APEX-CHAMP⁺ high-J CO observations of low-mass young stellar objects

I. The HH 46 envelope and outflow

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA e-mail: tvankempen@cfa.harvard.edu
³ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
⁴ Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121, Bonn, Germany
⁵ Nederlandse Onderzoeksschool Voor Astronomie (NOVA), PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ SRON Netherlands Institute for Space Research , PO Box 800, 9700 AV Groningen, The Netherlands

Received: 9 March 2009
Accepted: 12 May 2009

Abstract

Context. The spectacular outflow of HH 46/47 is driven by HH 46 IRS 1, an embedded Class I Young Stellar Object (YSO). Although much is known about this region from extensive optical and infrared observations, the properties of its protostellar envelope and molecular outflow are poorly constrained.

Aims. Our aim is to characterize the size, mass, density and temperature profiles of the protostellar envelope of HH 46 IRS 1 and its surrounding cloud material as well as the effect the outflow has on its environment.

Methods. The newly commisioned CHAMP⁺ and LABOCA arrays on the APEX telescope, combined with lower frequency line receivers, are used to obtain a large (5'5', 0.60.6 pc) continuum map and smaller (80''80'', 36 00036 000 AU) heterodyne maps in various isotopologues of CO and HCO⁺. The high-J lines of CO (6–5 and 7–6) and its isotopologues together with [C I] 2–1, observed with CHAMP⁺, are used to probe the warm molecular gas in the inner few hundred AU and in the outflowing gas. The data are interpreted with continuum and line radiative transfer models.

Results. Broad outflow wings are seen in CO low- and high-J lines at several positions, constraining the gas temperatures to a constant value of ~100 K along the red outflow axis and to ~60 K for the blue outflow. The derived outflow mass is of order 0.4–0.8 , significantly higher than previously found. The bulk of the strong high-J CO line emission has a surprisingly narrow width, however, even at outflow positions. These lines cannot be fit by a passively heated model of the HH 46 IRS envelope. We propose that it originates from photon heating of the outflow cavity walls by ultraviolet photons originating in outflow shocks and the accretion disk boundary layers. At the position of the bow shock itself, the UV photons are energetic enough to dissociate CO. The envelope mass of ~5  is strongly concentrated towards HH 46 IRS with a density power law of -1.8.

Conclusions. The fast mapping speed offered by CHAMP⁺ allows the use of high-J CO lines and their isotopes to generate new insights into the physics of the interplay between the molecular outflow and protostellar envelope around low-mass protostars. The UV radiation inferred from the high-J CO and [C I] data will affect the chemistry of other species.