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

TL;DR

This study used infrared color selection to discover 60 new Galactic Wolf-Rayet stars, refining detection methods, identifying X-ray counterparts, and analyzing their distribution, thereby expanding the known WR population and understanding their galactic placement.

Contribution

The paper introduces an improved infrared color selection technique for identifying Wolf-Rayet stars, increasing detection efficiency and revealing new WRs and their associations with stellar clusters.

Findings

Discovered 60 new Galactic WR stars, increasing total to 476.

Enhanced detection rate to ~40% when combining infrared selection with X-ray data.

Identified WR stars in previously unknown stellar clusters and associations.

Abstract

We have spectroscopically identified 60 Galactic Wolf-Rayet (WR) stars, including 38 nitrogen types (WN) and 22 carbon types (WC). Using photometry from the Spitzer/GLIMPSE and 2MASS databases, the WRs were selected via a method we have established that exploits their unique infrared colors, which is mainly the result of excess radiation from free-free scattering within their dense ionized winds. The selection criteria has been refined since our last report, and now yields WRs at a 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 subregions within the broad color space which yield WRs at a rate of >50%. Cross-correlation of WR candidates with archival X-ray point-source catalogs 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 that lies near the intersection of the Scutum-Centaurus Arm and the Galaxy's bar, in which five OB supergiants were also identified. In addition, two WC and four WN stars 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 a tracing of spiral arms and an enhanced WR surface density toward several massive-star formation sites (abridged).