Detection of Infall in the Protostar B335 with ALMA

Observations of the isolated globule B335 with ALMA have yielded absorption features against the continuum that are redshifted from the systemic velocity in both HCN and HCO$^+$ lines. These features provide unambiguous evidence for infall toward a central luminosity source. Previously developed models of inside-out collapse can match the observed line profiles of HCN and HCO$^+$ averaged over the central 50 AU. At the new distance of 100 pc, the inferred infall radius is 0.012 pc, the mass infall rate is $3 \times 10^{-6}$ solar masses per year, the age is 50,000 years, and the accumulated mass in the central zone is 0.15 solar masses, most of which must be in the star or in parts of a disk that are opaque at 0.8 mm. The continuum detection indicates an optically thin mass (gas and dust) of only $7.5\times 10^{-4}$ solar masses in the central region, consistent with only a very small disk mass.