The scatter of the M dwarf mass–radius relationship

Abstract

M dwarfs are prime targets in the hunt for habitable worlds around other stars. This is due to their abundance as well as their small radii and low masses and temperatures, which facilitate the detection of temperate, rocky planets in orbit around them. However, the fundamental properties of M dwarfs are difficult to constrain, often limiting our ability to characterise the planets they host. Here we test several theoretical relationships for M dwarfs by measuring 23 high precision, model-independent masses and radii for M dwarfs in binaries with white dwarfs. We find a large scatter in the radii of these low-mass stars, with 25 per cent having radii consistent with theoretical models while the rest are up to 12 per cent over-inflated. This scatter is seen in both partially-and fully-convective M dwarfs. No clear trend is seen between the over-inflation and age or metallicity, but there are indications that the radii of slowly rotating M dwarfs are more consistent with predictions, albeit with a similar amount of scatter in the measurements compared to more rapidly rotating M dwarfs. The sample of M dwarfs in close binaries with white dwarfs appears indistinguishable from other M dwarf samples, implying that common envelope evolution has a negligible impact on their structure. We conclude that theoretical and empirical mass-radius relationships lack the precision and accuracy required to measure the fundamental parameters of M dwarfs well enough to determine the internal structure and bulk composition of the planets they host.

WD1032 + 011, an inflated brown dwarf in an old eclipsing binary with a white dwarf

“…Both of these objects have large uncertainties on their radii, but also orbit active stars which may have had some effect on the measurement of the radius of the brown dwarf. Parsons et al (2018) found that the scatter in the Mdwarf Mass-Radius relationship was 6.2±4.8 per cent, with only about a quarter of M dwarfs being consistent with models. They determined that there was no trend with either age or metallicity as to which M dwarfs are inflated.…”

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

“…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.…”

“…We compare the effective temperatures for both stars derived in §3.2 to the known temperatures of the sample of M-Dwarfs listed in Parsons et al (2018). This is shown in Figure 12, along with theoretical tracks (Baraffe et al 2015) for stars of this mass range.…”

“…This increased and variable activity may have some effect on the size of the stars in the binary, contributing to a discrepancy between models and measured parameters. However there are very few known M-Dwarfs in eccentric orbits, Parsons et al (2018) list only 15 with e > 0.1, only three of which are double M-Dwarf systems. With so few of these types of systems known, it is difficult to quantify the significance of this effect, if any.…”

Accurate mass and radius determinations of a cool subdwarf in an eclipsing binary

“…SDSS J2355+0448 was originally identified as a white dwarf candidate15 and later re-discovered as part of our systematic search of white dwarf plus low-mass main sequence binaries within the SDSS and LAMOST spectroscopic data bases16, 17 (see the optical spectrum in Figure 1). Eclipsing binaries offer the opportunity to measure directly the masses and radii of the two components with unprecedented precision16, 19. This is the case for the cool subdwarf in SDSS J2355+0448.…”