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. For example, 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.
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 | CH+3 [93] |
l-C3H | Propynylidyne [94] | 37 | l-C3H+ [95] |
c-C3H | Cyclopropynylidyne [96] | 37 | — |
C3N | Cyanoethynyl [97] | 50 | C3N− [98] |
C3O | Tricarbon monoxide [94] | 52 | — |
C3S | Tricarbon sulfide [63] | 68 | — |
— | Hydronium | 19 | H3O+ [99] |
C2H2 | Acetylene [100] | 26 | — |
H2CN | Methylene amidogen [101] | 28 | H2CN+ [26] |
H2NC | Aminocarbyne [102] | 28 | — |
H2CO | Formaldehyde [91] | 30 | — |
H2CS | Thioformaldehyde [103] | 46 | — |
HCCN | — [104] | 39 | — |
HCCO | Ketenyl [105] | 41 | — |
— | Protonated hydrogen cyanide | 28 | HCNH+ [77] |
— | Protonated carbon dioxide | 45 | HOCO+ [106] |
HCNO | Fulminic acid [107] | 43 | — |
HOCN | Cyanic acid [108] | 43 | — |
CNCN | Isocyanogen [109] | 52 | — |
HOOH | Hydrogen peroxide [110] | 34 | — |
HNCO | Isocyanic acid [87] | 43 | — |
HNCN | Cyanomidyl radical [111] | 41 | — |
HNCS | Isothiocyanic acid [112] | 59 | — |
NH3 | Ammonia [113] | 17 | — |
HSCN | Thiocyanic acid [114] | 59 | — |
SiC3 | Silicon tricarbide [34] | 64 | — |
HMgNC | Hydromagnesium isocyanide [115] | 51.3 | — |
HNO2 | Nitrous acid [116] | 47 | — |
Molecule | Designation | Mass | Ions |
---|---|---|---|
— | Ammonium ion | 18 | NH+ 4 [118] [119] |
CH4 | Methane [120] | 16 | — |
CH3O | Methoxy radical [121] | 31 | — |
c-C3H2 | Cyclopropenylidene [27] [122] [123] | 38 | — |
l-H2C3 | Propadienylidene [123] | 38 | — |
H2CCN | Cyanomethyl [124] | 40 | — |
H2C2O | Ketene [87] | 42 | — |
H2CNH | Methylenimine [125] | 29 | — |
HNCNH | Carbodiimide [126] | 42 | — |
— | Protonated formaldehyde | 31 | H2COH+ [127] |
C4H | Butadiynyl [34] | 49 | C4H− [128] |
HC3N | Cyanoacetylene [27] [77] [129] [130] | 51 | — |
HCC-NC | Isocyanoacetylene [131] | 51 | — |
HCOOH | Formic acid [132] [129] | 46 | — |
NH2CN | Cyanamide [133] [134] | 42 | — |
NH2OH | Hydroxylamine [135] | 37 | — |
— | Protonated cyanogen | 53 | NCCNH+ [136] |
HC(O)CN | Cyanoformaldehyde [137] | 55 | — |
C5 | Linear C5 [138] | 60 | — |
SiC4 | Silicon-carbide cluster [53] | 92 | — |
SiH4 | Silane [139] | 32 | — |
Molecule | Designation | Mass | Ions |
---|---|---|---|
c-H2C3O | Cyclopropenone [141] | 54 | — |
E-HNCHCN | E-Cyanomethanimine [142] | 54 | — |
C2H4 | Ethylene [143] | 28 | — |
CH3CN | Acetonitrile [87] [144] [145] | 40 | — |
CH3NC | Methyl isocyanide [144] | 40 | — |
CH3OH | Methanol [87] [146] | 32 | — |
CH3SH | Methanethiol [147] | 48 | — |
l-H2C4 | Diacetylene [148] | 50 | — |
— | Protonated cyanoacetylene | 52 | HC3NH+ [77] |
HCONH2 | Formamide [140] | 44 | — |
HOCOOH | Carbonic acid [149] | — | |
C5H | Pentynylidyne [63] | 61 | — |
C5N | Cyanobutadiynyl radical [150] | 74 | — |
HC2CHO | Propynal [151] | 54 | — |
HC4N | — [34] | 63 | — |
CH2CNH | Ketenimine [122] | 40 | — |
C5S | — [152] | 92 | — |
Molecule | Designation | Mass | Ions |
---|---|---|---|
c-C2H4O | Ethylene oxide [154] | 44 | — |
CH3C2H | Methylacetylene [27] | 40 | — |
H3CNH2 | Methylamine [155] | 31 | — |
CH2CHCN | Acrylonitrile [87] [144] | 53 | — |
HCCCHNH | Propargylimine [156] | 53 | — |
H2CHCOH | Vinyl alcohol [153] | 44 | — |
C6H | Hexatriynyl radical [63] | 73 | C6H− [123] [157] |
HC4CN | Cyanodiacetylene [87] [130] [144] | 75 | — |
HC4NC | Isocyanodiacetylene [158] | 75 | — |
HC5O | — [159] | 77 | — |
CH3CHO | Acetaldehyde [154] | 44 | — |
CH3NCO | Methyl isocyanate [160] | 57 | — |
HOCH2CN | Glycolonitrile [161] | 57 | — |
Molecule | Designation | Mass |
---|---|---|
H3CC2CN | Methylcyanoacetylene [163] | 65 |
HC3H2CN | Propargyl cyanide [164] | 65 |
H2COHCHO | Glycolaldehyde [165] | 60 |
(CHOH)2 | 1,2-ethenediol [166] | 60 |
HCOOCH3 | Methyl formate [87] [129] | 60 |
CH3COOH | Acetic acid [162] | 60 |
H2C6 | Hexapentaenylidene [148] | 74 |
CH2CHCHO | Propenal [122] | 56 |
CH2CCHCN | Cyanoallene [122] [163] | 65 |
CH3CHNH | Ethanimine [167] | 43 |
C2H3NH2 | Vinylamine [168] | 43 |
C7H | Heptatrienyl radical [169] | 85 |
NH2CH2CN | Aminoacetonitrile [170] | 56 |
(NH2)2CO | Urea [171] | 60 |
Molecule | Designation | Mass | Ions |
---|---|---|---|
CH3C4H | Methyldiacetylene [172] | 64 | — |
CH3OCH3 | Dimethyl ether [173] | 46 | — |
CH3CH2CN | Propionitrile [87] [144] | 55 | — |
CH3CONH2 | Acetamide [122] [140] [134] | 59 | — |
CH3CH2OH | Ethanol [174] | 46 | — |
C8H | Octatetraynyl radical [175] | 97 | C8H− [176] [177] |
HC7N | Cyanohexatriyne or Cyanotriacetylene [113] [178] [179] | 99 | — |
CH3CHCH2 | Propylene (propene) [180] | 42 | — |
CH3CH2SH | Ethyl mercaptan [181] | 62 | — |
CH3NHCHO | N-methylformamide [134] |
Atoms | Molecule | Designation | Mass | Ions |
---|---|---|---|---|
10 | (CH3)2CO | Acetone [87] [182] | 58 | — |
10 | (CH2OH)2 | Ethylene glycol [183] [184] | 62 | — |
10 | CH3CH2CHO | Propanal [122] | 58 | — |
10 | CH3OCH2OH | Methoxymethanol [185] | 62 | — |
10 | CH3C5N | Methylcyanodiacetylene [122] | 89 | — |
10 | CH3CHCH2O | Propylene oxide [186] | 58 | — |
11 | NH2CH2CH2OH | Ethanolamine [187] | 61 | — |
11 | HC8CN | Cyanotetraacetylene [178] | 123 | — |
11 | C2H5OCHO | Ethyl formate [188] | 74 | — |
11 | CH3COOCH3 | Methyl acetate [189] | 74 | — |
11 | CH3C6H | Methyltriacetylene [122] [172] | 88 | — |
12 | C6H6 | Benzene [148] | 78 | — |
12 | C3H7CN | n-Propyl cyanide [188] | 69 | — |
12 | (CH3)2CHCN | iso-Propyl cyanide [190] [191] | 69 | — |
13 | CH3OCH2CH2OH | 2-methoxyethanol [192] | 76 | — |
13 | C 6H 5CN | Benzonitrile [193] | 104 | — |
13 | HC10CN | Cyanopentaacetylene [178] | 147 | — |
17 | C9H8 | Indene [9] | 116 | — |
19 | C10H7CN | 1-cyanonaphthalene [8] | 153 | — |
19 | C10H7CN | 2-cyanonaphthalene [8] | 153 | — |
60 | C60 | Buckminsterfullerene (C60 fullerene) [194] | 720 | C+ 60 [195] [196] [197] |
70 | C70 | C70 fullerene [194] | 840 | — |
These molecules all contain one or more deuterium atoms, a heavier isotope of hydrogen.
Atoms | Molecule | Designation |
---|---|---|
2 | HD | Hydrogen deuteride [198] [199] |
3 | H2D+, HD+ 2 | Trihydrogen cation [198] [199] |
3 | HDO, D2O | Heavy water [200] [201] |
3 | DCN | Hydrogen cyanide [202] |
3 | DCO | Formyl radical [202] |
3 | DNC | Hydrogen isocyanide [202] |
3 | N2D+ | — [202] |
3 | NHD, ND2 | Amidogen [203] |
4 | NH2D, NHD2, ND3 | Ammonia [199] [204] [205] |
4 | HDCO, D2CO | Formaldehyde [199] [206] |
4 | DNCO | Isocyanic acid [207] |
5 | NH3D+ | Ammonium ion [208] [209] |
6 | NH 2CDO; NHDCHO | Formamide [207] |
7 | CH2DCCH, CH3CCD | Methylacetylene [210] [211] |
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 [212] |
4 | MgCCH | Magnesium monoacetylide [152] |
4 | NCCP | Cyanophosphaethyne [152] |
5 | H2NCO+ | — [213] |
6 | SiH3CN | Silyl cyanide [152] |
10 | H2NCH2COOH | Glycine [214] [215] |
10 | C2H5NH2 | Ethylamine [168] |
12 | CO(CH2OH)2 | Dihydroxyacetone [216] [217] |
12 | C2H5OCH3 | Ethyl methyl ether [218] |
18 | C 10H+ 8 | Naphthalene cation [219] |
24 | C24 | Graphene [220] |
24 | C14H10 | Anthracene [221] [222] |
26 | C16H10 | Pyrene [221] |
27 | C11H12N2O2 | Tryptophan [223] [224] [225] |
R Horologii is a red giant star approximately 760 light-years away in the southern constellation of Horologium. It is a Mira variable with a period of 404.83 days, ranging from apparent magnitude 4.7 to 14.3—one of the largest ranges in brightness known of stars in the night sky visible to the unaided eye. The star is losing mass at the rate of 5.9×10−7 M☉·y−1.
CW Leonis or IRC +10216 is a variable 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-inch 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.
HD 114762 b is a small red dwarf star, in the HD 114762 system, formerly thought to be a massive gaseous extrasolar planet, approximately 126 light-years (38.6 pc) away in the constellation of Coma Berenices. This optically undetected companion to the late F-type main-sequence star HD 114762 was discovered in 1989 by Latham, et al., and confirmed in an October 1991 paper by Cochran, et al. It was thought to be the first discovered exoplanet
T Cephei is a Mira variable star in the constellation Cepheus. Located approximately 600 light-years distant, it varies between magnitudes 5.2 and 11.3 over a period of around 388 days.
Propynylidyne is a chemical compound that has been identified in interstellar space.
DP Leonis is a binary star system in the equatorial constellation of Leo. It is a variable star that ranges in apparent visual magnitude from 17.5 down to 19. The system is located at a distance of approximately 990 light-years from the Sun based on parallax. It is a cataclysmic variable star of the AM Herculis-type also known as polars. The system comprises an eclipsing white dwarf and red dwarf in tight orbit and an extrasolar planet. This eclipsing variable was discovered by P. Biermann and associates in 1982 as the optical counterpart to the EINSTEIN X-ray source E1114+182.
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.
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.
GJ 1151 is a star located in the northern circumpolar constellation of Ursa Major at a distance of 26.2 light-years from the Sun. It has a reddish hue and is too faint to be visible to the naked eye with an apparent visual magnitude of 14.0 The star is moving closer with a radial velocity of −36 km/s, and has a relatively large proper motion, traversing the celestial sphere at a rate of 1.815″·yr−1.
VHS J125601.92–125723.9 is a young triple brown dwarf system located in the constellation Corvus approximately 69.0 light-years from the Sun. The system consists of the equal-mass binary VHS J1256–1257AB and the distant planetary-mass companion VHS 1256–1257 b. In 2022, a continuous radio emission from the radiation belts surrounding VHS J1256–1257 was detected.
Tricarbon monosulfide (C3S) or tricarbon sulfur is a reactive molecular substance that has been detected in outer space. Tricarbon monosulfide is a heterocumulene or thiocumulene, consisting of a straight chain of three carbon atoms and a terminal sulfur atom.
A circumplanetary disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a planet. They are reservoirs of material out of which moons may form. Such a disk can manifest itself in various ways.
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.
EX Lupi is a young, single T-Tauri star in the southern constellation of Lupus. An irregular variable, it is the prototype of young, low-mass eruptive stars named EXors, with EX Lupi being this object's variable star designation. At its minimal activity level, EX Lupi resembles a classical T-Tauri star of the M0 dwarf type. The low latitude of this star, at a declination of −40°, makes it difficult for northern observers to view. Based on parallax measurements, it is located at a distance of about 505 light years from the Sun. The star lies next to a gap in the Lupus cloud complex, a star forming region.
HD 210056, also known as HR 8432, is a solitary orange hued star located in the southern circumpolar constellation Octans. Eggen (1993) listed it as a member of the old disk population.
HD 196737, also designated as HR 7893, is a solitary orange hued star located in the southern constellation Microscopium. It has an apparent magnitude of 5.47, allowing it to be faintly visible to the naked eye. The object is located relatively close at a distance of 241 light years based on Gaia DR3 parallax measurements, but is receding with a heliocentric radial velocity of 14.2 km/s. At its current distance, HD 196737's brightness is diminished by 0.14 magnitudes due to interstellar dust. It has an absolute magnitude of 1.17.
{{citation}}
: CS1 maint: multiple names: authors list (link){{cite journal}}
: CS1 maint: numeric names: authors list (link)there is no firm support for the tryptophan assignment
there is no compelling evidence for the tryptophan assignment