6 Discussion

Green et al. (2001) have discussed the evolutionary origin of subdwarf B stars in view of findings regarding the distribution of binary periods and companions found in other surveys (Saffer et al. 2001). They deduce that sdB stars with spectral lines from a cooler companion invariably have periods longer than a year, while very short-period sdBs have essentially invisible companions. The deduction is that both groups are produced by Roche lobe overflow/mass transfer from low-mass stars near the tip of the red giant branch. If the initial binary is sufficiently wide and the secondary is sufficiently massive and able to accept the dynamic mass transfer of the first couple of tenths of a solar mass without filling its Roche lobe, then the initial mass ratio may be reduced sufficiently to allow stable mass transfer and avoid a common-envelope phase. In this case the orbital period would remain long, and the secondary would increase in mass, becoming a blue straggler (BS) with $\mathrel{\raise1.16pt\hbox{$>$ }\kern-7.0pt \lower3.06pt\hbox{{$\scriptstyle \sim$ }}}$ $1\mbox{\,$M_{\odot}$ }$, as observed by ourselves. An important question will be to determine accurately the upper and lower limits on both masses and periods for such sdB+BS binaries. Green $\&$ Liebert (2001) suggest that binaries with less massive secondaries would form a common-envelope and end up either merging or as short-period sdB+MS systems.

The division of sdBs into long- and short-period binaries suggests a reason for the difference in helium abundance between the two samples. While low-helium (and -metal) abundances are known to be a result of atmospheric diffusion in high-gravity stars within a certain temperature range, external forces may partially disrupt these. Tidal interaction due to a binary companion will be much stronger in a short-period than in a long-period system. Unless the sdB star is rotating completely synchronously, tidal effects will operate on timescales shorter than diffusion ($\sim$10⁵ years) and may dilute the chemical separation. Extremely low-hydrogen abundances would therefore be seen preferentially in long-period sdB binaries.

The presence of sdBs within the sample with helium abundances significantly greater than normal (e.g. PG0229+064, PG0240+046) may be a consequence of their belonging to sdB group (1) - apparently single stars (Saffer et al. 2001). It is interesting that no sdB star with y>0.03 is known to be a short-period binary (Maxted et al. 2001). Since sdB stars are known with extremely high helium abundances (cf. Jeffery et al. 1987), we suggest that these could have an entirely separate origin, being the products of helium plus helium white dwarf mergers (Iben 1990). Evidence for such a conclusion is provided by the extreme helium star V652Her (Jeffery et al. 2001), considered to be strong evidence for such a merger product evolving to become an isolated helium main-sequence star (Saio & Jeffery 2000). When it becomes a subdwarf, diffusion will inevitably modify the initially helium-rich atmosphere. However, with a much more limited reservoir of hydrogen, extremely low helium abundances will be difficult to achieve. Consequently sdBs produced by mergers could be expected to show a very wide range of helium abundances.