The Accretion Disk and White Dwarf in the Short‐Period Dwarf Novae TY Piscium and V436 Centauri during Quiescence

Abstract

ABSTRACTThe short‐period dwarf novae TY Psc and V436 Cen are SU UMa systems with very similar orbital periods, similar recurrence times for normal outbursts (∼23 days) and superoutbursts (∼340 days), and nearly identical outburst amplitudes. We have carried out high‐gravity model atmosphere and accretion disk synthetic spectra from the grid of Wade & Hubeny. The best‐fit stellar model spectrum, from spectral slope and line fitting, is a white dwarf photosphere having Teff = 25,000 K, log g = 8, and essentially solar chemical abundances, while the best‐fit optically thick accretion disk model, from spectral slope fitting, has Mwd = 0.55 M⊙, M˙ = 10-9.5 M⊙ yr−1, and an inclination i = 18°. The implied accretion rate is almost certainly too large for dwarf nova quiescence. The predicted fluxes using parameters from the photosphere and disk spectral slope fitting models reveal enormous differences compared with the observed luminosity using a reasonable distance estimate. For TY Psc, the predicted accretion disk luminosity is ∼100 times too luminous, while the stellar luminosity is too luminous by a factor of ∼10. For V436 Cen, the best‐fit high‐gravity model photosphere, from spectral slope fitting, yields Teff = 24,000 K, log g = 8, and essentially solar abundance, while the best‐fit accretion disk models, from spectral slope fitting, yield Mwd = 0.8 M⊙, M˙ = 10-10 M⊙ yr−1, and i = 75°. The presence of broad absorption troughs at unusual wavelength positions suggests the presence of an absorption curtain (upper disk atmosphere) in V436 Cen. The temperatures we have for TY Psc and V436 Cen are higher than normal for the accreting white dwarfs in dwarf novae below the period gap. This could indicate that the systems were not in the deepest level of quiescence when they were observed.