2020ApJ...891...61Y

The collapse of a protostellar envelope results in the growth of a protostar and the development of a protoplanetary disk, playing a critical role during the early stages of star formation. Characterizing the gas infall in the envelope constrains the dynamical models of star formation. We present unambiguous signatures of infall, probed by optically thick molecular lines, toward an isolated embedded protostar, BHR 71 IRS1. The three-dimensional radiative transfer calculations indicate that a slowly rotating infalling envelope model following the "inside-out" collapse reproduces the observations of both HCO⁺J=4→3 and CS J=7→6 lines, as well as the low-velocity emission of the HCN J=4→3 line. The envelope has a model-derived age of 12,000 ± 3000 yr after the initial collapse. The envelope model underestimates the high-velocity emission at the HCN J=4→3 and H¹³CN J=4→3 lines, where outflows or a Keplerian disk may contribute. The ALMA observations serendipitously discover the emission of complex organic molecules (COMs) concentrated within a radius of 100 au, indicating that BHR 71 IRS1 harbors a hot corino. Eight species of COMs are identified, including CH₃OH and CH₃OCHO, along with H₂CS, SO₂ and HCN v₂ = 1. The emission of methyl formate and ¹³C-methanol shows a clear velocity gradient within a radius of 50 au, hinting at an unresolved Keplerian rotating disk.