Diffuse interstellar bands

Much astronomical work relies on the study of spectra - the light from astronomical objects dispersed using a prism or, more usually, a diffraction grating. A typical stellar spectrum will consist of a continuum, containing absorption lines, each of which is attributed to a particular atomic energy level transition in the atmosphere of the star.

The appearances of all astronomical objects are affected by extinction, the absorption and scattering of photons by the interstellar medium. Relevant to DIBs is interstellar absorption, which predominantly affects the whole spectrum in a continuous way, rather than causing absorption lines. In 1922, though, astronomer Mary Lea Heger

{{cite journal

| last=Heger | first=M. L.

| year=1922

| title=Further study of the sodium lines in class B stars

| journal=Lick Observatory Bulletin

| volume=10 | issue=337 | pages=141–148

| bibcode=1922LicOB..10..141H

| doi=10.5479/ADS/bib/1922LicOB.10.141H

| doi-access=free

}} first observed a number of line-like absorption features which seemed to be interstellar in origin.

Their interstellar nature was shown by the fact that the strength of the observed absorption was roughly proportional to the extinction, and that in objects with widely differing radial velocities the absorption bands were not affected by Doppler shifting, implying that the absorption was not occurring in or around the object concerned.

{{cite journal

|last=Herbig |first=G. H.

|year=1995

|title=The Diffuse Interstellar Bands

|journal=Annual Review of Astronomy and Astrophysics

|volume= 33|pages=19–73

|bibcode=1995ARA&A..33...19H

|doi= 10.1146/annurev.aa.33.090195.000315

}}

{{cite journal

|last=Krelowski |first=J.

|year=1989

|title=Diffuse interstellar bands - An observational review

|journal=Astronomische Nachrichten

|volume=310 |issue=4 |pages=255–263

|bibcode=1989AN....310..255K

|doi=10.1002/asna.2113100403

}}

{{cite journal

|last=Sollerman |first=J.

|display-authors=etal

|year=2005

|title=Diffuse Interstellar Bands in NGC 1448

|journal=Astronomy and Astrophysics

|volume=429|issue=2 |pages=559–567

|arxiv=astro-ph/0409340

|bibcode=2005A&A...429..559S

|doi=10.1051/0004-6361:20041465

|s2cid=18036448

}} The name diffuse interstellar band, or DIB for short, was coined to reflect the fact that the absorption features are much broader than the normal absorption lines seen in stellar spectra.

The first DIBs observed were those at wavelengths 578.0 and 579.7 nanometers (visible light corresponds to a wavelength range of 400 - 700 nanometers). Other strong DIBs are seen at 628.4, 661.4 and 443.0 nm. The 443.0 nm DIB is particularly broad at about 1.2 nm across - typical intrinsic stellar absorption features are 0.1 nm or less across.

Later spectroscopic studies at higher spectral resolution and sensitivity revealed more and more DIBs; a catalogue of them in 1975 contained 25 known DIBs, and a decade later the number known had more than doubled. The first detection-limited survey was published by Peter Jenniskens and Xavier Desert in 1994 (see Figure above),

{{cite journal

|last1=Jenniskens |first1=P.

|last2=Desert |first2=F.-X.

|year=1994

|title=A survey of diffuse interstellar bands (3800-8680 A)

|journal=Astronomy and Astrophysics Supplement Series

|volume=106 |pages=39

|bibcode=1994A&AS..106...39J

}} which led to the first conference on The Diffuse Interstellar Bands at the University of Colorado in Boulder on May 16–19, 1994. Today circa 500 have been detected.

In recent years, very high resolution spectrographs on the world's most powerful telescopes have been used to observe and analyse DIBs.

{{cite journal

|last1=Fossey |first1=S. J.

|last2=Crawford |first2=I. A.

|year=2000

|title=Observing with the Ultra-High-Resolution Facility at the Anglo-Australian Telescope: Structure of Diffuse Interstellar Bands

|journal=Bulletin of the American Astronomical Society

|volume=32 |pages=727

|bibcode=2000AAS...196.3501F

}} Spectral resolutions of 0.005 nm are now routine using instruments at observatories such as the European Southern Observatory at Cerro Paranal, Chile, and the Anglo-Australian Observatory in Australia, and at these high resolutions, many DIBs are found to contain considerable sub-structure.

{{cite journal

|last1=Jenniskens |first1= P.

|last2=Desert |first2=F. X.

|year=1993

|title=Complex Structure in Two Diffuse Interstellar Bands

|journal=Astronomy and Astrophysics

|volume=274 |pages=465

|bibcode=1993A&A...274..465J

}}

{{cite journal

|last1=Galazutdinov |first1= G.

|display-authors=etal

|year=2002

|title=Fine structure of profiles of weak diffuse interstellar bands

|journal=Astronomy and Astrophysics

|volume= 396|issue= 3|pages=987–991

|bibcode=2002A&A...396..987G

|doi= 10.1051/0004-6361:20021299

|doi-access=free

}}

The great problem with DIBs, apparent from the earliest observations, was that their central wavelengths did not correspond with any known spectral lines of any ion or molecule, and so the material which was responsible for the absorption could not be identified. A large number of theories were advanced as the number of known DIBs grew, and determining the nature of the absorbing material (the 'carrier') became a crucial problem in astrophysics.

One important observational result is that the strengths of most DIBs are not strongly correlated with each other. This means that there must be many carriers, rather than one carrier responsible for all DIBs. Also significant is that the strength of DIBs is broadly correlated with the interstellar extinction. Extinction is caused by interstellar dust; however, DIBs, are not likely to be caused by dust grains.

The existence of sub-structure in DIBs supports the idea that they are caused by molecules. Substructure results from band heads in the rotational band contour and from isotope substitution. In a molecule containing, say, three carbon atoms, some of the carbon will be in the form of the carbon-13 isotope, so that while most molecules will contain three carbon-12 atoms, some will contain two ¹²C atoms and one ¹³C atom, much less will contain one ¹²C and two ¹³C, and a very small fraction will contain three ¹³C molecules. Each of these forms of the molecule will create an absorption line at a slightly different rest wavelength.

The most likely candidate molecules for producing DIBs are thought to be large carbon-bearing molecules, which are common in the interstellar medium. Polycyclic aromatic hydrocarbons, long carbon-chain molecules such as polyynes, and fullerenes are all potentially important.

{{cite journal

|last1=Ehrenfreund |first1=P.

|year=1999

|title=The Diffuse Interstellar Bands as evidence for polyatomic molecules in the diffuse interstellar medium

|journal=Bulletin of the American Astronomical Society

|volume=31 |pages=880

|bibcode=1999AAS...194.4101E

}} These types of molecule experience rapid and efficient deactivation when excited by a photon, which both broadens the spectral lines and makes them stable enough to exist in the interstellar medium.{{cite journal|title=Ultrafast Studies on the Photophysics of Matrix-Isolated Radical Cations of Polycyclic Aromatic Hydrocarbons|journal=The Journal of Physical Chemistry A|volume= 108 |issue=1|pages=25–31|doi=10.1021/jp021832h|bibcode = 2004JPCA..108...25Z |last1=Zhao|first1=Liang|last2=Lian|first2=Rui|last3=Shkrob|first3=Ilya A.|last4=Crowell|first4=Robert A.|last5=Pommeret|first5=Stanislas|last6=Chronister|first6=Eric L.|last7=Liu|first7=An Dong|last8=Trifunac|first8=Alexander D.|year=2004|s2cid=97499895}}{{cite journal|title=Fluorescence of the perylene radical cation and an inaccessible D0/D1 conical intersection: An MMVB, RASSCF, and TD-DFT computational study|journal=The Journal of Chemical Physics|volume= 132 |issue=4|page= 044306|doi=10.1063/1.3278545 |pmid=20113032|bibcode = 2010JChPh.132d4306T |last1=Tokmachev|first1=Andrei M.|last2=Boggio-Pasqua|first2=Martial|last3=Mendive-Tapia|first3=David|last4=Bearpark|first4=Michael J.|last5=Robb|first5=Michael A.|year=2010}}

{{As of|2021}} the only molecule confirmed to be a DIB carrier is the buckminsterfullerene ion, C₆₀⁺. Soon after Harry Kroto discovered fullerenes in the 1980s, he proposed that they could be DIB carriers.{{cite journal|bibcode=1988Sci...242.1139K|doi=10.1126/science.242.4882.1139|title=Space, Stars, C60, and Soot|year=1988|last1=Kroto|first1=H.|journal=Science|volume=242|issue=4882|pages=1139–1145|pmid=17799730|s2cid=22397657}} Kroto pointed out that the ionised form C₆₀⁺ was more likely to survive in the diffuse interstellar medium.{{cite conference|title=Chains and Grains in Interstellar Space|last=Kroto|first=H. W.|author-link=Harry Kroto|conference=Polycyclic Aromatic Hydrocarbons and Astrophysics|year=1987|publisher=Springer|series=NATO Advanced Study Institute Series C|volume=191|pages=197–206|editor-last=Leger|editor-first=Alain|bibcode=1987ASIC..191..197K|url=http://www.kroto.info/wp-content/uploads/2015/05/HWKroto-1987-discussing-the-possibiliy-of-C60-and-its-analogues-in-space-and-as-DIB-carriers.pdf|doi=10.1007/978-94-009-4776-4_17|isbn=978-94-010-8619-6}} However, the lack of a reliable laboratory spectrum of gas-phase C₆₀⁺ made this proposal difficult to test.{{cite journal|date=1993-08-13|title=Electronic and infrared spectra of C₆₀⁺ and C₆₀ in neon and argon matrices|journal=Chemical Physics Letters|volume=211|issue=2–3|pages=227–234|doi=10.1016/0009-2614(93)85190-Y|issn=0009-2614|last1=Fulara|first1=Jan|last2=Jakobi|first2=Michael|last3=Maier|first3=John P.|bibcode=1993CPL...211..227F}}

In the early 1990s, laboratory spectra of C₆₀⁺ were obtained by embedding the molecule in solid ices, which showed strong bands in the near-infrared. In 1994, Bernard Foing and Pascale Ehrenfreund detected new DIBs with wavelengths close to those in the laboratory spectra, and argued that the difference was due to an offset between the gas-phase and solid-phase wavelengths.{{cite journal|bibcode=1994Natur.369..296F|doi=10.1038/369296a0|title=Detection of two interstellar absorption bands coincident with spectral features of C₆₀⁺|year=1994|last1=Foing|first1=B. H.|last2=Ehrenfreund|first2=P.|journal=Nature|volume=369|issue=6478|pages=296–298|s2cid=4354516}} However, this conclusion was disputed by other researchers, such as Peter Jenniskens, on multiple spectroscopic and observational grounds.{{cite journal|bibcode=1997A&A...327..337J|url=http://aa.springer.de/papers/7327001/2300337/small.htm|title=Diffuse interstellar bands near 9600Å: Not due to C₆₀⁺ yet|last1=Jenniskens|first1=P.|last2=Mulas|first2=G.|last3=Porceddu|first3=I.|last4=Benvenuti|first4=P.|journal=Astronomy and Astrophysics|year=1997|volume=327|page=337}}

A laboratory gas-phase spectrum of C₆₀⁺ was obtained in 2015 by a group led by John Maier.{{cite journal|last1=Maier|first1=J. P.|last2=Gerlich|first2=D.|last3=Holz|first3=M.|last4=Campbell|first4=E. K.|date=July 2015|title=Laboratory confirmation of C₆₀⁺ as the carrier of two diffuse interstellar bands|journal=Nature|volume=523|issue=7560|pages=322–323|doi=10.1038/nature14566|issn=1476-4687|pmid=26178962|bibcode=2015Natur.523..322C|s2cid=205244293}} Their results matched the band wavelengths that had been observed by Foing and Ehrenfreund in 1994. Three weaker bands of C₆₀⁺ were found in interstellar spectra soon afterwards, resolving one of the earlier objections raised by Jenniskens.{{cite journal|last1=Campbell|first1=E. K.|last2=Holz|first2=M.|last3=Maier|first3=J. P.|last4=Gerlich|first4=D.|last5=Walker|first5=G. A. H.|last6=Bohlender|first6=D.|date=2016|title=Gas Phase Absorption Spectroscopy of C₆₀⁺ and C₇₀⁺ in a Cryogenic Ion Trap: Comparison with Astronomical Measurements|journal=The Astrophysical Journal|volume=822|issue=1|pages=17|doi=10.3847/0004-637X/822/1/17|bibcode=2016ApJ...822...17C|s2cid=29848456 |issn=0004-637X |doi-access=free }} New objections were raised by other researchers,{{cite journal|bibcode=2017MNRAS.465.3956G|doi=10.1093/mnras/stw2948|title=C₆₀⁺ – looking for the bucky-ball in interstellar space|year=2017|last1=Galazutdinov|first1=G. A.|last2=Shimansky|first2=V. V.|last3=Bondar|first3=A.|last4=Valyavin|first4=G.|last5=Krełowski|first5=J.|journal=Monthly Notices of the Royal Astronomical Society|volume=465|issue=4|pages=3956–3964|doi-access=free |arxiv=1612.08898}} but by 2019 the C₆₀⁺ bands and their assignment had been confirmed by multiple groups of astronomers{{cite journal|bibcode=2018A&A...614A..28L|doi=10.1051/0004-6361/201832647|title=The EDIBLES survey II. The detectability of C₆₀⁺ bands|year=2018|last1=Lallement|first1=R.|last2=Cox|first2=N. L. J.|last3=Cami|first3=J.|last4=Smoker|first4=J.|last5=Fahrang|first5=A.|last6=Elyajouri|first6=M.|last7=Cordiner|first7=M. A.|last8=Linnartz|first8=H.|last9=Smith|first9=K. T.|last10=Ehrenfreund|first10=P.|last11=Foing|first11=B. H.|journal=Astronomy & Astrophysics|volume=614|pages=A28|arxiv=1802.00369|s2cid=106399567}}{{cite journal|last1=Cordiner|first1=M.|last2=Linnartz|first2=H.|last3=Cox|first3=N.|last4=Cami|first4=J.|last5=Najarro|first5=F.|last6=Proffitt|first6=C.|last7=Lallement|first7=R.|last8=Ehrenfreund|first8=P.|last9=Foing|first9=B.|last10=Gull|first10=T.|last11=Sarre|first11=P.|last12=Charnley|first12=S.|date=2019|title=Confirming Interstellar C₆₀⁺ Using the Hubble Space Telescope|journal=The Astrophysical Journal Letters|volume=875|issue=2|pages=L28|doi=10.3847/2041-8213/ab14e5|issn=2041-8205|arxiv=1904.08821|bibcode=2019ApJ...875L..28C|s2cid=121292704 |doi-access=free }} and laboratory chemists.{{cite journal|bibcode=2017ApJ...846..168S|doi=10.3847/1538-4357/aa82bc|title=C₆₀⁺ and the Diffuse Interstellar Bands: An Independent Laboratory Check|year=2017|last1=Spieler|first1=Steffen|last2=Kuhn|first2=Martin|last3=Postler|first3=Johannes|last4=Simpson|first4=Malcolm|last5=Wester|first5=Roland|last6=Scheier|first6=Paul|last7=Ubachs|first7=Wim|last8=Bacalla|first8=Xavier|last9=Bouwman|first9=Jordy|last10=Linnartz|first10=Harold|journal=The Astrophysical Journal|volume=846|issue=2|page=168|arxiv=1707.09230|s2cid=119425018 |doi-access=free }}

{{Portal|Astronomy}}

• List of interstellar and circumstellar molecules

{{reflist}}

• [https://web.archive.org/web/20050310020146/http://www.daviddarling.info/encyclopedia/D/DIB.html Entry in the Encyclopedia of Astrobiology, Astronomy, and Spaceflight]
• [http://leonid.arc.nasa.gov/DIBcatalog.html Diffuse Interstellar Band Catalog]

{{Molecules detected in outer space}}

{{DEFAULTSORT:Diffuse Interstellar Band}}

Category:Astrochemistry

Category:Interstellar media

Category:Astronomical spectroscopy

Category:Unsolved problems in astronomy