Spitzer/IRS investigation of MIPSGAL 24 microns compact bubbles

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${}^3\mathrm{c}{\mathrm{m}}^{-3}$ 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 $\sim 170$ and $\sim 1750$ K. Its mass is about $10^{-3}{\mathrm{M}}_{\odot }$ and its mass loss is about $10^{-6}{\mathrm{M}}_{\odot }/yr$. 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 ($\sim 300$ and $\sim 1250$ K) arise from the inside of the MB while its outer shell emits a colder dust component ($\sim 75$ K). The distance to the MB remains highly uncertain. Its mass is about $10^{-3}{\mathrm{M}}_{\odot }$ and its mass loss is about $10^{-5}{\mathrm{M}}_{\odot }/yr$.

accepted for publication in ApJ

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