NGC 7538 IRS2 in [NeII]: Shell and Cavity Kinematics of a Compact HII Region

TL;DR

This study uses high-resolution [NeII] spectral imaging to analyze the complex shell and cavity kinematics of the compact HII region NGC 7538 IRS2, revealing the influence of multiple stars and cloud structure.

Contribution

It provides the first detailed kinematic model of IRS2's cavities and demonstrates the role of lower-mass stars and cloud clumpiness in shaping HII region dynamics.

Findings

Ionized gas flows along cavity walls.

Cavities are associated with B-type stars.

Outflows from the O star binary are confined.

Abstract

NGC 7538 IRS2 is a compact HII region and recent star formation source, with a shell morphology, lying on the border of the visible HII region NGC 7538. We present a spectral cube of the [NeII] 12.8 micron emission line obtained with the TEXES spectrometer on Gemini North with velocity resolution approximately 4 km/s and angular resolution 0.3". The kinematics of the data cube show ionized gas flowing along multiple cavity walls. We have simulated the kinematics and structure of IRS2 with a model of superimposed cavities created by outflows from embedded stars in a cloud with density gradients. Most of the cavities, including the largest that dominates IRS2 structure, are associated with B-type stars; the outflow of the bright ionizing O star binary IRS2a/b is small in extent and lies in a high-density clump. The IRS2 model shows that the behavior of an HII region is not a matter of only the most massive star present; cloud clumpiness and activity of lower mass stars may determine the structure and kinematics.