Helium star

Table 6.2 gives an overview of some of the stages of stellar evolution where carbon and/or s-process anomalies occur (see Fig. 3.37). The C/O ratio increases down the series. In addition to the types listed there, there are infrared carbon stars such as IRC +10216,1 proto-planetary nebulae and a whole zoo of peculiar carbon stars, including J stars (strong 13C as in the case of HD 52432 shown in Fig. 1.7) and hydrogen-deficient carbon stars which can be cool, e.g. R Cor Bor, RY Sag and HD 137613 shown in Fig. 1.7, or hot (when they look like extreme helium stars) such stars may have lost their envelopes by binary mass transfer, or they may be born-again AGB stars. 

Woosley SE, Langer N, Weaver TA (1995) The presupemova evolution and explosion of helium stars that experience mass loss. Fresenius Astrophys J 448 315-338 Woosley SE, Weaver TA (1995) The evolution and explosion of massive stars. II. Explosive hydrodynamics and nucleosynthesis. Astrophys J Suppl 101 181-235... 

Table 3 summarizes the results of an analysis of four extremely helium-rich luminous sdOs (Husfeld, 1986 Husfeld et al., in preparation). Here, only upper limits for the hydrogen abundances can be given as no traces of this element can be found in the spectra. Consequently, helium appears as the most abundant element. Significantly overabundant are also carbon (with one exception LSE 263) and nitrogen. Silicon is effectively unaltered. This abundance pattern compares well with the abundances found in the extreme helium stars of spectral type B (given in col. 6 of Table 3). However, it should be stressed that the carbon depletion in LSE 263 makes this star a peculiar object in its class.

Figure 1. Lines of constant mass and varying chemical composition for the computed Wolf-Rayet models of 3, 5, 7, 10, 15, 20, SO, 40, and 60Mq in the HR diagram (solid lines). The pure helium stars are connected through a dashed line, while the extreme helium poor stars are connected through a dotted line. Also the HRD positions after applying a correction for the partly optically thick stellar wind on the effective temperature are shown. Furthermore, the theoretical zero age main sequence (ZAMS) is indicated, together with schematic evolutionary tracks for stars of 15, 30, and IOOMq. The crosses and circles correspond to HRD positions of observed WNE and WC stars, respectively, according to Smith and Willis (198S, Astron. Astrophys. Suppl. 54,229).

Light curves of exploding bare helium stars are also calculated to see whether the observed Type lb supernova light curves can be accounted for. 

In addition to SN 1987A, explosion of bare helium stars is calculated to examine whether and under what conditions Type Ib-like light curves are realized. 

Finally, we tried a numerical experiment where 56Ni exists only in the outermost 0.2 M layer beneath the surface. The escape of X-rays and 7-rays are significant and the resulting optical light curve (the dash-dotted curve in Fig. 11) is as narrow as SN 1964L. Such a composition inversion is not realistic for helium stars but a natural outcome from the off-center detonation in accreting white dwarfs. 

The slowest light curve of SN lb such as SN 1984L may be accounted for by the helium star... 

To clarify whether a separate class of SN Ic exists, observations and modeling of nebula phase spectra of SN Ic would be crucial. Also the mechanism of extensive mass loss and mixing required from the helium star model need to be studied. 

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