Post-outburst radio observation of the region around McNeil's nebula (V1647 Orionis)

Abstract

We present post-outburst (∼100 days after outburst) radio continuum observation of the region (∼30' x 30') around McNeil's nebula (V1647 Orionis). The observations were carried out using the Giant Metrewave Radio Telescope (GMRT), India, at 1272 MHz on 2004 Feb. 14.5 UT. Although 8 sources have been detected within a circular diameter of 25' centred on V1647 Ori, we did not detect any radio continuum emission from McNeil's nebula. We assign a 5σ upper limit of 0.15 mJy/beam for V1647 Ori where the beam size is 5.6 x 2.7. Even at higher frequencies of 4.9 and 8.5 GHz (VLA archival data), no radio emission has been detected from this region. Three scenarios, namely emission from a homogeneous HII region, ionised stellar wind and shock-ionised gas, are explored in the light of our GMRT upper-limit. For the case of a homogeneous HII region, the radius of the emitting region is constrained to be ≤26 AU corresponding to a temperature ≥2500 K, which is consistent with the reported radio and Ha emission. In the ionised stellar wind picture, our upper limit of radio emission translates to M/ν∞ < 1.2-1.8 x 10 -10 M ○. yr -1 km -1 s. On the other hand, if the stellar wind shocks the dense neutral (molecular) cloud, the radio upper limit implies that the fraction of the wind encountering the dense obstacle is <50%. Based on a recent measurement of X-ray outburst and later monitoring, the expected radio emission has been estimated. Using our radio limit, the radius (≤36 AU) and electron density (≥7.2 × 10 7 cm -3 ) of the radio emitting plasma have been constrained using a two phase medium in pressure equilibrium for a volume filling factor of 0.9.