Infall and outflow within 400 AU from a high-mass protostar

Abstract

Observational signatures of infalling envelopes and outflowing material in early stages of protostellar evolution and at small radii from the protostar are essential to progress in the understanding of the mass-accretion process in star formation. In this Letter, we report a detailed study on the accretion and outflow structure around a protostar in the well-known high-mass star-forming region AFGL 5142. We focus on the mm source MM–1, which exhibits hot-core chemistry, radio continuum emission, and strong water (H2O) and methanol (CH3OH) masers. Remarkably, our Very Long Baseline Interferometry (VLBI) observations of molecular masers over six years provided us with the 3D velocity field of circumstellar molecular gas with a resolution of 0.001–0.005′′ and at radii < 0.′′23 (or 400 AU) from the protostar. In particular, our measurements of CH3OH maser emission for the first time provided a direct measurement of the infall of a molecular envelope (radius of 300 AU and velocity of 5 km s-1) onto an intermediate- to high-mass protostar. We estimate an infall rate of 6 × 10-4 n8 M⊙ yr-1, where n8 is the ambient volume density in units of 108 cm-3 (required for maser excitation). In addition, our measurements of the H2O maser (and radio continuum) emission identify a collimated bipolar molecular outflow (and ionized jet) from MM–1. The evidence of simultaneous accretion and outflow at small spatial scales makes AFGL 5142 an extremely compelling target for high-angular resolution studies of high-mass star formation.