Abstract
Mass transfer and accretion are very important to understand the evolution and observational properties of cataclysmic variables (CVs). Due to the lack of an accretion disk, eclipsing profiles of polars are the best source to study the character of mass transfer in CVs. By analyzing long-term photometric variations in the eclipsing polar HU Aqr, the property of mass transfer and accretion are investigated. The correlation between the brightness state change and the variation of the ingress profile suggests that both the accretion hot spot and the accretion stream are produced instantaneously. The observations clearly show that it is the variation of mass transfer causing the brightness state changes that is a direct evidence of variable mass transfer in a CV. It is shown that it is the local dark-spot activity near the L1 point to cause the change of the mass transfer rather than the activity cycles of the cool secondary star. Our results suggest that the evolution of CVs is more complex than that predicted by the standard model and we should consider the effect of variable mass accretion in nova and dwarf nova outbursts.
Highlights
Cataclysmic variables (CVs) are semi-detached binaries where a low-mass component star transfers mass to a white dwarf (e.g., Warner 1995)
The observations clearly show that it is the variation of mass transfer causing the brightness state changes that is a direct evidence of variable mass transfer in a cataclysmic variables (CVs)
Our results suggest that the evolution of CVs is more complex than that predicted by the standard model and we should consider the effect of variable mass accretion in nova and dwarf nova outbursts
Summary
Cataclysmic variables (CVs) are semi-detached binaries where a low-mass (spectral types of K and M) component star transfers mass to a white dwarf (e.g., Warner 1995). CVs as a mass transferring binary, the AML should keep the secondary filling the critical Roche lobe and transferring mass to the white dwarf continuously. Mass transfer and accretion in CVs are very important to understand their evolution and observational properties (e.g., the novae and dwarf-novae outbursts). Light components from the accretion stream and spot on the white dwarf can be isolated. The eclipse times of eclipsing polars can be determined with a high precision because the accretion stream and the hot spot on the white dwarf can be isolated during the eclipses. Very smallamplitude cyclic changes in the observed-calculated (OC) diagram can be detected They are good targets to search for planets and brown dwarfs orbiting CVs by analyzing the light travel time effect. Some examples discovered by our group are those orbiting the detached white-dwarf binaries QS Vir and RR Cae (Qian et al 2010b, 2012a) and the subdwarf B-type binary NY Vir (Qian et al 2012b)
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