Constraining the Infalling Envelope Models of Embedded Protostars: BHR
  71 and its Hot Corino
  The collapse of the protostellar envelope results in the growth of the
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$^{+}$ $J=4\rightarrow3$ and CS $J=7\rightarrow6$ lines, and the low
velocity emission of the HCN $J=4\rightarrow3$ line. The envelope has a
model-derived age of 12000$\pm$3000 years after the initial collapse. The
envelope model underestimates the high velocity emission at the HCN
$J=4\rightarrow3$ and H$^{13}$CN $J=4\rightarrow3$ 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 CH$_{3}$OH and CH$_{3}$OCHO, along with
H$_{2}$CS, SO$_{2}$ and HCN $v_{2}=1$. The emission of methyl formate and
$^{13}$C-methanol shows a clear velocity gradient within a radius of 50 au,
hinting at an unresolved Keplerian rotating disk.
