ASKAP EMU Radio Detection of the Reflection Nebula VdB-80 in the Monoceros Crossbones Filamentary Structure

TL;DR

This paper reports the discovery of a new HII region named Lagotis associated with the reflection nebula VdB-80, using ASKAP EMU survey data, and analyzes its properties and relation to the surrounding molecular cloud structure.

Contribution

First detection of the HII region Lagotis in the VdB-80 reflection nebula using ASKAP EMU data, providing detailed physical parameters and its placement within the Monoceros Crossbones filament.

Findings

Lagotis is a previously unidentified HII region with a flux density of 30.2 mJy.

The HII region has a size of approximately 0.76 by 0.68 parsecs.

The region's properties are consistent with ionization by star HD 46060.

Abstract

We present a new radio detection from the Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) survey associated with the reflection nebula (RN) VdB-80. The radio detection is determined to be a previously unidentified HII region, now named Lagotis. The RN is located towards Monoceros, centred in the molecular cloud feature known as the `Crossbones'. The 944 MHz EMU image shows a roughly semicircular HII region with an integrated flux density of 30.2$\pm$0.3 mJy. The HII region is also seen at 1.4 GHz by NVSS, yielding an estimated spectral index of 0.65$\pm$0.51, consistent with thermal radio emission. Gaia DR3 and 2MASS data give a distance to the stars associated with the HII region of $\sim$960 pc. This implies a size of 0.76$\times$0.68($\pm$0.09) pc for the HII region. We derive an HII region electron density of the bright radio feature to be 26 cm$^{-3}$, requiring a Lyman-alpha photon flux of $10^{45.6}$ s$^{-1}$, which is consistent with the expected Lyman flux of HD 46060, the B2II type star which is the likely ionising star of the region. The derived distance to this region implies that the Crossbones feature is a superposition of two filamentary clouds, with Lagotis embedded in the far cloud.