Red Eyes on Wolf-Rayet Stars: 60 New Discoveries via Infrared Color Selection

Abstract

We have spectroscopically identified 60 Galactic Wolf-Rayet stars (WRs), including 38 nitrogen types (WN) and 22 carbon types (WC). Using photometry from the Spitzer /GLIMPSE and Two-Micron All-Sky Survey (2MASS) databases, the new WRs were selected via a method we have established that exploits their unique infrared colors, which is mainly the result of excess radiation generated by free-free scattering within their dense ionized winds. The selection criterion has been refined since the last report, resulting in a WR detection rate of ≈20% in spectroscopic follow-up of candidates that comprise a broad color space defined by the color distribution of all known WRs having B > 14 mag. However, there are smaller regions within this color space which yield WRs at a rate of >50% in spectroscopic follow-up. Candidates which are not WRs are mainly Be stars, which is possibly attributable to the physical similarities between the free-free emission parameters of Be disks and WR winds. As an additional selection experiment, the list of WR candidates was cross-correlated with archival X-ray point-source catalogs, which increases the WR detection rate of the broad color space to ≈40%; ten new WR X-ray sources have been found, in addition to a previously unrecognized X-ray counterpart to a known WR. The extinction values, distances, and galactocentric radii of all new WRs are calculated using the method of spectroscopic parallax. Although the majority of the new WRs have no obvious association with stellar clusters, two WC8 stars reside in a previously unknown massive-star cluster, in which five OB supergiants were also identified. The new system lies at an estimated distance of ≈ 6.1 kpc, near the intersection of the Scutum-Centaurus Arm with the Galaxy's bar. In addition, two WC and four WN stars, all but one of which are X-ray sources, were identified in association with the stellar clusters Danks 1 and 2. A WN9 star has also been associated with the cluster [DBS2003] 179. This work brings the total number of known Galactic WRs to 476, or ≈7%-8% of the total empirically estimated population. An examination of their Galactic distribution reveals an approximate tracing of spiral arms and an enhanced WR surface density toward several massive-star formation sites. Subject headings: arXiv:1105.5134v3 [astro-ph.GA]

Spatial distribution of Galactic Wolf–Rayet stars and implications for the global population

“…We note that < 5% of WR stars discovered since the year 2011 are brighter than KS= 8 mag (Mauerhan et al 2011;Shara et al 2012;Smith et al 2012;Chené et al 2013). Therefore, we adopt this as the current completeness limit in systemic magnitude.. Figure 11 presents predicted magnitude distributions for WR populations containing 450, 550 and 650 stars between RG = 3-15 kpc plus 250 central stars (RG < 3 kpc).…”

“…There are 9 currently identified WR stars located within the boundary of the giant HII region G305.4+0.1, of which only 4 reside in the two central clusters Danks 1 & 2 (see Figure 16 of Mauerhan et al 2011, andDavies et al 2012a). This is rather surprising, as one may expect to find these stars -as descendants of massive progenitors -at the centre of these star clusters due to relaxation of their orbits.…”

“…comm. ), (f) Crowther et al (2006b), (g) Mauerhan et al (2011), (h) Figer, McLean & Najarro (1997), (i) Roman-Lopes (2011c) …”

“…Shara et al (1991Shara et al ( , 1999 applied similar methods to push the extent of the known Galactic population beyond 5 kpc from the Sun, and extension to near-IR wavelengths has facilitated yet deeper investigation of the Galactic disk (Shara et al 2009(Shara et al , 2012. Another distinctive feature of WR stars -the near-IR excess caused by free-free emission in their winds -has been exploited to yield further discoveries (Homeier et al 2003;Hadfield et al 2007;Mauerhan, Van Dyk & Morris 2011). These efforts, alongside several serendipitous discoveries (e.g., Clark et al 2005, Mauerhan et al 2010a, have brought the recognised Galactic WR star population to ∼635 as of March 2014 1 .…”

“…By way of example, three WR stars have been discovered since 2006 between the RA's of WR 20 and WR 21. The first, discovered by Mauerhan et al (2011), is assigned WR 20-1, while two further discoveries from Roman-Lopes, Barba, & Morrell (2011a) are assigned WR 20-2 and -3 (RA ordered).…”