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

Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: joergensen@strw.leidenuniv.nl

Received: 11 February 2004
Accepted: 14 May 2004

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 mm is well-fit by an envelope model based on previously reported submillimeter continuum images down to the sensitivity of the interferometer without introducing a disk/compact source, in contrast to what is seen for other protostellar objects. A velocity gradient in dense material close to the central protostar 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  central object. Velocity gradients in the propagation direction of the outflow suggest a clear interaction between the outflowing material and “quiescent” core. Significant differences are observed between the emission morphologies of various molecular species. The C¹⁸O interferometer 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 1.5 cm^-3, which is also required to reproduce previously reported 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.