The dusty Nebula surrounding HR Car: a Spitzer view

TL;DR

This study uses Spitzer mid-IR observations to analyze the dust and gas in the nebula around LBV star HR Car, revealing a photodissociation region, dust formation during outburst, and gas-phase iron enhancement.

Contribution

First mid-IR spectral analysis of HR Car's nebula showing evidence of a PDR and dust formation during LBV outburst, contrasting with more evolved LBVs.

Findings

Detection of a photodissociation region in the nebula.

Evidence of dust formation during the LBV outburst.

Gas-phase Fe abundance enhancement suggesting shocks.

Abstract

We present mid-IR observations of the Galactic Luminous Blue Variable (LBV) HR Car and its associated nebula carried out with the Spitzer Space Telescope using both IRAC and IRS, as part of a GTO program aimed to study stellar ejecta from evolved stars. Our observations reveal a rich mid-IR spectrum of the inner nebula showing both solid state and atomic gas signatures. Strong low-excitation atomic fine structure lines such as $ 26.0 \mu$m [\ion{Fe}{2}] and $ 34.8 \mu$m [\ion{Si}{2}], indicate, for the first time, the presence of a PDR in this object class. While the physics and chemistry of the low-excitation gas appears to be dominated by photodissociation, a possible contribution due to shocks can be inferred from the evidence of gas phase Fe abundance enhancement. The presence of amorphous silicates, inferred from the observed characteristic broad feature at $10 \mu$m located in the inner nebula, suggests that dust has formed during the LBV outburst. This is in contrast with the detection of crystalline dust in other probably more evolved Galactic LBVs, which is similar to the crystalline dust observed in red supergiants. This has been considered to be evidence of dust production during evolutionary phases prior to the outburst.