Astronomical Applications of Remote Sensing:
A Search for Diffuse Interstellar Band Absorption Features
Jennifer Nicklaus
An Undergraduate Thesis
Submitted in Partial Fulfillment for the Requirements of
Bachelor of Arts
in
Geography and Earth Science
April 15, 2011
Abstract
At first glance geography and astronomy have nothing in common, but upon further review,
astronomy shares many of the same principles of remote sensing. Both rely on data obtained
indirectly from distant sources and both their data comes in the form of electromagnetic (EM)
radiation. The field of spectroscopy is of particular importance to both; used in climate and
atmospheric studies and in almost every aspect of astronomy. Spectroscopy is the study of the
interaction of matter with EM radiation, and astronomers use it to identify the molecular
components of stars and the tenuous gasses between them and Earth. As light from the stars
passes through interstellar material, the material absorbs certain wavelengths and adds absorption
features to the spectrum that reaches Earth. One particular class of these absorption features is
called Diffuse Interstellar Bands, or DIBs, and both their chemical source and formation site
have remained a mystery since their discovery over 60 years ago. The eclipse of the binary star
Epsilon Aurigae presented a unique opportunity to search for the DIB formation site in
circumstellar material by analyzing its spectrum for changes in absorption at known DIB
wavelengths through the first half of the eclipse which began in August 2009. To carry out this
investigation high-resolution spectroscopic data from February 2009 to April 2010 were
analyzed using the software Image Reduction and Analysis Facility (IRAF). Analysis showed no
changes in absorption near DIB wavelengths over this period, leading to the conclusion that the
formation site of DIBs is not in circumstellar material such that in the Epsilon Aurigae system.
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Table of Contents
1. Introduction
1.1. Remote Sensing and Spectroscopy
1.2. Equipment
1.3. Diffuse Interstellar Bands
1.4. Study Area
2. Data Acquisition
2.1. ARC Echelle Spectrograph
3. Objective and Methods
4. Analysis and Results
5. Discussion and Conclusion
6. Acknowledgements
7. References
3
List of Figures
1) Basic principles of spectroscopy. 5
2) The process by which light from distant stars is affected by interstellar material… 5
3) The ARC Echelle Telescope at Apache Point Observatory, NM. 7
4) Comparison of the climates at the sites of some of the world’s best telescopes. 7
5) Spectrum of DIB star HD183143 compared to the non DIB star β Orionis. 8
6) Infrared images from the early stage of the current eclipse of Epsilon Aurigae. 10
7) Scale model of Epsilon Aurigae. 10
8) An artist’s rendition of the Epsilon Aurigae system. 11
9) Model of Epsilon Aurigae showing light intensity over the course of the eclipse. 12
10) A simple model of diffraction grating. 13
11) Light after it has been sent through a diffraction grating. 14
12) Configuration of the University of Queensland Spectrograph, similar to ARCES. 14
13) Complete spectrum of Epsilon Aurigae (Left), magnified view of smaller portions… 15
14) Stellar potassium line (7699Å) plotted with a 0.25 step. 19
15) Stellar potassium line (7699Å) plotted as combinations of three spectra each from… 19
16) Stellar sodium line (5889.95Å) plotted with a 0.25 step. 20
17) Stellar sodium line (5889.95Å) plotted as combinations of three spectra each from… 20
18) 5780.48Å DIB plotted with a 0.03 step. 21
19) 5780.48Å DIB plotted as combinations of three spectra each from before eclipse… 21
20) 6065.28Å DIB plotted with a 0.03 step. 22
21) 6065.28Å DIB plotted as combinations of three spectra each from before eclipse… 22
22) 6195.98Å DIB plotted as combinations of three spectra each from before eclipse… 23
23) 6613.62Å DIB plotted as combinations of three spectra each from before eclipse… 23
24) 6660.71Å DIB plotted with a 0.01 step. 24
25) 6660.71Å DIB plotted as combinations of three spectra each from before eclipse… 24
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1. Introduction
1.1. Remote Sensing and Spectroscopy
At its most basic, remote sensing is the collection of data through indirect contact.
Remote sensing has many applications in geography and the earth sciences, from climatology to
urban geography. Every object above a temperature of absolute zero emits electromagnetic
radiation, and this radiation is recorded by remote sensing equipment. Everything from solarimaging satellites to the human eye uses remote sensing to gather data, and remote sensing is the
only way to gather data about distant stars for astronomical research. Spectroscopy (or
spectrometry) is the study of the interaction of matter with electromagnetic radiation and is
widely used in both astronomy and earth science. Every atom absorbs and emits a specific
wavelength of electromagnetic radiation, and these properties allow the chemical composition of
Earth’s atmosphere, distant stars, and interstellar clouds to be determined.
Clouds of cold matter between Earth and stars add their own individual absorption lines
to the emission spectra of the stars, and identifying the atoms and molecules responsible for
those absorption features tells researchers what the clouds are made of. Fig. 1 shows these
processes, including a heated gas forming emission lines and a cooled gas forming absorption
lines. In geographic remote sensing research such as monitoring greenhouse gas concentrations
it is possible to ascertain minute changes in the concentration of gasses by analyzing changes in
the emission spectrum of the Earth’s radiated energy passing through the atmosphere, and the
research proposed here applies the same principles to stellar processes. Fig. 2 illustrates the
process by which EM radiation from distant stars reaches Earth and shows how interstellar
material adds absorption lines to the star’s original spectrum, much the same as Earth’s
atmosphere adds absorption features to the spectrum. The Earth’s emission spectrum has
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changed as atmospheric concentrations of CO2 and other greenhouse gasses has increased, and
the absorption lines in stellar spectra caused by interstellar clouds would also be expected to
change if the density or position of the cloud relative to the line of sight were to change.
Figure 1. Basic principles of spectroscopy.
Figure 2. The process by which light from distant stars is affected by interstellar material before
reaching monitoring equipment on Earth.
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1.2. Equipment
Data for astronomical research such as this can only be acquired with sophisticated
telescopes and equipment, and this particular data was gathered with the ARC Echelle
Spectrograph (ARCES) at Apache Point Observatory in New Mexico (Fig. 3). Climate and
atmospheric conditions play a large role in the quality of astronomical data, and understanding
their effect is the key to successful observatory site selection. The most obvious requirement for
a successful observatory is cloud-free skies but high humidity and atmospheric turbulence,
known as poor seeing conditions, cause blurry images and result in poor data quality. Especially
in light of the quest for ever-larger telescopes, finding locations with excellent seeing conditions
is very important. For example, testing for the site of the Giant Magellan Telescope, one of the
largest telescopes presently under construction, began in 2005 and is still in progress today
(Thomas-Osip, 2009).
Most of the best observatories are at high altitudes in dry climates such as the American
Southwest and Chilean Atacama Desert (Fig. 4). Some, such as Mauna Kea, are located above
an inversion layer which also decreases the amount of atmospheric turbulence. In the case of
Mauna Kea, the inversion layer creates a boundary between the humid, warm maritime air near
the ocean and the dry, cool air of the upper atmosphere (http://www.ifa.hawaii.edu/mko/
about_maunakea.htm). The climate classifications in which the majority of the observatories in
Fig. 4 are located are arid desert and steppe because these have the lowest amounts of
precipitation and the highest percentage of clear skies. Apache Point is in southern New Mexico
in the Sacramento Mountains and is at an elevation of almost 2800 meters, placing it above the
most turbulent lower atmosphere. The site also has relatively low dust and aerosol
concentrations and is far enough from any large cities that it does not suffer from much light
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pollution (http://www.apo.nmsu.edu). Locations with better seeing conditions produce more
accurate and clearer data that can be analyzed to a higher spectral resolution.
Figure 3. The ARC Echelle Telescope at Apache Point Observatory, NM.
Figure 4. Comparison of the climates at the sites of some of the world’s best telescopes.
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1.3. Diffuse Interstellar Bands
Diffuse Interstellar Bands, or DIBs, are relatively broad spectroscopic absorption features
that range from one or two angstroms to tens of angstroms wide (Maier, 2010) and several
hundred of them have been identified (Fig. 5). DIB absorption features are known to be
associated with interstellar material and their absorption is characterized by diffuse profiles that
do not resolve into the sharp band structure expected of molecular absorption in a very cold and
tenuous environment. DIB absorption features have so far proven impossible to re-create in a
laboratory setting, but several theories in the last decade have included C7- (McCall, 2000),
H2CCC (Maier, 2010), naphthalene (Iglesias-Groth, 2008), anthracene (Iglesias-Groth, 2010),
and C60+. The paper of McCall et al (2000) was one of the earliest examined here and theorized
that C7- could be a source of several DIBs, but as stated in the paper it has two main problems;
first, the lack of correlation between the DIBs that all seem related to C7-, and second, the
differences between the observed DIB wavelength and the laboratory wavelength. A few
individual DIBs seem to correlate with polycyclic aromatic hydrocarbons (PAHs) (Snow, 2001)
as well, but none of the results are certain. DIBs feature strength correlates very roughly with
atomic hydrogen and gas column densities, but neither their chemical carriers nor formation site
have been positively identified.
Figure 5. Spectrum of DIB star HD183143 compared to the non DIB star β Orionis.
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1.4. Study Area
The question of the formation site of DIBs led to the question of whether it could be
present in circumstellar material such as that in the Epsilon Aurigae system. Epsilon Aurigae is
an eclipsing binary star in which the secondary (less bright) object, a dusty disk, almost
completely obscures the brightest (primary) star every 27 years. The secondary object orbits the
primary star in the plane of the field of view so the eclipse is essentially total (Fig. 6), however,
the primary star is too luminous for its light to be completely blocked across all wavelengths.
The consensus view of Epsilon Aurigae pairs a primary F0 supergiant star of approximately
7800K with a radius between .25AU and 1AU and a secondary object surrounded by a large,
flattened disk-like cloud, shown in Fig. 7. Fig. 8 is an artist's rendition of the system. There are
two opposing models for the masses of the primary and secondary stars; the high-mass model
that shows the primary as a standard 15M⊙ supergiant and the secondary as an object of 13M⊙ ,
and the low-mass model shows the primary as a 1M⊙ post-asymptotic giant branch star and the
secondary with a mass of 5M⊙ (Guinan, 2002). The eclipse of Epsilon Aurigae occurs
approximately once every 27 years and lasts for around 2 years. The primary star also pulsates
with a period of approximately 100 days (Kemp, 1986), which further complicates analysis of its
spectrum. Although there is some agreement on the specifics of the primary star, we are only
able to make educated guesses at the properties of the secondary. Because the disk surrounding
it is optically thick, the secondary star is for all intents and purposes invisible.
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Figure 6. Infrared images from the early stage of the current eclipse of Epsilon Aurigae.
Figure 7. Scale model of Epsilon Aurigae.
11
Figure 8. An artist’s rendition of the Epsilon Aurigae system.
The disk surrounding the secondary object is cool, at around 475K (Guinan, 2002), and has a
radius and thickness of 4AU and 0.9AU respectively (Leadbeater, 2010). Most system models
include a central opening in the disk because this best explains the brightening seen midway
through the eclipse, shown in Fig. 9, but the nature of the object at its center is much debated.
The disk is seen almost edge-on, is tilted 2 degrees with respect to its orbit (Guinan, 2002), and
completely obscures the secondary object, allowing us to see none of its spectrum. This lack of a
detectable spectrum means that the 13M⊙ secondary star in the high-mass model cannot be a
single star, because its spectrum would be easily visible through the cloud. To explain this
discrepancy, the secondary star in this model is thought to actually be two stars, each of 6.5M⊙,
which would be roughly 10% as luminous as the single star and would explain why it is so
under-luminous (Guinan, 2002). The mid-eclipse brightening has increased over time, which
could be a sign of evolvement within the disk structure (Guinan, 2002). It has also been proposed
by Ferluga (1990) and Leadbeater & Stencel (2010) that the disk includes a system of rings due
12
to the step-like changes in neutral potassium (7699Å) seen as the eclipse progresses, which could
suggest that the secondary is a protoplanetary disk (Guinan, 2002) that may someday form
planets.
Figure 9. Model of Epsilon Aurigae showing light intensity over the course of the eclipse.
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2. Data Acquisition
2.1. ARC Echelle Spectrograph
The ARC Echelle Spectrograph (ARCES) is mounted on the 3.5-meter Astrophysical
Research Consortium telescope and works by directing light to a diffraction grating, a panel with
thousands of slits or grooves per inch that diffracts and splits the light similar to a prism. The
diffracted light is then read by a computer sensor to be recorded and digitized. Fig. 10 is a
simple illustration of diffraction grating and Fig. 11 is the output from diffraction grating
redirected to project outside the telescope. A schematic showing the configuration of a
spectrograph similar to the ARC Echelle is shown in Fig. 12. Absorption lines are clearly visible
in Fig. 11 as dark spots in the otherwise bright spectrum, and a computer measures the intensity
of each column of pixels to plot a spectrum (Fig. 13). Although difficult to see, Fig. 11 is made
up of dozens of rows, each is a small segment of the whole spectrum. Only once the computer
analyzes them and strings them together does an image like Fig. 11 turn into the detailed
spectrum in Fig. 13. When looking at the entire spectrum (Fig. 13, left) it is impossible to clearly
identify individual features, but changing the range of the x-axis allows small features to be
distinguished (Fig. 13, right). The data is recorded in Flexible Image Transport System files
(.fits), which is the standard format for spectrographic data. The researchers at Apache Point
then sent the files to Dr. Julie Dahlstrom, who reduced the data and provided it for this project.
14
Figure 10. A simple model of diffraction grating.
Figure 11. Light after it has been sent through a diffraction grating.
15
Figure 12. Configuration of the University of Queensland Spectrograph, similar to ARCES.
Figure 13. Complete spectrum of Epsilon Aurigae (Left), magnified view of smaller portions of
the spectrum (Right).
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3. Objective and Methods
The goal of this research was to use spectroscopy and remote sensing to determine if the
eclipsing body in the Epsilon Aurigae system is a formation site of DIBs. If the disk is a
formation site, the DIB features should show appreciable changes as the eclipse progresses. To
achieve this goal, spectra from Epsilon Aurigae before the eclipse and at various points during
the eclipse ingress were compared, and these spectra were also compared to the spectrum of a
star without DIBs. The eclipse began around August 2009, and the earliest Epsilon Aurigae
spectra used were from February 2009 so as to begin analysis with data completely free of the
influence of the eclipsing body. Stars are too different to ever be perfect comparisons but Alpha
Leporis is also a F0 star and is the most similar to the primary of Epsilon Aurigae, so it was
chosen as the non-DIB comparison star for this analysis. Time constraints and data availability
limited the temporal extent of analysis to between February 2009 and April 2010, but spectra
were taken on an approximately monthly basis during this period. Hundreds of DIBs have been
identified, but it would be far outside the scope of this research to study them all so only a few of
them were analyzed.
Analysis was performed using the program Image Reduction and Analysis Facility, or IRAF.
The primary method of analysis was using the IRAF specplot function to plot multiple spectra
together and then visually comparing them. The spectra were superimposed in the plot both with
and without a vertical offset, or step. Imarith and sarith are IRAF functions used to perform
arithmetic on individual and multiple spectra respectively, and a combination of both was used to
remove the stellar absorption lines from the spectrum of Epsilon Aurigae by dividing an Epsilon
Aurigae spectrum by the spectrum without DIBs. Because Alpha Leporis does not have DIB
features, the only features removed from the Epsilon Aurigae spectrum were stellar in origin.
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IRAF can be used to perform atmospheric correction to remove features created by Earth’s
atmosphere, but since the DIBs studied do not appear near strong atmospheric features it was
deemed unnecessary. Sarith was also used to combine several Epsilon Aurigae spectra from the
same eclipse stage, which reduced the amount of noise and increased the quality of the resulting
spectrum.
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4. Analysis and Results
Removing stellar lines from the Epsilon Aurigae spectra did not prove useful because
most of the DIBs studied were not influenced by stellar features, but combining several spectra
from various stages of the eclipse produced more concise plots. The changes to stellar lines
(Figures 14 to 17) as the eclipse progress are strong and clear but the DIB features (Figures 18 to
25) show essentially no change. While only a few of the hundreds of DIBs were studied here, the
only one that shows any appreciable variation is the stellar blend DIB at 6065.28Å (Fig. 20, 21).
However, the velocity shifts seen in the 6065Å DIB were occurring before the eclipse and seem
to coincide with the proposed 100-day pulsation period of the primary star (Kemp, 1986). If
there were DIB carriers in the circumstellar material of the disk there would be changes in the
DIBs on a scale similar to those of the stellar lines, and even more variation would be present if
the model that postulates the existence of rings in the disk is correct. If ring model is correct and
the formation site of DIBs were in the circumstellar material the expected results would be not
just the steady change of the stellar lines but changes that increase and decrease in magnitude as
the rings pass between the observer and the primary star.
19
Figure 14. Stellar potassium line (7699Å) plotted with a 0.25 step.
Figure 15. Stellar potassium line (7699Å) plotted as combinations of three spectra each from
before eclipse, entering eclipse, and mid-eclipse stages.
20
Figure 16. Stellar sodium line (5889.95Å) plotted with a 0.25 step.
Figure 17. Stellar sodium line (5889.95Å) plotted as combinations of three spectra each from
before eclipse, entering eclipse, and mid-eclipse stages.
21
Figure 18. 5780.48Å DIB plotted with a 0.03 step.
Figure 19. 5780.48Å DIB plotted as combinations of three spectra each from before eclipse,
eclipse ingress, and mid-eclipse.
22
Figure 20. 6065.28Å DIB plotted with a 0.03 step.
Figure 21. 6065.28Å DIB plotted as combinations of three spectra each from before eclipse,
eclipse ingress, and mid-eclipse.
23
Figure 22. 6195.98Å DIB plotted as combinations of three spectra each from before eclipse,
eclipse ingress, and mid-eclipse.
Figure 23. 6613.62Å DIB plotted as combinations of three spectra each from before eclipse,
eclipse ingress, and mid-eclipse.
24
Figure 24. 6660.71Å DIB plotted with a 0.01 step.
Figure 25. 6660.71Å DIB plotted as combinations of three spectra each from before eclipse,
eclipse ingress, and mid-eclipse.
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5. Discussion and Conclusion
It might not be obvious, but many facets of astronomy and geography are closely related.
Without geographic analysis of potential sites there would be no definite way to determine the
best locations for observatories, and although spectroscopy was first pioneered by physicists,
today it is used by geographers in remote sensing, atmospheric science, and a variety of other
research topics. Astronomical spectroscopy is essentially remote sensing, applied to spatial
phenomenon, and the source of DIBs is one of its most enduring mysteries. The eclipse of
Epsilon Aurigae was the perfect opportunity to search for their formation site in the circumstellar
material of the eclipsing disk.
The various data analysis methods used here and comparisons of many days worth of
spectra all show no appreciable change in any of the DIBs studied. The stellar lines show
changes that would have been seen in the DIBs if they were being formed in the disk, but if any
changes are taking place they are so small as to be insignificant. The conclusions formed from
this research should be confirmed by completing analysis of spectra from the remainder of the
eclipse, but the lack of change during ingress makes it unlikely that the results would change
during egress. Several other studies, including that of stellar cluster M17 by Hanson, Howarth,
and Conti (1997), have looked for DIBs in circumstellar material and also concluded that
circum- and inter-stellar material does not contain DIBs. The lack of change attributable to the
eclipse of Epsilon Aurigae in the DIBs studied here leads to the conclusion that their formation
site is not in the circumstellar material of the secondary disk, and this coupled with the lack of
results from other similar studies strongly indicates that the formation site of DIBs is not present
in circumstellar material. Although some DIBs have been shown to correlate well with C2 and
PAH molecules, the source of the vast majority of them remains unknown. While this research
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has failed to pinpoint a source of DIBs it did rule out a possible location, which will hopefully
lead to a successful search for their carrier in other locations.
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6. Acknowledgements
I would like to thank everyone who lent their knowledge and support to this thesis, especially Dr.
Julie Dahlstrom, Dr. Don York (The University of Chicago), Dr. Joy Mast, and Dr. Matt Zorn.
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Figure 1. http://www.amateurspectroscopy.com/Spectroscope.htm.
Figure 2. Courtesy Dr. Julie Dahlstrom.
Figure 3. Courtesy Dr. Julie Dahlstrom.
Figure 4. Created using ArcMap10.
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