An Ultraviolet to Mid-Infrared Study of the Physical and Wind Properties of HD 164270 (WC9) and Comparison to BD +30 3639 ([WC9])

We present new Spitzer IRS observations of HD 164270 (WC9, WR103). A quantitative analysis of the UV, optical, near-, and mid-IR spectra of HD 164270 is presented, allowing for line blanketing and wind clumping, revealing T_* ~ 48 kK, log L/L_☉ ~ 4.9, and ~ 10^-5 M_☉ yr^-1 for a volume filling factor of f ~ 0.1. Our models predict that He is partially recombined in the outer stellar wind, such that recent radio-derived mass-loss rates of WC9 stars have been underestimated. We obtain C/He ~ 0.2 and O/He ~ 0.01 by number from optical diagnostics. Mid-IR fine-structure lines of [Ne II] 12.8 μm and [S III] 18.7 μm are observed, with [Ne III] 15.5 μm and [S IV] 10.5 μm absent. From these we obtain Ne/He ~ Ne⁺/He = 2.2 × 10^-3 by number, 7 times higher than the solar value (as recently derived by Asplund et al.), and S/He ~ S²⁺/He = 5.1 × 10^-5 by number. From a comparison with similar results for other WC subtypes we conclude that WC9 stars are as chemically advanced as earlier subtypes. We consider why late WC stars are exclusively observed in high-metallicity environments. In addition, we compare the UV/optical/mid-IR spectroscopic morphology of HD 164270 with the planetary nebula central star BD +30 3639 ([WC9]). Their UV and optical signatures are remarkably similar, such that our quantitative comparisons confirm similarities in stellar temperature, wind densities, and chemistry first proposed by Smith & Aller, in spite of completely different evolutionary histories, with HD 164270 presently a factor of 10 more massive than BD +30 3639. At mid-IR wavelengths, the dust from the dense young nebula of BD +30 3639 completely dominates its appearance, in contrast with HD 164270.