Non-LTE Abundances in OB stars: Preliminary Results for 5 Stars in the Outer Galactic Disk

The aim of this study is to analyse and determine elemental abundances for a large sample of distant B stars in the outer Galactic disk in order to constrain the chemical distribution of the Galactic disk and models of chemical evolution of the Galaxy. Here, we present preliminary results on a few stars along with the adopted methodology based on securing simultaneous O and Si ionization equilibria with consistent NLTE model atmospheres. The chemical distribution of B stars in the outer Galactic disk is presently poorly probed, based on only a few abundance results for B distant stars (e.g., Daflon et al. 2004). In order to enlarge the number of studied stars and to better represent the chemical distribution of the outer Galactic disk, we obtained high-resolution echelle spectra for a sample of 136 OB stars located towards the Galactic anti-center using the MIKE spectrograph on the 6.5m Magellan Clay telescope. A subsample of 50 sharp-lined B stars has been selected for the abundance analysis. High resolution, high signal-to-noise spectra of 3 well studied main-sequence B stars (HD 61068, HD 63922 and HD 74575) were added to the sample in order to test our adopted methodology. We use an iterative method to obtain simultaneously the stellar parameters (effective temperature (Teff), surface gravity (logg), microturbulence, and abundances of Si and O) based on non-LTE synthesis of H, He, Si, and O profiles. The synthetic spectra are computed using SYNSPEC (Hubeny & Lanz 2011), which interpolates in a grid of non-LTE model atmospheres computed with TLUSTY (Hubeny & Lanz 1995) and detailed atomic models of O (69, 219 and 41 levels for O I, II and III, respectively) and Si (70, 122 and 53 levels for Si II, III and IV, respectively). The adopted method, based on Hunter et al. (2007), consists of the following steps: (a) Initial values for the stellar parameters are set from the stellar spectral type. (b) Ionization balance of Si II/III/IV and/or O I/II/III provides Teff and abundances of Si and O. (c) logg is obtained from fits of the pressure broadened wings of the Balmer lines Hα, Hβ and Hγ. (d) Microturbulence is defined by requiring that the Si III line abundances are inde- pendent of the line strength (equivalent width).