Diffuse interstellar bands

Short description: Absorption features in astronomical spectra

Relative strengths of observed diffuse interstellar bands

Diffuse interstellar bands (DIBs) are absorption features seen in the spectra of astronomical objects in the Milky Way and other galaxies. They are caused by the absorption of light by the interstellar medium. Circa 500 bands have now been seen, in ultraviolet, visible and infrared wavelengths.[1]

The origin of most DIBs remains unknown, with common suggestions being polycyclic aromatic hydrocarbons and other large carbon-bearing molecules.[2][3] Only one DIB carrier has been identified: ionised buckminsterfullerene (C60+), which is responsible for several DIBs in the near-infrared.[4] The carriers of most DIBs remain unidentified.

## Contents

* 1 Discovery and history
* 2 The nature of the carriers
* 3 Identification of C60+ as a carrier
* 4 See also
* 5 References
* 6 External links

## Discovery and history

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[5] 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.[6][7][8] 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),[9] 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.[10] 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.[11][12]

## The nature of the carriers

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.[6][13] 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.[14][15]

## Identification of C60+ as a carrier

As of 2021[update] 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.[16] Kroto pointed out that the ionised form C60+ was more likely to survive in the diffuse interstellar medium.[17][16] However, the lack of a reliable laboratory spectrum of gas-phase C60+ made this proposal difficult to test.[18]

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.[19] However, this conclusion was disputed by other researchers, such as Peter Jenniskens, on multiple spectroscopic and observational grounds.[20]

A laboratory gas-phase spectrum of C60+ was obtained in 2015 by a group led by John Maier.[21] Their results matched the band wavelengths that had been observed by Foing and Ehrenfreund in 1994.[21] Three weaker bands of C60+ were found in interstellar spectra soon afterwards, resolving one of the earlier objections raised by Jenniskens.[22] New objections were raised by other researchers,[23] but by 2019 the C60+ bands and their assignment had been confirmed by multiple groups of astronomers[24][25] and laboratory chemists.[26]

## See also

* List of interstellar and circumstellar molecules

## References

1. ↑ "ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) - Merging Observations and Laboratory Data". 2016-03-29. https://ntrs.nasa.gov/search.jsp?R=20160004200.
2. ↑ Bierbaum, Veronica M.; Keheyan, Yeghis; Page, Valery Le; Snow, Theodore P. (January 1998). "The interstellar chemistry of PAH cations". Nature 391 (6664): 259–260. doi:10.1038/34602. PMID 9440689. Bibcode: 1998Natur.391..259S.
3. ↑ Snow, Theodore P. (2001-03-15). "The unidentified diffuse interstellar bands as evidence for large organic molecules in the interstellar medium". Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 57 (4): 615–626. doi:10.1016/S1386-1425(00)00432-7. PMID 11345242. Bibcode: 2001AcSpA..57..615S.
4. ↑ Campbell, E. K.; Holz, M.; Gerlich, D.; Maier, J. P. (2015). "Laboratory confirmation of C60+ as the carrier of two diffuse interstellar bands". Nature 523 (7560): 322–3. doi:10.1038/nature14566. PMID 26178962. Bibcode: 2015Natur.523..322C.
5. ↑ Heger, M. L. (1922). "Further study of the sodium lines in class B stars". Lick Observatory Bulletin 10 (337): 141–148. doi:10.5479/ADS/bib/1922LicOB.10.141H. Bibcode: 1922LicOB..10..141H.
6. ↑ 6.0 6.1 Herbig, G. H. (1995). "The Diffuse Interstellar Bands". Annual Review of Astronomy and Astrophysics 33: 19–73. doi:10.1146/annurev.aa.33.090195.000315. Bibcode: 1995ARA&A..33...19H.
7. ↑ Krelowski, J. (1989). "Diffuse interstellar bands - An observational review". Astronomische Nachrichten 310 (4): 255–263. doi:10.1002/asna.2113100403. Bibcode: 1989AN....310..255K.
8. ↑ Sollerman, J. (2005). "Diffuse Interstellar Bands in NGC 1448". Astronomy and Astrophysics 429 (2): 559–567. doi:10.1051/0004-6361:20041465. Bibcode: 2005A&A...429..559S.
9. ↑ Jenniskens, P.; Desert, F.-X. (1994). "A survey of diffuse interstellar bands (3800-8680 A)". Astronomy and Astrophysics Supplement Series 106: 39. Bibcode: 1994A&AS..106...39J.
10. ↑ Fossey, S. J.; Crawford, I. A. (2000). "Observing with the Ultra-High-Resolution Facility at the Anglo-Australian Telescope: Structure of Diffuse Interstellar Bands". Bulletin of the American Astronomical Society 32: 727. Bibcode: 2000AAS...196.3501F.
11. ↑ Jenniskens, P.; Desert, F. X. (1993). "Complex Structure in Two Diffuse Interstellar Bands". Astronomy and Astrophysics 274: 465. Bibcode: 1993A&A...274..465J.
12. ↑ Galazutdinov, G. (2002). "Fine structure of profiles of weak diffuse interstellar bands". Astronomy and Astrophysics 396 (3): 987–991. doi:10.1051/0004-6361:20021299. Bibcode: 2002A&A...396..987G.
13. ↑ Ehrenfreund, P. (1999). "The Diffuse Interstellar Bands as evidence for polyatomic molecules in the diffuse interstellar medium". Bulletin of the American Astronomical Society 31: 880. Bibcode: 1999AAS...194.4101E.
14. ↑ Zhao, Liang; Lian, Rui; Shkrob, Ilya A.; Crowell, Robert A.; Pommeret, Stanislas; Chronister, Eric L.; Liu, An Dong; Trifunac, Alexander D. (2004). "Ultrafast Studies on the Photophysics of Matrix-Isolated Radical Cations of Polycyclic Aromatic Hydrocarbons". The Journal of Physical Chemistry A 108 (1): 25–31. doi:10.1021/jp021832h. Bibcode: 2004JPCA..108...25Z.
15. ↑ Tokmachev, Andrei M.; Boggio-Pasqua, Martial; Mendive-Tapia, David; Bearpark, Michael J.; Robb, Michael A. (2010). "Fluorescence of the perylene radical cation and an inaccessible D0/D1 conical intersection: An MMVB, RASSCF, and TD-DFT computational study". The Journal of Chemical Physics 132 (4): 044306. doi:10.1063/1.3278545. PMID 20113032. Bibcode: 2010JChPh.132d4306T.
16. ↑ 16.0 16.1 Kroto, H. (1988). "Space, Stars, C60, and Soot". Science 242 (4882): 1139–1145. doi:10.1126/science.242.4882.1139. PMID 17799730. Bibcode: 1988Sci...242.1139K.
17. ↑ Kroto, H. W. (1987). "Chains and Grains in Interstellar Space". in Leger, Alain. Polycyclic Aromatic Hydrocarbons and Astrophysics. 191. Springer. pp. 197–206. doi:10.1007/978-94-009-4776-4_17. ISBN 978-94-010-8619-6. Bibcode: 1987ASIC..191..197K. 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.
18. ↑ Fulara, Jan; Jakobi, Michael; Maier, John P. (1993-08-13). "Electronic and infrared spectra of C60+ and C60 in neon and argon matrices". Chemical Physics Letters 211 (2–3): 227–234. doi:10.1016/0009-2614(93)85190-Y. ISSN 0009-2614. Bibcode: 1993CPL...211..227F.
19. ↑ Foing, B. H.; Ehrenfreund, P. (1994). "Detection of two interstellar absorption bands coincident with spectral features of C60+". Nature 369 (6478): 296–298. doi:10.1038/369296a0. Bibcode: 1994Natur.369..296F.
20. ↑ Jenniskens, P.; Mulas, G.; Porceddu, I.; Benvenuti, P. (1997). "Diffuse interstellar bands near 9600Å: Not due to C60+ yet". Astronomy and Astrophysics 327: 337. Bibcode: 1997A&A...327..337J. http://aa.springer.de/papers/7327001/2300337/small.htm.
21. ↑ 21.0 21.1 Maier, J. P.; Gerlich, D.; Holz, M.; Campbell, E. K. (July 2015). "Laboratory confirmation of C60+ as the carrier of two diffuse interstellar bands". Nature 523 (7560): 322–323. doi:10.1038/nature14566. ISSN 1476-4687. PMID 26178962. Bibcode: 2015Natur.523..322C.
22. ↑ Campbell, E. K.; Holz, M.; Maier, J. P.; Gerlich, D.; Walker, G. A. H.; Bohlender, D. (2016). "Gas Phase Absorption Spectroscopy of C60+ and C70+ in a Cryogenic Ion Trap: Comparison with Astronomical Measurements". The Astrophysical Journal 822 (1): 17. doi:10.3847/0004-637X/822/1/17. ISSN 0004-637X. Bibcode: 2016ApJ...822...17C.
23. ↑ Galazutdinov, G. A.; Shimansky, V. V.; Bondar, A.; Valyavin, G.; Krełowski, J. (2017). "C60+ – looking for the bucky-ball in interstellar space". Monthly Notices of the Royal Astronomical Society 465 (4): 3956–3964. doi:10.1093/mnras/stw2948. Bibcode: 2017MNRAS.465.3956G.
24. ↑ Lallement, R.; Cox, N. L. J.; Cami, J.; Smoker, J.; Fahrang, A.; Elyajouri, M.; Cordiner, M. A.; Linnartz, H. et al. (2018). "The EDIBLES survey II. The detectability of C60+ bands". Astronomy & Astrophysics 614: A28. doi:10.1051/0004-6361/201832647. Bibcode: 2018A&A...614A..28L.
25. ↑ Cordiner, M.; Linnartz, H.; Cox, N.; Cami, J.; Najarro, F.; Proffitt, C.; Lallement, R.; Ehrenfreund, P. et al. (2019). "Confirming Interstellar C60+ Using the Hubble Space Telescope". The Astrophysical Journal Letters 875 (2): L28. doi:10.3847/2041-8213/ab14e5. ISSN 2041-8205. Bibcode: 2019ApJ...875L..28C.
26. ↑ Spieler, Steffen; Kuhn, Martin; Postler, Johannes; Simpson, Malcolm; Wester, Roland; Scheier, Paul; Ubachs, Wim; Bacalla, Xavier et al. (2017). "C60+ and the Diffuse Interstellar Bands: An Independent Laboratory Check". The Astrophysical Journal 846 (2): 168. doi:10.3847/1538-4357/aa82bc. Bibcode: 2017ApJ...846..168S.

## External links

* Entry in the Encyclopedia of Astrobiology, Astronomy, and Spaceflight
* Diffuse Interstellar Band Catalog

* v
* t
* e

Molecules detected in outer space

Molecules|

| Diatomic|

* Aluminium monochloride
* Aluminium monofluoride
* Aluminium monoxide
* Argonium
* Carbon monophosphide
* Carbon monosulfide
* Carbon monoxide
* Carborundum
* Cyanogen radical
* Diatomic carbon
* Fluoromethylidynium
* Helium hydride ion
* Hydrogen chloride
* Hydrogen fluoride
* Hydrogen (molecular)
* Hydroxyl radical
* Iron(II) oxide
* Magnesium monohydride cation
* Methylidyne radical
* Nitric oxide
* Nitrogen (molecular)
* Nitrogen monohydride
* Nitrogen sulfide
* Oxygen (molecular)
* Phosphorus monoxide
* Phosphorus mononitride
* Potassium chloride
* Silicon carbide
* Silicon mononitride
* Silicon monoxide
* Silicon monosulfide
* Sodium chloride
* Sodium iodide
* Sulfur monohydride
* Sulfur monoxide
* Titanium oxide

|
|

Triatomic|

* Aluminium hydroxide
* Aluminium isocyanide
* Amino radical
* Carbon dioxide
* Carbonyl sulfide
* CCP radical
* Chloronium
* Diazenylium
* Dicarbon monoxide
* Disilicon carbide
* Ethynyl radical
* Formyl radical
* Hydrogen cyanide (HCN)
* Hydrogen isocyanide (HNC)
* Hydrogen sulfide
* Hydroperoxyl
* Iron cyanide
* Isoformyl
* Magnesium cyanide
* Magnesium isocyanide
* Methylene radical
* N2H+
* Nitrous oxide
* Nitroxyl
* Ozone
* Phosphaethyne
* Potassium cyanide
* Protonated molecular hydrogen
* Sodium cyanide
* Sodium hydroxide
* Silicon carbonitride
* c-Silicon dicarbide
* SiNC
* Sulfur dioxide
* Thioformyl
* Thioxoethenylidene
* Titanium dioxide
* Tricarbon
* Water

Four
atoms|

* Acetylene
* Ammonia
* Cyanic acid
* Cyanoethynyl
* Cyclopropynylidyne
* Formaldehyde
* Fulminic acid
* HCCN
* Hydrogen peroxide
* Hydromagnesium isocyanide
* Isocyanic acid
* Isothiocyanic acid
* Ketenyl
* Methylene amidogen
* Methyl radical
* Propynylidyne
* Protonated carbon dioxide
* Protonated hydrogen cyanide
* Silicon tricarbide
* Thioformaldehyde
* Tricarbon monoxide
* Tricarbon sulfide
* Thiocyanic acid

Five
atoms|

* Ammonium ion
* Butadiynyl
* Carbodiimide
* Cyanamide
* Cyanoacetylene
* Cyanoformaldehyde
* Cyanomethyl
* Cyclopropenylidene
* Formic acid
* Isocyanoacetylene
* Ketene
* Methane
* Methoxy radical
* Methylenimine
* Propadienylidene
* Protonated formaldehyde
* Protonated formaldehyde
* Silane
* Silicon-carbide cluster

Six
atoms|

* Acetonitrile
* Cyanobutadiynyl radical
* E-Cyanomethanimine
* Cyclopropenone
* Diacetylene
* Ethylene
* Formamide
* HC4N
* Ketenimine
* Methanethiol
* Methanol
* Methyl isocyanide
* Pentynylidyne
* Propynal
* Protonated cyanoacetylene

Seven
atoms|

* Acetaldehyde
* Acrylonitrile
* Vinyl cyanide
* Cyanodiacetylene
* Ethylene oxide
* Glycolonitrile
* Hexatriynyl radical
* Methylacetylene
* Methylamine
* Methyl isocyanate
* Vinyl alcohol

Eight
atoms|

* Acetic acid
* Aminoacetonitrile
* Cyanoallene
* Ethanimine
* Glycolaldehyde
* Heptatrienyl radical
* Hexapentaenylidene
* Methylcyanoacetylene
* Methyl formate
* Propenal

Nine
atoms|

* Acetamide
* Cyanohexatriyne
* Cyanotriacetylene
* Dimethyl ether
* Ethanol
* Methyldiacetylene
* Octatetraynyl radical
* Propene
* Propionitrile

Ten
atoms
or more|

* Acetone
* Benzene
* Benzonitrile
* Buckminsterfullerene (C60, C60+, fullerene, buckyball)
* C70 fullerene
* Cyanodecapentayne
* Cyanopentaacetylene
* Cyanotetra-acetylene
* Ethylene glycol
* Ethyl formate
* Methyl acetate
* Methyl-cyano-diacetylene
* Methyltriacetylene
* Propanal
* n-Propyl cyanide
* Pyrimidine

Deuterated
molecules|

* Ammonia
* Ammonium ion
* Formaldehyde
* Formyl radical
* Heavy water
* Hydrogen cyanide
* Hydrogen deuteride
* Hydrogen isocyanide
* Methylacetylene
* N2D+
* Trihydrogen cation

Unconfirmed|

* Anthracene
* Dihydroxyacetone
* Ethyl methyl ether
* Glycine
* Graphene
* H2NCO+
* Linear C5
* Naphthalene cation
* Phosphine
* Pyrene
* Silylidine

Related|

* Abiogenesis
* Astrobiology
* Astrochemistry
* Atomic and molecular astrophysics
* Chemical formula
* Circumstellar envelope
* Cosmic dust
* Cosmic ray
* Cosmochemistry
* Diffuse interstellar band
* Earliest known life forms
* Extraterrestrial life
* Extraterrestrial liquid water
* Forbidden mechanism
* Homochirality
* Intergalactic dust
* Interplanetary medium
* Interstellar medium
* Photodissociation region
* Iron–sulfur world theory
* Kerogen
* Molecules in stars
* Nexus for Exoplanet System Science
* Organic compound
* Outer space
* PAH world hypothesis
* Panspermia
* Polycyclic aromatic hydrocarbon (PAH)
* RNA world hypothesis
* Spectroscopy
* Tholin

* Category:Astrochemistry
*   *   *

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