Molecular Astrophysics

National Radio Astronomy Observatory

Molecular Astrophysics concerns the study of emission from molecules in space. Lew Snyder recently presented a list of the 110 currently known interstellar molecules. These molecules have large numbers of observable transitions. To find specific frequencies, try Herb Pickett's Molecular Spectroscopy Home Page or Frank Lovas' list of recommended rest frequencies. Tom Kuiper has put together an explanation of molecular radio spectroscopy for emission lines. Lines may also be observed in absorption--for example the highly redshifted lines seen against the gravitationally lensed quasar PKS1830-211. High energy radiation, such as ultraviolet light, can break the molecular bonds which hold atoms in molecules. In general then, molecules are found in cool astrophysical environments. The most massive objects in our Galaxy are giant clouds of molecules and dust, creatively named Giant Molecular Clouds. In these clouds, and smaller versions of them, stars and planets are formed. One of the primary fields of study of molecular astrophysics then, is star and planet formation. Molecules may be found in many environments, however, from stellar atmospheres to those of planetary satellites. Most of these locations are cool, and molecular emission is most easily studied via photons emitted when the molecules make transitions between low rotational energy states. One molecule, comprised of the abundant carbon and oxygen atoms, and very stable against dissociation into atoms, is carbon monoxide, CO. The wavelength of the photon emitted when the CO molecules falls from its lowest excited state to its zero energy, or ground, state is 2.6mm, or 115 gigahertz (billion hertz). This frequency is a thousand times higher than typical FM radio frequencies. At these high frequencies, molecules in the Earth's atmosphere can block transmissions from space, and telescopes must be located in dry (water is an important atmospheric blocker), high sites. Radio telescopes must have very accurate surfaces to produce high fidelity images. NRAO pioneered development of accurate antennas and high frequency receivers, and the development of molecular astrophysics, with the 11m radio telescope. In 1982, the surface of the 11m was replaced with a much more accurate 12m surface, pictured above.

Examples of objects studied in Molecular Astrophysics

Protostars, just forming from their parent clouds.

Protostars like that in the dark cloud L1157 jettison material from their poles as they spin down into a stable star. In this image, of the 2.6mm transition of CO taken with the NRAO 12m, blueshifted gas, moving towards us at speeds of 10,000 miles per hour, is color-coded blue, while gas moving away from us is coded red. Green stains the quiescent gas in the cloud. The protostar itself is not visible in this image. It might best be studied through emission from warm dust closely associated with it, which radiates in the millimeter and submillimeter continuum.

Common emission mechanisms in molecular astrophysics.

Continuum emission: modified blackbody from cold dust

Dust and gas toward the protostar VLA1623-2418.
As for L1157, red and blue in the image show redshifted and blueshifted jets of CO (the J=2-1 line at 1.3mm, observed by the IRAM 30m telescope in this case) emanating from the poles of a protostar. The region of the star itself appears in the yellow in this image, which shows continuum emission at 1.3mm. Note other clumps to the northeast, possible the sites of future star formation.

Spectral line emission: rotational transitions from molecular gas (CO, CS, HCO+, H2O etc.) and fine-structure transitions from atomic species (CI)

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