The Low State Temperature Distribution and First Chemical Abundances of White Dwarfs in Polars

Abstract

During the low optical brightness states of AM Herculis systems (polars) when accretion has declined to a very low value, the underlying magnetic white dwarf photosphere can be modelled without the complication of thermal bremstrahlung and cyclotron emission from the luminous accretion column. The far ultraviolet spectra can be modelled with high gravity solar composition photospheres. In this way, I present new temperatures and the first chemical abundance estimates for the white dwarfs in three selected polars from the IUE NEWSIPS archive. For the white dwarf in V834 Cen with Teff = 16,000$K, Si/H = 0.1 solar, C/H = 0.5 solar, for BY Cam, Teff = 17,000 K, Si/H = 0.1, C/H = 5 solar and for RX J1313-32, Teff = 22,000 K, Si/H = 0.1 solar, C/H = 0.1 solar. The temperature distribution of 24 white dwarfs in polars with known temperatures above and below the period gap is compared with the distribution of the white dwarf temperatures in dwarf novae during quiescence. In both cases, the magnetic white dwarfs in polars are significantly cooler than the non-magnetics. For all CV white dwarfs, magnetic and non-magnetic with Teff < 12,500 K, 91% of the objects (10 out of 11) are magnetics in polars. This suggests that long term accretion heating and cooling of white dwarfs in polars differs from the effects of long term accretion in non-magnetic disk accretors.