Physics
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000hst..prop.8620g&link_type=abstract
HST Proposal ID #8620
Physics
Hst Proposal Id #8620 Hot Stars
Scientific paper
4 Dra is a unique triple system containing a magnetic cataclysmic variable {AM Her-type} and an M3 III giant. Even though the M-giant completely dominates the optical emission of the system, we can learn much from 4 Dra about the accretion physics in and the evolution of AM Her stars, because: {a} 4 Dra is the second brightest AM Her star in the ultraviolet, {b} 4 Dra is one of the very few cataclysmic variables with a good HIPPARCOS parallax {d 180 pc}, and, in addition, {c} 4 Dra is so far the only bright CV for which an estimate of the age, 10^8 yrs could be derived. We propose an in-depth HST/STIS echelle study of the AM Her star in 4 Dra. Our scientific goals are twofold. {1} We will derive the fundamental properties of the accreting magnetic white dwarf, such as its photospheric temperature, the temperature and the size {lateral extent} of the accretion-heated pole cap, and the chemical composition of the accretion-enriched atmosphere. {2} We will use the HST/STIS echelle spectra for Doppler mapping of the UV line emission. The HST maps of LineCIV1550 and LineHeII1640 emission will probe the velocity field and ionization structure in the complete accretion flow, including the irradiated face of the donor star, the ballistic gas stream from that star to the white dwarf magnetosphere, the threading region between the stream and magnetosphere, the magnetically controlled flow down to the standoff shock, and the surrounding heated regions at the white dwarf surface.
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