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Register a new userGeneral Model for Light curves of Chromospherically Active Binary Stars
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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.
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New high-resolution spectra, of the chromospherically active binary system CF Tuc, taken at the Mt. John University Observatory in 2007, were analyzed using two methods: cross-correlation and Fourier--based disentangling. As a result, new radial velocity curves of both components were obtained. The resulting orbital elements of CF Tuc are: $a_{1}{sin}i$=$0.0254pm0.0001$ AU, $a_{2}{sin}i$=$0.0228pm0.0001$ AU, $M_{1}{sin}i$=$0.902pm0.005$ $M_{odot}$, and $M_{2}{sin}i$=$1.008pm0.006$ $M_{odot}$. The cooler component of the system shows H$alpha$ and CaII H & K emissions. Our spectroscopic data and recent $BV$ light curves were solved simultaneously using the Wilson-Devinney code. A dark spot on the surface of the cooler component was assumed to explain large asymmetries observed in the light curves. The following absolute parameters of the components were determined: $M_{1}$=$1.11pm0.01$ $M_{odot}$, $M_{2}$=$1.23pm0.01$ $M_{odot}$, $R_{1}$=$1.63pm0.02$ $R_{odot}$, $R_{2}$=$3.60pm0.02$ $R_{odot}$, $L_{1}$=$3.32pm0.51$ $L_{odot}$ and $L_{2}$=$3.91pm0.84$ $L_{odot}$. The orbital period of the system was studied using the O-C analysis. The O-C diagram could be interpreted in terms of either two abrupt changes or a quasi-sinusoidal form superimposed on a downward parabola. These variations are discussed by reference to the combined effect of mass transfer and mass loss, the Applegate mechanism and also a light-time effect due to the existence of a massive third body (possibly a black hole) in the system. The distance to CF Tuc was calculated to be $89pm6$ pc from the dynamic parallax, neglecting interstellar absorption, in agreement with the Hipparcos value.
RAVE, the unbiased magnitude limited survey of the southern sky stars, contained 456,676 medium-resolution spectra at the time of our analysis. Spectra cover the CaII IRT range which is a known indicator of chromospheric activity. Our previous work (Matijeviv{c} et al. 2012) classified all spectra using locally linear embedding. It identified 53,347 cases with a suggested emission component in calcium lines. Here we use a spectral subtraction technique to measure the properties of this emission. Synthetic templates are replaced by the observed spectra of non-active stars to bypass the difficult computations of non-LTE profiles of the line cores and stellar parameter dependence. We derive both the equivalent width of the excess emission for each calcium line on a 5AA wide interval and their sum EW_IRT for ~44,000 candidate active dwarf stars with S/N>20 and with no respect to the source of their emission flux. From these ~14,000 show a detectable chromospheric flux with at least 2sigma confidence level. Our set of active stars vastly enlarges previously known samples. Atmospheric parameters and in some cases radial velocities of active stars derived from automatic pipeline suffer from systematic shifts due to their shallower calcium lines. We re-estimate the effective temperature, metallicity and radial velocities for candidate active stars. The overall distribution of activity levels shows a bimodal shape, with the first peak coinciding with non-active stars and the second with the pre main-sequence cases. The catalogue will be publicly available with the next RAVE public data releases.
As part of an All-Sky Automated Survey for SuperNovae (ASAS-SN) search for sources with large flux decrements, we discovered a transient where the quiescent, stellar source, ASASSN-V J192114.84+624950.8, rapidly decreased in flux by $sim55%$ ($sim0.9$ mag) in the g-band. The textit{TESS} light curve revealed that the source is a highly eccentric, eclipsing binary. Fits to the light curve using textsc{phoebe} find the binary orbit to have $e=0.79$, $P_{rm orb}=18.462~text{days}$, and $i=88.6^{circ}$ and the ratios of the stellar radii and temperatures to be $R_2/R_1 = 0.71$ and $T_{e,2}/T_{e,1} = 0.82$. Both stars are chromospherically active, allowing us to determine their rotational periods of $P_1=1.52$ days and $P_2=1.79$ days, respectively. A LBT/MODS spectrum shows that the primary is a late-G or early-K type dwarf. Fits to the SED show that the luminosities and temperatures of the two stars are $L_1 = 0.48~L_{sun}$, $T_1= 5050~K$, $L_2 = 0.12~L_{sun}$, and $T_{2} = 4190~K$. We conclude that ASASSN-V J192114.84+624950.8 consists of two chromospherically active, rotational variable stars in a highly elliptical eclipsing orbit.
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.
This paper describes a multiwavelengh optical study of chromospheres in two X-ray/EUV selected active binary stars with strong H_alpha emission, V789 Mon (2RE J0725-002) and GZ Leo (2RE J1101+223). The goal of the study is to determine radial velocities and fundamental stellar parameters in chromospherically active binary systems in order to include them in the activity-rotation and activity-age relations. We carried out high resolution echelle spectroscopic observations and applied spectral subtraction technique in order to measure emission excesses due to chromosphere. The detailed study of activity indicators allowed us to characterize the presence of different chromospheric features in these systems and enabled to include them in a larger activity-rotation survey. We computed radial velocities of the systems using cross correlation with the radial velocity standards. The double-line spectral binarity was confirmed and the orbital solutions improved for both systems. In addition, other stellar parameters such as: spectral types, projected rotational velocities (vsini), and the equivalent width of the lithium LiI 6707.8 AA absorption line were determined.
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