Spitzer/IRS investigation of MIPSGAL 24 microns compact bubbles

Abstract

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 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 [O IV], [Ne III], [Ne V], [S III] and [S IV]. We identify them as planetary nebulae with a density of a few 10^3 cm^(−3) 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 (2MASS 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_⊙ and its mass loss is about 10^(−6) M_⊙/yr. The second dust-rich MB has recently been suggested as a Be/B[e]/LBV 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_⊙ and its mass loss is about 10^(−5) M_⊙/yr.

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