This is a list of molecules that have been detected in the interstellar medium and circumstellar envelopes, grouped by the number of component atoms. The chemical formula is listed for each detected compound, along with any ionized form that has also been observed.
The molecules listed below were detected through astronomical spectroscopy. Their spectral features arise because molecules either absorb or emit a photon of light when they transition between two molecular energy levels. The energy (and thus the wavelength) of the photon matches the energy difference between the levels involved. Molecular electronic transitions occur when one of the molecule's electrons moves between molecular orbitals, producing a spectral line in the ultraviolet, optical or near-infrared parts of the electromagnetic spectrum. Alternatively, a vibrational transition transfers quanta of energy to (or from) vibrations of molecular bonds, producing signatures in the mid- or far-infrared. Gas-phase molecules also have quantised rotational levels, leading to transitions at microwave or radio wavelengths. [1]
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 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]
Molecule | Designation | Mass | Ions |
---|---|---|---|
CH3 | Methyl radical [92] | 15 | — |
l-C3H | Propynylidyne [16] [93] | 37 | l-C3H+ [94] |
c-C3H | Cyclopropynylidyne [95] | 37 | — |
C3N | Cyanoethynyl [96] | 50 | C3N− [97] |
C3O | Tricarbon monoxide [93] | 52 | — |
C3S | Tricarbon sulfide [16] [63] | 68 | — |
— | Hydronium | 19 | H3O+ [98] |
C2H2 | Acetylene [99] | 26 | — |
H2CN | Methylene amidogen [100] | 28 | H2CN+ [27] |
H2NC | Aminocarbyne [101] | 28 | — |
H2CO | Formaldehyde [91] | 30 | — |
H2CS | Thioformaldehyde [102] | 46 | — |
HCCN | — [103] | 39 | — |
HCCO | Ketenyl [104] | 41 | — |
— | Protonated hydrogen cyanide | 28 | HCNH+ [77] |
— | Protonated carbon dioxide | 45 | HOCO+ [105] |
HCNO | Fulminic acid [106] | 43 | — |
HOCN | Cyanic acid [107] | 43 | — |
CNCN | Isocyanogen [108] | 52 | — |
HOOH | Hydrogen peroxide [109] | 34 | — |
HNCO | Isocyanic acid [87] | 43 | — |
HNCN | Cyanomidyl radical [110] | 41 | — |
HNCS | Isothiocyanic acid [111] | 59 | — |
NH3 | Ammonia [16] [112] | 17 | — |
HSCN | Thiocyanic acid [113] | 59 | — |
SiC3 | Silicon tricarbide [16] | 64 | — |
HMgNC | Hydromagnesium isocyanide [114] | 51.3 | — |
HNO2 | Nitrous acid [115] | 47 | — |
Molecule | Designation | Mass | Ions |
---|---|---|---|
— | Ammonium ion | 18 | NH+ 4 [117] [118] |
CH4 | Methane [119] | 16 | — |
CH3O | Methoxy radical [120] | 31 | — |
c-C3H2 | Cyclopropenylidene [28] [121] [122] | 38 | — |
l-H2C3 | Propadienylidene [122] | 38 | — |
H2CCN | Cyanomethyl [123] | 40 | — |
H2C2O | Ketene [87] | 42 | — |
H2CNH | Methylenimine [124] | 29 | — |
HNCNH | Carbodiimide [125] | 42 | — |
— | Protonated formaldehyde | 31 | H2COH+ [126] |
C4H | Butadiynyl [16] | 49 | C4H− [127] |
HC3N | Cyanoacetylene [16] [28] [77] [128] [129] | 51 | — |
HCC-NC | Isocyanoacetylene [130] | 51 | — |
HCOOH | Formic acid [131] [128] | 46 | — |
NH2CN | Cyanamide [132] [133] | 42 | — |
NH2OH | Hydroxylamine [134] | 37 | — |
— | Protonated cyanogen | 53 | NCCNH+ [135] |
HC(O)CN | Cyanoformaldehyde [136] | 55 | — |
C5 | Linear C5 [137] | 60 | — |
SiC4 | Silicon-carbide cluster [53] | 92 | — |
SiH4 | Silane [138] | 32 | — |
Molecule | Designation | Mass | Ions |
---|---|---|---|
c-H2C3O | Cyclopropenone [140] | 54 | — |
E-HNCHCN | E-Cyanomethanimine [141] | 54 | — |
C2H4 | Ethylene [142] | 28 | — |
CH3CN | Acetonitrile [87] [143] [144] | 40 | — |
CH3NC | Methyl isocyanide [143] | 40 | — |
CH3OH | Methanol [87] [145] | 32 | — |
CH3SH | Methanethiol [146] | 48 | — |
l-H2C4 | Diacetylene [16] [147] | 50 | — |
— | Protonated cyanoacetylene | 52 | HC3NH+ [77] |
HCONH2 | Formamide [139] | 44 | — |
C5H | Pentynylidyne [16] [63] | 61 | — |
C5N | Cyanobutadiynyl radical [148] | 74 | — |
HC2CHO | Propynal [149] | 54 | — |
HC4N | — [16] | 63 | — |
CH2CNH | Ketenimine [121] | 40 | — |
C5S | — [150] | 92 | — |
Molecule | Designation | Mass | Ions |
---|---|---|---|
c-C2H4O | Ethylene oxide [152] | 44 | — |
CH3C2H | Methylacetylene [28] | 40 | — |
H3CNH2 | Methylamine [153] | 31 | — |
CH2CHCN | Acrylonitrile [87] [143] | 53 | — |
HCCCHNH | Propargylimine [154] | 53 | — |
H2CHCOH | Vinyl alcohol [151] | 44 | — |
C6H | Hexatriynyl radical [16] [63] | 73 | C6H− [122] [155] |
HC4CN | Cyanodiacetylene [87] [129] [143] | 75 | — |
HC4NC | Isocyanodiacetylene [156] | 75 | — |
HC5O | — [157] | 77 | — |
CH3CHO | Acetaldehyde [16] [152] | 44 | — |
CH3NCO | Methyl isocyanate [158] | 57 | — |
HOCH2CN | Glycolonitrile [159] | 57 | — |
Molecule | Designation | Mass |
---|---|---|
H3CC2CN | Methylcyanoacetylene [161] | 65 |
HC3H2CN | Propargyl cyanide [162] | 65 |
H2COHCHO | Glycolaldehyde [163] [164] | 60 |
(CHOH)2 | 1,2-ethenediol [165] | 60 |
HCOOCH3 | Methyl formate [87] [128] [164] | 60 |
CH3COOH | Acetic acid [160] | 60 |
H2C6 | Hexapentaenylidene [16] [147] | 74 |
CH2CHCHO | Propenal [121] | 56 |
CH2CCHCN | Cyanoallene [121] [161] | 65 |
CH3CHNH | Ethanimine [166] | 43 |
C2H3NH2 | Vinylamine [167] | 43 |
C7H | Heptatrienyl radical [168] | 85 |
NH2CH2CN | Aminoacetonitrile [169] | 56 |
(NH2)2CO | Urea [170] | 60 |
Molecule | Designation | Mass | Ions |
---|---|---|---|
CH3C4H | Methyldiacetylene [171] | 64 | — |
CH3OCH3 | Dimethyl ether [172] | 46 | — |
CH3CH2CN | Propionitrile [16] [87] [143] | 55 | — |
CH3CONH2 | Acetamide [121] [139] [133] | 59 | — |
CH3CH2OH | Ethanol [173] | 46 | — |
C8H | Octatetraynyl radical [174] | 97 | C8H− [175] [176] |
HC7N | Cyanohexatriyne or Cyanotriacetylene [16] [112] [177] [178] | 99 | — |
CH3CHCH2 | Propylene (propene) [179] | 42 | — |
CH3CH2SH | Ethyl mercaptan [180] | 62 | — |
CH3NHCHO | N-methylformamide [133] |
Atoms | Molecule | Designation | Mass | Ions |
---|---|---|---|---|
10 | (CH3)2CO | Acetone [87] [181] | 58 | — |
10 | (CH2OH)2 | Ethylene glycol [182] [183] | 62 | — |
10 | CH3CH2CHO | Propanal [121] | 58 | — |
10 | CH3OCH2OH | Methoxymethanol [184] | 62 | — |
10 | CH3C5N | Methylcyanodiacetylene [121] | 89 | — |
10 | CH3CHCH2O | Propylene oxide [185] | 58 | — |
11 | NH2CH2CH2OH | Ethanolamine [186] | 61 | — |
11 | HC8CN | Cyanotetraacetylene [16] [177] | 123 | — |
11 | C2H5OCHO | Ethyl formate [187] | 74 | — |
11 | CH3COOCH3 | Methyl acetate [188] | 74 | — |
11 | CH3C6H | Methyltriacetylene [121] [171] | 88 | — |
12 | C6H6 | Benzene [147] | 78 | — |
12 | C3H7CN | n-Propyl cyanide [187] | 69 | — |
12 | (CH3)2CHCN | iso-Propyl cyanide [189] [190] | 69 | — |
13 | C 6H 5CN | Benzonitrile [191] | 104 | — |
13 | HC10CN | Cyanopentaacetylene [177] | 147 | — |
17 | C9H8 | Indene [9] | 116 | — |
19 | C10H7CN | 1-cyanonaphthalene [8] | 153 | — |
19 | C10H7CN | 2-cyanonaphthalene [8] | 153 | — |
60 | C60 | Buckminsterfullerene (C60 fullerene) [192] | 720 | C+ 60 [193] [194] [195] |
70 | C70 | C70 fullerene [192] | 840 | — |
These molecules all contain one or more deuterium atoms, a heavier isotope of hydrogen.
Atoms | Molecule | Designation |
---|---|---|
2 | HD | Hydrogen deuteride [196] [197] |
3 | H2D+, HD+ 2 | Trihydrogen cation [196] [197] |
3 | HDO, D2O | Heavy water [198] [199] |
3 | DCN | Hydrogen cyanide [200] |
3 | DCO | Formyl radical [200] |
3 | DNC | Hydrogen isocyanide [200] |
3 | N2D+ | — [200] |
3 | NHD, ND2 | Amidogen [201] |
4 | NH2D, NHD2, ND3 | Ammonia [197] [202] [203] |
4 | HDCO, D2CO | Formaldehyde [197] [204] |
4 | DNCO | Isocyanic acid [205] |
5 | NH3D+ | Ammonium ion [206] [207] |
6 | NH 2CDO; NHDCHO | Formamide [205] |
7 | CH2DCCH, CH3CCD | Methylacetylene [208] [209] |
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.
Atoms | Molecule | Designation |
---|---|---|
2 | SiH | Silylidine [74] |
4 | PH3 | Phosphine [210] |
4 | MgCCH | Magnesium monoacetylide [150] |
4 | NCCP | Cyanophosphaethyne [150] |
5 | H2NCO+ | — [211] |
4 | SiH3CN | Silyl cyanide [150] |
10 | H2NCH2COOH | Glycine [212] [213] |
10 | C2H5NH2 | Ethylamine [167] |
12 | CO(CH2OH)2 | Dihydroxyacetone [214] [215] |
12 | C2H5OCH3 | Ethyl methyl ether [216] |
18 | C 10H+ 8 | Naphthalene cation [217] |
24 | C24 | Graphene [218] |
24 | C14H10 | Anthracene [219] [220] |
26 | C16H10 | Pyrene [219] |
Gliese 876 is a red dwarf approximately 15 light-years away from Earth in the constellation of Aquarius. It is one of the closest known stars to the Sun confirmed to possess a planetary system with more than two planets, after Gliese 1061, YZ Ceti, Tau Ceti, and Luyten's Star; as of 2018, four extrasolar planets have been found to orbit the star. The planetary system is also notable for the orbital properties of its planets. It is the only known system of orbital companions to exhibit a near-triple conjunction in the rare phenomenon of Laplace resonance. It is also the first extrasolar system around a normal star with measured coplanarity. While planets b and c are located in the system's habitable zone, they are giant planets believed to be analogous to Jupiter.
26 Andromedae, abbreviated 26 And, is a binary star system in the constellation Andromeda. 26 Andromedae is the Flamsteed designation. It has a combined apparent visual magnitude of 6.10, which is near the lower limit of visibility to the naked eye. The distance to this system can be estimated from its annual parallax shift of 5.35 mas, which yields a distance of about 600 light years. At that distance, the visual magnitude of the stars is diminished from an extinction of 0.04 due to interstellar dust. The system is moving further from the Earth with a heliocentric radial velocity of +3.3 km/s.
Nu1 Boötis (ν1 Boötis) is an orange-hued star in the northern constellation of Boötes. It has an apparent visual magnitude of +5.02, which indicates the star is faintly visible to the naked eye. Based upon an annual parallax shift of 3.89 mas as seen from Earth, it is located roughly 840 light years distant from the Sun. At that distance, the visual magnitude of the star is diminished by an extinction of 0.13 due to interstellar dust.
HD 4628 is a main sequence star in the equatorial constellation of Pisces. It has a spectral classification of K2.5 V and an effective temperature of 5,055 K, giving it an orange-red hue with a slightly smaller mass and girth than the Sun. HD 4628 lies at a distance of approximately 24.3 light years from the Sun based on parallax. The apparent magnitude of 5.7 is just sufficient for this star to be viewed with the unaided eye. The star appears to be slightly older than the Sun—approximately 5.4 billion years in age. The surface activity is low and, based upon the detection of UV emission, it may have a relatively cool corona with a temperature of one million K.
Xi Cassiopeiae is a blue-white hued binary star system in the northern constellation of Cassiopeia. It has an apparent visual magnitude of +4.81 and thus is faintly visible to the naked eye. Based upon an annual parallax shift of 2.28 mas as seen from Earth, this system is located roughly 1,400 light years from the Sun. At that distance, the visual magnitude of the system is diminished by an extinction factor of 0.20 due to interstellar dust. It is advancing in the general direction of the Sun with a radial velocity of roughly −10.6 km/s.
Omega Cassiopeiae is a binary star system in the northern constellation of Cassiopeia. It has a combined apparent visual magnitude of +4.99, which means it is a faint star but visible to the naked eye. Based upon an annual parallax shift of 4.65 mas as seen from Earth, this system is located roughly 730 light years from the Sun. At that distance, the visual magnitude is diminished by an extinction of 0.16 due to interstellar dust.
CW Leonis or IRC +10216 is a carbon star that is embedded in a thick dust envelope. It was first discovered in 1969 by a group of astronomers led by Eric Becklin, based upon infrared observations made with the 62 inches (1.6 m) Caltech Infrared Telescope at Mount Wilson Observatory. Its energy is emitted mostly at infrared wavelengths. At a wavelength of 5 μm, it was found to have the highest flux of any object outside the Solar System.
Hydrogen isocyanide is a chemical with the molecular formula HNC. It is a minor tautomer of hydrogen cyanide (HCN). Its importance in the field of astrochemistry is linked to its ubiquity in the interstellar medium.
HD 100546, is a star 316.4 light-years from Earth. It is orbited by an approximately 20 MJ exoplanet at 6.5 AU, although further examination of the disk profile indicate it might be a more massive object such as a brown dwarf or more than one planet. The star is surrounded by a circumstellar disk from a distance of 0.2 to 4 AU, and again from 13 AU out to a few hundred AU, with evidence for a protoplanet forming at a distance of around 47 AU.
Sagittarius B2 is a giant molecular cloud of gas and dust that is located about 120 parsecs (390 ly) from the center of the Milky Way. This complex is the largest molecular cloud in the vicinity of the core and one of the largest in the galaxy, spanning a region about 45 parsecs (150 ly) across. The total mass of Sgr B2 is about 3 million times the mass of the Sun. The mean hydrogen density within the cloud is 3000 atoms per cm3, which is about 20–40 times denser than a typical molecular cloud.
36 Ursae Majoris is a double star in the northern constellation of Ursa Major. With an apparent visual magnitude of 4.8, it can be seen with the naked eye in suitable dark skies. Based upon parallax measurements, this binary lies at a distance of 42 light-years from Earth.
U Hydrae is a single star in the equatorial constellation of Hydra, near the northern constellation border with Sextans. It is a semiregular variable star of sub-type SRb, with its brightness ranging from visual magnitude (V) 4.7 to 5.2 over a 450-day period, with some irregularity. This object is located at a distance of approximately 680 light years from the Sun based on parallax. It is drifting closer with a radial velocity of −26 km/s.
23 Orionis is a double star located around 1,200 light-years away from the Sun in the equatorial constellation of Orion. It is visible to the naked eye as a dim, blue-white-hued point of light with a combined apparent visual magnitude of 4.99. The pair are moving away from the Earth with a heliocentric radial velocity of +18 km/s, and they are members of the Orion OB1 association, subgroup 1a.
HD 165634 is a star in the southern constellation of Sagittarius. It has a yellow hue and is faintly visible to the naked eye with apparent visual magnitude of 4.56. The star is located at a distance of approximately 339 light years from the Sun based on parallax, but is drifting closer with a radial velocity of −5 km/s. It has an absolute magnitude of −0.53.
69 Virginis is a single star in the zodiac constellation of Virgo, located about 259 light years away. It is visible to the naked eye as a faint orange-hued star with an apparent visual magnitude of 4.76, although it is a suspected variable that may range in magnitude from 4.75 down to 4.79. This object is moving closer to the Earth with a heliocentric radial velocity of −13 km/s. The light from this star is polarized due to intervening interstellar dust.
HD 126053 is the Henry Draper Catalogue designation for a star in the equatorial constellation of Virgo. It has an apparent magnitude of 6.25, which means it is faintly visible to the naked eye. According to the Bortle scale, it requires dark suburban or rural skies to view. Parallax measurements made by the Hipparcos spacecraft provide an estimated distance of 57 light years to this star. It is drifting closer with a heliocentric radial velocity of −19.2 km/s.
λ Librae is the Bayer designation for a binary star system in the zodiac constellation of Libra. It can be faintly seen with the naked eye, having an apparent visual magnitude of 5.03. With an annual parallax shift of 10.54 mas, it is roughly 310 light years from the Sun. At that distance, the visual magnitude of this system is diminished by an extinction factor of 0.22 due to interstellar dust. It is 0.1 degree north of the ecliptic.
HD 73882 is a visual binary system with the components separated by 0.6″ and a combined spectral class of O8. One of stars is an eclipsing binary system. The period of variability is listed as both 2.9199 days and 20.6 days, possibly due to the secondary being a spectroscopic binary star.
V4332 Sagittarii is a nova-like event in the constellation of Sagittarius. It was discovered February 24, 1994 at an apparent visual magnitude of 8.9 by Japanese amateur astronomer Minoru Yamamoto from Okazaki, Aichi, then confirmed by K. Hirosawa. Initially designated Nova Sagittarii 1994 #1, it was given the variable star designation V4332 Sgr. A spectra of the event taken March 4 lacked the characteristic features of a classical nova, with the only emission lines being of the Balmer series. Subsequent spectra showed a rapid decline in luminosity and a change of spectral type over a period of five days. By 2003, the object was ~1500 times less luminous than at peak magnitude and showed a spectrum of an M-type star.
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