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Register a new userPeriod changes and four-color light curves of the active overcontact binary v396 monocerotis
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This paper analyzes the first secured four color light curves of V396 Mon using the 2003 version of the WD code. It is confirmed that V396 Mon is a shallow W-type contact binary system with a mass ratio $q=2.554(pm0.004)$ and a degree of contact factor $f=18.9%(pm1.2%)$. A period investigation based on all available data shows that the period of the system includes a long-term decrease ($dP/dt=-8.57times{10^{-8}}$ days/year) and an oscillation ($A_3=0.^{d}0160$; $T_3=42.4,years$). They are caused by angular momentum loss (AML) and light-time effect, respectively. The suspect third body perhaps is a small M-type star (about 0.31 solar mass). Though some proofs show that this system has strong magnetic activity, through analyzing we found that the Applegate mechanism cannot explain the periodic changes. This binary is an especially important system according to Qians statistics of contact binaries as its mass ratio lies near the proposed pivot point about which the physical structure of contact binaries supposedly oscillate.
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Light curves in the B, V, and I_c passbands have been obtained for the type II Cepheids V154 in M3 and V42 and V84 in M5. Alternating cycle behavior, similar to that seen among RV Tauri variables, is confirmed for V84. Old and new observations, spanning more than a century, show that V154 has increased in period while V42 has decreased in period. V84, on the other hand, has shown large, erratic changes in period that do not appear to reflect the long term evolution of V84 through the HR diagram.
The starspots on the surface of many chromospherically active binary stars concentrate on long--lived active longitudes separated by 180 degrees. The activity shifts between these two longitudes, the flip-flop events, have been observed in single stars like FK Comae and binary stars like $sigma$ Geminorum. Recently, interferometry has revealed that ellipticity may at least partly explain the flip-flop events in $sigma$ Geminorum. This idea was supported by the double peaked shape of the long--term mean light curve of this star. Here, we show that the long--term mean light curves of fourteen chromospherically active binaries follow a general model which explains the connection betweenm orbital motion, starspot distribution changes, ellipticity and flip ~events. Surface differential rotation is probably weak in these stars, because the interference of two constant period waves may explain the observed light curve changes. These two constant periods are the active longitude period $(P_{mathrm{act}})$ and the orbital period $(P_{mathrm{orb}})$. We also show how to apply the same model to single stars, where only the value of $P_{mathrm{act}}$ is known. Finally, we present a tentative interference hypothesis about the origin of magnetic fields in all spectral types of stars.
We present Period-Luminosity and Period-Luminosity-Color relations at maximum-light for Mira variables in the Magellanic Clouds using time-series data from the Optical Gravitational Lensing Experiment (OGLE-III) and {it Gaia} data release 2. The maximum-light relations exhibit a scatter typically up to $sim 30%$ smaller than their mean-light counterparts. The apparent magnitudes of Oxygen-rich Miras at maximum-light display significantly smaller cycle-to-cycle variations than at minimum-light. High-precision photometric data for Kepler Mira candidates also exhibit stable magnitude variations at the brightest epochs while their multi-epoch spectra display strong Balmer emission lines and weak molecular absorption at maximum-light. The stability of maximum-light magnitudes for Miras possibly occurs due to the decrease in the sensitivity to molecular bands at their warmest phase. At near-infrared wavelengths, the Period-Luminosity relations of Miras display similar dispersion at mean and maximum-light with limited time-series data in the Magellanic Clouds. A kink in the Oxygen-rich Mira Period-Luminosity relations is found at 300 days in the $VI$-bands which shifts to longer-periods ($sim 350$~days) at near-infrared wavelengths. Oxygen-rich Mira Period-Luminosity relations at maximum-light provide a relative distance modulus, $Delta mu = 0.48pm0.08$~mag, between the Magellanic Clouds with a smaller statistical uncertainty than the mean-light relations. The maximum-light properties of Miras can be very useful for stellar atmosphere modeling and distance scale studies provided their stability and the universality can be established in other stellar environments in the era of extremely large telescopes.
V752 Cen is a triple-lined spectroscopic contact binary. Its multi-color light curves were obtained in the years 1971 and 2018, independently. Photometric analyses reveal that the two sets of light curves produce almost consistent results. It contains a W-subtype totally eclipsing binary, and its mass ratio and fill-out factor are $q = 3.35(1)$ and $f = 29(2),%$. The absolute elements of its two component stars were determined to be $M_{1} = 0.39(2)M_odot$, $M_{2} = 1.31(7)M_odot$, $R_{1} = 0.77(1)R_odot$, $R_{2} = 1.30(2)R_odot$, $L_{1} = 0.75(3)L_odot$ and $L_{2} = 2.00(7)L_odot$. The period of V752 Cen is 0.37023198 day. The 0.37-d period remained constant from its first measurement in 1971 until the year 2000. However, it changed suddenly around the year 2000 and has been increasing continuously at a rate of $dP/dt=+5.05times{10^{-7}}daycdot year^{-1}$ since then, which can be explained by mass transfer from the less massive component star to the more massive one with a rate of $frac{dM_{2}}{dt}=2.52times{10^{-7}}M_odot/year$. The period variation of V752 Cen over the 48 years in which the period has been monitored is really unusual, and is potentially related to effects from the possible presence of a nearby third star or of a pair of stars in a second binary.
Rotational modulation of stellar light curves due to dark spots encloses information on spot properties and, thus, on magnetic activity. In particular, the decay of the autocorrelation function (ACF) of light curves is presumed to be linked to spot/active-region lifetimes, given that some coherence of the signal is expected throughout their lifetime. In the literature, an exponential decay has been adopted to describe the ACF. Here, we investigate the relation between the ACF and the active-region lifetimes. For this purpose, we produce artificial light curves of rotating spotted stars with different observation, stellar, and spot properties. We find that a linear decay and respective timescale better represent the ACF than the exponential decay. We therefore adopt a linear decay. The spot/active-region timescale inferred from the ACF is strongly restricted by the observation length of the light curves. For 1-year light curves our results are consistent with no correlation between the inferred and the input timescales. The ACF decay is also significantly affected by differential rotation and spot evolution: strong differential rotation and fast spot evolution contribute to a more severe underestimation of the active-region lifetimes. Nevertheless, in both circumstances the observed timescale is still correlated with the input lifetimes. Therefore, our analysis suggests that the ACF decay can be used to obtain a lower limit of the active-region lifetimes for relatively long-term observations. However, strategies to avoid or flag targets with fast active-region evolution or displaying stable beating patterns associated with differential rotation should be employed.
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