Nitrogen chemistry and depletion in starless cores

Abstract

Aims. We investigated the chemistry of nitrogen-containing species, principally isotopologues of CN, HCN, and HNC, in a sample of pre-protostellar cores. Methods. We used the IRAM 30 m telescope to measure the emission in rotational and hyperfine transitions of CN, HCN, 13 CN, H 13 CN, HN 13 C, and HC 15 N in L 1544, L 183, Oph D, L 1517B, L 310. The observations were made along axial cuts through the dust emission peak, at a number of regularly-spaced offset positions. The observations were reduced and analyzed to obtain the column densities, using the measurements of the less abundant isotopic variants in order to minimize the consequences of finite optical depths in the lines. The observations were compared with the predictions of a free-fall gravitational collapse model, which incorporates a non-equilibrium treatment of the relevant chemistry. Results. We found that CN, HCN, and HNC remain present in the gas phase at densities well above that at which CO depletes on to grains. The CN:HCN and the HNC:HCN abundance ratios are larger than unity in all the objects of our sample. Furthermore, there is no observational evidence for large variations of these ratios with increasing offset from the dust emission peak and hence with density. Whilst the differential freeze-out of CN and CO can be understood in terms of the current chemistry, the behaviour of the CN:HCN ratio is more difficult to explain. Models suggest that most nitrogen is not in the gas phase but may be locked in ices. Unambiguous conclusions require measurements of the rate coefficients of the key neutral-neutral reactions at low temperatures.

The CN/C15N isotopic ratio towards dark clouds

“…The 13 CN:H 13 CN ratios are thus 0.6 and 1.7 for L1498 and L1544 respectively, which are in harmony with ratios found by Hily-Blant et al (2010b) towards several starless cores.…”

“…This is likely due to the L1544 lines presenting two closeby velocity components (Hily-Blant et al 2010b), which are not resolved in the C 15 N spectrum. These two velocity components may be related to the collapse signature recently evidenced by the inverse P-Cygni profile of a water line (Caselli et al 2012).…”

“…The spectroscopic properties of both species and observed transitions are summarized in Table 1. Towards L1544, the 13 CN spectrum is taken from Hily-Blant et al (2010b), and towards L1498, it is taken from unpublished spectra of Padovani et al (2011). Data reduction was done using the CLASS software (Hily-Blant et al 2005).…”

The interstellar gas-phase chemistry of HCN and HNC

“…(Hirota et al 1998) were not able to evaluate the HNC abundance and therefore only reported HCN abundances. Recently (Hily-Blant et al 2010) determined CN, HCN and HNC abundances in various starless cores. Using an LTE analysis on H 13 CN and H 13 NC isotopomers, the authors found much smaller HCN and HNC abundances in dark cloud L134N than earlier observations.…”

Weeds: a CLASS extension for the analysis of millimeter and sub-millimeter spectral surveys

“…Finally it can compute model predictions at the LTE, as often needed to identify lines in spectra close to the confusion limit. Weeds has already been successfully used to analyze part of the IRAS 16293-2422 survey obtained with Herschel-HIFI Hily-Blant et al 2010), and we expect that it will be useful for future spectral surveys with this instrument as well. We think that it will become a standard tool for analyzing spectral surveys obtained with single dish ground based telescopes such as the IRAM-30 m. Yet, Weeds is not limited to the analysis of single dish observations.…”