Abstract

Star formation processes such as infall, accretion, and outflows increase the complexity of molecules, allowing us to use those molecules to probe the physical environments where stars form.  The most deeply embedded protostars present particularly rich spectra of molecules due to their dense envelope and active infall and outflows, making them best probed by molecular spectroscopy.  Stars form via the infall of mass from a core, but direct evidence for such infall has been elusive. The most direct probe of infall is redshifted absorption against the central continuum source, which is best shown in dense gas tracers, such as HCO+ and HCN.  Our ALMA observations of these two molecules show such redshifted absorption toward an isolated embedded protostar, BHR71. Both lines show a similar redshifted absorption profile, indicative of infall. We model the line profiles with 3D radiative transfer calculations to constrain the kinematics properties of the collapsing envelope.  Together with a parameterized chemical abundance profile, we successfully reproduce the infall signature from the HCO+ line. The observation matches a younger envelope, which is consistent with the outflow dynamical age, than the ones constrained by the previous dust model. We also found emissions of complex organic molecules (COMs), revealing the "hot corinos" nature of BHR71.  We found that complex organic molecules emit from a compact region centered on the continuum source, corresponding to ~70-90 AU. Several COMs, such as methanol and methyl formate, show clear signatures of rotation, which is consistent with a ring from a part of rotating infalling envelope. The similar abundances of COMs toward isolated protostars, such as B335 and L483, suggests that the warm temperature at the inner region passively release the COMs rather than actively modifies the chemical composition of COMs.