effective rotation rates in the range 5-65 GHz with
a trend toward lower rotational frequency as the radius of the molecule increases. The resulting curves reproduce the recent observations of anomalous
microwave emission in the Perseus molecular complex and in the dark cloud
LDN 1622 (see fig.2).
4 Conclusions
The cross sections obtained for single fullerenes and buckyonions reproduce
laboratory measurements for carbon onions and the behaviour of the interstellar medium UV extinction curve. The prominent band predicted between
5 and 6 eV in the theoretical cross sections can explain the position and width
observed for the 2175 Å bump. They also reproduce the rise in the extinction
curve at higher energies and other well known diffuse interstellar bands (like
the 4429 Å band which is apparently correlated with the UV bump). ISM
densities of 0.2 and 0.08 ppm are inferred for single fullerenes and buckyonions
respectively (very similar to the densities of fullerenes observed in meteorites
0.1 ppm). These results are consistent with estimates for the carbon budget
in the ISM. Fullerenes and buckyonions could be the most abundant form of
carbon in the ISM.
Finally, it is possible to understand the Perseus molecular complex and
the dark cloud LDN 1622 microwave anomalous emission in terms of electric
dipole emission of fulleranes if these molecules follow a size distribution similar
to that proposed in the study of the UV extinction bump. The dominant
microwave emission would be associated in both cases to the smaller fulleranes.
References
1.
2.
3.
4.
H.W. kroto. Science 243,1139-1143 (1988)
Becker, L., Bunch, T.E. 1997, Meteoritics & Planetary Science, 32, 479
Pizzarello, S. et al. 2001, Science, 293, 2236
Foing, B.F. & Ehrenfreund, P. 1994, Nature, 369, 296
6
Susana Iglesias-Groth
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
Cabioc’h, T., Rivière, J.P., Delafond, J. & Mater, J. 1995, Science, 30, 4787
D. Ugarte, Astrophys. J. 443, L85 (1995).
P. Jenniskens, J.M. Greenberg, Astron. Astrophys. 274, 439 (1993)
G.H. Herbig, Ann. Rev. Astron. Astrophys. 33, 19 (1995)
J. Cioslowski, Electronic Structure Calculations on Fullerenes and their Derivatives (Oxford University Press, New York, 1995).
Iglesias-Groth, S, Ruiz, A., Bretón, J. & Gómez-Llorente, J.M , Journal of
Chemical Physics, 116, 1648 (2002)
S. Iglesias-Groth, MA Thesis, Universidad de La Laguna , Spain (2003)
Iglesias-Groth, S, Ruiz, A., Bretón, J. & Gómez-Llorente, J.M , Journal of
Chemical Physics 118, 7103 (2003).
Iglesias-Groth, S. 2004, ApJ, 608, L37
Ruiz, A., Bretón, J. & Gómez-Llorente, J.M, J. Chem. Phys. 2004, 120, 6163
Berkowitz, J., 1999, Chem. Phys, 111, 1446.
Chhowalla, M , Wang, H, Sano, N, Teo, K.B.K., Lee, S.B. & Amaratunga, A.J.
2003, Phys. Rev. Lett., 90, 15
A. Léger, L. dH́endecourt, L. Verstrate & W. Schmidt, Astron. Astrophys. 203,
145 (1988).
Webster, A.S., 1991, Nature, 352, 412
Webster, A.S.1992, A&A, 257, 750
Webster, A.S 1993a, MNRAS, 263, 385
Webster, A.S 1993b, MNRAS, 265, 421
Kogut, A et. al. 1996, ApJ, 460, 1
Leitch, E.M., Readhead, A.C.S., Pearson, T.J. & Myers, S.T. 1997, ApJ, 486,
L23
de Oliveira-Costa, A. et al. 1999, ApJ, 527, L9
de Oliveira-Costa, A. et al. 2002, ApJ, 567, 363
de Oliveira-Costa, A. et al. 2004, ApJ, 606, L89
Finkbeiner, D.P., Langston, G.I & Minter, A.H. 2004, ApJ, 617, 350
Fernández-Cerezo, S. et al. 2006, MNRAS,370, 15
Draine, B.T. & Lazarian, A. 1998a, ApJ, 494, L19
Draine, B.T. & Lazarian, A. 1998b, ApJ, 508, 157
Finkbeiner, D.P., Schlegel, D.J., Frank, C. & Heiles, C. 2002, ApJ, 566, 898
Casassus, S., Readhead, A.C.S., Pearson, T.J., Nyman, L.A, Shepherd, M.C.
& Bronfman, L. 2004, ApJ, 603, 599
Watson,R.A. et. al 2005, ApJ, 624, L89
Casassus, S., Cabrera, G., Forster, F., Pearson, T.J, Readhead, A.C.S & Dickinson, C., 2006, ApJ, 639, 951
Watson,R.A. et. al 2006, in preparation
Iglesias-Groth, S. 2005, ApJ, 632, L25
Iglesias-Groth, S. 2006, MNRAS, 368, 1925
Iglesias-Groth, S. 2007, in preparation