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Register a new userDiscovery of a Highly Eccentric, Chromospherically Active Binary: ASASSN-V J192114.84+624950.8
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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.
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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.
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 report and characterize a white-light superflare on a previously undiscovered M dwarf detected by the ASAS-SN survey. Employing various color-magnitude and color-spectral type relationships, we estimate several stellar parameters, including the quiescent V-band magnitude, from which we derive a flare amplitude of $Delta V sim 10$. We determine an r-band absolute magnitude of $M_{r} = 11.4$, consistent with a mid-M dwarf, and an approximate distance to the source of $2.2$ kpc. Using classical-flare models, we infer a flare energy of $E_{V} simeq (4.1pm 2.2)times 10^{36}$ ergs, making this one of the strongest flares documented on an M dwarf.
The eclipsing binary T-Cyg1-12664 was observed both spectroscopically and photometrically. Radial velocities of both components and ground-based VRI light curves were obtained. The Keplers R-data and radial velocities for the system were analysed simultaneously. Masses and radii were obtained as 0.680$pm$0.021 M$_{odot}$ and 0.613$pm$0.007 R$_{odot}$for the primary and 0.341$pm$0.012M$_{odot}$ and 0.897$pm$0.012R$_{odot}$ for the secondary star. The distance to the system was estimated as 127$pm$14 pc. The observed wave-like distortion at out-of-eclipse is modeled with two separate spots on the more massive star, which is also confirmed by the Ca {sc ii} K and H emission lines in its spectra. Locations of the components in the mass-radius and mass-effective temperature planes were compared with the well-determined eclipsing binaries low-mass components as well as with the theoretical models. While the primary stars radius is consistent with the main-sequence stars, the radius of the less massive component appears to be 2.8 times larger than that of the main-sequence models. Comparison of the radii of low-mass stars with the models reveals that the observationally determined radii begin to deviate from the models with a mass of 0.27 Msun and suddenly reaches to maximum deviation at a mass of 0.34 Msun. Then, the deviations begin to decrease up to the solar mass. The maximum deviation seen at a mass of about 0.34 Msun is very close to the mass of fully convective stars as suggested by theoretical studies. A third star in the direction of the eclipsing pair has been detected from our VRI images. The observed infrared excess of the binary is most probably arisen from this star which may be radiated mostly in the infrared bands.
The analysis of eclipsing binaries containing non-radial pulsators allows: i) to combine two different and independent sources of information on the internal structure and evolutionary status of the components, and ii) to study the effects of tidal forces on pulsations. KIC 3858884 is a bright Kepler target whose light curve shows deep eclipses, complex pulsation patterns with pulsation frequencies typical of {delta} Sct, and a highly eccentric orbit. We present the result of the analysis of Kepler photometry and of high resolution phaseresolved spectroscopy. Spectroscopy yielded both the radial velocity curves and, after spectral disentangling, the primary component effective temperature and metallicity, and line-of-sight projected rotational velocities. The Kepler light curve was analyzed with an iterative procedure devised to disentangle eclipses from pulsations which takes into account the visibility of the pulsating star during eclipses. The search for the best set of binary parameters was performed combining the synthetic light curve models with a genetic minimization algorithm, which yielded a robust and accurate determination of the system parameters. The binary components have very similar masses (1.88 and 1.86 Msun) and effective temperatures (6800 and 6600 K), but different radii (3.45 and 3.05 Rsun). The comparison with the theoretical models evidenced a somewhat different evolutionary status of the components and the need of introducing overshooting in the models. The pulsation analysis indicates a hybrid nature of the pulsating (secondary) component, the corresponding high order g-modes might be excited by an intrinsic mechanism or by tidal forces.
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