The hot subdwarf B+ white dwarf binary KPD 1930+2752 - A supernova type Ia progenitor candidate

Context. The nature of the progenitors of type Ia supernovae is still under debate. KPD 1930+ 2752 is one of the best SN Ia progenitor candidates known today. The object is a double degenerate system consisting of a subluminous B star ( sdB) and a massive white dwarf (WD). Maxted et al. (2000) conclude that the system mass exceeds the Chandrasekhar mass. This conclusion, however, rests on the assumption that the sdB mass is 0.5 M(circle dot). However, recent binary population synthesis calculations suggest that the mass of an sdB star may range from 0.3 M(circle dot) to more than 0.7 M(circle dot). Aims. It is therefore important to measure the mass of the sdB star simultaneously with that of the white dwarf. Since the rotation of the sdB star is tidally locked to the orbit, the inclination of the system can be constrained if the sdB radius and the projected rotational velocity can be measured with high precision. An analysis of the ellipsoidal variations in the light curve allows the constraints derived from spectroscopy to be tightened. Methods. We derived the mass-radius relation for the sdB star from a quantitative spectral analysis of 150 low-resolution spectra obtained with the Calar Alto 2.2 m telescope using metal-rich, line-blanketed LTE model atmospheres with and without NLTE line formation. The projected rotational velocity was determined for the first time from 200 high-resolution spectra obtained with the Keck I 10 m and with the ESO-VLT 8.2 m telescopes. In addition a reanalysis of the published light curve was performed. Results. The atmospheric and orbital parameters were measured with unprecedented accuracy. In particular the projected rotational velocity v(rot) sin i = 92.3 +/- 1.5 km s(-1) was determined. Assuming the companion to be a white dwarf, the mass of the sdB is limited between 0.45 M(circle dot) and 0.64 M(circle dot) and the corresponding total mass of the system ranges from 1.33 M(circle dot) to 2.04 M(circle dot). This constrains the inclination to i > 68 degrees. The photometric analysis allows the parameters to be constrained even more. A neutron star companion can be ruled out and the mass of the sdB is limited to the range between 0.45 M(circle dot) and 0.52 M(circle dot). The total mass of the system ranges from 1.36 M(circle dot) to 1.48 M(circle dot) and hence is likely to exceed the Chandrasekhar mass. The inclination angle is 80 degrees and the light curve shows weak and shallow signs of eclipses. A high-precision light curve is needed in order to accurately measure these eclipses. So KPD 1930+ 2752 qualifies as an excellent double degenerate supernova Ia progenitor candidate. Conclusions.