Spitzer/InfraRed Spectrograph Investigation of MIPSGAL 24 μm Compact Bubbles

The MIPSGAL 24 μm 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 four 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 [O IV], [Ne III], [Ne V], [S III], and [S IV]. We identify them as planetary nebulae with a density of a few 10³ cm^-3 and a central white dwarf of gsim200,000 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 (Two Micron All Sky Survey and IRAC) broadband images. The first dust-rich MB matches a Wolf-Rayet star of ~60,000 K at 7.5 kpc with dust components of ~170 and ~1750 K. Its mass is about 10^-3 M _sun and its mass loss is about 10^-6 M _sun yr^-1. The second dust-rich MB has recently been suggested as a Be/B[e]/luminous blue variable candidate. The gas lines of [Fe II] as well as hot continuum components (~300 and ~1250 K) arise from the inside of the MB while its outer shell emits a colder dust component (~75 K). The distance to the MB remains highly uncertain. Its mass is about 10^-3 M _sun and its mass loss is about 10^-5 M _sun yr^-1.