Strong irradiation of protostellar cores in Corona Australis^⋆,⋆⋆

¹ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 København K, Denmark
e-mail: jlindberg@snm.ku.dk
² Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 København Ø, Denmark

Received: 15 May 2012
Accepted: 19 September 2012

Abstract

Context. The importance of the physical environment in the evolution of newly formed low-mass stars remains an open question. In particular, radiation from nearby more massive stars may affect both the physical and chemical structure of these kinds of young stars.

Aims. We constrain the physical characteristics of a group of embedded low-mass protostars in Corona Australis in the vicinity of the young luminous Herbig Be star R CrA.

Methods. Millimetre wavelength maps of molecular line and continuum emission towards the low-mass star forming region IRS7 near R CrA from the Submillimeter Array (SMA) and Atacama Pathfinder Experiment (APEX) are presented. The maps show the distribution of 18 lines from 7 species (H₂CO, CH₃OH, HC₃N, c-C₃H₂, HCN, CN and SiO) on scales from 3′′ to 60′′ (400–8000 AU). Using a set of H₂CO lines, we estimate the temperatures and column densities in the region using both LTE and non-LTE methods. The results are compared with 1-D radiative transfer modelling of the protostellar cores. These models constrain which properties of the central source, protostellar envelope, and surrounding radiation field can give rise to the observed line and continuum emission.

Results. Most of the H₂CO emission from the regions emerges from two elongated (~6000 AU long) narrow (<1500 AU) ridges dominating the emission picked up in both interferometric and single-dish measurements. The temperatures inferred from the H₂CO lines are no less than  ~30 K and more likely 50–60 K, and the line emission peaks are offset by  ~2500 AU from the location of the embedded protostars. These temperatures can not be explained by the heating from the young stellar objects (YSOs) themselves. Irradiation by the nearby Herbig Be star R CrA could, however, explain these high temperatures. The elevated temperatures can in turn impact the physical and chemical characteristics of protostars, in particular, lead to enhanced abundances of typical tracers of photon dominated regions (PDRs) such as seen in single-dish line surveys of embedded protostars in the region.