Resolving the Structure and Kinematics of the BN Object at $0\rlap.{"}2$ Resolution

TL;DR

This study uses high-resolution 7 mm observations to analyze the structure and kinematics of the BN object in Orion, detecting radio recombination lines for the first time and revealing velocity gradients suggestive of outflow activity.

Contribution

First detection of radio recombination lines from the BN object at 7 mm, providing new insights into its velocity structure and ionized outflow dynamics.

Findings

Detected H53α recombination line from BN with LSR velocity of 20.1 km/s.

Evidence of a velocity gradient indicating possible ionized outflow.

Radio continuum emission appears optically thick, but line emission suggests optical thinness.

Abstract

We present sensitive 7 mm observations of the H53$\alpha$ recombination line and adjacent continuum, made toward the Orion BN/KL region. In the continuum we detect the BN object, the radio source I (GMR I) and the radio counterpart of the infrared source n (Orion-n). Comparing with observations made at similar angular resolutions but lower frequency, we discuss the spectral indices and angular sizes of these sources. In the H53$\alpha$ line we only detect the BN object. This is the first time that radio recombination lines are detected from this source. The LSR radial velocity of BN from the H53$\alpha$ line, $v_{LSR} = 20.1 \pm 2.1$ km s$^{-1}$, is consistent with that found from previous studies in near-infared lines. While the continuum emission is expected to have considerable optical depth at 7 mm, the observed H53$\alpha$ line emission is consistent with an optically-thin nature and we discuss possible explanations for this apparent discrepancy. There is evidence of a velocity gradient, with the NE part of BN being redshifted by $\sim 10$ km s$^{-1}$ with respect to the SW part. This is consistent with the suggestion of Jiang et al. that BN may be driving an ionized outflow along that direction.