The dusty Nebula surrounding HR Car: a Spitzer view

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.

💡 Research Summary

The authors present a comprehensive mid‑infrared study of the Galactic luminous blue variable (LBV) HR Car and its surrounding nebula using the Spitzer Space Telescope’s Infrared Array Camera (IRAC) and Infrared Spectrograph (IRS). The IRAC images reveal the nebular morphology at wavelengths up to 8 µm, while the low‑resolution IRS spectra (5–38 µm) uncover a rich mixture of solid‑state features and atomic fine‑structure lines.

A broad emission bump centered near 10 µm is identified as amorphous silicate dust. Its presence indicates that dust condensation occurred during the most recent LBV outburst, when rapidly cooling ejecta formed non‑crystalline silicate grains. This contrasts with other, presumably more evolved Galactic LBVs (e.g., AG Car, R 127) where crystalline silicate features dominate and are thought to have originated in earlier evolutionary phases such as the red‑supergiant stage.

The spectrum also displays strong low‑excitation fine‑structure lines: