Complex organic molecules and cosmic ray ionisation rate towards the massive protostar Cepheus A HW2

Abstract

Cosmic rays (CRs) are important drivers for molecular chemistry in star-forming regions, and laboratory experiments have shown that CRs can stimulate the release of complex organic molecules (COMs) such as methanol. Observationally, this has primarily been tested in cold, low-mass cores, so studying how CRs affect COM formation in a high-mass star-forming environment is of great interest. We performed a high-sensitivity wide-band spectral line survey with the Onsala 20 m telescope towards the high-mass protostar Cepheus A HW2, which is known to host an ionised jet. Consistent with previous studies, two primary velocity components ($-11$ km s$^{-1}$ and $-5$ km s$^{-1}$) were identified. Column densities and relative abundances of the detected ions and COMs were estimated from rotational diagrams, single transitions and RADEX grid searches (CH$_3$OH: $1.6\times10^{-9}$, CH$_3$CN: $5.9\times10^{-11}$, t-HCOOH: $7.9\times10^{-11}$, H$_2$CCO: $1.7\times10^{-11}$, CH$_3$CHO: $1.9\times10^{-11}$, CH$_3$OCHO: $7.6\times10^{-10}$ at $-11$ km s$^{-1}$). Deuterium fractions were also estimated (in range $0.002-0.3$ at $-11$ km s$^{-1}$), and the volume density of molecular hydrogen ($2.6\times10^5$ cm$^{-3}$ at $-11$ km s$^{-1}$) was constrained from the RADEX grid searches. Electron fractions and CR ionisation rates (CRIR, $6.8\times10^{-17}$ s$^{-1}$ at $-11$ km s$^{-1}$, $\leq9.2\times10^{-19}$ s$^{-1}$ at $-5$ km s$^{-1}$) were estimated through analytic chemistry using different ions as probes. The gas-grain chemical code Nautilus reproduced the observed abundances of CH$_3$OH, CH$_3$CN, HCO$^+$, N$_2$H$^+$ at the observed density, temperature and CRIR within the uncertainty of the model. The results indicate that the CR ionisation rate of the kinematic component associated with most of the COMs' emission in the region is locally enhanced.