Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002mnras.336..265m&link_type=abstract
Monthly Notice of the Royal Astronomical Society, Volume 336, Issue 1, pp. 265-275.
Astronomy and Astrophysics
Astronomy
12
Accretion, Accretion Discs, Binaries: General, Stars: Evolution, Stars: General, Stars: Individual: Ae Aqr, White Dwarfs
Scientific paper
A possible evolution for the enigmatic cataclysmic variable AE Aquarii is considered that may put into context the long orbital period and short white dwarf rotation period compared with other DQ Her systems. It has been shown that mass transfer could have been initiated when the secondary KIV-V star was already somewhat evolved when it established Roche lobe contact. In this initial phase the orbital period of the system was probably Porb,i~ 8.5 h, and the white dwarf rotation period P*,i > 1 h. Mass transfer in the form of diamagnetic gas blobs will result in an initial discless accretion process, resulting in an efficient drain of the binary orbital angular momentum. Since the initial mass ratio of the binary was probably qi~ 0.8, a high mass transfer rate and a slow expansion of the Roche lobe of the secondary star followed, accompanied by a fast expanding secondary following the mass loss. This could have resulted in the KIV-V secondary flooding its Roche surface, causing a run-away mass transfer of that lasted for approximately , during which time the binary expanded to an orbital period of approximately Porb~ 11 h. During this phase the mass accretion rate on to the surface of the white dwarf most probably exceeded the critical value for stable nuclear burning , which could have resulted in AE Aqr turning into an ultrasoft X-ray source. The high mass transfer terminated when a critical mass ratio of qcrit= 0.73 was reached. Disc torques spun-up the white dwarf to a period close to 33 s within the time-scale before the high mass transfer shut down when qcrit was reached. The decrease in the mass loss of the secondary allowed it to re-establish hydrostatic equilibrium on the dynamical time-scale (fraction of a day). From this point when qcrit is reached the mass transfer and binary evolution proceed at a slower rate since mass transfer from the secondary star is driven by magnetic braking of the secondary on a time-scale , which is the same as the thermal time-scale tth~ 6.3 × 107 yr, i.e. the time-scale on which the secondary shrinks to restore its perturbed thermal equilibrium after the high mass loss. The significantly lower mass transfer in this phase will result in mass ejection from the system. This propeller-ejector action erodes the rotational kinetic energy of the white dwarf, channelling it into mass ejection and non-thermal activity, which explains the non-thermal outbursts that are observed at radio wavelengths and occasionally also at TeV energies.
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