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Register a new userAnalysis of a Kepler Light Curve of the Novalike Cataclysmic Variable KIC 8751494
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Authors
Taichi Kato
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We analyzed a Kepler light curve of KIC 8751494, a recently recognized novalike cataclysmic variable in the Kepler field. We detected a stable periodicity of 0.114379(1) d, which we identified as being the binarys orbital period. The stronger photometric period around 0.12245 d, which had been detected from the ground-based observation, was found to be variable, and we identified this period as being the positive superhump period. This superhump period showed short-term (10-20 d) and strong variations in period most unexpectedly when the object entered a slightly faint state. The fractional superhump excess varied as large as ~30%. The variation of the period very well traced the variation of the brightness of the system. The time-scales of this variation of the superhump period was too slow to be interpreted as the variation caused by the change in the disk radius due to the thermal disk instability. We interpreted that the period variation was caused by the varying pressure effect on the period of positive superhumps. This finding suggests that the pressure effect, in at least novalike systems, plays a very important (up to ~30% in the precession rate) role in producing the period of the positive superhumps. We also described a possible detection of the negative superhumps with a varying period of 0.1071-0.1081 d in the Q14 run of the Kepler data. We also found that the phase of the velocity variation of the emission lines reported in the earlier study is compatible with the SW Sex-type classification. Further, we introduced a new two-dimentional period analysis using least absolute shrinkage and selection operator (Lasso) and showed superior advantage of this method.
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The ratios $r_{01}$ and $r_{10}$ of small to large separations of KIC 2837475 primarily exhibit an increase behavior in the observed frequency range. The calculations indicate that only the models with overshooting parameter $delta_{rm ov}$ between approximately 1.2 and 1.6 can reproduce the observed ratios $r_{01}$ and $r_{10}$ of KIC 2837475. The ratios $r_{01}$ and $r_{10}$ of the frequency separations of p-modes with inner turning points that are located in the overshooting region of convective core can exhibit an increase behavior. The frequencies of the modes that can reach the overshooting region decrease with the increase in $delta_{rm ov}$. Thus the ratio distributions are more sensitive to $delta_{rm ov}$ than to other parameters. The increase behavior of the KIC 2837475 ratios results from a direct effect of the overshooting of convective core. The characteristic of the ratios provides a strict constraint on stellar models. Observational constraints point to a star with $M=1.490pm0.018$ $M_{odot}$, $R=1.67pm0.01$ $R_{odot}$, age $=2.8pm0.4$ Gyr, and $1.2lesssim$ $delta_{rm ov}$ $lesssim1.6$ for KIC 2837475.
Novalike cataclysmic variables have persistently high mass transfer rates and prominent steady state accretion disks. We present an analysis of infrared observations of twelve novalikes obtained from the Two Micron All Sky Survey, the Spitzer Space Telescope, and the Wide-field Infrared Survey Explorer All Sky Survey. The presence of an infrared excess at >3-5 microns over the expectation of a theoretical steady state accretion disk is ubiquitous in our sample. The strength of the infrared excess is not correlated with orbital period, but shows a statistically significant correlation (but shallow trend) with system inclination that might be partially (but not completely) linked to the increasing view of the cooler outer accretion disk and disk rim at higher inclinations. We discuss the possible origin of the infrared excess in terms of emission from bremsstrahlung or circumbinary dust, with either mechanism facilitated by the mass outflows (e.g., disk wind/corona, accretion stream overflow, and so on) present in novalikes. Our comparison of the relative advantages and disadvantages of either mechanism for explaining the observations suggests that the situation is rather ambiguous, largely circumstantial, and in need of stricter observational constraints.
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