The stellar rotation–activity relationship in fully convective M dwarfs

Abstract

The coronal activity-rotation relationship is considered to be a proxy for the underlying stellar dynamo responsible for magnetic activity in solar and late-type stars. While this has been studied in considerable detail for partly-convective stars that are believed to operate an interface dynamo, it is poorly unconstrained in fully-convective stars that lack the necessary shear layer between radiative core and the convective envelope. We present new X-ray observations of 19 slowly-rotating fully-convective stars with rotation periods from the MEarth Project. We use these to calculate X-ray luminosities (or upper limits for undetected sources) and combine these with existing measurements from Wright & Drake (2016). We confirm the existence of fullyconvective stars in the X-ray unsaturated regime and find that these objects follow the same rotation-activity relationship seen for partly-convective stars. We measure the power-law slope of the relationship between Rossby number (the ratio of the rotation period to the convective turnover time) and the fractional X-ray luminosity for X-ray unsaturated fully-convective stars for the first time, and find it to be consistent with that found for partly-convective stars. We discuss this implications of this result for our understanding of stellar magnetic dynamos in fully-and partly-convective stars. Finally, we also use this data to improve empirical estimates of the convective turnover time for fully-convective stars.

Exploring the M-dwarf Luminosity–Temperature–Radius relationships using Gaia DR2

“…The sample is divided into stars that lie in the saturated (triangles) and unsaturated (circles) regimes. This transitions is marked by a grey dashed line drawn at L X /L bol 3 × 10 −4 , which corresponds to the lower limit from Wright et al (2018). The left pane shows a spurious correlation in our sample between L X /L bol and T SED , which is corrected for in the right plot using a linear fit (black dotted line).…”

Photometric rotation periods for 107 M dwarfs from the APACHE survey

“…With TESS now beginning to survey the northern hemisphere, the study presented here is particularly timely, as all of the bright M dwarfs in the sample objective of this work are prime targets for the potential identification of transiting small planets amenable to high-precision mass determination and further atmospheric characterization measurements (TESS mission Level 1 Requirement). Our sample of rotation periods can also be used to further our understanding of important issues of stellar astrophysics pertaining to partly and fully convective stars, such as differences in rotational evolution (e.g., Gilhool et al 2018, and references therein) and the age-rotation-activity relation (e.g., Wright et al 2018;González-Álvarez et al 2019).…”

Magnetic Inflation and Stellar Mass. V. Intensification and Saturation of M-dwarf Absorption Lines with Rossby Number

“…The saturation values are similar to what is seen in chromospheric saturation of M dwarf stars. Newton et al (2017) found a value of Ro sat = 0.21 ± 0.02 for saturation of Hα with Rossby number, and Wright et al (2018) found a value of Ro sat = 0.14 +0.08 −0.04 for X-ray emission for fully convective M dwarfs. Douglas et al (2014) found a value of 0.11 +0.02 )−0.03 for K and M dwarfs in the Hyades cluster.…”

“…The saturation is observed in sun-like stars (e.g. Pallavicini et al 1981;Wilson 1966) as well as M dwarfs stars on either side of the fully convective boundary (Newton et al 2017;Wright et al 2018).…”