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Register a new userGalactic Evolved Massive Stars Discovered by Their Infrared Emission
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Determining the Galactic distribution and numbers of massive stars, such as Wolf-Rayet stars (WRs), is hampered by intervening Galactic or local circumstellar dust obscuration. In order to probe such regions of the Galaxy we can use infrared observations, which provide a means for finding such hidden populations through the dust. The availability of both 2MASS and Spitzer/GLIMPSE large-scale survey data provides infrared colours from 1.25 to 8$mu$m for a large fraction of the inner Galactic plane. In 2005 we initiated a pilot study of the combined set of infrared colours for two GLIMPSE fields and showed that WRs typically occupy a sparsely populated region of the colour space. We followed up 42 of our WR candidates spectroscopically in the near-infrared, and with limited additional observations of some of these candidates in the optical. Six new WRs, four late-type WN and two late-type WC stars, were discovered as a result. Of the remaining $sim$86% of the sample, five appear to be O-type stars. 21 stars are likely of type Be, and 10 stars appear to be of late-type, or possibly young stellar objects, which have contaminated the infrared color space. The survey is generally unbiased towards clusters or field stars, and the new WRs found are in both the field and in and around the RCW 49 region (including cluster Westerlund 2). In this work, and in our other recent work, we show that the infrared broad-band colours to be the most efficient means of identifying (particularly, dust-obscured) candidate massive stars, notably WRs.
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We present results of the analysis of a sample of 22 stars of spectral types from O7 to B5 and luminosity classes I-V for which Spitzer/IRS spectra are available. The IRS spectra of these stars are examined for signs of excess infrared (IR) emission by comparison with stellar atmospheric spectra. We find that the spectra of half of the studied stars are dominated by excess emission in the far-IR, including all six super- and bright giants. In order to examine the origin of the far-IR excess, we supplement the Spitzer data with optical high-resolution echelle spectroscopy ($lambda/Delta lambda sim 10^5$), near-IR high-contrast coronagraphic imaging taken with the SPHERE instrument at VLT with a spatial resolution of 0.05, and WISE and Herschel photometry. In the optical region, we detect various absorption and emission lines (H$alpha$, CIII, and NIII) irrespective of the far-IR excess. Pfund($alpha$) and Humphrey($alpha$) lines are observed at the same time as the far-IR excess. These lines are stronger in stars with far-IR excess than in stars without excess. A scattered-light disk in the central r < 2.5 region of the far-IR excess stars HD149404, HD151804, and HD154368 can be excluded from H band imaging down to a 1$sigma$ contrast of $F(r)/F_{*} sim 10^{-6}$. The far-IR excess is fit either by a free-free component from ionized gas as for the winds of hot stars or a large (1pc) circumstellar dust shell. The putative dust envelopes required to explain the excess have a visual extinction as low as a few hundred $mu$-mag.
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