Radio continuum imaging of the R Coronae Austrinae star-forming region with the ATCA

¹ Observatory, PO Box 14, 00014 University of Helsinki, Finland e-mail: oskari.miettinen@helsinki.fi
² Dept. of Physics, Georgia Southern University, Statesboro, GA 30460-8031, USA

Received: 3 January 2008
Accepted: 28 April 2008

Abstract

Aims. The aim of this study is to investigate the nature of radio sources associated with young stellar objects (YSOs) belonging to the R CrA cluster. By combining the centimetre radio data with the wealth of shorter wavelength data accumulated recently we wish to refine estimates of the evolutionary stages of the YSOs.

Methods. The region was imaged at 3, 6, and 20 cm using the Australia Telescope Compact Array. Fluxes and spectral indices for the brightest radio sources were derived from these observations. Some of the 3 and 6 cm data were obtained simultaneously, which is useful for reliable spectral index determinations of variable sources.

Results. Eight pointlike sources were detected. Seven of them can be assigned to YSOs, which have counterparts in the X-rays, infrared or submm. One of the YSOs, Radio Source 9, is a Class 0 candidate, and another, IRS 7B, is suggested to be in the Class 0/I transition stage. IRS 7B is associated with extended radio lobes at 6 and 20 cm. The lobes may have a gyrosynchrotron emission component, which could be understood in terms of Fermi accleration in shocks. The Class I objects detected here seem to be a mixed lot. One of these, the wide binary IRS 5, shows a negative spectral index, rapid variability, and a high degree of circular polarisation with  ≈ 33% on one of the days of observation. These signs of magnetic activity suggest that at least one of the binary components has advanced beyond the Class I stage. The radio source without YSO assigment, Radio Source 5, has been suggested to be a brown dwarf. The radio properties, in particular its persistent strong emission, do not support this classification.

Conclusions. The radio characteristics of the detected YSOs roughly agree with the scheme where the dominant emission mechanism changes with age. The heterogeneity of the Class I group can possibly be explained by a drastic decline in the jet activity during this stage, which also changes the efficiency of free-free absorption around the protostar.