Detection of non-thermal emission from the massive protostellar jet HH80-81 at low radio frequencies using GMRT

Abstract

Low radio frequencies are favourable for the identification of emission from non-thermal processes such as synchrotron emission. The massive protostellar jet associated with IRAS 18162-2048 (also known as the HH80-81 system) has been imaged at low radio frequencies: 325, 610 and 1300 MHz, using the Giant Metrewave Radio Telescope, India. This is the first instance of detection of non-thermal emission from a massive protostellar jet at such low radio frequencies. The central region displays an elongated structure characteristic of the jet. In addition, the associated Herbig-Haro objects such as HH80, HH81, HH80N, and other condensations along the inner regions of the jet exhibit negative spectral indices. The spectral indices of most condensations are ∼ −0.7, higher than the value of −0.3 determined earlier using high frequency measurements. The magnetic field values derived using radio flux densities in the present work, under the assumption of equipartition near minimum energy condition, lie in the range 116 − 180 µG. We probe into the hard X-ray nature of a source that has been attributed to HH80, in an attempt to reconcile the non-thermal characteristics of radio and X-ray measurements. The flux densities of condensations at 610 MHz, measured nearly 11 yrs apart, display variability that could be ascribed to the cooling of condensations, and emphasize the importance of coeval or nearly-coeval measurements for estimation of spectral indices.

Radio jets from young stellar objects

“…6). Very recent GMRT observations of HH 80-81 (Vig et al 2018) at 325 and 610 MHz found negative spectral indices steeper than previous studies at higher frequencies. This has been interpreted as indicating that an important free-free contribution is present at high frequencies even in the non-thermal radio knots.…”

Particle acceleration in the Herbig–Haro objects HH 80 and HH 81

“…One possible interpretation is that this emission could be due to inverse Compton scattering (IC) of thermal infrared from the ambient cloud, produced by relativistic particles accelerated in the reverse shock, or present in the molecular cloud (e.g. Vig et al 2018). However, to identify the processes that can produce this radiation, a deeper and detailed study is required, using higher sensitivity X-ray images.…”

“…Despite being located at large distances from the protostar, HH objects also seem to present a non-thermal component of radio emission. This can be inferred from the detection of very negative spectral indices (α = −0.3 a −0.7; e.g., Marti et al 1993, Vig et al 2018 measured in low angular resolution observations. Kinematic studies of HH 80, HH 81, and HH 80N (hereafter HH 80/81/80N), reveal proper motions of ∼200-400 km s −1 (Masqué et al 2015, Heathcote et al 1998, while the velocity of the jet material is about 1000 km s −1 (Marti et al 1995).…”

Investigating Particle Acceleration in the Wolf–Rayet Bubble G2.4+1.4

“…This acceleration mechanism is ubiquitous in sources with supersonic outflows, such as supernovae remnants (e.g. Dubner & Giacani 2015), Herbig-Haro objects (Rodríguez-Kamenetzky et al 2019), proto-stellar jets (Vig et al 2018), compact young stellar clusters (Yang et al 2018), and microquasars (Mirabel & Rodríguez 1994). In OB and WR systems, the detection of synchrotron emission is strongly correlated with binarity (Dougherty & Williams 2000;De Becker et al 2006).…”