X-ray induced chemistry of water and related molecules in low-mass
  protostellar envelopes
  Recent water line observations toward several low-mass protostars suggest low
water gas fractional abundances in the inner warm envelopes. Water destruction
by X-rays has been proposed to influence the water abundances in these regions,
but the detailed chemistry, including the nature of alternative oxygen
carriers, is not yet understood. In this study, we aim to understand the impact
of X-rays on the composition of low-mass protostellar envelopes, focusing
specifically on water and related oxygen bearing species. We compute the
chemical composition of two low-mass protostellar envelopes using a 1D
gas-grain chemical reaction network, under various X-ray field strengths.
According to our calculations, outside the water snowline, the water gas
abundance increases with $L_{\mathrm{X}}$. Inside the water snowline, water
maintains a high abundance of $\sim 10^{-4}$ for small $L_{\mathrm{X}}$, with
water and CO being the dominant oxygen carriers. For large $L_{\mathrm{X}}$,
the water gas abundances significantly decrease just inside the water snowline
(down to $\sim10^{-8}-10^{-7}$) and in the innermost regions ($\sim10^{-6}$).
For these cases, the O$_{2}$ and O gas abundances reach $\sim 10^{-4}$ within
the water snowline, and they become the dominant oxygen carriers. The HCO$^{+}$
and CH$_{3}$OH abundances, which have been used as tracers of the water
snowline, significantly increase/decrease within the water snowline,
respectively, as the X-ray fluxes become larger. The abundances of some other
dominant molecules, such as CO$_{2}$, OH, CH$_{4}$, HCN, and NH$_{3}$, are also
affected by strong X-ray fields, especially within their own snowlines. These
X-ray effects are larger in lower density envelope models. Future observations
of water and related molecules (using e.g., ALMA and ngVLA) will access the
regions around protostars where such X-ray induced chemistry is effective.
