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Register a new userA highly unequal-mass eclipsing M-dwarf binary in the WFCAM Transit Survey
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In this paper we present the discovery of a highly unequal-mass eclipsing M-dwarf binary, providing a unique constraint on binary star formation theory and on evolutionary models for low-mass binary stars. The binary is discovered using high- precision infrared light curves from the WFCAM Transit Survey (WTS) and has an orbital period of 2.44 d. We find stellar masses of M1 = 0.53 (0.02) Msun and M2 = 0.143 (0.006) Msun (mass ratio 0.27), and radii of R1 = 0.51 (0.01) Rsun and R2 = 0.174 (0.006) Rsun. This puts the companion in a very sparsely sampled and important late M-dwarf mass-regime. Since both stars share the same age and metallicity and straddle the theoretical boundary between fully and partially convective stellar interiors, a comparison can be made to model predictions over a large range of M-dwarf masses using the same model isochrone. Both stars appear to have a slightly inflated radius compared to 1 Gyr model predictions for their masses, but future work is needed to properly account for the effects of star spots on the light curve solution. A significant, subsynchronous, ~2.56 d signal with ~2% peak-to-peak amplitude is detected in the WFCAM light curve, which we attribute to rotational modulation of cool star spots. We propose that the subsynchronous rotation is either due to a stable star-spot complex at high latitude on the (magnetically active) primary (i.e. differential rotation), or to additional magnetic braking, or to interaction of the binary with a third body or circumbinary disk during its pre-main-sequence phase.
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We report on the discovery of four ultra-short period (P<0.18 days) eclipsing M-dwarf binaries in the WFCAM Transit Survey. Their orbital periods are significantly shorter than of any other known main-sequence binary system, and are all significantly below the sharp period cut-off at P~0.22 days as seen in binaries of earlier type stars. The shortest-period binary consists of two M4 type stars in a P=0.112 day orbit. The binaries are discovered as part of an extensive search for short-period eclipsing systems in over 260,000 stellar lightcurves, including over 10,000 M-dwarfs down to J=18 mag, yielding 25 binaries with P<0.23 days. In a popular paradigm, the evolution of short period binaries of cool main-sequence stars is driven by loss of angular momentum through magnetised winds. In this scheme, the observed P~0.22 day period cut-off is explained as being due to timescales that are too long for lower-mass binaries to decay into tighter orbits. Our discovery of low-mass binaries with significantly shorter orbits implies that either these timescales have been overestimated for M-dwarfs, e.g. due to a higher effective magnetic activity, or that the mechanism for forming these tight M-dwarf binaries is different from that of earlier type main-sequence stars.
We announce the discovery of a new eclipsing hot subdwarf B + M dwarf binary, EC 10246-2707, and present multi-colour photometric and spectroscopic observations of this system. Similar to other HW Vir-type binaries, the light curve shows both primary and secondary eclipses, along with a strong reflection effect from the M dwarf; no intrinsic light contribution is detected from the cool companion. The orbital period is 0.1185079936 +/- 0.0000000009 days, or about three hours. Analysis of our time-series spectroscopy reveals a velocity semi-amplitude of K_1 = 71.6 +/- 1.7 km/s for the sdB and best-fitting atmospheric parameters of Teff = 28900 +/- 500 K, log g = 5.64 +/- 0.06, and log[N(He)/N(H)] = -2.5 +/- 0.2. Although we cannot claim a unique solution from modeling the light curve, the best-fitting model has an sdB mass of 0.45 Msun and a cool companion mass of 0.12 Msun. These results are roughly consistent with a canonical-mass sdB and M dwarf separated by a ~ 0.84 Rsun. We find no evidence of pulsations in the light curve and limit the amplitude of rapid photometric oscillations to < 0.08%. Using 15 years of eclipse timings, we construct an O-C diagram but find no statistically significant period changes; we rule out |P-dot| > 7.2 x 10^(-12). If EC 10246-2707 evolves into a cataclysmic variable, its period should fall below the famous CV period gap.
YY Gem is a short-period eclipsing binary system containing two nearly identical, rapidly rotating, very active early-M dwarfs. This binary represents an important benchmark system for calibrating empirical relations between fundamental properties of low-mass stars and for testing theories of interior structure and evolution of these objects. Both components of YY Gem exhibit inflated radii, which has been attributed to poorly understood magnetic activity effects. Despite a long history of magnetic activity studies of this system no direct magnetic field measurements have been made for it. Here we present a comprehensive characterisation of the surface magnetic field in both components of YY Gem. We reconstructed the global field topologies with the help of a tomographic inversion technique applied to high-resolution spectropolarimetric data. This analysis revealed moderately complex global fields with a typical strength of 200-300 G and anti-aligned dipolar components. A complementary Zeeman intensification analysis of the disentangled intensity spectra showed that the total mean field strength reaches 3.2-3.4 kG in both components of YY Gem. We used these results together with other recent magnetic field measurements of M dwarfs to investigate the relation between the global and small-scale fields in these stars. We also assessed predictions of competing magnetoconvection interior structure models developed for YY Gem, finding that only one of them anticipated the surface field strength compatible with our observations. Results of our star spot mapping of YY Gem do not support the alternative family of theoretical stellar models which attempts to explain the radii inflation by postulating a large spot filling factor.
We report the discovery of KELT J041621-620046, a moderately bright (J$sim$10.2) M dwarf eclipsing binary system at a distance of 39$pm$3 pc. KELT J041621-620046 was first identified as an eclipsing binary using observations from the Kilodegree Extremely Little Telescope (KELT) survey. The system has a short orbital period of $sim$1.11 days and consists of components with M$_1$ = $0.447^{-0.047}_{+0.052},M_odot$ and M$_2$ = $0.399^{-0.042}_{+0.046},M_odot$ in nearly circular orbits. The radii of the two stars are R$_1$ = $0.540^{-0.032}_{+0.034},R_odot$ and R$_2$ = $0.453pm0.017,R_odot$. Full system and orbital properties were determined (to $sim$10% error) by conducting an EBOP global modeling of the high precision photometric and spectroscopic observations obtained by the KELT Follow-up Network. Each star is larger by 17-28% and cooler by 4-10% than predicted by standard (non-magnetic) stellar models. Strong H$alpha$ emission indicates chromospheric activity in both stars. The observed radii and temperature discrepancies for both components are more consistent with those predicted by empirical relations that account for convective suppression due to magnetic activity.
We observed spectroscopically the eclipsing binary system T-Cyg1-01385 in order to determine physical properties of the components. The double-lined nature of the system is revealed for the first time and the radial velocities are obtained for both stars. We have derived masses, radii and luminosities for both components. Analyses of the radial velocities and the KeplerCam and the T$r$ES light curves yielded masses of M$_1$=1.059$pm$0.032 Msun ~and M$_2$=0.342$pm$0.017 {Msun} and radii of R$_1$=1.989$pm$0.022 {Rsun} and R$_2$=0.457$pm$0.013 {Rsun}. Locations of the low-mass companion in the mass-radius and mass-effective temperature planes and comparison with the other low-mass stars show that the secondary star appears just at the transition from partially to fully convective interiors for the M dwarfs. When compared to stellar evolution models, the luminosities and effective temperatures of the components are consistent with Z=0.004 and an age of about 6 Gyr. A distance to the system was calculated as d=355$pm$7 pc using the BV and JHK magnitudes.
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