VISIR/VLT and VLA joint imaging analysis of the circumstellar nebula around IRAS~18576+0341

TL;DR

This study combines high-resolution mid-infrared and radio imaging to analyze the structure of the circumstellar nebula around IRAS 18576+0341, revealing contrasting morphologies of dust and ionized gas.

Contribution

First joint imaging analysis of the nebula around IRAS 18576+0341 using VISIR and VLA, highlighting morphological differences between dust and ionized gas components.

Findings

The nebula shows a symmetric dust shell contrasting with an asymmetric ionized gas envelope.

Mid-infrared and radio maps are in excellent agreement, indicating free-free emission from ionized material.

Dust temperature and optical depth are relatively uniform across the nebula.

Abstract

High spatial and sensitivity images of the Luminous Blue Variable IRAS 18576+0341 were obtained using the mid infrared imager VISIR at the Very Large Telescope and the Very Large Array interferometer. The resulting mid-infrared continuum maps show a similar clumpy and approximately circular symmetric nebula, which contrasts sharply with the asymmetry that characterizes the ionized component of the envelope, as evidenced from the radio and [Ne II] line images obtained with comparable spatial resolution. In particular, there is excellent overall agreement between the 12.8 micron map and the radio images, consistent with free-free emission from circumstellar ionized material surrounding a central stellar wind. The color temperature and optical depth maps obtained from mid-infrared images show only slight fluctuations, suggesting quite uniform dust characteristics over the dust shell. We explore various possibilities to understand the cause of the different morphology of the dusty and gaseous component of the circumstellar envelope which are compatible with the observations.