Hydrogen cyanide and isocyanide in prestellar cores

Abstract

Aims. We studied the abundance of HCN, H 13 CN, and HN 13 C in a sample of prestellar cores, in order to search for species associated with high density gas. Methods. We used the IRAM 30 m radiotelescope to observe along the major and the minor axes of L1498, L1521E, and TMC 2, three cores chosen on the basis of their CO depletion properties. We mapped the J = 1 → 0 transition of HCN, H 13 CN, and HN 13 C towards the source sample plus the J = 1 → 0 transition of N 2 H + and the J = 2 → 1 transition of C 18 O in TMC 2. We used two different radiative transfer codes, making use of recent collisional rate calculations, in order to determine more accurately the excitation temperature, leading to a more exact evaluation of the column densities and abundances. Results. We find that the optical depths of both H 13 CN(1−0) and HN 13 C(1−0) are non-negligible, allowing us to estimate excitation temperatures for these transitions in many positions in the three sources. The observed excitation temperatures are consistent with recent computations of the collisional rates for these species and they correlate with hydrogen column density inferred from dust emission. We conclude that HCN and HNC are relatively abundant in the high density zone, n(H 2 ) ∼ 10 5 cm −3 , where CO is depleted. The relative abundance [HNC]/[HCN] differs from unity by at most 30% consistent with chemical expectations. The three hyperfine satellites of HCN(1−0) are optically thick in the regions mapped, but the profiles become increasingly skewed to the blue (L1498 and TMC 2) or red (L1521E) with increasing optical depth suggesting absorption by foreground layers.

Radiative Transfer and Molecular Data for Astrochemistry

“…Previously, HCN rates were used for HNC, which turns out not to be accurate at all. Recent observations of the H 13 CN/HN 13 C ratio in a sample of dense cores are in good agreement with the newly calculated collision rates (Padovani et al 2011b). …”

The origin of gas-phase HCO and CH3O radicals in prestellar cores

“…If the actual T ex is above 4 K, this will not change the column density values very much. We cannot, however, completely exclude T ex values between 3 and 4 K because such values have already been observed in other species (Padovani et al 2011;Hily-Blant et al 2013). Should the excitation temperatures actually be below 4 K, the column densities would be higher than those quoted in Table 3.…”

The CN/C15N isotopic ratio towards dark clouds

“…The 13 CN column density may also be compared with the H 13 CN column density, which is 1.8 ×10 12 cm −2 towards L1498 (Padovani et al 2011) and 4.6 ×10 12 cm −2 towards L1544 (HB13). 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.…”

“…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).…”