The SOFIA Massive (SOMA) Star Formation Q-band follow-up I. Carbon-chain chemistry of intermediate-mass protostars

Abstract

Evidence for similar chemical characteristics around low- and high-mass protostars has been found: in particular, a variety of carbon-chain species and complex organic molecules (COMs) are formed around them. On the other hand, the chemical compositions around intermediate-mass (IM; $2M_{\odot }<m_{\ast }<8M_{\odot }$) protostars have not been studied with large samples. In particular, it is unclear the extent to which carbon-chain species are formed around them. We aim to obtain the chemical compositions, particularly focusing on carbon-chain species, towards a sample of IM protostars. We have conducted Q-band (31.5-50 GHz) line survey observations towards eleven mainly intermediate-mass protostars with the Yebes 40 m radio telescope. The target protostars were selected from a sub-sample of the source list of the SOFIA Massive (SOMA) Star Formation project. Nine carbon-chain species (HC${}_3$N, HC${}_5$N, C${}_3$H, C${}_4$H, $linear-$H${}_2$CCC, $cyclic-$C${}_3$H${}_2$, CCS, C${}_3$S, and CH${}_3$CCH), three COMs (CH${}_3$OH, CH${}_3$CHO, and CH${}_3$CN), H${}_2$CCO, HNCO, and four simple sulfur (S)-bearing species (${}^{13}$CS, C${}^{34}$S, HCS${}^{+}$, H${}_2$CS) have been detected. The rotational temperatures of HC${}_5$N are derived to be $\sim 20-30$ K in three IM protostars and they are very similar compared to those around low- and high-mass protostars. These results indicate that carbon-chain molecules are formed in lukewarm ($\sim 20-30$ K) gas around the IM protostars by the Warm Carbon-Chain Chemistry (WCCC) process. Carbon-chain formation occurs ubiquitously in the warm gas around protostars across a wide range of stellar masses. Carbon-chain molecules and COMs coexist around most of the target IM protostars, which is similar to the situation in low- and high-mass protostars. The chemical characteristics around protostars are common in the low-, intermediate- and high-mass regimes.

arXiv commentAccepted for publication in the Astronomy and Astrophysics (A&A)

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