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1994PASP..106..209P - Publ. Astron. Soc. Pac., 106, 209-238 (1994/March-0)
The DQ Herculis stars.
PATTERSON J.
Abstract (from CDS):
The observed Lx/Lv values are surprisingly low for a rapidly accreting white dwarf, suggesting that most of the accretion energy is not radiated in a strong shock above the magnetic pole. The fluxes can be more satisfactorily explained if most of the radial infall energy manages to bypass the shock and deposit itself directly in the white dwarf photosphere, where it should emerge as EUV radiation. This also provides an adequate source of ionizing photons to power the high-excitation optical and UV emission lines. This is probablythe DQ Her analogue to the famous "soft X-ray excess" in AM Her stars. However, unlike the AM Her case, this radiation has not been directly observed, so the analogy must not (yet) be embraced too firmly.
There is some conventional wisdom today which segregates the short-period from the long-period DQ Her stars. But the observational grounds for this distinction are slim, except in one respect: X-ray emission from short-period systems appears to be weaker and softer. This must be due to the shallower depth of the potential well, and/or the greater difficulty the fast rotators have in enforcing radial accretion flow.
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Simbad objects: 18
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