On the radio-X-ray connection in young stellar objects in the Orion nebula cluster
1 University of Vienna, Department of Astrophysics, Türkenschanzstraße 17, 1180 Vienna, Austria
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MA 02138 Cambridge, USA
Received: 17 October 2012
Accepted: 16 January 2013
Context. Both X-ray and radio observations offer insight into the high-energy processes of young stellar objects (YSOs). The observed thermal X-ray emission can be accompanied by both thermal and nonthermal radio emission. Due to variability, simultaneous X-ray and radio observations are a priori required, but only a comparably small number of YSOs have been studied in this way. Results have been inconclusive due to the even smaller number of YSOs that were simultaneously detected in X-ray and radio observations.
Aims. We use archival X-ray and radio observations of the Orion nebula cluster (ONC) to significantly enlarge the sample size of known YSOs with both X-ray and radio detections.
Methods. We study the ONC using multi-epoch non-simultaneous archival Chandra X-ray and NRAO Very Large Array (VLA) single-band radio data. The multiple epochs allow us to reduce the impact of variability by obtaining approximated quiescent fluxes.
Results. We find that only a small fraction of the X-ray sources (7%) have radio counterparts, even if 60% of the radio sources have X-ray counterparts. YSOs with detections in both bands thus constitute a small minority of the cluster. The radio flux density is typically too low to distinguish thermal and nonthermal radio sources. Only a small fraction of the YSOs with detections in both bands are compatible with the empirical “Güdel-Benz” (GB) relation. Most of the sources not compatible with the GB relation are proplyds, and thus likely thermal sources, but only a fraction of the proplyds is detected in both bands, such that the role of these sources is inconclusive.
Conclusions. While the radio sources appear to be globally unrelated to the X-ray sources, the X-ray dataset clearly is much more sensitive than the radio data. We find tentative evidence that known non-thermal radio sources and saturated X-ray sources are indeed close to the empirical relation, even if skewed to higher radio luminosities, as they are expected to be. Most of the sources that are clearly incompatible with the empirical relation are proplyds that could instead plausibly be thermal radio sources. The newly expanded Karl G. Jansky Very Large Array with its significantly enhanced continuum sensitivity is beginning to provide an ideal tool for addressing this issue. Combined X-ray and radio studies of YSOs using older VLA data are clearly limited by the typically low signal-to-noise of the radio detections, providing insufficient information to disentangle thermal and nonthermal sources.
Key words: radio continuum: stars / stars: pre-main sequence / X-rays: stars / stars: protostars
© ESO, 2013