Protostellar discs fed by dense collapsing gravomagneto sheetlets

Abstract

Stars form from the gravitational collapse of turbulent, magnetized molecular cloud cores. Our non-ideal MHD simulations reveal that the intrinsically anisotropic magnetic resistance to gravity during the core collapse naturally generates dense gravomagneto sheetlets within inner protostellar envelopes – disrupted versions of classical sheet-like pseudo-discs. They are embedded in a magnetically dominant background, where less dense materials flow along the local magnetic field lines and accumulate in the dense sheetlets. The sheetlets, which feed the disc predominantly through its upper and lower surfaces, are the primary channels for mass and angular momentum transfer from the envelope to the disc. The protostellar disc inherits a small fraction (up to 10 per cent) of the magnetic flux from the envelope, resulting in a disc-averaged net vertical field strength of 1–10 mG and a somewhat stronger toroidal field, potentially detectable through ALMA Zeeman observations. The inherited magnetic field from the envelope plays a dominant role in disc angular momentum evolution, enabling the formation of gravitationally stable discs in cases where the disc field is relatively well-coupled to the gas. Its influence remains significant even in marginally gravitationally unstable discs formed in the more magnetically diffusive cases, removing angular momentum at a rate comparable to or greater than that caused by spiral arms. The magnetically driven disc evolution is consistent with the apparent scarcity of prominent spirals capable of driving rapid accretion in deeply embedded protostellar discs. The dense gravomagneto sheetlets observed in our simulations may correspond to the ‘accretion streamers’ increasingly detected around protostars.