Hydrogen: abundance

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.

The evolution of the carbon abundance at the surface of both components of a mass-exchanging (Algol-type) binary is examined (fig. 1). Distinction is made between case B and case AB (fig. 2) of mass transfer, in view of the different timescales involved. In the mass accreting component thermohaline mixing is adopted when matter with decreasing hydrogen abundance is deposited on the surface. 

In the model by Maeder (1988), the red supergiant is peeled off until the atmospheric hydrogen abundance is low enough to make the star blue. The models of Maeder evolve along a line of constant Z in Fig.2 The same is true for the models of Wood (1988). 

This yields a silicon abundance on the astronomical scale of log Aast(Si) = 7.546 and a hydrogen abundance on the meteoritic scale of log A et(H) = 10.45 or 2.84 X 10 ° which is given in Table 3. Anders and Grevesse (1989) calculated a value of 1.554 for the ratio of solar to meteoritic abrmdances, which leads to a hydrogen abundance of 2.97 X 10 ° on the meteoritic scale. Lodders (2003) used a conversion factor of 1.540 based on the ratio of photospheric and meteoritic sihcon. 

Samuelson R. E., Hand R. A., Kunde V. G., and Maguire W. C. (1981) Mean molecular-weight and hydrogen abundance of Titan s atmosphere. Nature 292, 688-693. 

As the hydrogen abundance drops and the 3He and 4He abundances increase, additional reactions (Table 5) come into play and eventually the p-p II and p-p III chains dominate over p-p I. 

When the core hydrogen abundance drops to Xc < 0.5, any further increases in Tc fail to compensate for the drop in energy generation and the whole star starts to contract. As nuclear reactions are extinguished, the convective core vanishes, but only when Xc gravitational energy, with the core contracting on a thermal timescale tx 2 x 106 years. 

Early models suggest at least two phases of mass transfer necessary to explain the very low surface hydrogen abundances e.g. [J27]. The current best model for the evolution of v Sgr [9] is that it began as a f 0+3 Mq, f 50 d binary in which the envelope was blown to infinity, with little change of orbit, as the more massive star approached both Roche Lobe overflow and the Cepheid instability strip simultaneously. 

The HOx radicals also regulate the O3 level in the Martian atmosphere via catalytic cycles analogous to those in the terrestrial stratosphere. However, on Venus catalytic cycles involving chlorine for the re-combination of CO2 are required because the hydrogen abundance is too low for catalytic cycles involving OH to be as efficient as they are on Mars. 

Christensen-Dalsgaard (1998) obtains Yo = 0.2713 and Zo = 0.0196, Bahcall et al. (2001) report for their standard solar model Yo = 0.2735. Figure 1 shows the hydrogen abundance in the present-day Sun as a function of distance from the solar center. The... 

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