THE DETERMINATION OF ELEMENT ABUNDANCES IN THE SOLAR NEIGHBORHOOD FROM B-TYPE STELLAR SPECTRA .1. ARGON

Using the coudé spectrograph and CCD camera on the coudé feed telescope at the Kitt Peak National Observatory, we have obtained high-resolution (≃0.09 Å FWHM), high signal-to-noise (≃200) observations of weak A II absorption lines formed in the atmospheres of main-sequence early-type stars. These have been analyzed using LTE model atmosphere techniques to derive a mean argon-to-hydrogen abundance ratio (on the scale log [H] = 12) of log [A] = 6.49, which we believe is accurate to ±0.1 dex. As the stellar atmospheres should be uncontaminated by the products of interior reactions, this abundance should be that of the interstellar medium from which the stars formed some 106-107 yrs ago, and hence reflects the current chemical composition of the solar neighborhood. Our result is compatible with that of Veck and Parkinson, who found log [A] = 6.38+0.18-0.30, but has a smaller observational uncertainty. In addition, our abundance estimate is significantly different from that of Withbroe (log [A] ≃ 6.65), which is often adopted in interstellar medium studies of argon, and implies zero argon depletions for the unreddened sightlines to α Vir and λ Sco.