The Atmospheric Composition of the Hot Pre-Degenerate Star H1504+65 Revisited

Abstract

Nousek et al. (1986) have recently announced the discovery of a remarkable hot compact star, H1504+65, which appears to have an atmosphere devoid of hydrogen and helium. Although the object seems significantly hotter than the related PG1159 stars, its suggested atmospheric composition remains at variance with the He-rich composition inferred for the latter stars (Wesemael, Green, and Liebert 1985). Nousek et al. (1986) have brought forward convincing arguments against a H-dominated atmosphere in H1504+65; it seems that the object is definitely not an extremely hot DA star. On the other hand, the case against a He-rich atmosphere is much weaker and is based on the following circumstantial arguments: (1) no helium lines are detected in the spectrum of H1504+65 (the detection limit is ≈ 0 5 Å), (2) the ultraviolet continuum slope is steeper than that predicted from pure helium modelsaxtrapolated to effective temperatures larger than the upper limits of available grids (Te = 200 000 K for log g = 7 ), and (3) the EXOSAT observations appear to conflict with the He interpretation, although suitable models (with high effective temperatures) are not readily available in this case also. Because of the lack of appropriate models, Nousek et al. (1986) have been careful not to rule out completely the possibility of a He-rich atmosphere for H1504+65. In particular, they admit that He II lines could probably be undetectable if H1504+65 had an effective temperature substantially larger than Te = 150 000 K. Nevertheless, they have found it tempting to hypothesize an atmosphere devoid of both hydrogen and helium. Coupled with the presence of weak C IV and O VI features in the spectrum of H1504+65, this has led to the suggestion that H1504+65 is actually a bare C/O nucleus (cf. Shipman 1987). Although this suggestion is interesting, it has undeservedly remained unchallenged and we have felt it appropriate to reexamine the case against a He-rich atmosphere with the help of further modelling efforts.