Neon Abundances from a SpitzerIRS Survey of Wolf-Rayet Stars.pdf

a r X i v : 0 7 0 7 .2 7 7 0 v 1 [ a s t r o - p h ] 1 8 J u l 2 0 0 7 Neon Abundances from a Spitzer/IRS Survey of Wolf-Rayet Stars R. Ignace Department of Physics, Astronomy, Geology, East Tennessee State University, Johnson City, TN 37614 ignace@etsu.edu J. P. Cassinelli, G. Tracy, E. Churchwell Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706 joecas@astro.wisc.edu, gatracy@wisc.edu, ebc@astro.wisc.edu H. J. G. L. M. Lamers Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands H.J.G.L.M.Lamers@astro.uu.nl ABSTRACT We report on neon abundances derived from Spitzer high resolution spectral data of eight Wolf-Rayet (WR) stars using the forbidden line of [Ne III] 15.56 microns. Our targets include four WN stars of subtypes 4–7, and four WC stars of subtypes 4–7. We derive ion fraction abundances γ of Ne2+ for the winds of each star. The ion fraction abundance is a product of the ionization fraction Qi in stage i and the abundance by number AE of element E relative to all nuclei. Values generally consistent with solar are obtained for the WN stars, and values in excess of solar are obtained for the WC stars. Subject headings: stars: abundances — stars: mass-loss — stars: winds — stars: Wolf-Rayet 1. Introduction Wolf-Rayet (WR) stars continue to attract the attention of many researchers because they are evolved massive stars that are useful for test- ing stellar evolutionary models and because they have extremely massive winds (Lamers et al. 1991; Meynet Maeder 2003). We report on infrared observations of [Ne III] 15.56 microns for eight WR stars, each of a different spectral subtype (see Tab. 1), obtained with the Spitzer Infrared Spec- trograph (IRS) using the Short-High (SH) mode in the 10–20 micron band. Details about the Spitzer telescope and particularly the IRS instrument can be found in Houck et al. (2004). Our program targeted four nitrogen-rich WN stars, and four