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 [\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 103cm−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 $10^{-3}\
\rm{M_\odot}anditsmasslossisabout10^{-6}\ \rm{M_\odot/yr}. The second
dust-rich MB has recently been suggested as a Be/B[e]/LBV candidate. The gas
lines of [\ion{Fe}{2}] as well as hot continuum components (\sim300and\sim1250K)arisefromtheinsideoftheMBwhileitsoutershellemitsacolderdustcomponent(\sim75K).ThedistancetotheMBremainshighlyuncertain.Itsmassisabout10^{-3}\ \rm{M_\odot}anditsmasslossisabout10^{-5}\ \rm{M_\odot/yr}$.Comment: accepted for publication in Ap