Hydrogen cyanide and isocyanide in prestellar cores

Abstract

We studied the abundance of HCN, H13CN, and HN13C in a sample of prestellar cores, in order to search for species associated with high density gas. We used the IRAM 30m 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, H13CN, and HN13C towards the source sample plus the J=1-0 transition of N2H+ and the J=2-1 transition of C18O 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. We find that the optical depths of both H13CN(1-0) and HN13C(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(H2) about 10^5 cm-3, where CO is depleted. The relative abundance [HNC]/[HCN] differs from unity by at most 30 per cent 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.

13 pages, 19 figures, to be published in Astronomy and Astrophysics

References in corpus (6)

• Cold Dark Clouds: The Initial Conditions for Star Formation
• On the internal structure of starless cores. II. A molecular survey of L1498 and L1517B
• On the frequency of N2H+ and N2D+
• Spatial mapping of ices in the Oph-F core: A direct measurement of CO depletion and the formation of CO2
• The abundances of nitrogen-containing molecules during pre-protostellar collapse
• CN in prestellar cores

Cited by in corpus (34)

• Chemistry of Dark Clouds: Databases, Networks, and Models
• Astrochemistry of dust, ice and gas: introduction and overview
• Interstellar chemistry of nitrogen hydrides in dark clouds
• The HNC/HCN Ratio in Star-Forming Regions
• The 15N-enrichment in dark clouds and Solar System objects
• Chemical Evolution in High-Mass Star-Forming Regions: Results from the MALT90 Survey
• Isotopic Anomalies in Primitive Solar System Matter: Spin-state Dependent Fractionation of Nitrogen and Deuterium in Interstellar Clouds
• The thermal reactivity of HCN and NH3 in interstellar ice analogues
• Evolution and excitation conditions of outflows in high-mass star-forming regions
• The origin of gas-phase HCO and CH3O radicals in prestellar cores
• Investigating the Minimum Energy Principle in Searches for New Molecular Species - the Case of H${}_2$C${}_3$O Isomers
• A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds
• Nitrogen fractionation in high-mass star-forming cores across the Galaxy
• The CN/C15 N isotopic ratio towards dark clouds
• Nitrogen and hydrogen fractionation in high-mass star forming cores from observations of HCN and HNC
• Detection of the HC${}_3$NH${}^{+}$ and HCNH${}^{+}$ ions in the L1544 pre-stellar core
• Abundance of HCN and its C and N isotopologues in L1498
• Physical properties and chemical composition of the cores in the California molecular cloud
• Chemical tracers in proto-brown dwarfs: CN, HCN, and HNC observations
• The chemical structure of young high-mass star-forming clumps: (I) Deuteration
• Carbon isotope fractionation and depletion in TMC1
• A Search for O_2 in CO-depleted Molecular Cloud Cores with Herschel
• Molecules in η Carinae
• Exploring HNC and HCN line emission as probes of the protoplanetary disk temperature
• The evolutionary status of dense cores in the NGC 1333 IRAS 4 region
• Nitrogen fractionation towards a pre-stellar core traces isotope-selective photodissociation
• Radial molecular abundances and gas cooling in starless cores
• Similar levels of deuteration in the pre-stellar core L1544 and the protostellar core HH211
• Young starless cores embedded in the magnetically dominated Pipe Nebula. II. Extended dataset
• Dark cloud-type chemistry in PDRs with moderate UV field
• Sampling Molecular Gas in the Helix Planetary Nebula: Variation in HNC/HCN with UV Flux
• Chemical compositions of five Planck cold clumps
• What did the seahorse swallow? APEX 170 GHz observations of the chemical conditions in the Seahorse infrared dark cloud
• HCN (1-0) opacity of outflowing gas in Arp 220W