Abstract
ABSTRACT We present a newly enlarged census of the compact radio population towards the Orion Nebula Cluster (ONC) using high-sensitivity continuum maps (3–10 $\mu$Jy beam−1) from a total of ∼30-h centimetre-wavelength observations over an area of ∼20 × 20 arcmin2 obtained in the C-band (4–8 GHz) with the Karl G. Jansky Very Large Array (VLA) in its high-resolution A-configuration. We thus complement our previous deep survey of the innermost areas of the ONC, now covering the field of view of the Chandra Orion Ultra-deep Project (COUP). Our catalogue contains 521 compact radio sources of which 198 are new detections. Overall, we find that 17 per cent of the (mostly stellar) COUP sources have radio counterparts, while 53 per cent of the radio sources have COUP counterparts. Most notably, the radio detection fraction of X-ray sources is higher in the inner cluster and almost constant for r > 3 arcmin (0.36 pc) from θ1 Ori C, suggesting a correlation between the radio emission mechanism of these sources and their distance from the most massive stars at the centre of the cluster, e.g. due to increased photoionisation of circumstellar discs. The combination with our previous observations 4 yr prior lead to the discovery of fast proper motions of up to ∼373 km s−1 from faint radio sources associated with ejecta of the OMC1 explosion. Finally, we search for strong radio variability. We found changes in flux density by a factor of ≲5 within our observations and a few sources with changes by a factor >10 on long time-scales of a few years.
Highlights
The advent of wide-band centimetre-wavelength observing capabilities has enabled a new era of stellar radio astronomy, including observations of radio counterparts of young stellar objects (YSOs)
Other studies, using the Very Large Array (VLA) towards the Orion Nebula Cluster (ONC) have focused in the inner cluster covering similar areas usually reaching rms noise levels above 30 μJy beam−1 limiting the number of detections from a few tens to up to 175 sources (Sheehan et al 2016)
An absolute uncertainty in peak flux densities of 5 per cent has been estimated based on systematic variability using a non-variable test-case. This uncertainty has been added in quadrature with the uncertainties from the 2D-Gaussian fit (IMFIT) already corrected by the primary beam response. In this new census of compact radio sources towards the ONC, we report 198 new sources not previously reported at these frequencies
Summary
The advent of wide-band centimetre-wavelength observing capabilities has enabled a new era of stellar radio astronomy, including observations of radio counterparts of young stellar objects (YSOs). These show evidence for both thermal free–free emission from ionized material and non-thermal (gyro-)synchrotron emission from magnetospheric activity (Dulk 1985; Gudel 2002). The improved sensitivity of the VLA enables the detection of non-stellar emission like that from jets and outflows (Forbrich et al 2016; Bally et al 2020), enabling astrometric studies as presented here. We use the excellent sensitivity even on short time-scales to continue our study of YSO radio variability, motivated by the findings of extreme variability on short timescales (factor >138 in less than 2 d and a factor of 10 in less than 30 min) in our previous deep ONC pointing (Forbrich et al 2017)
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