Sometimes a transition can involve more than one of these types of energy level e.g. ro-vibrational spectroscopy changes both the rotational and vibrational energy level. Occasionally all three occur together, as in the Phillips band of C2 (diatomic carbon), in which an electronic transition produces a line in the near-infrared, which is then split into several vibronic bands by a simultaneous change in vibrational level, which in turn are split again into rotational branches.[2]
The spectrum of a particular molecule is governed by the selection rules of quantum chemistry and by its molecular symmetry. Some molecules have simple spectra which are easy to identify, whilst others (even some small molecules) have extremely complex spectra with flux spread among many different lines, making them far harder to detect.[3] Interactions between the atomic nuclei and the electrons sometimes cause further hyperfine structure of the spectral lines. If the molecule exists in multiple isotopologues (versions containing different atomic isotopes), the spectrum is further complicated by isotope shifts.
Detection of a new interstellar or circumstellar molecule requires identifying a suitable astronomical object where it is likely to be present, then observing it with a telescope equipped with a spectrograph working at the required wavelength, spectral resolution and sensitivity. The first molecule detected in the interstellar medium was the methylidyne radical (CH•) in 1937, through its strong electronic transition at 4300 angstroms (in the optical).[4] Advances in astronomical instrumentation have led to increasing numbers of new detections. From the 1950s onwards, radio astronomy began to dominate new detections, with sub-mm astronomy also becoming important from the 1990s.[3]
The inventory of detected molecules is highly biased towards certain types which are easier to detect: e.g. radio astronomy is most sensitive to small linear molecules with a high molecular dipole.[3] The most common molecule in the Universe, H2 (molecular hydrogen), is completely invisible to radio telescopes because it has no dipole;[3] its electronic transitions are too energetic for optical telescopes, so detection of H2 required ultraviolet observations with a sounding rocket.[5] Vibrational lines are often not specific to an individual molecule, allowing only the general class to be identified. For example, the vibrational lines of polycyclic aromatic hydrocarbons (PAHs) were identified in 1984,[6] showing the class of molecules is very common in space,[7] but it took until 2021 to identify any specific PAHs through their rotational lines.[8][9]
One of the richest sources for detecting interstellar molecules is Sagittarius B2 (Sgr B2), a giant molecular cloud near the centre of the Milky Way. About half of the molecules listed below were first found in Sgr B2, and many of the others have been subsequently detected there.[10] A rich source of circumstellar molecules is CW Leonis (also known as IRC +10216), a nearby carbon star, where about 50 molecules have been identified.[11] There is no clear boundary between interstellar and circumstellar media, so both are included in the tables below.
The discipline of astrochemistry includes understanding how these molecules form and explaining their abundances. The extremely low density of the interstellar medium is not conducive to the formation of molecules, making conventional gas-phase reactions between neutral species (atoms or molecules) inefficient. Many regions also have very low temperatures (typically 10 kelvin inside a molecular cloud), further reducing the reaction rates, or high ultraviolet radiation fields, which destroy molecules through photochemistry.[12] Explaining the observed abundances of interstellar molecules requires calculating the balance between formation and destruction rates using gas-phase ion chemistry (often driven by cosmic rays), surface chemistry on cosmic dust, radiative transfer including interstellar extinction, and sophisticated reaction networks.[13] The use of molecular lines to determine the physical properties of astronomical objects is known as molecular astrophysics.
Molecules
The following tables list molecules that have been detected in the interstellar medium or circumstellar matter, grouped by the number of component atoms. Neutral molecules and their molecular ions are listed in separate columns; if there is no entry in the molecule column, only the ionized form has been detected. Designations (names of molecules) are those used in the scientific literature describing the detection; if none was given that field is left empty. Mass is listed in atomic mass units. Deuterated molecules, which contain at least one deuterium (2H) atom, have slightly different masses and are listed in a separate table. The total number of unique species, including distinct ionization states, is indicated in each section header.
Most of the molecules detected so far are organic. The only detected inorganic molecule with five or more atoms is SiH4.[14] Molecules larger than that all have at least one carbon atom, with no N−N or O−O bonds.[14]
Evidence for the existence of the following molecules has been reported in the scientific literature, but the detections either are described as tentative by the authors, or have been challenged by other researchers. They await independent confirmation.
^Chaffee, Frederick H.; Lutz, Barry L.; Black, John H.; Vanden Bout, Paul A.; Snell, Ronald L. (1980). "Rotational fine-structure lines of interstellar C2 toward Zeta Persei". The Astrophysical Journal. 236: 474. Bibcode:1980ApJ...236..474C. doi:10.1086/157764.
^Leger, A.; Puget, J. L. (1984). "Identification of the "unidentified" IR emission features of interstellar dust ?". Astronomy and Astrophysics. 137: L5. Bibcode:1984A&A...137L...5L.
^ abcMcGuire, Brett A.; Loomis, Ryan A.; Burkhardt, Andrew M.; Lee, Kin Long Kelvin; Shingledecker, Christopher N.; Charnley, Steven B.; Cooke, Ilsa R.; Cordiner, Martin A.; Herbst, Eric; Kalenskii, Sergei; Siebert, Mark A.; Willis, Eric R.; Xue, Ci; Remijan, Anthony J.; McCarthy, Michael C. (19 March 2021). "Detection of two interstellar polycyclic aromatic hydrocarbons via spectral matched filtering". Science. 371 (6535): 1265–1269. arXiv:2103.09984. Bibcode:2021Sci...371.1265M. doi:10.1126/science.abb7535. PMID33737489. S2CID232269920.
^Cummins, S. E.; Linke, R. A.; Thaddeus, P. (1986), "A survey of the millimeter-wave spectrum of Sagittarius B2", Astrophysical Journal Supplement Series, 60: 819–878, Bibcode:1986ApJS...60..819C, doi:10.1086/191102
^Brown, Laurie M.; Pais, Abraham; Pippard, A. B. (1995), "The physics of the interstellar medium", Twentieth Century Physics (2nd ed.), CRC Press, p. 1765, ISBN978-0-7503-0310-1
^ abcCernicharo, J.; Guelin, M. (1987), "Metals in IRC+10216 - Detection of NaCl, AlCl, and KCl, and tentative detection of AlF", Astronomy and Astrophysics, 183 (1): L10–L12, Bibcode:1987A&A...183L..10C
^Ziurys, L. M.; Apponi, A. J.; Phillips, T. G. (1994), "Exotic fluoride molecules in IRC +10216: Confirmation of AlF and searches for MgF and CaF", Astrophysical Journal, 433 (2): 729–732, Bibcode:1994ApJ...433..729Z, doi:10.1086/174682
^Tenenbaum, E. D.; Ziurys, L. M. (2009), "Millimeter Detection of AlO (X2Σ+): Metal Oxide Chemistry in the Envelope of VY Canis Majoris", Astrophysical Journal, 694 (1): L59–L63, Bibcode:2009ApJ...694L..59T, doi:10.1088/0004-637X/694/1/L59
^Souza, S. P; Lutz, B. L (1977). "Detection of C2 in the interstellar spectrum of Cygnus OB2 number 12 /VI Cygni number 12/". The Astrophysical Journal. 216: L49. Bibcode:1977ApJ...216L..49S. doi:10.1086/182507.
^Lambert, D. L.; Sheffer, Y.; Federman, S. R. (1995), "Hubble Space Telescope observations of C2 molecules in diffuse interstellar clouds", Astrophysical Journal, 438: 740–749, Bibcode:1995ApJ...438..740L, doi:10.1086/175119
^ abAdams, Walter S. (1941), "Some Results with the COUDÉ Spectrograph of the Mount Wilson Observatory", Astrophysical Journal, 93: 11–23, Bibcode:1941ApJ....93...11A, doi:10.1086/144237
^ abGuelin, M.; Cernicharo, J.; Paubert, G.; Turner, B. E. (1990), "Free CP in IRC + 10216", Astronomy and Astrophysics, 230: L9–L11, Bibcode:1990A&A...230L...9G
^ abcDopita, Michael A.; Sutherland, Ralph S. (2003), Astrophysics of the diffuse universe, Springer-Verlag, ISBN978-3-540-43362-0
^Dent, W.R.F.; Wyatt, M.C.; Roberge, A.; Augereau, J.-C.; Casassus, S.; Corder, S.; Greaves, J.S.; de Gregorio-Monsalvo, I; Hales, A.; Jackson, A.P.; Hughes, A. Meredith; Lagrange, A.-M; Matthews, B.; Wilner, D. (March 6, 2014). "Molecular Gas Clumps from the Destruction of Icy Bodies in the β Pictoris Debris Disk". Science. 343 (6178): 1490–1492. arXiv:1404.1380. Bibcode:2014Sci...343.1490D. doi:10.1126/science.1248726. PMID24603151. S2CID206553853.
^Latter, W. B.; Walker, C. K.; Maloney, P. R. (1993), "Detection of the Carbon Monoxide Ion (CO+) in the Interstellar Medium and a Planetary Nebula", Astrophysical Journal Letters, 419: L97, Bibcode:1993ApJ...419L..97L, doi:10.1086/187146
^Wagenblast, R.; et al. (January 1993), "On the origin of NH in diffuse interstellar clouds", Monthly Notices of the Royal Astronomical Society, 260 (2): 420–424, Bibcode:1993MNRAS.260..420W, doi:10.1093/mnras/260.2.420
^Ziurys, L. M. (1987), "Detection of interstellar PN - The first phosphorus-bearing species observed in molecular clouds", Astrophysical Journal Letters, 321 (1 Pt 2): L81–L85, Bibcode:1987ApJ...321L..81Z, doi:10.1086/185010, PMID11542218
^Tenenbaum, E. D.; Woolf, N. J.; Ziurys, L. M. (2007), "Identification of phosphorus monoxide (X 2 Pi r) in VY Canis Majoris: Detection of the first PO bond in space", Astrophysical Journal Letters, 666 (1): L29–L32, Bibcode:2007ApJ...666L..29T, doi:10.1086/521361, S2CID121424802
^Tenenbaum, E. D.; Ziurys, L. M. (2010), "Exotic Metal Molecules in Oxygen-rich Envelopes: Detection of AlOH (X1Σ+) in VY Canis Majoris", Astrophysical Journal, 712 (1): L93–L97, Bibcode:2010ApJ...712L..93T, doi:10.1088/2041-8205/712/1/L93
^ abcdIrvine, William M.; et al. (1988), "Newly detected molecules in dense interstellar clouds", Astrophysical Letters and Communications, 26: 167–180, Bibcode:1988ApL&C..26..167I, PMID11538461
^Halfen, D. T.; Clouthier, D. J.; Ziurys, L. M. (2008), "Detection of the CCP Radical (X 2Πr) in IRC +10216: A New Interstellar Phosphorus-containing Species", Astrophysical Journal, 677 (2): L101–L104, Bibcode:2008ApJ...677L.101H, doi:10.1086/588024
^Whittet, Douglas C. B.; Walker, H. J. (1991), "On the occurrence of carbon dioxide in interstellar grain mantles and ion-molecule chemistry", Monthly Notices of the Royal Astronomical Society, 252: 63–67, Bibcode:1991MNRAS.252...63W, doi:10.1093/mnras/252.1.63
^Zack, L. N.; Halfen, D. T.; Ziurys, L. M. (June 2011), "Detection of FeCN (X 4Δi) in IRC+10216: A New Interstellar Molecule", The Astrophysical Journal Letters, 733 (2): L36, Bibcode:2011ApJ...733L..36Z, doi:10.1088/2041-8205/733/2/L36
^Hollis, J. M.; Jewell, P. R.; Lovas, F. J. (1995), "Confirmation of interstellar methylene", Astrophysical Journal, Part 1, 438: 259–264, Bibcode:1995ApJ...438..259H, doi:10.1086/175070
^Snyder, L. E.; Buhl, D. (1971), "Observations of Radio Emission from Interstellar Hydrogen Cyanide", Astrophysical Journal, 163: L47–L52, Bibcode:1971ApJ...163L..47S, doi:10.1086/180664
^ abSchilke, P.; Benford, D. J.; Hunter, T. R.; Lis, D. C., Phillips, T. G.; Phillips, T. G. (2001), "A Line Survey of Orion-KL from 607 to 725 GHz", Astrophysical Journal Supplement Series, 132 (2): 281–364, Bibcode:2001ApJS..132..281S, doi:10.1086/318951{{citation}}: CS1 maint: multiple names: authors list (link)
^Schilke, P.; Comito, C.; Thorwirth, S. (2003), "First Detection of Vibrationally Excited HNC in Space", The Astrophysical Journal, 582 (2): L101–L104, Bibcode:2003ApJ...582L.101S, doi:10.1086/367628
^ abSchenewerk, M. S.; Snyder, L. E.; Hjalmarson, A. (1986), "Interstellar HCO - Detection of the missing 3 millimeter quartet", Astrophysical Journal Letters, 303: L71–L74, Bibcode:1986ApJ...303L..71S, doi:10.1086/184655
^Womack, M.; Ziurys, L. M.; Wyckoff, S. (1992), "A survey of N2H(+) in dense clouds - Implications for interstellar nitrogen and ion-molecule chemistry", Astrophysical Journal, Part 1, 387: 417–429, Bibcode:1992ApJ...387..417W, doi:10.1086/171094
^Hollis, J. M.; et al. (1991), "Interstellar HNO: Confirming the Identification - Atoms, ions and molecules: New results in spectral line astrophysics", Atoms, 16: 407–412, Bibcode:1991ASPC...16..407H
^Ziurys, L. M.; et al. (1994), "Detection of interstellar N2O: A new molecule containing an N-O bond", Astrophysical Journal Letters, 436: L181–L184, Bibcode:1994ApJ...436L.181Z, doi:10.1086/187662
^Hollis, J. M.; Rhodes, P. J. (November 1, 1982), "Detection of interstellar sodium hydroxide in self-absorption toward the galactic center", Astrophysical Journal Letters, 262: L1–L5, Bibcode:1982ApJ...262L...1H, doi:10.1086/183900
^Goldsmith, P. F.; Linke, R. A. (1981), "A study of interstellar carbonyl sulfide", Astrophysical Journal, Part 1, 245: 482–494, Bibcode:1981ApJ...245..482G, doi:10.1086/158824
^Phillips, T. G.; Knapp, G. R. (1980), "Interstellar Ozone", American Astronomical Society Bulletin, 12: 440, Bibcode:1980BAAS...12..440P
^ abcdefghijJohansson, L. E. B.; et al. (1984), "Spectral scan of Orion A and IRC+10216 from 72 to 91 GHz", Astronomy and Astrophysics, 130 (2): 227–256, Bibcode:1984A&A...130..227J
^ abIrvine, W. M.; et al. (1984), "Confirmation of the Existence of Two New Interstellar Molecules: C3H and C3O", Bulletin of the American Astronomical Society, 16: 877, Bibcode:1984BAAS...16..877I
^Mangum, J. G.; Wootten, A. (1990), "Observations of the cyclic C3H radical in the interstellar medium", Astronomy and Astrophysics, 239: 319–325, Bibcode:1990A&A...239..319M
^Bell, M. B.; Matthews, H. E. (1995), "Detection of C3N in the spiral arm gas clouds in the direction of Cassiopeia A", Astrophysical Journal, Part 1, 438: 223–225, Bibcode:1995ApJ...438..223B, doi:10.1086/175066
^Minh, Y. C.; Irvine, W. M.; Brewer, M. K. (1991), "H2CS abundances and ortho-to-para ratios in interstellar clouds", Astronomy and Astrophysics, 244: 181–189, Bibcode:1991A&A...244..181M, PMID11538284
^Guelin, M.; Cernicharo, J. (1991), "Astronomical detection of the HCCN radical - Toward a new family of carbon-chain molecules?", Astronomy and Astrophysics, 244: L21–L24, Bibcode:1991A&A...244L..21G
^Minh, Y. C.; Irvine, W. M.; Ziurys, L. M. (1988), "Observations of interstellar HOCO(+) - Abundance enhancements toward the Galactic center", Astrophysical Journal, Part 1, 334 (1): 175–181, Bibcode:1988ApJ...334..175M, doi:10.1086/166827, PMID11538465
^Frerking, M. A.; Linke, R. A.; Thaddeus, P. (1979), "Interstellar isothiocyanic acid", Astrophysical Journal Letters, 234: L143–L145, Bibcode:1979ApJ...234L.143F, doi:10.1086/183126
^ abNguyen-Q-Rieu; Graham, D.; Bujarrabal, V. (1984), "Ammonia and cyanotriacetylene in the envelopes of CRL 2688 and IRC + 10216", Astronomy and Astrophysics, 138 (1): L5–L8, Bibcode:1984A&A...138L...5N
^Coutens, A.; Ligterink, N. F. W.; Loison, J.-C.; Wakelam, V.; Calcutt, H.; Drozdovskaya, M. N.; Jørgensen, J. K.; Müller, H. S. P.; Van Dishoeck, E. F.; Wampfler, S. F. (2019). "The ALMA-PILS survey: First detection of nitrous acid (HONO) in the interstellar medium". Astronomy & Astrophysics. 623: L13. arXiv:1903.03378. Bibcode:2019A&A...623L..13C. doi:10.1051/0004-6361/201935040. S2CID119274002.
^Butterworth, Anna L.; et al. (2004), "Combined element (H and C) stable isotope ratios of methane in carbonaceous chondrites", Monthly Notices of the Royal Astronomical Society, 347 (3): 807–812, Bibcode:2004MNRAS.347..807B, doi:10.1111/j.1365-2966.2004.07251.x
^Cernicharo, J.; Tercero, B.; Fuente, A.; Domenech, J. L.; Cueto, M.; Carrasco, E.; Herrero, V. J.; Tanarro, I.; Marcelino, N.; Roueff, E.; Gerin, M.; Pearson, J. (18 June 2013). "Detection of the Ammonium Ion in Space". The Astrophysical Journal. 771 (1): L10. arXiv:1306.3364. Bibcode:2013ApJ...771L..10C. doi:10.1088/2041-8205/771/1/L10. S2CID118461954.
^Lacy, J. H.; et al. (1991), "Discovery of interstellar methane - Observations of gaseous and solid CH4 absorption toward young stars in molecular clouds", Astrophysical Journal, 376: 556–560, Bibcode:1991ApJ...376..556L, doi:10.1086/170304
^Cernicharo, J.; Marcelino, N.; Roueff, E.; Gerin, M.; Jiménez-Escobar, A.; Muñoz Caro, G. M. (2012), "Discovery of the Methoxy Radical, CH3O, toward B1: Dust Grain and Gas-phase Chemistry in Cold Dark Clouds", The Astrophysical Journal Letters, 759 (2): L43–L46, Bibcode:2012ApJ...759L..43C, doi:10.1088/2041-8205/759/2/L43, S2CID95954921
^Irvine, W. M.; et al. (1988), "Identification of the interstellar cyanomethyl radical (CH2CN) in the molecular clouds TMC-1 and Sagittarius B2", Astrophysical Journal Letters, 334 (2): L107–L111, Bibcode:1988ApJ...334L.107I, doi:10.1086/185323, PMID11538463
^ abcLiu, S.-Y.; Mehringer, D. M.; Snyder, L. E. (2001), "Observations of Formic Acid in Hot Molecular Cores", Astrophysical Journal, 552 (2): 654–663, Bibcode:2001ApJ...552..654L, doi:10.1086/320563
^ abWalmsley, C. M.; Winnewisser, G.; Toelle, F. (1990), "Cyanoacetylene and cyanodiacetylene in interstellar clouds", Astronomy and Astrophysics, 81 (1–2): 245–250, Bibcode:1980A&A....81..245W
^Hollis, J. M.; et al. (2006), "Cyclopropenone (c-H2C3O): A New Interstellar Ring Molecule", Astrophysical Journal, 642 (2): 933–939, Bibcode:2006ApJ...642..933H, doi:10.1086/501121
^Guelin, M.; Neininger, N.; Cernicharo, J. (1998), "Astronomical detection of the cyanobutadiynyl radical C_5N", Astronomy and Astrophysics, 335: L1–L4, arXiv:astro-ph/9805105, Bibcode:1998A&A...335L...1G
^Irvine, W. M.; et al. (1988), "A new interstellar polyatomic molecule - Detection of propynal in the cold cloud TMC-1", Astrophysical Journal Letters, 335 (2): L89–L93, Bibcode:1988ApJ...335L..89I, doi:10.1086/185346, PMID11538462
^ abMehringer, David M.; et al. (1997), "Detection and Confirmation of Interstellar Acetic Acid", Astrophysical Journal Letters, 480 (1): L71, Bibcode:1997ApJ...480L..71M, doi:10.1086/310612
^ abLovas, F. J.; et al. (2006), "Hyperfine Structure Identification of Interstellar Cyanoallene toward TMC-1", Astrophysical Journal Letters, 637 (1): L37–L40, Bibcode:2006ApJ...637L..37L, doi:10.1086/500431
^Guelin, M.; et al. (1997), "Detection of a new linear carbon chain radical: C7H", Astronomy and Astrophysics, 317: L37–L40, Bibcode:1997A&A...317L...1G
^Cernicharo, J.; Guelin, M. (1996), "Discovery of the C8H radical", Astronomy and Astrophysics, 309: L26–L30, Bibcode:1996A&A...309L..27C
^Brünken, S.; et al. (2007), "Detection of the Carbon Chain Negative Ion C8H− in TMC-1", Astrophysical Journal, 664 (1): L43–L46, Bibcode:2007ApJ...664L..43B, doi:10.1086/520703
^Kroto, H. W.; et al. (1978), "The detection of cyanohexatriyne, H (C≡ C)3CN, in Heiles's cloud 2", The Astrophysical Journal, 219: L133–L137, Bibcode:1978ApJ...219L.133K, doi:10.1086/182623
^Lis, D. C.; et al. (2002), "Detection of Triply Deuterated Ammonia in the Barnard 1 Cloud", Astrophysical Journal, 571 (1): L55–L58, Bibcode:2002ApJ...571L..55L, doi:10.1086/341132.
^Turner, B. E. (1990), "Detection of doubly deuterated interstellar formaldehyde (D2CO) - an indicator of active grain surface chemistry", Astrophysical Journal Letters, 362: L29–L33, Bibcode:1990ApJ...362L..29T, doi:10.1086/185840.
^Gerin, M.; et al. (1992), "Interstellar detection of deuterated methyl acetylene", Astronomy and Astrophysics, 253 (2): L29–L32, Bibcode:1992A&A...253L..29G.
^Widicus Weaver, S. L.; Blake, G. A. (2005), "1,3-Dihydroxyacetone in Sagittarius B2(N-LMH): The First Interstellar Ketose", Astrophysical Journal Letters, 624 (1): L33–L36, Bibcode:2005ApJ...624L..33W, doi:10.1086/430407
^Iglesias-Groth, S.; et al. (2008-09-20), "Evidence for the Naphthalene Cation in a Region of the Interstellar Medium with Anomalous Microwave Emission", The Astrophysical Journal Letters, 685 (1): L55–L58, arXiv:0809.0778, Bibcode:2008ApJ...685L..55I, doi:10.1086/592349, S2CID17190892 - This spectral assignment has not been independently confirmed, and is described by the authors as "tentative" (page L58).
^On Earth, the dominant isotope of argon is 40Ar, so ArH+ would have a mass of 41 amu. However, the interstellar detection was of the 36ArH+isotopologue, which has a mass of 37 amu.