Sun-bathing around low-mass protostars: APEX-CHAMP+ observations of high-J CO

TL;DR

This study maps high-J CO lines around low-mass protostars, revealing UV heating effects and shock-heated gas, emphasizing the importance of spatial data for understanding star formation environments.

Contribution

First high-resolution maps of high-J CO lines around low-mass protostars, highlighting UV heating and shock effects in their surroundings.

Findings

UV photons dominate warm gas emission along outflow cavities.

Passive heating contributes at the protostar position.

Shock-heated gas is prominent in more massive Class 0 outflows.

Abstract

We present the first maps of high-excitation CO J=6-5 and 7-6 and isotopologue lines over 2'-5' regions at 10" resolution toward low-mass protostars to probe the origin of the warm gas in their surroundings. The data were obtained using the CHAMP+ 650/850 GHz heterodyne array receiver on APEX. Surprisingly strong quiescent extended narrow-line high-J 12CO 6-5 and 7-6 emission is seen toward all protostars, suggesting that heating by UV photons along the outflow cavity dominates the emission. At the source position itself, passive heating of the collapsing inner envelope by the luminosity of the source also contributes. The UV photons are generally not energetic enough to dissociate CO since the [C I] 2-1 emission, also probed by our data, is weak except at the bow shock at the tip of the outflow. The extended UV radiation is produced by the star-disk boundary layer as well as the jet- and bow-shocks, and will also affect the chemistry of species such as H2O and HCN around the outflow axis. Shock-heated warm gas characterized by broad CO line profiles is seen only toward the more massive Class 0 outflows. Outflow temperatures, estimated from the CO 6-5/3-2 line wing ratios, are ~100 K. These data illustrate the importance of getting spatial information to characterize the physical processes in YSO surroundings, important to interpret future Herschel and ALMA data.