Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011hsa6.conf..442t&link_type=abstract
Highlights of Spanish Astrophysics VI, Proceedings of the IX Scientific Meeting of the Spanish Astronomical Society (SEA), held
Astronomy and Astrophysics
Astrophysics
Scientific paper
Star formation occurs in the densest and coldest parts of molecular clouds. In these regions, neither H_2 nor He emit appreciable radiation, and because of that, the study of the cold star-forming gas depends critically on the observation of low-abundance tracer species like CS, HCN, or NH_3. In recent years, evidence has emerged that many of these tracers suffer significant changes in abundance during the process of core contraction, and because of that, we now know that the observational appearance of many star-forming regions changes systematically as the gas becomes denser. Most of these chemical changes result from the fact that at low temperatures and high densities, molecules stick to the dust grains and cannot evaporate back into the gas phase. As a result, classical dense gas tracers like CS and HCN freeze out under the very same gas conditions that they were supposed to trace, and thus become unreliable tools to study cold, star-forming gas. Fortunately, a small number of species seem to benefit from the freeze out of CO, and they increase in abundance, at least in the range of typical densities of low-mass starless cores. Nitrogen-bearing molecules like NH_3, in particular, have become favored tracers of core material, given that their abundance increases in CO-depleted regions. Some deuterated ions, like H_2D+, also favor CO-depleted gas, and their abundance enhancement under these conditions is responsible for the high degree of deuterium fractionation observed in some star-forming regions. This new understanding of dense core chemistry offers a new tool to study dense core evolution, and a number of searches to identify cores at the earliest stages of contraction are in progress.
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