Imaging chemical differentiation around the low-mass protostar L483-mm

Abstract

This paper presents a millimeter wavelength aperture-synthesis study of the spatial variations of the chemistry in the envelope around the deeply embedded low-mass protostar L483-mm on 1000 AU (5") scales. Lines of 8 molecular species including CN, C18O, CS, C34S, HCN, H13CN, HCO+ and N2H+ have been observed using the Owens Valley Radio Observatory Millimeter Array. Continuum emission at 2.7-3.4 millimeter is well-fit by a pure envelope model down to the sensitivity of the interferometer without introducing a disk/compact source, in contrast to what is seen for other protostars. A velocity gradient in the densest material is traced by HCN, CS and N2H+, and is perpendicular to the large-scale CO outflow, with a pattern consistent with rotation around a 1 solar mass central object. Velocity gradients in the propagation direction of the outflow suggest a clear interaction between the outflowing material and "quiescent" core. The C18O observations are fit with a "drop" abundance profile where CO is frozen-out in a region of the envelope with temperatures lower than 40 K and densities higher than 1.5e5 cm^{-3}, which is also required to reproduce single-dish observations. The N2H+ emission strongly resembles that of NH3 and is found to be absent toward the central continuum source. This is a direct consequence of the high CO abundances in the inner region as illustrated by a chemical model for the L483 envelope. The observed CN emission forms a spatial borderline between the outflowing and quiescent material probed by, respectively, HCO+ and N2H+, and also shows intermediate velocities compared to these two species. A scenario is suggested in which CN is enhanced in the walls of an outflow cavity due to the impact of UV irradiation either from the central protostellar system or related to shocks caused by the outflow.