V-348 SAGITTARII - ANALYSIS OF THE ABSORPTION-SPECTRUM

The highly variable star V 348 Sgr has been variously described as a h ot RCrB star, an extreme helium star and a cool Wolf-Rayet ([WC11]) ce ntral star of a planetary nebula. Despite the Wolf-Rayet nature of its spectrum, sufficient pure absorption lines are visible to allow the L TE analysis of its photosphere presented in this paper. The weak C IV lambda 5801,12 Angstrom doublet and the strong C III lambda 4647,50,51 Angstrom blend provide a sensitive measure of the carbon abundance. T his analysis finds the surface parameters of V 348 Sgr to be T-eff = 2 2 000 K, log g less than or equal to 2.7, n(c) = 0.10, log n(N) = -3.7 , log n(O) = -3.5 and log n(Si) -5.1, where n are abundances by number fraction. These results are in reasonable agreement with a recent non -LTE analysis of the Wolf-Rayet spectrum (Leuenhagen & Hamann 1994 [LH 94]). The hydrogen abundance could not be determined from the present analysis; adopting n(H) = 0.20 after LH94, the helium abundance become s n(He) = 0.70. The model flux distribution can be used to estimate th e separate contributions of the interstellar and non-variable circumst ellar extinction towards V 348 Sgr; the former corresponds to a redden ing E(B)-v = 0.63. Previous high carbon abundance models had predicted a strong CII opacity edge in the UV; the edge is sufficiently diluted by line-blanketing that the models now give a good fit to the observe d flux distribution. V 348 Sgr is confirmed to have a surface carbon a bundance much higher than that encountered in extreme helium stars (ty pically n(C) = 0.01), but typical of other cool Wolf-Rayet central sta rs. However, the N/O and O/Si ratios are within the ranges observed in EHe and hot RCrB stars, implying that V 348 Sgr and other [WC-L] star s may represent the C-rich extremity of an ubiquitous helium-star prod uction mechanism. The analysis demonstrates the presence of CNO, 3 alp ha and C-12(alpha, gamma)O-16 prod ucts, as well as a substantial hydr ogen remnant, which imply that considerable disruption of the stellar surface layers must have occurred. Consequences for evolutionary model s are considered.