SPITZER/INFRARED SPECTROGRAPH INVESTIGATION OF MIPSGAL 24 μm COMPACT BUBBLES: LOW-RESOLUTION OBSERVATIONS

Abstract

We present Spitzer/IRS low resolution observations of 11 compact circumstellar bubbles from the MIPSGAL 24 µm 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 µm 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 µm, [Ne v] 14.3 and 24.3 µm, [Ne iii] 15.5 µm), and the [O iv] line accounts for 50 to almost 100% of the 24 µm 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.

Searching for OH maser emission towards the MIPSGAL compact Galactic bubbles

“…We have showed that at 24 µm a large sample of known post-AGB stars appear as very bright point sources. On the other hand, almost all the MBs classified as PN are 'disks' or 'rings' (Figure 2; Nowak et al 2014). We infer that the most probable morphology for a PPN MB would be that of a 'disk'.…”

High-resolution Very Large Array observations of 18 MIPSGAL bubbles

“…The categories most populated are 'ring' and 'disk' nebulae, accounting for 339 objects out of 428. 9 As previously discussed, Nowak et al (2014) found that more than 80 per cent of classified 'ring' and 'disk' bubbles are PNe, and that few other PNe are found in other categories. It is possible to estimate that the entire bubbles sample should contain about 300 PNe.…”

“…This can be ascribed to the homogeneity of the original 24-µm sample, consisting of objects with a similar angular extension and shape. We recall that Nowak et al (2014) pointed out that almost all bubbles classified as PNe show a disk or ring morphology. PNe with very different 24-µm shape were excluded a priori by Mizuno et al (2010).…”

“…The main hypothesis about the nature of the bubbles is that they are different types of evolved stars. In fact, follow up studies have found among them planetary nebulae (PNe), supernova remnants, Wolf-Rayet (WR) stars, luminous blue-variable E-mail:ingallinera@oact.inaf.it 1 http://mipsgal.ipac.caltech.edu/ (LBV) stars, asymptotic giant branch (AGB) stars, and so on (see Nowak et al 2014 for a review). However, currently, only about 30 per cent of the bubbles are classified.…”

MN48: a new Galactic bona fide luminous blue variable revealed bySpitzerand SALT

“…MN48 is prominent at 24 µm, in all IRAC and WISE 2 http://simbad.harvard.edu/simbad/ 3 Spitzer spectroscopy of circumstellar nebulae around evolved massive stars showed that their emission is dominated by the dust continuum emission (Flagey et al 2011;Nowak et al 2014). Detection of the nebula around MN48 at longer wavelengths (e.g.…”