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