Spitzerobservations of a circumstellar nebula around the candidate luminous blue variable MWC 930

Abstract

Context. The source MWC 930 is a star just ∼2 • above the Galactic plane whose nature is not clear and has not been studied in detail so far. While a post-asymptotic giant branch classification was proposed in the past, studies of its optical spectrum and photometry pointed toward strong variability, therefore, the object was reclassified as a luminous blue variable (LBV) candidate. Aims. The LBVs typically undergo phases of strong mass loss in the form of eruptions that can create shells of ejecta around the star. Our goal is to search for the presence of such a circumstellar nebula in MWC 930 and investigate its properties. Methods. To do so, we make use of space-based infrared data from our Spitzer campaign performed with the InfraRed Array Camera (IRAC) and the InfraRed Spectrograph (IRS), as well as data from optical and infrared (IR) surveys. Results. In our Spitzer images, we clearly detect an extended shell around MWC 930 at wavelengths longer than 5 μm. The midinfrared spectrum is dominated by the central star and mostly shows forbidden lines of [FeII] with an underlying continuum that decreases with wavelengths up to ∼15 μm and then inverts its slope, displaying a second peak around 60 μm, which is evidence of cold dust grains formed in a past eruption. By modeling the spectral energy distribution, we identify two central components, beside the star and the outer shell. These extra sources of radiation are interpreted as material close to the central star, which may be due to a recent ejection. Features of C-bearing molecules or grains are not detected.

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

“…These spectra are very similar to those of other dust-rich massive stars observed with Spitzer/IRS (e.g. the LBV and LBV candidates HR Car, HD 168625, G79.29+0.46, andMWC 930, Umana et al 2009, 2010;Agliozzo et al 2014;Cerrigone et al 2014).…”

“…These spectra are very similar to those of other dust-rich massive stars observed with Spitzer/IRS (e.g. the LBV and LBV candidates HR Car, HD 168625, G79.29+0.46, andMWC 930, Umana et al 2009, 2010;Agliozzo et al 2014;Cerrigone et al 2014). The spectra of the central stars in MB3955 and MB4121 are much poorer in terms of gas lines, with only a few emission lines detected towards both of them (see Figure 4 and Table 5).…”

Confirmation of the Luminous Blue Variable Status of MWC 930

“…As we discussed in [3], the presence of a far-IR excess in the spectral energy distribution of MWC 930 was a hint towards the LBV evolutionary status. Recently an extended shell around the object was discovered in IR images taken by the Spitzer Space Observatory and the WISE mission at wavelengths longer than 5 µm [22]. This finding is consistent with the idea of a past giant eruption of MWC 930.…”

WS1: one more new Galactic bona fide luminous blue variable

“…MWC 930 was suggested as a cLBV quite long ago (Miroshnichenko et al 2005) and its LBV status was confirmed this year (Miroshnichenko et al 2014). Discovery of a circular shell around MWC 930 (Cerrigone et al 2014) lends an additional support to the LBV status of this star.…”