Nitrogen-bearing molecules in prestellar cores

Details Nitrogen-bearing molecules in prestellar cores Nitrogen-bearing molecules in prestellar cores

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.282p&link_type=abstract

The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole

Astronomy and Astrophysics

Astronomy

Scientific paper

Tracers which are found in the high density gas phase (nh=10^5-10^6 cm-3) are fundamental to study the kinematics of the central regions of prestellar cores. The main issue for molecular lines is depletion which increases rapidly at these high densities and low temperatures. Tafalla et al. (2006, A&A, 455, 577) found that N_2H^+ and NH_3 have a spatial distribution similar to optically thin dust emission, but Hily-Blant et al. (2010, A&A, 513, 41) also obtained evidence for the existence in the gas phase of species containing both carbon and nitrogen at n>10^5 cm-3, where CO has disappeared. The implication of this is that at least some form of carbon may remain in the gas phase. Our approach to this problem, which has not been fully exploited to date, is to make use of the hyperfine splitting present in essentially all low lying transitions of N-containing species. Besides abundant species as CN and HCN, we observed H13CN(1-0) and HN13C(1-0) toward a sample of prestellar cores which have been chosen on the basis of the CO depletion properties (Bacmann et al. 2002, A&A, 389, L6). The observation of the more optically thin HCN and HNC isotopologues allowed us to a better determination of the excitation temperatures. These values fall on the theoretical curve expected by two different radiative transfer codes (RADEX, see van der Tak et al. 2007, A&A, 468, 627, and the Monte Carlo code in Tafalla et al. 2002, ApJ, 569, 815), representing a pretty confirmation of the recent calculation of HCN and HNC collisional rates (Sarrasin et al. 2010, MNRAS, 404, 518, and Dumouchel et al. 2010, MNRAS, 406, 2488). One important result is that we found an abundance ratio [HN13C]/[H13CN] of about 1 in all the sources, in strong agreement with detailed chemical models (e.g. Herbst et al. 2000, MNRAS, 311, 869) and we also noticed a qualitatively correlation between the values of the excitation temperatures and the H_2 column density: this could mean that the so-called depletion hole is not present or better that the abundance decreases toward the centre, but without reaching the value of zero.

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