Spitzer/IRS investigation of MIPSGAL 24 microns compact bubbles

TL;DR

This study uses Spitzer/IRS observations to analyze four MIPSGAL 24 micron compact bubbles, revealing their diverse natures as planetary nebulae, Wolf-Rayet star, and a possible Be/B[e]/LBV candidate through mid-IR spectral modeling.

Contribution

First detailed mid-IR spectral analysis of MIPSGAL bubbles to determine their physical nature and classify their types based on gas and dust emission features.

Findings

Two MBs are planetary nebulae with high-ionization gas lines.

One MB is a Wolf-Rayet star with dust components at different temperatures.

Another MB is a candidate Be/B[e]/LBV star with complex gas and dust emission.

Abstract

The MIPSGAL 24 $\mu$m Galactic Plane Survey has revealed more than 400 compact-extended objects. Less than 15% of these MIPSGAL bubbles (MBs) are known and identified as evolved stars. We present Spitzer observations of 4 MBs obtained with the InfraRed Spectrograph to determine the origin of the mid-IR emission. We model the mid-IR gas lines and the dust emission to infer physical conditions within the MBs and consequently their nature. Two MBs show a dust-poor spectrum dominated by highly ionized gas lines of [\ion{O}{4}], [\ion{Ne}{3}], [\ion{Ne}{5}], [\ion{S}{3}] and [\ion{S}{4}]. We identify them as planetary nebulae with a density of a few 10$^3\ \rm{cm^{-3}}$ and a central white dwarf of $\gtrsim 200,000$ K. The mid-IR emission of the two other MBs is dominated by a dust continuum and lower-excitation lines. Both of them show a central source in the near-IR (2MASS and IRAC) broadband images. The first dust-rich MB matches a Wolf-Rayet star of $\sim 60,000$ K at 7.5 kpc with dust components of $\sim170$ and $\sim1750$ K. Its mass is about $10^{-3}\ \rm{M_\odot}$ and its mass loss is about $10^{-6}\ \rm{M_\odot/yr}$. The second dust-rich MB has recently been suggested as a Be/B[e]/LBV candidate. The gas lines of [\ion{Fe}{2}] as well as hot continuum components ($\sim300$ and $\sim1250$ K) arise from the inside of the MB while its outer shell emits a colder dust component ($\sim75$ K). The distance to the MB remains highly uncertain. Its mass is about $10^{-3}\ \rm{M_\odot}$ and its mass loss is about $10^{-5}\ \rm{M_\odot/yr}$.