2018MNRAS.477.5312Q

Ions and electrons play an important role in various stages of the star formation process. By following the magnetic field of their environment and interacting with neutral species, they slow down the gravitational collapse of the proto-star envelope. This process (known as ambipolar diffusion) depends on the ionization degree, which can be derived from the HCO⁺ abundance. We present a study of HCO⁺ and its isotopologues (H¹³CO⁺ , HC¹⁸O⁺ , DCO⁺ , and D¹³CO⁺) in the low-mass proto-star IRAS16293-2422. The structure of this object is complex, and the HCO⁺emission arises from the contribution of a young NW-SE outflow, the proto-stellar envelope, and the foreground cloud. We aim at constraining the physical parameters of these structures using all the observed transitions. For the young NW-SE outflow, we derive T_kin= 180-220 K and n(H₂) = (4-7)x 10⁶ cm^–3 with an HCO⁺abundance of (3-5)x 10^–9. Following previous studies, we demonstrate that the presence of a cold (T_kin<= 30 K) and low density [n(H₂) <= 1 x 10⁴ cm^–3] foreground cloud is also necessary to reproduce the observed line profiles. We have used the gas-grain chemical code NAUTILUS to derive the HCO⁺ abundance profile across the envelope and the external regions where X(HCO⁺) >= 1 x 10^–9 dominate the envelope emission. From this, we derive an ionization degree of 10^–8.9 <= x( e) <= 10^–7.9. The ambipolar diffusion time-scale is ∼5 times the free-fall time-scale, indicating that the magnetic field starts to support the source against gravitational collapse and the magnetic field strength is estimated to be 6-46µG.