Hydromagnetic accretion shocks around low-mass protostars

We present theoretical arguments for the existence of an accretion shock driven by magnetic fields around low-mass protostars. Our analysis is based on the current theory for low-mass stars formed in isolation. In this theory, the magnetic field is trapped by the freely falling accreting matter until the density is high enough for matter and field to decouple from each other. We propose that the decoupled magnetic field diffuses outward as more and more flux is brought in by accreting matter, driving a hydromagnetic shock wave away from the central protostar. The radius of the shock is typically on the order of a few thousand AU near the end of the main protostellar accretion phase around solar mass stars. We also suggest that the accreting material behind the shock is turbulent; otherwise, the magnetic field in the postshock region could almost support the accreting matter against gravity, a situation prone to interchange-type instabilities. We identify the postshock region with the 10(3) AU size flattened structure around HL Tauri, and predict that the magnetic field strength there is on the order of 1 mG. If confirmed by observations, the shock and its associated structure will lend strong support to the current paradigm-for low-mass star formation in isolation invoking gradual loss of magnetic support of precollapsing molecular clouds. Our analytic treatment of the problem provides a guide for future numerical simulations of the main accretion phase of star formation including magnetic field. Its implication for the so-called magnetic flux problem is also discussed.