The detection of binary companions to subdwarf B stars

Intermediate dispersion spectroscopy of a sample of 40 hot subdwarf B stars between 5500 and 9000Å has been obtained. The sample includes a large fraction of targets which have been studied photometrically. Seven targets show strong lines arising from the infrared Ca  ii triplet, the unequivocal signature of a cool companion. The positive Ca  ii identifications include known photometric binaries and new targets; all are associated with a photometric red excess. Assuming a canonical value for the subdwarf luminosity, all of the detected companions are overluminous compared with the main sequence. The detection procedure indicates an improved and more secure method for the measurement of the binary frequency of hot subdwarfs.     

Gravitational lensing in eclipsing binary stars

I consider the effect of the gravitational deflection of light upon the light curves of eclipsing binary stars, focusing mainly upon systems containing at least one white dwarf component. In absolute terms the effects are small, however they are strongest at the time of secondary eclipse when the white dwarf transits its companion, and act to reduce the depth of this feature. If not accounted for, this may lead to under-estimation of the radius of the white dwarf compared with that of its companion. I show that the effect is significant for plausible binary parameters, and that it leads to ∼25 per cent reduction in the transit depth in the system KPD 1930+2752. The reduction of eclipse depth is degenerate with the stellar radius ratio, and therefore cannot be used to establish the existence of lensing. A second-order effect of the light bending is to steepen the ingress and egress features of the secondary eclipse relative to the primary eclipse, although it will be difficult to see this in practice. I consider also binaries containing neutron stars and black holes. I conclude that, although relatively large effects are possible in such systems, a combination of rarity, faintness and intrinsic variability makes it unlikely that lensing will be detectable in them.     

Low- and intermediate-mass close binary evolution and the initial–final mass relation – III. Conservative case with convective overshooting and non-conservative case without overshooting

We have completed our series of calculation of case B evolution of Population I close binaries by including a conservative case with convective overshooting and a non-conservative case without convective overshooting in this paper. To apply the results easily to binary population synthesis, we fit two formulae for the remnant mass as a function of the initial mass of the primary, the radius of the primary at the onset of Roche lobe overflow (RLOF) and the initial mass ratio, with errors less than 1.9 per cent and 3.6 per cent for the conservative and non-conservative cases, respectively. We have also made comparisons between the results in this paper and those of previous studies in this series to examine the effects of our assumptions.
We find that the remnant mass depends on when RLOF begins in the Hertzsprung gap and the dependence increases with the mass of the primary. Helium–carbon–oxygen white dwarfs are formed in this series of calculation. Both the non-conservative assumption and convective overshooting make RLOF more stable: some binaries with initial mass ratios   q _i= 4.0  are dynamically stable during RLOF in all cases except for the conservative case without overshooting. From these models, we have discussed potential electron-degenerate oxygen–neon cores. Finally, we show the conditions under which one evolution scenario may be more likely than the others according to the results of previous authors.     

氦白矮星与小质量主序星并合及脉动热亚矮星的形成

sdBVrs型热亚矮星是一类特殊的热亚矮星,观测显示它们同时具有p模和g模的脉动.目前这一类热亚矮星的演化起源仍然缺乏完备的解释.对氦白矮星与小质量主序星并合模型进行详细计算表明,并合模型的结果符合sdBVrs型热亚矮星的表面有效温度、重力加速度等观测特征,并且能够激发出稳定的p模和g模脉动.因此,氦白矮星与小质量主序星并合是形成sdBVrs型热亚矮星可能的渠道之一.     

Confirmation of eclipses in KPD 0422+5421, a binary containing a white dwarf and a subdwarf B star

We report additional photometric CCD observations of KPD 0422+5421, a binary with an orbital period of 2.16 h which contains a subdwarf B star (sdB) and a white dwarf. There are two main results of this work. First, the light curve of KPD 0422+5421 contains two distinct periodic signals, the 2.16-h ellipsoidal modulation discovered by Koen, Orosz & Wade and an additional modulation at 7.8 h. This 7.8-h modulation is clearly not sinusoidal: the rise time is about 0.25 in phase, whereas the decay time is 0.75 in phase. Its amplitude is roughly half of the amplitude of the ellipsoidal modulation. Secondly, after the 7.8-h modulation is removed, the light curve folded on the orbital period clearly shows the signature of the transit of the white dwarf across the face of the sdB star and the signature of the occultation of the white dwarf by the sdB star. We have used the Wilson–Devinney code to model the light curve to obtain the inclination, the mass ratio and the Ω potentials, and a Monte Carlo code to compute confidence limits on interesting system parameters. We find component masses of     and     ( M _total     , 68 per cent confidence limits). If we impose an additional constraint and require the computed mass and radius of the white dwarf to be consistent with a theoretical mass–radius relation, we find     and     (68 per cent confidence limits). In this case the total mass of the system is less than 1.4 M_⊙ at the 99.99 per cent confidence level. We briefly discuss possible interpretations of the 7.8-h modulation and the importance of KPD 0422+5421 as a member of a rare class of evolved binaries.     

The population in the background of open clusters: tracer of the Norma-Cygnus arm

We present colour–magnitude diagrams of open clusters, located in the range  112° < l < 252°  , manifesting stellar populations in the background of clusters. Some of the populations are found to be located beyond the Perseus arm and may be the part of the Norma-Cygnus (outer) arm. The outer arm seems to be continued from   l ∼ 120°  to ∼235°. The background populations follow the downward warp of the Galactic plane around   l ∼ 240°  .     

Merged binary white dwarf evolution: rapidly accreting carbon–oxygen white dwarfs and the progeny of extreme helium stars

We have examined the evolution of merged low-mass double white dwarfs that become luminous helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a carbon–oxygen (CO) white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. When the mass above the helium-burning shell becomes small enough, the star evolves blueward almost horizontally in the Hertzsprung–Russell diagram. The theoretical models for the merger of a 0.6-M_⊙ CO white dwarf with a 0.3-M_⊙ He white dwarf agree very well with the observed locations of extreme helium stars in the  log  T _eff–log  g   diagram, with their observed rates of blueward evolution, and with luminosities and masses obtained from their pulsations. Together with predicted merger rates for  CO+He  white dwarf pairs, the evolutionary time-scales are roughly consistent with the observed numbers of extreme helium stars. Predicted surface carbon and oxygen abundances can be consistent with the observed values if carbon and oxygen produced in the helium shell during a previous asymptotic giant branch phase are assumed to exist in the helium zone of the initial CO white dwarfs. These results establish the  CO+He  white dwarf merger as the best, if not only, viable model for the creation of extreme helium stars and, by association, the majority of R Coronae Borealis stars.     

Radial velocity measurements of white dwarfs

We present 594 radial velocity measurements for 71 white dwarfs obtained during our search for binary white dwarfs and not reported elsewhere. We identify three excellent candidate binaries, which require further observations to confirm our preliminary estimates for their orbital periods, and one other good candidate. We investigate whether our data support the existence of a population of single, low-mass (≲0.5 M_⊙) white dwarfs (LMWDs). These stars are difficult to explain using standard models of stellar evolution. We find that a model with a mixed single/binary population is at least ~20 times more likely to explain our data than a pure binary population. This result depends on assumed period distributions for binary LMWDs, assumed companion masses and several other factors. Therefore, the evidence in favour of the existence of a population of single LMWDs is not sufficient, in our opinion, to firmly establish the existence of such a population, but does suggest that extended observations of LMWDs to obtain a more convincing result would be worthwhile.