Chemistry of C₃ and carbon chain molecules in DR21(OH)

Abstract

Context. C₃ is the smallest pure carbon chain detected in the dense environment of star-forming regions, although diatomic C₂ is detected in diffuse clouds. Measurement of the abundance of C₃ and the chemistry of its formation in dense star-forming regions has remained relatively unexplored.
Aims: We aim to identify the primary C₃ formation routes in dense star-forming regions following a chemical network producing species like CCH and c-C₃H₂ in the star-forming cores associated with DR21(OH), a high-mass star-forming region.
Methods: We observed velocity resolved spectra of four ro-vibrational far-infrared transitions of C₃ between the vibrational ground state and the low-energy ν₂ bending mode at frequencies between 1654-1897 GHz using HIFI on board Herschel, in DR21(OH). Several transitions of CCH and c-C₃H₂ were also observed with HIFI and the IRAM 30 m telescope. Rotational temperatures and column densities for all chemical species were estimated. A gas and grain warm-up model was used to obtain estimates of densities and temperatures of the envelope. The chemical network in the model was used to identify the primary C₃ forming reactions in DR21(OH).
Results: We detected C₃ in absorption in four far-infrared transitions, P(4), P(10), Q(2), and Q(4). The continuum sources MM1 and MM2 in DR21(OH), though spatially unresolved, are sufficiently separated in velocity to be identified in the C₃ spectra. All C₃ transitions are detected from the embedded source MM2 and the surrounding envelope, whereas only Q(4) and P(4) are detected toward the hot core MM1. The abundance of C₃ in the envelope and MM2 is ~6 × 10^-10 and ~3 × 10^-9, respectively. For CCH and c-C₃H₂, we only detect emission from the envelope and MM1. The observed CCH, C₃ and c-C₃H₂ abundances are most consistent with a chemical model with n_H2 ~ 5 × 10⁶ cm^-3, a post-warm-up dust temperature T_max = 30 K, and a time of ~0.7-3 Myr.
Conclusions: Post-warm-up gas phase chemistry of CH₄ released from the grain at t ~ 0.2 Myr and lasting for 1 Myr can explain the observed C₃ abundance in the envelope of DR21(OH), and no mechanism involving photodestruction of PAH molecules is required. The chemistry in the envelope is similar to the warm carbon chain chemistry found in lukewarm corinos. We interpret the observed lower C₃ abundance in MM1 as compared to MM2 and the envelope to be due to the destruction of C₃ in the more evolved MM1. The timescale for the chemistry derived for the envelope is consistent with the dynamical timescale of 2 Myr derived for DR21(OH) in other studies.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia, with important participation from NASA.