Galactic Evolved Massive Stars Discovered by Their Infrared Emission
MMassive Stars: From α to Ω Rhodes, Greece, 10-14 June 2013Massive Stars: From α to Ω Rhodes, Greece, 10-14 June 2013
Galactic Evolved Massive Stars Discovered byTheir Infrared Emission
A. P. Marston , J. Mauerhan , S. Van Dyk , M. Cohen and P. Morris ESA/ESAC, PO Box 78, 28691 Villanueva de la Ca˜nada, Madrid, Spain Steward Observatory, U. Arizona, Tucson, AZ 85721-0065, USA Spitzer Science Center/Caltech, 220-6, Pasadena, CA 91125, USA Monterey Institute for Research in Astronomy, 200 8th Street, Marina, CA 93933, USA NASA Herschel Science Center/Caltech, 100-22, Pasadena, CA 91125, USA
Abstract
Determining the Galactic distribution and numbers of massive stars, such as Wolf-Rayetstars (WRs), is hampered by intervening Galactic or local circumstellar dust obscuration. Inorder to probe such regions of the Galaxy we can use infrared observations, which provide ameans for finding such hidden populations through the dust. The availability of both 2MASSand Spitzer/GLIMPSE large-scale survey data provides infrared colours from 1.25 to 8 µ mfor a large fraction of the inner Galactic plane. In 2005 we initiated a pilot study of thecombined set of infrared colours for two GLIMPSE fields and showed that WRs typicallyoccupy a sparsely populated region of the colour space. We followed up 42 of our WRcandidates spectroscopically in the near-infrared, and with limited additional observationsof 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 ∼
86% of the sample, fiveappear 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 arein 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 tobe the most efficient means of identifying (particularly, dust-obscured) candidate massivestars, notably WRs.
Wolf-Rayet (WR) stars represent the later evolutionary stages of massive stars (single or inbinaries) with initial main sequence masses larger than 20 M (cid:12) . The exact evolutionary pathsfor such high mass stars are not well established and there are several possibilities dependingon various factors such as initial mass, rotation rate and magnetic field (See Langer, 2012) a r X i v : . [ a s t r o - ph . S R ] S e p Galactic Evolved Stars from Their Infrared Emission for a recent overview of the situation. Clearly, better statistical information covering a rangeof stellar parameters can help in the understanding of likely evolutionary scenarios for verymassive stars. In the last 10 years or so the number of known WR stars has more thandoubled (Shara et al, 2009; ? ; Hadfield et al, 2007; Mauerhan et al, 2011), with more than430 galactic WR stars now known. This article reports on early, previously unreported, workin identifying WR stars in the galaxy. The strong winds and significant mass loss of WR stars lead to a power-law spectrum excessfrom the wind with spectral index of 2.7 to 3.2 which is in addition to the photosphericemission (Morris et al, 1993). This becomes particularly notable in the near- to mid-infrared(See for more details Mauerhan et al, 2011). The Spitzer/GLIMPSE survey of the galacticplane at galactic latitudes − < b < +1 deg provides infrared fluxes for tens of millionsof sources in the wavelength range 3.8 µ m to 8 µ m benjamin03. The 2MASS survey covers thewhole sky at the near-infrared JHK s wavebands. Combining GLIMPSE and 2MASS coloursallows a colour-colour diagram of sources where WR stars are in a clearly defined colour spacecontaining a relatively low percentage of objects (Figure 1). The reddening vector shows thatWR stars remain in a clearly distinguishable region of the colour-colour diagram.Figure 1: Colour-colour diagram of 2 deg by 2 deg segment of the GLIMPSE catalogue, centredon the H II region RCW 49, cross-matched (with a 1 (cid:48)(cid:48) match radius) with the highest-reliability 2MASS point sources ( ∼ < l <
320 deg. These were followed up with spectra taken with the SOFI instrumenton board the New Technology Telescope (NTT) on La Silla, Chile, effectively covering the arston et al.
Left : The normalized H and K SOFI/NTT spectra for the new WN10 star dis-covered in this program, G321.0331-00.4274
Right:
Normalized optical spectrum of the samestar taken at the CTIO 4m Blanco telescope and RC Spec.1.8 to 2.5 µ m spectral range in June 2005. This covers a number diagnostic H, He and Cline wavelengths for identifying WN and WC subtypes of WR stars. Since only spectralline diagnostics were needed, flux calibration was not required and weather conditions werevariable. Further optical spectroscopy followup was performed on the CTIO 4m Blancotelescope using the RC spectrometer in March 2006. Our spectral follow-up revealed a ∼
15% success rate for finding WR stars. A total of 6 newWR stars were found from 42 candidate stars viewed. Four of these new WR stars were foundaway from the Westerlund 2 cluster/RCW49 field while two further WR stars were found inthe RCW49 region outer regions. These include 2 WC stars and 4 WN stars, including thenew WN10 star G321.0331-00.4274 (see Figure 2).In total, 80% of stars showed emission lines. Most of these are Be (or sometimes O)stars. Examples are shown in Fig.A1 of Mauerhan et al (2011) where the H and K bandspectra are dominated by HI emission lines.
Later work has improved the colour selection criterion of the method used here Mauerhanet al (2011) and a significant number of candidates are still to be looked at. Roman-Lopes(2011a,b); Roman-Lopes et al (2011) have presented the discovery SOFI infrared spectra of5 of the 6 WR stars discovered in this study using SOFI archival data. These are referredto as WR20aa, WR20c, WR60aa, WR67a, and WR67b. Identifying WR star candidates viatheir infrared colours has since been improved (Hadfield et al, 2007; Mauerhan et al, 2009,
Galactic Evolved Stars from Their Infrared Emission in situ or very near by. Alternately,there is a large fraction of WR stars forming a population of fast moving, runaway stars inthe galaxy. Something that may well be resolved by upcoming GAIA observations.