AVLA search for embedded young stellar objects and protostellar candidates in L1630

Abstract

Five molecular cloud cores in L1630 have been observed with the VLA in C configuration at a wavelength of 3.5 cm. These cores were identified in molecular line emission by a previous study, and contain numerous clumps which are excellent candidates for sites of low-mass star formation. To a 5σ limit of 0.1 mJy, corresponding to an 8.4-GHz luminosity of 1.9 × 1016 erg s−1 Hz−1 (uncorrected for primary beam attenuation), most (>75 per cent) of the clumps within these cores do not have an associated radio continuum source. Of the three likely associations, two were discovered in previous surveys of LBS 23 (HH 24–26), one of which (SSV 61) shows evidence for radio variability, while the third source is spatially coincident with one of the clumps in LBS 18 and may be a new class 0 protostar candidate. Another new discovery is that HH 26IR is probably detected at the 4σ level. Radio emission has also been detected from two of the early-type stars exciting the NGC 2068 reflection nebula (HD 38563C and S). These results are in agreement with earlier work which proposes that, in (the northern part of) L1630, a large fraction of the molecular gas is not directly involved with star formation at the current epoch, although there is evidence that low-mass star formation is in the early stages at a number of locations. A tentative suggestion is that perhaps only young stellar objects in the earliest and latest phases of protostellar evolution exhibit detectable radio continuum emission. The earliest emission may be due to bremsstrahlung from an ionized jet, which declines as a result of a decreasing accretion rate or outflow efficiency. The later emission is due to gyrosynchrotron processes arising as a consequence of a magnetic field in the vicinity of the star—disc interface. Alternatively, a combination of the source geometry and optical depth may be responsible for such an apparent correlation. Further observations of a larger sample are required to test whether this correlation is seen in a wider view of low-mass star formation.