Seminars at FGG

A systematic study of the spectral energy distribution of WZ Sge-stars. Revealing new candidates for period-bouncers."

Speaker: Vitaly Neustroev (Finnish Centre for Astronomy with ESO (FINCA) Finland)

Date and time: 2018-09-04 13:00

According to standard evolutionary theory, cataclysmic variables (CV) evolve from longer to shorter orbital periods until a minimum period P_min is reached. The period bouncers are such CVs that have passed beyond P_min and are evolving back toward longer periods, with the donor star now extremely dim. This has long been predicted to be the "graveyard" and current state of 70% of all CVs, though only about a dozen of more or less robust candidates for such period bouncer systems have been identified until now, out of a few thousand of known CVs. CVs of the WZ Sge-type have been long considered as potential period bouncer candidates. However, only very few of recently discovered WZ Sge-type stars were observed spectroscopically in quiescence due to their faintness (19–22 mag). Thus, the lack of information on many of WZ Sge-type stars does not allow us to put restrictions on their system parameters and to confirm or deny their period bounce nature. The determination of system parameters requires many hours of observations with very large telescopes. This explains why direct (spectral) evidence for brown-dwarf donors exists only for a couple of CVs. We recently took a novel, simpler yet equally valuable approach based on multicolour broadband photometry, without the recourse to spectroscopy, to reveal the best candidates for the period bouncers. By adopting such an approach, we have already studied ~30 WZ Sge-type stars (more stars will be observed during the upcoming NOT observations) and have found solid evidence for very low-mass donor stars of very low luminosity in several of them. In this presentation, I will discuss the various methods by which such post period-minimum CVs can be recognized, and will highlight recent progress in this field.