Unveiling the detailed density and velocity structures of the protostellar core B335

Abstract

We present an observational study of the protostellar core B335 harboring a low-mass Class 0 source. The observations of the H13CO+(J=1-0) line emission were carried out using the Nobeyama 45 m telescope and Nobeyama Millimeter Array. Our combined image of the interferometer and single-dish data depicts detailed structures of the dense envelope within the core. We found that the core has a radial density profile of n(r) prop. r^-p and a reliable difference in the power-law indices between the outer and inner regions of the core: p~2 for r >= 4000 AU and p ~ 1.5 for r <= 4000 AU}. The dense core shows a slight overall velocity gradient of ~1.0 km s^-1 over the scale of 20,000 AU across the outflow axis. We believe that this velocity gradient represents a solid-body-like rotation of the core. The dense envelope has a quite symmetrical velocity structure with a remarkable line broadening toward the core center, which is especially prominent in the position-velocity diagram across the outflow axis. The model calculations of position-velocity diagrams do a good job of reproducing observational results using the collapse model of an isothermal sphere in which the core has an inner free-fall region and an outer region conserving the conditions at the formation stage of a central stellar object. We derived a central stellar mass of ~0.1 M_sun, and suggest a small inward velocity, v(r>r_inf) ~ 0 km s^-1 in the outer core at >= 4000 AU. We concluded that our data can be well explained by gravitational collapse with a quasi-static initial condition, such as Shu's model, or by the isothermal collapse of a marginally critical Bonnor-Ebert sphere.