2011ApJ...741....4F

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/cm3 and a central white dwarf of ≳ 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 (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_{x2609} and its mass loss is about 10^–6 M_{x2609}/yr. 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_{x2609} and its mass loss is about 10^–5 M_{x2609}/yr.