Far-Ultraviolet Spectroscopy of Magnetic Cataclysmic Variables

We have obtained Hubble Space Telescope (HST) STIS data for a total of 11 polars as part of a program aimed at compiling a homogeneous database of high-quality far-ultraviolet (FUV) spectra for a large number of cataclysmic variables (CVs). Of the 11 polars, eight were found in a state of low accretion activity (V347 Pav, VV Pup, V834 Cen, BL Hyi, MR Ser, ST LMi, RX J1554.2+2721, and V895 Cen) and three in a state of high activity (CD Ind, AN UMa, and UW Pic). The STIS spectra of the low-state polars unambiguously reveal the photospheric emission of their white dwarf (WD) primaries. We have used pure hydrogen WD models to fit the FUV spectra of the low-state systems (except RX J1554.2+2721, which is a high-field polar) in order to measure the WD effective temperatures. In all cases, the fits could be improved by adding a second component, which is presumably due to residual accretion onto the magnetic pole of the WD. The WD temperatures obtained range from 10,800 to 14,300 K for log g = 8.0. Our analysis more than doubles the number of polars with accurate WD effective temperatures. Comparing the WD temperatures of polars with those of nonmagnetic CVs, we find that at any given orbital period the WDs in polars are colder than those in nonmagnetic CVs. The temperatures of polars below the period gap are consistent with gravitational radiation as the only active angular momentum loss mechanism. The differences in WD effective temperatures between polars and nonmagnetic CVs are significantly larger above the period gap, suggesting that magnetic braking in polars might be reduced by the strong field of the primary. We derive distance estimates to the low-state systems from the flux scaling factors of our WD model fits. Combining these distance measurements with those from the literature, we establish a lower limit on the space density of polars of 1.3 × 10-6 pc-3.