Monday, July 22, 2013

1307.5135 (Zhibin Dai et al.)

The updated photometries and orbital period analysis for polar AM Herculis on the upper edge of period gap    [PDF]

Zhibin Dai, Shengbang Qian, Linjia Li
All 21 new optical light curves including 5 ones obtained in photometries of 2009 and 16 ones detected from the AAVSO covering from 1977 to 2011 demonstrate 16 new primary light minimum times in high state, and 7 new-found low-state transient events covering from 2006 to 2009 consisting of 5 gaussian shape events and 2 events in exponential form with decay timescales of $<0^{\rm d}.005$, which is one order of magnitude less than the previous X-ray flare event. In the state transition, the two special events that "disrupted event" with an amplitude of $\sim2^{\rm m}$ and a duration of $\sim72$ min and R-band continuing twin events larger than all known R-band flares in M-type red dwarfs are detected. All 45 available high-state data covering over 35 years construct an updated O-C diagram of \objectname{AM Herculis}, which shows significant sine-like variation with a period of 12-15 yr and an amplitude of 6-9 min clearly. Using the inspected physical parameters of the donor star, the secular variation in O-C diagram cannot be interpreted by any decided angular momentum loss mechanism, but can satisfy the condition $\tau_{\dot{\rm M}_{2}}\simeq\tau_{\rm KH}>>\tau_{\dot{\rm R}_{\rm 2}}$, which is required by numerical calculations of the secular evolution of CVs. In order to explain the prominent periodic modulation, three plausible mechanisms that spot motion, light travel-time effect and magnetic active cycles, are discussed in detail.
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