Collisional excitation and non-LTE modelling of interstellar chiral propylene oxide

TL;DR

This paper presents quantum calculations of collisional excitation rates for propylene oxide with helium and hydrogen, and models its detection in interstellar space to understand its abundance and chemistry.

Contribution

It provides the first theoretical collisional cross sections for propylene oxide with helium and applies non-LTE radiative transfer modeling to interpret interstellar observations.

Findings

Detected propylene oxide in Sgr B2(N) at ~10 K

Derived column density of ~3e12 cm-2

Estimated fractional abundance of ~2.5e-11

Abstract

The first set of theoretical cross sections for propylene oxide (CH3CHCH2O) colliding with cold He atoms has been obtained at the full quantum level using a high-accuracy potential energy surface. By scaling the collision reduced mass, rotational rate coefficients for collisions with para-H2 are deduced in the temperature range 5-30 K. These collisional coefficients are combined with radiative data in a non-LTE radiative transfer model in order to reproduce observations of propylene oxide made towards the Sagittarius B2(N) molecular cloud with the Green Bank and Parkes radio telescopes. The three detected absorption lines are found to probe the cold (~ 10 K) and translucent (nH ~ 2000 cm-3) gas in the outer edges of the extended Sgr B2(N) envelope. The derived column density for propylene oxide is Ntot ~ 3e12 cm-2, corresponding to a fractional abundance relative to total hydrogen of ~ 2.5e-11. The present results are expected to help our understanding of the chemistry of propylene oxide, including a potential enantiomeric excess, in the cold interstellar medium.