First detection of Circular Polarization in radio continuum towards a Massive Protostar

TL;DR

This paper reports the first detection of circular polarization in radio continuum towards a massive protostar, revealing magnetic fields of at least 20-35 G and suggesting mechanisms like gyrosynchrotron emission or Faraday conversion.

Contribution

It provides the first measurement of circular polarization in radio emission from a massive protostar and estimates its magnetic field strength, offering new insights into star formation models.

Findings

Detected 3-5% circular polarization in a massive protostar

Estimated magnetic field strength of at least 20-35 G near the protostar

Proposed mechanisms include gyrosynchrotron emission and Faraday conversion

Abstract

Polarization measurements provide strong constraints on magnetic fields in star-forming systems. While magnetic field estimates of a few kiloGauss (kG) have been obtained near the surface of low-mass protostars, there are no analogous measurements in the immediate vicinity of the surface of massive protostars. We report the measurement of radio continuum circular polarization (CP) towards a massive protostar IRAS 18162-2048 for the first time wielding Karl G. Jansky Very Large Array (VLA) observations. The fractional CP varies between $3-5\%$ across the observed frequency range of $4-6$ GHz. We consider multiple hypotheses for the production of CP and propose (i) gyrosynchrotron emission and (ii) Faraday conversion due to turbulence in the magnetic medium - both driven by mildly relativistic electrons as plausible mechanisms. We estimate, for the first time, a magnetic field $B\gtrsim20-35$ G close to the massive protostar. The Lorentz factor of the low energy electrons is estimated to be in the range $\gamma_{min}\sim5-7$ for gyrosynchrotron emission and $80-100$ for Faraday conversion from our observations. The magnetic field estimate can provide important constraints to the formation models of massive stars.