Magnetic inhibition of convection and the fundamental properties of low-mass stars

Abstract

When determining absolute ages of identifiably young stellar populations, results strongly depend on which stars are studied. Cooler (K, M) stars typically yield ages that are systematically younger than warmer (A, F, G) stars by a factor of two. I explore the possibility that these age discrepancies are the result of magnetic inhibition of convection in cool young stars by using magnetic stellar evolution isochrones to determine the age of the Upper Scorpius subgroup of the Scorpius-Centaurus OB Association. A median age of 10 Myr consistent across spectral types A through M is found, except for a subset of F-type stars that appear significantly older. Agreement is shown for ages derived from the Hertzsprung-Russell diagram and from the empirical mass-radius relationship defined by eclipsing multiple-star systems. Surface magnetic field strengths required to produce agreement are of order 2.5 kG and are predicted from a priori estimates of equipartition values. A region in the HR diagram is identified that plausibly connects stars whose structures are weakly influenced by the presence of magnetic fields with those whose structures are strongly influenced by magnetic fields. The models suggest this region is characterized by stars with rapidly thinning outer convective envelopes where the radiative core mass is greater than 75% of the total stellar mass. Furthermore, depletion of lithium predicted from magnetic models appears in better agreement with observed lithium equivalent widths than predictions from non-magnetic models. These results suggest that magnetic inhibition of convection plays an important role in the early evolution of low-mass stars and that it may be responsible for noted age discrepancies in young stellar populations.

Apparent Non-Coevality Among the Stars in Upper Scorpio: Resolving the Problem Using a Model of Magnetic Inhibition of Convection

“…We note that Feiden (2016) has also recently applied a different model of magneto-convection to two of the low-mass stars which we propose to model in USco. We will compare our results with Feiden's in the Discussion (Section 6, below).…”

“…Recently, Feiden (2016) has applied the Feiden-Chaboyer model in order to address the age discrepancy in USco between high-mass and low-mass stars. Feiden's derived a consistent age of 10 Myr for highand low-mass stars in USco.…”

Hints for Small Disks around Very Low Mass Stars and Brown Dwarfs∗

“…To cover a large range in stellar mass, we combine the evolutionary tracks from Baraffe et al (2015) and Feiden (2016) as in Pascucci et al (2016). To find corresponding luminosities and temperatures for each stellar mass, we use the 2 Myr isochrone because this age is a good match to the age of the star-forming regions we consider in this study.…”

M Dwarf Rotation from the K2 Young Clusters to the Field. I. A Mass–Rotation Correlation at 10 Myr

“…One potential resolution to this discrepancy is that magnetic activity and starspots have altered the fundamental parameters of lower-mass stars (e.g., Feiden & Chaboyer 2013;Somers & Pinsonneault 2015a;MacDonald & Mullan 2017;Somers & Stassun 2017), making them appear younger in the HR diagram as has been noted in other young clusters (Jackson & Jeffries 2014;Jackson et al 2016;Jeffries et al 2017). When considering these effects, Feiden (2016) found a consensus age of 10Myr for the cluster-we adopt this age.…”