Interstellar formaldehyde

Last updated

Interstellar formaldehyde (a topic relevant to astrochemistry) was first discovered in 1969 by L. Snyder et al. using the National Radio Astronomy Observatory. Formaldehyde (H2CO) was detected by means of the 111 - 110 ground state rotational transition at 4830 MHz. [1] On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON). [2] [3]


Initial discovery

Formaldehyde was first discovered in interstellar space in 1969 by L. Snyder et al. using the National Radio Astronomy Observatory. H2CO was detected by means of the 111 - 110 ground state rotational transition at 4830 MHz. [4]

Formaldehyde was the first polyatomic organic molecule detected in the interstellar medium and since its initial detection has been observed in many regions of the galaxy. [5] The isotopic ratio of [12C]/[13C] was determined to be about or less than 50% in the galactic disk. [6] Formaldehyde has been used to map out kinematic features of dark clouds located near Gould's Belt of local bright stars. [7] In 2007, the first H2CO 6 cm maser flare was detected. [8] It was a short duration outburst in IRAS 18566 + 0408 that produced a line profile consistent with the superposition of two Gaussian components, which leads to the belief that an event outside the maser gas triggered simultaneous flares at two different locations. [8] Although this was the first maser flare detected, H2 masers have been observed since 1974 by Downes and Wilson in NGC 7538. [9] Unlike OH, H2O, and CH3OH, only five galactic star forming regions have associated formaldehyde maser emission, which has only been observed through the 110 → 111 transition. [9]

According to Araya et al., H2CO are different from other masers in that they are weaker than most other masers (such as OH, CH3OH, and H2O) and have only been detected near very young massive stellar objects. [10] Unlike OH, H2O, and CH3OH, only five galactic star forming regions have associated formaldehyde maser emission, which has only been observed through the 110 → 111 transition. [11] Because of the widespread interest in interstellar formaldehyde it has recently been extensively studied, yielding new extragalactic sources, including NGC 253, NGC 520, NGC 660, NGC 891, NGC 2903, NGC 3079, NGC 3628, NGC 6240, NGC 6946, IC 342, IC 860, Arp 55, Arp 220, M82, M83, IRAS 10173+0828, IRAS 15107+0724, and IRAS 17468+1320. [12]

Interstellar reactions

The gas-phase reaction that produces formaldehyde possesses modest barriers and is too inefficient to produce the abundance of formaldehyde that has been observed. [13] One proposed mechanism for the formation is the hydrogenation of CO ice, shown below. [13]

H + CO → HCO + H → H2CO (rate constant=9.2*10−3 s−1)

This is the basic production mechanism leading to H2CO; there are several side reactions that take place with each step of the reaction that are based on the nature of the ice on the grain according to David Woon. [13] The rate constant presented is for the hydrogenation of CO. The rate constant for the hydrogenation of HCO was not provided as it was much larger than that of the hydrogenation of CO, likely because HCO is a radical. [14] Awad et al. mention that this is a surface level reaction only and only the monolayer is considered in calculations; this includes the surface within cracks in the ice. [14]

Formaldehyde is relatively inactive in gas phase chemistry in the interstellar medium. Its action is predominantly focused in grain-surface chemistry on dust grains in interstellar clouds [15] ,. [16] Reactions involving formaldehyde have been observed to produce molecules containing C-H, C-O, O-H, and C-N bonds. [16] While these products are not necessarily well known, Schutte et al. believe these to be typical products of formaldehyde reactions at higher temperatures, polyoxymethylene, methanolamine, methanediol, and methoxyethanol for example (see Table 2 [15] ). Formaldehyde is believed to be the primary precursor for most of the complex organic material in the interstellar medium, including amino acids. [16] Formaldehyde most often reacts with NH3, H2O, CH3OH, CO, and itself, H2CO [15] ,. [16] The three dominating reactions are shown below. [15]

H2CO + NH3 → amine (when [NH3]:[H2CO] > .2)
H2CO + H2O → diols (always dominate as [H2O] > [H2CO])
H2CO + H2CO → [-CH2-O-]n (catalyzed by NH3 when [NH3]:[H2CO] > .005)

There is no kinetic data available for these reactions as the entire reaction is not verified nor well understood. These reactions are believed to take place during warm-up of the ice on grains which releases the molecules to react. These reactions begin at temperatures as low as 40K - 80K but may take place at even lower temperatures.

Note that many other reactions are listed on the UMIST RATE06 database.

Importance of observation

Formaldehyde appears to be a useful probe for astrochemists due to its low reactivity in the gas phase and to the fact that the 110 - 111 and 211 - 212 K-doublet transitions are rather clear. Formaldehyde has been used in many capacities and to investigate many systems including,

Rotational spectrum

A rotational spectrum of H2CO at the ground state vibrational level at 30 K. H2CO rot spec.png
A rotational spectrum of H2CO at the ground state vibrational level at 30 K.
The rotational energy level diagram of H2CO at 30 K shown with ortho/para splitting. H2co energydiagram.png
The rotational energy level diagram of H2CO at 30 K shown with ortho/para splitting.

Above is the rotational spectrum at the ground state vibrational level of H2CO at 30 K. This spectrum was simulated using Pgopher and S-Reduction Rotational constants from Muller et al. [18] The observed transitions are the 6.2 cm 111 - 110 and 2.1 cm 212 - 211 K-doublet transitions. At right is the rotational energy level diagram. The ortho/para splitting is determined by the parity of Ka, ortho if Ka is odd and para if Ka is even. [17]

Related Research Articles

Astrochemistry is the study of the abundance and reactions of molecules in the Universe, and their interaction with radiation. The discipline is an overlap of astronomy and chemistry. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is also called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds is of special interest, because it is from these clouds that solar systems form.

Dark nebula Type of interstellar cloud so dense that it obscures the visible wavelengths of light from objects behind it

A dark nebula or absorption nebula is a type of interstellar cloud that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebulae. The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of molecular clouds. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Isolated small dark nebulae are called Bok globules. Like other interstellar dust or material, things it obscures are only visible using radio waves in radio astronomy or infrared in infrared astronomy.

Helix Nebula Planetary nebula in the constellation Aquarius

The Helix Nebula is a planetary nebula (PN) located in the constellation Aquarius. Discovered by Karl Ludwig Harding, probably before 1824, this object is one of the closest to the Earth of all the bright planetary nebulae. The distance, measured by the Gaia mission, is 655±13 light-years. It is similar in appearance to the Cat's Eye Nebula and the Ring Nebula, whose size, age, and physical characteristics are similar to the Dumbbell Nebula, varying only in its relative proximity and the appearance from the equatorial viewing angle. The Helix Nebula has sometimes been referred to as the "Eye of God" in pop culture, as well as the "Eye of Sauron".

Black Eye Galaxy

The Black Eye Galaxy is a relatively isolated spiral galaxy located 17 million light-years away in the northern constellation of Coma Berenices. It was discovered by Edward Pigott in March 1779, and independently by Johann Elert Bode in April of the same year, as well as by Charles Messier in 1780. A dark band of absorbing dust in front of the galaxy's bright nucleus gave rise to its nicknames of the "Black Eye" or "Evil Eye" galaxy. M64 is well known among amateur astronomers because of its appearance in small telescopes.

Hydroxyl radical Neutral form of the hydroxide ion (OH−)

The hydroxyl radical, OH, is the neutral form of the hydroxide ion (OH). Hydroxyl radicals are highly reactive (easily becoming hydroxy groups) and consequently short-lived. They form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. It is also an important radical formed in radiation chemistry, since it leads to the formation of hydrogen peroxide and oxygen, which can enhance corrosion and SCC in coolant systems subjected to radioactive environments. Hydroxyl radicals are also produced during UV-light dissociation of H2O2 (suggested in 1879) and likely in Fenton chemistry, where trace amounts of reduced transition metals catalyze peroxide-mediated oxidations of organic compounds.

Messier 106

Messier 106 is an intermediate spiral galaxy in the constellation Canes Venatici. It was discovered by Pierre Méchain in 1781. M106 is at a distance of about 22 to 25 million light-years away from Earth. M106 contains an active nucleus classified as a Type 2 Seyfert, and the presence of a central supermassive black hole has been demonstrated from radio-wavelength observations of the rotation of a disk of molecular gas orbiting within the inner light-year around the black hole. NGC 4217 is a possible companion galaxy of Messier 106. A Type II supernova was observed in M106 in May 2014.

Asymptotic giant branch Stars powered by fusion of hydrogen and helium in shell with an inactive core of carbon and oxygen

The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars late in their lives.

Trihydrogen cation

The trihydrogen cation or protonated molecular hydrogen is a cation with formula H+
, consisting of three hydrogen nuclei (protons) sharing two electrons.

Astrophysical maser

An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. This emission may arise in molecular clouds, comets, planetary atmospheres, stellar atmospheres, or various other conditions in interstellar space.


A megamaser is a type of astrophysical maser, which is a naturally occurring source of stimulated spectral line emission. Megamasers are distinguished from astrophysical masers by their large isotropic luminosity. Megamasers have typical luminosities of 103 solar luminosities (L), which is 100 million times brighter than masers in the Milky Way, hence the prefix mega. Likewise, the term kilomaser is used to describe masers outside the Milky Way that have luminosities of order L, or thousands of times stronger than the average maser in the Milky Way, gigamaser is used to describe masers billions of times stronger than the average maser in the Milky Way, and extragalactic maser encompasses all masers found outside the Milky Way. Most known extragalactic masers are megamasers, and the majority of megamasers are hydroxyl (OH) megamasers, meaning the spectral line being amplified is one due to a transition in the hydroxyl molecule. There are known megamasers for three other molecules: water (H2O), formaldehyde (H2CO), and methine (CH).

Ethynyl radical

The ethynyl radical is an organic compound with the chemical formula C≡CH. It is a simple molecule that does not occur naturally on Earth but is abundant in the interstellar medium. It was first observed by electron spin resonance isolated in a solid argon matrix at liquid helium temperatures in 1963 by Cochran and coworkers at the Johns Hopkins Applied Physics Laboratory. It was first observed in the gas phase by Tucker and coworkers in November 1973 toward the Orion Nebula, using the NRAO 11-meter radio telescope. It has since been detected in a large variety of interstellar environments, including dense molecular clouds, bok globules, star forming regions, the shells around carbon-rich evolved stars, and even in other galaxies.

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.

Propynylidyne is a chemical compound that has been identified in interstellar space.


Diazenylium is the chemical N2H+, an inorganic cation that was one of the first ions to be observed in interstellar clouds. Since then, it has been observed for in several different types of interstellar environments, observations that have several different scientific uses. It gives astronomers information about the fractional ionization of gas clouds, the chemistry that happens within those clouds, and it is often used as a tracer for molecules that are not as easily detected (such as N2). Its 1–0 rotational transition occurs at 93.174 GHz, a region of the spectrum where Earth's atmosphere is transparent and it has a significant optical depth in both cold and warm clouds so it is relatively easy to observe with ground-based observatories. The results of N2H+ observations can be used not only for determining the chemistry of interstellar clouds, but also for mapping the density and velocity profiles of these clouds.

Protonated hydrogen cyanide Chemical compound

HCNH+, also known as protonated hydrogen cyanide, is a molecular ion of astrophysical interest.

Cyanopolyynes are a group of chemicals with the chemical formula HC
(n = 3,5,7,...). Structurally, they are polyynes with a cyano group covalently bonded to one of the terminal acetylene units. A rarely seen group of molecules both due to the difficulty in production and the unstable nature of the paired groups, the cyanopolyynes have been observed as a major organic component in interstellar clouds. This is believed to be due to the hydrogen scarcity of some of these clouds. Interference with hydrogen is one of the reason for the molecule's instability due to the energetically favorable dissociation back into hydrogen cyanide and acetylene.


Imidogen is an inorganic compound with the chemical formula NH. Like other simple radicals, it is highly reactive and consequently short-lived except as a dilute gas. Its behavior depends on its spin multiplicity, i.e. the triplet versus singlet ground state.

DR 21

DR 21 is a large molecular cloud located in the constellation Cygnus, discovered in 1966 as a radio continuum source by Downes and Rinehart. DR 21 is located about 6,000 light-years (1,800 pc) from Earth and extends for 80 light-years (25 pc). The region contains a high rate of star formation and is associated with the Cygnus X star forming region. It has an estimated mass of 1,000,000 M.

NGC 1386

NGC 1386 is a spiral galaxy located in the constellation Eridanus. It is located at a distance of circa 53 million light years from Earth, which, given its apparent dimensions, means that NGC 1386 is about 50,000 light years across. It is a Seyfert galaxy, the only one in Fornax Cluster.


  1. Snyder, L. E., Buhl, D., Zuckerman, B., & Palmer, P. 1969, Phys. Rev. Lett., 22, 679
  2. Zubritsky, Elizabeth; Neal-Jones, Nancy (11 August 2014). "RELEASE 14-038 - NASA's 3-D Study of Comets Reveals Chemical Factory at Work". NASA . Retrieved 12 August 2014.
  3. Cordiner, M.A.; et al. (11 August 2014). "Mapping the Release of Volatiles in the Inner Comae of Comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) Using the Atacama Large Millimeter/Submillimeter Array". The Astrophysical Journal . 792 (1): L2. arXiv: 1408.2458 . Bibcode:2014ApJ...792L...2C. doi:10.1088/2041-8205/792/1/L2. S2CID   26277035.
  4. Snyder, L. E., Buhl, D., Zuckerman, B., & Palmer, P. 1969, Phys. Rev. Lett., 22, 679
  5. Zuckerman, B.; Buhl, D.; Palmer, P.; Snyder, L. E. 1970, Astrophysical Journal, 160, 485
  6. 1 2 Henkel, C.; Guesten, R.; Gardner, F. F. 1985, Astronomy and Astrophysics, 143, 148
  7. Sandqvist, A.; Tomboulides, H.; Lindblad, P. O. 1988, Astronomy and Astrophysics, 205, 225
  8. 1 2 Araya, E. _et al_. 2007, Astrophysical Journal, 654, L95
  9. 1 2 Hoffman, I. M.; Goss, W. M.; Palmer, P. 2007, Astrophysical Journal, 654, 971
  10. 1 2 3 Araya et al. 2007, Astrophysical Journal, 669, 1050
  11. Hoffman, I. M.; Goss, W. M.; Palmer, P. 2007, Astrophysical Journal, 654, 971
  12. 1 2 3 4 J. G. Mangum et al. 2008, Astrophysical Journal, 673, 832.
  13. 1 2 3 Woon, D.E. 2002, Astrophysical Journal, 569, 541.
  14. 1 2 Awad et al. 2005, Astrophysical Journal, 626, 262.
  15. 1 2 3 4 W. A. Schutte et al. 1993, Science, 259, 1143.
  16. 1 2 3 4 W. A. Schutte et al. 1993, Icarus, 104, 118.
  17. 1 2 M. Tudorie et al. 2006, Astronomy and Astrophysics, 453, 755.
  18. H. S. P. Muller et al. 2000, Journal of Molecular Spectroscopy, 200, 143.