Chemistry of C_3 and carbon chain molecules in DR21(OH)

Abstract

Context. C_3 is the smallest pure carbon chain detected in the dense environment of star-forming regions, although diatomic C_2 is detected in diffuse clouds. Measurement of the abundance of C_3 and the chemistry of its formation in dense star-forming regions has remained relatively unexplored. Aims. We aim to identify the primary C_3 formation routes in dense star-forming regions following a chemical network producing species like CCH and c-C_(3)H_2 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 ν_2 bending mode at frequencies between 1654–1897 GHz using HIFI on board Herschel, in DR21(OH). Several transitions of CCH and c-C_(3)H_2 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 C_3 forming reactions in DR21(OH). Results. We detected C_3 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 C_3 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-C_(3)H_2, we only detect emission from the envelope and MM1. The observed CCH, C_3  and c-C_(3)H_2 abundances are most consistent with a chemical model with nH_2 ~ 5 × 10^6 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 C_3 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 C_3 abundance in MM1 as compared to MM2 and the envelope to be due to the destruction of C_3 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.

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