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Register a new userThe effective temperature -- radius relationship of M-dwarfs
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M-dwarf stars provide very favourable conditions to find habitable worlds beyond our solar system. The estimation of the fundamental parameters of the transiting exoplanets rely on the accuracy of the theoretical predictions for radius and effective temperature of the host M-dwarf, hence the importance of multiple empirical tests of very low-mass star (VLM) models, the theoretical counterpart of M-dwarfs. Recent determinations of mass, radius and effective temperature of a sample of M-dwarfs of known metallicity have disclosed a supposed discontinuity in the effective temperature-radius diagram corresponding to a stellar mass of about 0.2Mo, that has been ascribed to the transition from partially convective to fully convective stars. In this paper we compare existing VLM models to these observations, and find that theory does not predict any discontinuity at around 0.2Mo, rather a smooth change of slope of the effective temperature-radius relationship around this mass value. The appearance of a discontinuity 5is due to naively fitting the empirical data with linear segments. Also, its origin is unrelated to the transition to fully convective structures. We find that this feature is instead an empirical signature for the transition to a regime where electron degeneracy provides an important contribution to the stellar EOS, and constitutes an additional test of the consistency of the theoretical framework for VLM models.
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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.
Precise and accurate parameters for late-type (late K and M) dwarf stars are important for characterization of any orbiting planets, but such determinations have been hampered by these stars complex spectra and dissimilarity to the Sun. We exploit an empirically calibrated method to estimate spectroscopic effective temperature ($T_{rm{eff}}$) and the Stefan-Boltzmann law to determine radii of 183 nearby K7-M7 single stars with a precision of 2-5%. Our improved stellar parameters enable us to develop model-independent relations between $T_{rm{eff}}$ or absolute magnitude and radius, as well as between color and $T_{rm{eff}}$. The derived $T_{rm{eff}}$-radius relation depends strongly on [Fe/H], as predicted by theory. The relation between absolute $K_S$ magnitude and radius can predict radii accurate to $simeq$3%. We derive bolometric corrections to the $VR_CI_CgrizJHK_S$ and Gaia passbands as a function of color, accurate to 1-3%. We confront the reliability of predictions from Dartmouth stellar evolution models using a Markov Chain Monte Carlo to find the values of unobservable model parameters (mass, age) that best reproduce the observed effective temperature and bolometric flux while satisfying constraints on distance and metallicity as Bayesian priors. With the inferred masses we derive a semi-empirical mass-absolute magnitude relation with a scatter of 2% in mass. The best-agreement models over-predict stellar $T_{rm{eff}}$s by an average of 2.2% and under-predict stellar radii by 4.6%, similar to differences with values from low-mass eclipsing binaries. These differences are not correlated with metallicity, mass, or indicators of activity, suggesting issues with the underlying model assumptions e.g., opacities or convective mixing length.
We report on 13 new high-precision measurements of stellar diameters for low-mass dwarfs obtained by means of near-infrared long-baseline interferometry with PIONIER at the Very Large Telescope Interferometer. Together with accurate parallaxes from Gaia DR2, these measurements provide precise estimates for their linear radii, effective temperatures, masses, and luminosities. This allows us to refine the effective temperature scale, in particular towards the coolest M-dwarfs. We measure for late-type stars with enhanced metallicity slightly inflated radii, whereas for stars with decreased metallicity we measure smaller radii. We further show that Gaia DR2 effective temperatures for M-dwarfs are underestimated by $sim$ 8.2 % and give an empirical $M_{G}$-$T_{rm eff}$ relation which is better suited for M-dwarfs with $T_{rm eff}$ between 2600 and 4000 K. Most importantly, we are able to observationally identify a discontinuity in the $T_{rm eff}$-radius plane, which is likely due to the transition from partially convective M-dwarfs to the fully convective regime. We found this transition to happen between 3200 K and 3340 K, or equivalently for stars with masses $approx 0.23 M_{odot}$. We find that in this transition region the stellar radii are in the range from 0.18 to 0.42$R_{odot}$ for similar stellar effective temperatures.
Rotation periods from Kepler K2 are combined with projected rotation velocities from the WIYN 3.5-m telescope, to determine projected radii for fast-rotating, low-mass ($0.15 leq M/M_{odot} leq 0.6$) members of the Praesepe cluster. A maximum likelihood analysis that accounts for observational uncertainties, binarity and censored data, yields marginal evidence for radius inflation -- the average radius of these stars is $6pm4$ per cent larger at a given luminosity than predicted by commonly-used evolutionary models. This over-radius is smaller (at 2-sigma confidence) than was found for similar stars in the younger Pleiades using a similar analysis; any decline appears due to changes occurring in higher mass ($>0.25 M_{odot}$) stars. Models incorporating magnetic inhibition of convection predict an over-radius, but do not reproduce this mass dependence unless super-equipartition surface magnetic fields are present at lower masses. Models incorporating flux-blocking by starspots can explain the mass dependence but there is no evidence that spot coverage diminishes between the Pleiades and Praesepe samples to accompany the decline in over-radius. The fastest rotating stars in both Praesepe and the Pleiades are significantly smaller than the slowest rotators for which a projected radius can be measured. This may be a selection effect caused by more efficient angular momentum loss in larger stars leading to their progressive exclusion from the analysed samples. Our analyses assume random spin-axis orientations; any alignment in Praesepe, as suggested by Kovacs (2018), is strongly disfavoured by the broad distribution of projected radii.
The accuracy of theoretical mass, radius and effective temperature values for M-dwarf stars is an active topic of debate. Differences between observed and theoretical values have raised the possibility that current theoretical stellar structure and evolution models are inaccurate towards the low-mass end of the main sequence. To explore this issue we use the CHEOPS satellite to obtain high-precision light curves of eclipsing binaries with low mass stellar companions. We use these light curves combined with the spectroscopic orbit for the solar-type companion to measure the mass, radius and effective temperature of the M-dwarf star. Here we present the analysis of three eclipsing binaries. We use the pycheops data analysis software to fit the observed transit and eclipse events of each system. Two of our systems were also observed by the TESS satellite -- we similarly analyse these light curves for comparison. We find consistent results between CHEOPS and TESS, presenting three stellar radii and two stellar effective temperature values of low-mass stellar objects. These initial results from our on-going observing programme with CHEOPS show that we can expect to have ~24 new mass, radius and effective temperature measurements for very low mass stars within the next few years.
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