The EBLM Project

Abstract

We present 2271 radial velocity measurements taken on 118 single-line binary stars, taken over eight years with the CORALIE spectrograph. The binaries consist of F/G/K primaries and M dwarf secondaries. They were initially discovered photometrically by the WASP planet survey, as their shallow eclipses mimic a hot Jupiter transit. The observations we present permit a precise characterisation of the binary orbital elements and mass function. With modelling of the primary star, this mass function is converted to a mass of the secondary star. In the future, this spectroscopic work will be combined with precise photometric eclipses to draw an empirical mass/radius relation for the bottom of the mass sequence. This has applications in both stellar astrophysics and the growing number of exoplanet surveys around M dwarfs. In particular, we have discovered 34 systems with a secondary mass below 0.2 M , and so we will ultimately double the number of known very low-mass stars with well-characterised masses and radii. The quality of our data combined with the amplitude of the Doppler variations mean that we are able to detect eccentricities as small as 0.001 and orbital periods to sub-second precision. Our sample can revisit some earlier work on the tidal evolution of close binaries, extending it to low mass ratios. We find some exceptional binary systems that are eccentric at orbital periods below three days, while our longest circular orbit has a period of 10.4 days. Amongst our systems, we note one remarkable architecture in J1146-42 that boasts three stars within one astronomical unit. By collating the EBLM binaries with published WASP planets and brown dwarfs, we derive a mass spectrum with twice the resolution of previous work. We compare the WASP/EBLM sample of tightly bound orbits with work in the literature on more distant companions up to 10 AU. We note that the brown dwarf desert appears wider, as it carves into the planetary domain for our short-period orbits. This would mean that a significantly reduced abundance of planets begins at ∼3 M Jup , well before the deuterium-burning limit. This may shed light on the formation and migration history of massive gas giants.

Turbulent Viscosity Acting on the Equilibrium Tidal Flow in Convective Stars

“…This mechanism is usually invoked to explain the circularization and synchronization of binary systems containing low-mass or solar-like main-sequence stars (e.g. Zahn 1989;Meibom & Mathieu 2005;Meibom et al 2006;Van Eylen et al 2016;Lurie et al 2017;Triaud et al 2017;von Boetticher et al 2019), and evolved stars (e.g. Verbunt & Phinney 1995;Beck et al 2018;Price-Whelan & Goodman 2018).…”

Circumbinary planets -- the next steps

“…A unique aspect of this discovery, compared with the Kepler discoveries, is that the binary was already known and well characterised as a part of the EBLM (Triaud et al, 2017) and BEBOP (Martin et al, 2019) radial velocity surveys, and those measurements were vital to the planet's characterisation. TESS is unlikely to significantly break into new parameter spaces of circumbinary planets, due to the shortened observational timespans and inferior photometric precision to Kepler.…”

NGTS J214358.5−380102 – NGTS discovery of the most eccentric known eclipsing M-dwarf binary system

“…There are some examples of eclipsing binaries with similar orbital periods that do exhibit some eccentricity. Triaud et al (2017) obtained spectroscopic orbits of 100 low mass eclipsing binaries with a range of orbital periods. Figure 13 shows the eccentricity of these binary systems as a function Figure 13.…”

“…Figure 13 shows the eccentricity of these binary systems as a function Figure 13. Plot illustrating the relationship between the semi major axis of the binary and the eccentricity of the binary orbit for a sample of low mass eclipsing binaries described in Triaud et al 2017. NGTS J2143-38 is indicated in blue.…”

“…Whilst there are clearly examples of eccentricity in binary orbits with periods on the order of days, there are no examples of shorter orbital period binaries with greater eccentricity than NGTS J2143-38 . It is also important to note, that the objects in Triaud et al (2017) are F, G and K type stars with M-dwarf secondary companions, and therefore do differ in nature to double M-dwarf binaries like NGTS J2143-38 . Of the double-lined M-Dwarf binaries listed by Parsons et al (2018), none have a greater eccentricity than NGTS J2143-38 , regardless of the period or semi major axis of the systems.…”