C₂H in prestellar cores

Abstract

Aims. We study the abundance of C 2 H in prestellar cores both because of its role in the chemistry and because it is a potential probe of the magnetic field. We also consider the non-LTE behaviour of the N = 1−0 and N = 2−1 transitions of C 2 H and improve current estimates of the spectroscopic constants of C 2 H. Methods. We used the IRAM 30 m radiotelescope to map the N = 1−0 and N = 2−1 transitions of C 2 H towards the prestellar cores L1498 and CB246. Towards CB246, we also mapped the 1.3 mm dust emission, the J = 1−0 transition of N 2 H + and the J = 2−1 transition of C 18 O. We used a Monte Carlo radiative transfer program to analyse the C 2 H observations of L1498. We derived the distribution of C 2 H column densities and compared with the H 2 column densities inferred from dust emission. Results. We find that while non-LTE intensity ratios of different components of the N = 1−0 and N = 2−1 lines are present, they are of minor importance and do not impede C 2 H column density determinations based upon LTE analysis. Moreover, the comparison of our Monte-Carlo calculations with observations suggest that the non-LTE deviations can be qualitatively understood. For extinctions less than 20 visual magnitudes, we derive toward these two cores (assuming LTE) a relative abundance [C 2 H]/[H 2 ] of (1.0± 0.3)× 10 −8 in L1498 and (0.9 ± 0.3) × 10 −8 in CB246 in reasonable agreement with our Monte-Carlo estimates. For L1498, our observations in conjunction with the Monte Carlo code imply a C 2 H depletion hole of radius 9 × 10 16 cm similar to that found for other C-containing species. We briefly discuss the significance of the observed C 2 H abundance distribution. Finally, we used our observations to provide improved estimates for the rest frequencies of all six components of the C 2 H (1−0) line and seven components of C 2 H (2−1). Based on these results, we compute improved spectroscopic constants for C 2 H. We also give a brief discussion of the prospects for measuring magnetic field strengths using C 2 H.

Hydrogen cyanide and isocyanide in prestellar cores

“…strongest component, as found for the C 2 H(1−0) emission in the same core (Padovani et al 2009). The other two cores show ratios which deviate from the LTE curve revealing again the presence of optical depth effects.…”

Tracing the evolutionary stage of Bok globules: CCS and NH3

“…A possible explanation could be that both molecules do not trace exactly similar cloud regions, as in the cases found by Codella & Scappini (1998b) and Lai & Crutcher (2000) where the most dense parts of cores are preferentially detected in NH 3 , while CCS emission originates from a more extended surrounding region. These central depletion holes in evolved sources are also observed for other carbon-bearing species, such as the CCH radical (e.g., Beuther et al 2008;Padovani et al 2009), which resembles in many properties, and is thought to be a precursor of, CCS. Since both NH 3 and CCS have roughly comparable critical densities (∼10 3 -10 4 cm −3 ), a difference in the emitting region is most likely a result of different spatial distributions.…”

A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds

“…(d) Far distance; the distance ambiguity of the IRAS source was resolved by Sewilo et al (2004). Reitblat (1980); Padovani et al (2009);Spielfiedel et al (2012). (k) n crit at 20 K. (l) Lapinov et al (2007).…”