* Astronomy

Blobrana · L

RE: KPD 1930+2752

Blobrana · L

Title: Whole Earth Telescope Observations of the subdwarf B star KPD 1930+2752: A rich, short period pulsator in a close binary
Authors: M. D. Reed, S.L. Harms, S. Poindexter, A.-Y. Zhou, J.R. Eggen, M.A. Morris, A.C. Quint, S. McDaniel, A. Baran, N. Dolez, S. D. Kawaler, D. W. Kurtz, P. Moskalik, R. Riddle, S. Zola, R. H. Ostensen, J.-E. Solheim, S.O. Kepler, A. daCosta, J. L. Provencal, F. Mullally, D. W. Winget, M. Vuckovic, R. Crowe, D. Terry, R. Avila, B. Berkey, S. Stewart, J. Bodnarik, D. Bolton, P.-M. Binder, K. Sekiguchi, D. J. Sullivan, S.-L. Kim, W.-P. Chen, C.-W. Chen, H.-C. Lin, X.-J. Jian, H. Wu, J.-P. Gou, Z. Liu, E. Leibowitz, Y. Lipkin, C. Akan, R. Janulis, R. Pretorius, W. Ogloza, G. Stachowski, M. Paparo, R. Szabo, Z. Csubry, D. Zsuffa, R. Silvotti, S. Marinoni, I. Bruni, G. Vauclair, M. Chevreton, J.M. Matthews, C. Cameron, H. Pablo

KPD 1930+2752 is a short-period pulsating subdwarf B (sdB) star. It is also an ellipsoidal variable with a known binary period just over two hours. The companion is most likely a white dwarf and the total mass of the system is close to the Chandresakhar limit. In this paper we report the results of Whole Earth Telescope (WET) photometric observations during 2003 and a smaller multisite campaign from 2002. From 355 hours of WET data, we detect 68 pulsation frequencies and suggest an additional 13 frequencies within a crowded and complex temporal spectrum between 3065 and 6343 \mu Hz (periods between 326 and 157 s). We examine pulsation properties including phase and amplitude stability in an attempt to understand the nature of the pulsation mechanism. We examine a stochastic mechanism by comparing amplitude variations with simulated stochastic data. We also use the binary nature of KPD 1930+2752 for identifying pulsation modes via multiplet structure and a tidally-induced pulsation geometry. Our results indicate a complicated pulsation structure that includes short-period (\approx 16 h) amplitude variability, rotationally split modes, tidally-induced modes, and some pulsations which are geometrically limited on the sdB star.

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Blobrana · L

Title: The subdwarf B + white dwarf binary KPD 1930+2752 - a Supernova Type Ia progenitor candidate
Authors: S. Geier, S. Nesslinger, U. Heber, N. Przybilla, R. Napiwotzki, R.-P. Kudritzki

The nature of the progenitors of type Ia supernovae is still under controversial 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 and a massive white dwarf. 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 Mo. However, recent binary population synthesis calculations suggest that the mass of an sdB star may range from 0.3 Mo to more than 0.7 Mo. 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. An analysis of the ellipsoidal variations in the light curve allows to tighten the constraints derived from spectroscopy. We derive the mass-radius relation for the sdB star from a quantitative spectral analysis. The projected rotational velocity is determined for the first time from high-resolution spectra. In addition a reanalysis of the published light curve is performed. The atmospheric and orbital parameters are measured with unprecedented accuracy. In particular the projected rotational velocity vrotsini = 92.3 ±1.5 km/s is determined. The mass of the sdB is limited between 0.45 Mo and 0.52 Mo. The total mass of the system ranges from 1.36 Mo to 1.48 Mo and hence is likely to exceed the Chandrasekhar mass. So KPD 1930+2752 qualifies as an excellent double degenerate supernova Ia progenitor candidate.

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