An Empirical Correction for Activity Effects on the Temperatures, Radii, and Estimated Masses of Low-Mass Stars and Brown Dwarfs

Abstract

We present empirical relations for determining the amount by which the effective temperatures and radii-and therefore the estimated masses-of low-mass stars and brown dwarfs are altered due to chromospheric activity. We base our relations on a large set of low-mass stars in the field with Hα activity measurements, and on a set of low-mass eclipsing binaries with X-ray activity measurements from which we indirectly infer the Hα activity. Both samples yield consistent relations linking the amount by which an active object's temperature is suppressed, and its radius inflated, to the strength of its Hα emission. These relations are found to approximately preserve bolometric luminosity. We apply these relations to the peculiar brown dwarf eclipsing binary 2M0535−05, in which the active, higher-mass brown dwarf has a cooler temperature than its inactive, lower-mass companion. The relations correctly reproduce the observed temperatures and radii of 2M0535−05 after accounting for the Hα emission; 2M0535−05 would be in precise agreement with theoretical isochrones were it is inactive. The relations that we present are applicable to brown dwarfs and low-mass stars with masses below 0.8 M and for which the activity, as measured by the fractional Hα luminosity, is in the range −4.6 log L Hα /L bol −3.3. We expect these relations to be most useful for correcting radius and mass estimates of low-mass stars and brown dwarfs over their active lifetimes (few Gyr) and when the ages or distances (and therefore luminosities) are unknown. We also discuss the implications of this work for improved determinations of young cluster initial mass functions.

Masses and activity of AB Doradus B a/b

“…Providing our best proxy of the quiescent stellar atmosphere, we only used this spectrum in our modeling. We caution, however, that even low activity levels can influence the determined effective temperature for low-mass objects (e.g., Stassun et al 2012). Although the spectrum must be composed of two individual components, they are not clearly separated in the data.…”

The effect of star-spots on the ages of low-mass stars determined from the lithium depletion boundary

“…Ventura et al 1998;D'Antona, Ventura & Mazzitelli 2000). There is growing observational evidence from fast-rotating, magnetically active stars in tidally-locked eclipsing binaries, young clusters and the field, that magnetic activity may increase the radii of low-mass stars (Lopez-Morales 2007; Morales, Ribas & Jordi 2008;Jackson, Jeffries & Maxted 2009;Stassun et al 2012). The mechanism by which it does so is still unclear; but could include the magnetic stabilisation of the star against convection (Gough & Tayler 1966;Moss 1968;Mullan & Macdonald 2001;Feiden & Chaboyer 2013), a reduction in convective efficiency due to a turbulent dynamo (Feiden & Chaboyer 2014) or the blocking of emergent flux by dark, magnetic starspots (Spruit 1982;Spruit & Weiss 1986;Jackson & Jeffries 2014).…”

SOPHIE velocimetry ofKeplertransit candidates

“…If CoRoT-15 b appears to be inflated, WASP-30 b, LHS 6343 C and KOI-205 b have a radius compatible with their age, as predicted by models. 2M0535-05 A and B may have smaller radii than predicted because of their activity (Stassun et al, 2012).…”