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Register a new userThe Mass-Radius Relationship for Very Low Mass Stars: Four New Discoveries from the HATSouth Survey
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We report the discovery of four transiting F-M binary systems with companions between 0.1-0.2 Msun in mass by the HATSouth survey. These systems have been characterised via a global analysis of the HATSouth discovery data, combined with high-resolution radial velocities and accurate transit photometry observations. We determined the masses and radii of the component stars using a combination of two methods: isochrone fitting of spectroscopic primary star parameters, and equating spectroscopic primary star rotation velocity with spin-orbit synchronisation. These new very low mass companions are HATS550-016B (0.110 -0.006/+0.005 Msun, 0.147 -0.004/+0.003 Rsun), HATS551-019B (0.17 -0.01/+0.01 Msun, 0.18 -0.01/+0.01 Rsun), HATS551-021B (0.132 -0.005/+0.014 Msun, 0.154 -0.008/+0.006 Rsun), HATS553-001B (0.20 -0.02/+0.01 Msun, 0.22 -0.01/+0.01 Rsun). We examine our sample in the context of the radius anomaly for fully-convective low mass stars. Combining our sample with the 13 other well-studied very low mass stars, we find a tentative 5% systematic deviation between the measured radii and theoretical isochrone models.
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We measured the radii of 7 low and very low-mass stars using long baseline interferometry with the VLTI interferometer and its VINCI and AMBER near-infrared recombiners. We use these new data, together with literature measurements, to examine the luminosity- radius and mass-radius relations for K and M dwarfs. The precision of the new interferometric radii now competes with what can be obtained for double-lined eclipsing binaries. Interferometry provides access to much less active stars, as well as to stars with much better measured distances and luminosities, and therefore complements the information obtained from eclipsing systems. The radii of magnetically quiet late-K to M dwarfs match the predictions of stellar evolution models very well, providing direct confirmation that magnetic activity explains the discrepancy that was recently found for magnetically active eclipsing systems. The radii of the early K dwarfs are well reproduced for a mixing length parameter that approaches the solar value, as qualitatively expected.
The fundamental properties of low-mass stars are not as well understood as those of their more massive counterparts. The best method for constraining these properties, especially masses and radii, is to study eclipsing binary systems, but only a small number of late-type (M0 or later) systems have been identified and well-characterized to date. We present the discovery and characterization of six new M dwarf eclipsing binary systems. The twelve stars in these eclipsing systems have masses spanning 0.38-0.59 Msun and orbital periods of 0.6--1.7 days, with typical uncertainties of ~0.3% in mass and 0.5--2.0% in radius. Combined with six known systems with high-precision measurements, our results reveal an intriguing trend in the low-mass regime. For stars with M=0.35-0.80 Msun, components in short-period binary systems (P<1 day; 12 stars) have radii which are inflated by up to 10% (mean=4.8+/-1.0%) with respect to evolutionary models for low-mass main-sequence stars, whereas components in longer-period systems (>1.5 days; 12 stars) tend to have smaller radii (mean=1.7+/-0.7%). This trend supports the hypothesis that short-period systems are inflated by the influence of the close companion, most likely because they are tidally locked into very high rotation speeds that enhance activity and inhibit convection. In summary, very close binary systems are not representative of typical M dwarfs, but our results for longer-period systems indicate that the evolutionary models are broadly valid in the M~0.35-0.80 Msun regime.
We present the results of a precise near-infrared (NIR) radial velocity (RV) survey of 32 low-mass stars with spectral types K2-M4 using CSHELL at the NASA IRTF in the $K$-band with an isotopologue methane gas cell to achieve wavelength calibration and a novel iterative RV extraction method. We surveyed 14 members of young ($approx$ 25-150 Myr) moving groups, the young field star $varepsilon$ Eridani as well as 18 nearby ($<$ 25 pc) low-mass stars and achieved typical single-measurement precisions of 8-15 m s$^{-1}$ with a long-term stability of 15-50 m s$^{-1}$. We obtain the best NIR RV constraints to date on 27 targets in our sample, 19 of which were never followed by high-precision RV surveys. Our results indicate that very active stars can display long-term RV variations as low as $sim$ 25-50 m s$^{-1}$ at $approx$ 2.3125 $mu$m, thus constraining the effect of jitter at these wavelengths. We provide the first multi-wavelength confirmation of GJ 876 bc and independently retrieve orbital parameters consistent with previous studies. We recovered RV variability for HD 160934 AB and GJ 725 AB that are consistent with their known binary orbits, and nine other targets are candidate RV variables with a statistical significance of 3-5$sigma$. Our method combined with the new iSHELL spectrograph will yield long-term RV precisions of $lesssim$ 5 m s$^{-1}$ in the NIR, which will allow the detection of Super-Earths near the habitable zone of mid-M dwarfs.
Measurements of the physical properties of stars at the lower end of the main sequence are scarce. In this context we report masses, radii and surface gravities of ten very-low-mass stars in eclipsing binary systems, with orbital periods of the order of several days. The objects probe the stellar mass-radius relation in the fully convective regime, $M_star lesssim 0.35$ M$_odot$, down to the hydrogen burning mass-limit, $M_{mathrm{HB}} sim 0.07$ M$_odot$. The stars were detected by the WASP survey for transiting extra-solar planets, as low-mass, eclipsing companions orbiting more massive, F- and G-type host stars. We use eclipse observations of the host stars (TRAPPIST, Leonhard Euler, SPECULOOS telescopes), and radial velocities of the host stars (CORALIE spectrograph), to determine physical properties of the low-mass companions. Companion surface gravities are derived from the eclipse and orbital parameters of each system. Spectroscopic measurements of the host star effective temperature and metallicity are used to infer the host star mass and age from stellar evolution models. Masses and radii of the low-mass companions are then derived from the eclipse and orbital parameters of each system. The objects are compared to stellar evolution models for low-mass stars, to test for an effect of the stellar metallicity and orbital period on the radius of low-mass stars in close binary systems. Measurements are in good agreement with stellar models; an inflation of the radii of low-mass stars with respect to model predictions is limited to 1.6 $pm$ 1.2% in the fully convective regime. The sample of ten objects indicates a scaling of the radius of low-mass stars with the host star metallicity. No correlation between stellar radii and orbital periods of the binary systems is determined. A combined analysis with comparable objects from the literature is consistent with this result.
Very-low-mass stars can develop secularly unstable hydrogen-burning shells late in their life. Since the thermal pulses that go along are driven at the bottoms of very shallow envelopes, the stars luminosities and effective temperatures react strongly during a pulse cycle. Towards the end of the Galaxys stelliferous era, the hydrogen-shell flashing very-low-mass single stars should inflict an intricate light-show performed by the large population of previously inconspicuous dim stars. Unfortunately, this natural spectacle will discharge too late for mankind to indulge in. Not all is hopeless, though: In the case of close binary-star evolution, hydrogen-shell flashes of mass-stripped, very-low mass binary components can develop in a fraction of a Hubble time. Therefore, the Galaxy should be able put forth a few candidates that are going to evolve through a H-shell flash in a humanity-compatible time frame.
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