X-ray-induced chemistry of water and related molecules in low-mass protostellar envelopes

Context. Water is a key molecule in star- and planet-forming regions. Recent water line observations toward several low-mass protostars suggest low water gas fractional abundances (<10⁻⁶ with respect to total hydrogen density) in the inner warm envelopes (r < 10² au). Water destruction by X-rays is thought to influence the water abundances in these regions, but the detailed chemistry, including the nature of alternative oxygen carriers, is not yet understood.
Aims: Our aim is to understand the impact of X-rays on the composition of low-mass protostellar envelopes, focusing specifically on water and related oxygen-bearing species.
Methods: We computed the chemical composition of two proto-typical low-mass protostellar envelopes using a 1D gas-grain chemical reaction network. We varied the X-ray luminosities of the central protostars, and thus the X-ray ionization rates in the protostellar envelopes.
Results: The protostellar X-ray luminosity has a strong effect on the water gas abundances, both within and outside the H₂O snowline (T_gas ~ 10² K, r ~ 10² au). Outside, the water gas abundance increases with L_X, from ~10⁻¹⁰ for low L_X to ~10⁻⁸-10⁻⁷ at L_X > 10³⁰ erg s⁻¹. Inside, water maintains a high abundance of ~10⁻⁴ for L_X ≲ 10²⁹-10³⁰ erg s⁻¹, with water and CO being the dominant oxygen carriers. For L_X ≳ 10³⁰-10³¹ erg s⁻¹, the water gas abundances significantly decrease just inside the water snowline (down to ~10⁻⁸-10⁻⁷) and in the innermost regions with T_gas ≳ 250 K (~10⁻⁶). For these cases, the fractional abundances of O₂ and O gas reach ~10⁻⁴ within the water snowline, and they become the dominant oxygen carriers. In addition, the fractional abundances of HCO⁺ and CH₃OH, which have been used as tracers of the water snowline, significantly increase and decrease, respectively, within the water snowline as the X-ray fluxes become larger. The fractional abundances of some other dominant molecules, such as CO₂, OH, CH₄, HCN, and NH₃, are also affected by strong X-ray fields, especially within their own snowlines. These X-ray effects are larger in lower-density envelope models.
Conclusions: X-ray-induced chemistry strongly affects the abundances of water and related molecules including O, O₂, HCO⁺, and CH₃OH, and can explain the observed low water gas abundances in the inner protostellar envelopes. In the presence of strong X-ray fields, gas-phase water molecules within the water snowline are mainly destroyed with ion-molecule reactions and X-ray-induced photodissociation. 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.