Low-mass eclipsing binaries in the WFCAM Transit Survey : The persistence of the M-dwarf radius inflation problem

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We present the characterization of five new short-period low-mass eclipsing binaries (LMEBs) from the WFCAM Transit Survey. The analysis was performed by using the photometric WFCAM J-mag data and additional low- and intermediate-resolution spectroscopic data to obtain both orbital and physical properties of the studied sample. The light curves and the measured radial velocity curves were modelled simultaneously with the JKTEBOP code, with Markov chain MonteCarlo simulations for the error estimates. The best-model fit have revealed that the investigated detached binaries are in very close orbits, with orbital separations of 2.9 ≤ a ≤ 6.7R⊙ and short periods of 0.59 ≤ Porb ≤ 1.72 d, approximately. We have derived stellar masses between 0.24 and 0.72M⊙ and radii ranging from 0.42 to 0.67 R⊙. The great majority of the LMEBs in our sample has an estimated radius far from the predicted values according to evolutionary models. The components with derived masses of M < 0.6M⊙ present a radius inflation of ~9 per cent or more. This general behaviour follows the trend of inflation for partially radiative stars proposed previously. These systems add to the increasing sample of low-mass stellar radii that are not well-reproduced by stellarmodels. They further highlight the need to understand the magnetic activity and physical state of small stars. Missions like TESS will provide many such systems to perform high-precision radius measurements to tightly constrain low-mass stellar evolution models.Peer reviewe

Properties of ultra-cool dwarfs with Gaia. An assessment of the accuracy for the temperature determination

We aimed to assess the accuracy of the Gaia teff and logg estimates as derived with current models and observations. We assessed the validity of several inference techniques for deriving the physical parameters of ultra-cool dwarf stars. We used synthetic spectra derived from ultra-cool dwarf models to construct (train) the regression models. We derived the intrinsic uncertainties of the best inference models and assessed their validity by comparing the estimated parameters with the values derived in the bibliography for a sample of ultra-cool dwarf stars observed from the ground. We estimated the total number of ultra-cool dwarfs per spectral subtype, and obtained values that can be summarised (in orders of magnitude) as 400000 objects in the M5-L0 range, 600 objects between L0 and L5, 30 objects between L5 and T0, and 10 objects between T0 and T8. A bright ultra-cool dwarf (with teff=2500 K and \logg=3.5 will be detected by Gaia out to approximately 220 pc, while for teff=1500 K (spectral type L5) and the same surface gravity, this maximum distance reduces to 10-20 pc. The RMSE of the prediction deduced from ground-based spectra of ultra-cool dwarfs simulated at the Gaia spectral range and resolution, and for a Gaia magnitude G=20 is 213 K and 266 K for the models based on k-nearest neighbours and Gaussian process regression, respectively. These are total errors in the sense that they include the internal and external errors, with the latter caused by the inability of the synthetic spectral models (used for the construction of the regression models) to exactly reproduce the observed spectra, and by the large uncertainties in the current calibrations of spectral types and effective temperatures.Comment: 18 pages, 17 figures, accepted by Astronomy & Astrophysic

The Rotational Evolution of Young, Binary M Dwarfs

We have analysed K2 light curves for more than 3,000 low mass stars in the $\sim$8 Myr old Upper Sco association, the $\sim$125 Myr age Pleiades open cluster and the $\sim$700 Myr old Hyades and Praesepe open clusters to determine stellar rotation rates. Many of these K2 targets show two distinct periods, and for the lowest mass stars in these clusters virtually all of these systems with two periods are photometric binaries. The most likely explanation is that we are detecting the rotation periods for both components of these binaries. We explore the evolution of the rotation rate in both components of photometric binaries relative to one another and to non-photometric binary stars. In Upper Sco and the Pleiades, these low mass binary stars have periods that are much shorter on average and much closer to each other than would be true if drawn at random from the M dwarf single stars. In Upper Sco, this difference correlates strongly with the presence or absence of infrared excesses due to primordial circumstellar disks -- the single star population includes many stars with disks, and their rotation periods are distinctively longer on average than their binary star cousins of the same mass. By Praesepe age, the significance of the difference in rotation rate between the single and binary low mass dMs is much less, suggesting that angular momentum loss from winds for fully-convective zero-age main sequence stars erases memory of the rotation rate dichotomy for binary and single very low mass stars at later ages.Comment: accepted by A

Proper motions of young stars in Chamaeleon. I. A Virtual Observatory study of spectroscopically confirmed members

(abridged) We want to provide further evidence of the origin of the proposed stellar members of Chamaeleon and to identify interlopers from the foreground \epsilon Cha and \eta Cha associations. To this aim, we compile lists of spectroscopically confirmed members of Chamaeleon I and II, \epsilon Cha and \eta Cha, and of background objects in the same line of sight. Using Virtual Observatory tools, we cross-match these lists with the UCAC3 catalogue to get the proper motions of the objects. In the vector point diagram, we identify the different moving groups, and use this information to study the membership of proposed candidate members of the associations from the literature. For those objects with available radial velocities, we compute their Galactic space velocities. We look for correlations between the known properties of the objects and their proper motions. The members of the dark clouds exhibit clearly different proper motions from those of the foreground associations and of the background stars. The data suggest that Chamaeleon II could have different dynamical properties from Chamaeleon I. Although the two foreground clusters \epsilon and \eta Chamaeleontis constitute two different proper motion groups, they have similar spatial motions, which are different from the spatial motion of Chamaeleon I. On the other hand, the space motions of the Chamaeleon II stars look more similar to those of the foreground clusters than to the Chamaeleon I stars, but the numbers are low. Hence, with the available data it is unclear to what extent the stellar populations in both clouds are physically connected to each other. We find no correlations between the proper motions and the properties of the objects in either of the clouds