Spitzer/infrared spectrograph investigation of MIPSGAL 24 {\mu}m compact
  bubbles : Low resolution observations
  We present Spitzer/IRS low resolution observations of 11 compact
circumstellar bubbles from the MIPSGAL 24 {\mu}m Galactic Plane Survey. We find
that this set of MIPSGAL bubbles (MBs) is divided into two categories, and that
this distinction correlates with the morphologies of the MBs in the mid- IR.
The four MBs with central sources in the mid-IR exhibit dust-rich, low
excitation spectra, and their 24 {\mu}m emission is accounted for by the dust
continuum. The seven MBs without central sources in the mid-IR have spectra
dominated by high excitation gas lines (e.g., [O IV] 26.0 {\mu}m, [Ne V] 14.3
and 24.3 {\mu}m, [Ne III] 15.5 {\mu}m), and the [O IV] line accounts for 50 to
almost 100% of the 24 {\mu}m emission in five of them. In the dust-poor MBs,
the [Ne V] and [Ne III] line ratios correspond to high excitation conditions.
Based on comparisons with published IRS spectra, we suggest that the dust-poor
MBs are highly excited planetary nebulae with peculiar white dwarfs (e.g.,
[WR], novae) at their centers. The central stars of the four dust-rich MBs are
all massive star candidates. Dust temperatures range from 40 to 100 K in the
outer shells. We constrain the extinction along the lines of sight from the IRS
spectra. We then derive distance, dust masses, and dust production rate
estimates for these objects. These estimates are all consistent with the nature
of the central stars. We summarize the identifications of MBs made to date and
discuss the correlation between their mid-IR morphologies and natures.
Candidate Be/B[e]/LBV and WR stars are mainly "rings" with mid-IR central
sources, whereas PNe are mostly "disks" without mid-IR central sources.
Therefore we expect that most of the 300 remaining unidentified MBs will be
classified as PNe.
