The Stratification of Metals in Hot White Dwarf Atmospheres N.J. Dickinson, M.A. Barstow, I. Hubeny * * Steward Observatory, University of Arizona

Motivation Uncertainties in modelling metals in many WDs Metals affect T eff estimates -> evolutionary models, stellar atmosphere models etc Stellar structure, local environment, evolutionary processes, supernova chemistry etc Nathan 2

Method Models constructed using TLUSTY Spectra synthesised using SYNSPEC Line fitting achieved with XSPEC Look at PG and REJ Nathan 3

PG T eff =110,000K, log g=7.58 Barstow et al. (2003) couldn’t fit C, N, O, Si No sign of circumstellar material/ binary companion in UV data Nathan 4

Nathan 5 C IV doublet

REJ T eff = 44,350K log g=7.81 Holberg et al. (1999) found similar N V lines to PG ’s C IV ; fit with stratified super- abundance (N/H=5x10 -5, Barstow et al., 2003). Chayer et al. (2005) found smaller, homogeneous N abundance fit data; Schuh et al., Nathan 6

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Homogeneous N/H= 5x10 -5 Nathan 9

Stratified N/H= 5x10 -5 Nathan 10

N/H= 2.24x10 -7 ; Chayer et al ‘05 = 6.3x10 -7 Nathan 11

S Nathan 12 LTE, N/H=5x10 -5 NLTE, N/H=2.24x10 -7 H

PG ; Homogeneous C/H= 4.85x10 -6 Nathan 13

Homogeneous C/H= 1.2x10 -5 Nathan 14

Stratified C/H= 1.2x10 -5 Nathan 15

Conclusions Stratifying metals as in Holberg et al., 1999 is not a effective as once thought When NLTE was included, lower homogeneous abundances fit REJ ’s N V lines well PG C IV probably not stratified in a slab; stellar winds? Nathan 16