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

Abstract

Two eclipsing binaries in the USco association have recently yielded precise values of masses and radii for 4 low-mass members of the association. Standard evolution models would require these dM4.5 -dM5 stars to have ages which are younger than the ages of more massive stars in the association by factors which appear (in extreme cases) to be as large as ~3. Are the stars in the association therefore noncoeval? We suggest that the answer is No: by incorporating the effects of magnetic inhibition of convective onset, we show that the stars in USco can be restored to coevality provided that the 4 lowmass member stars have vertical surface fields in the range 200 -700 G. Fields of such magnitude have already been measured on the surface of certain solar-type stars in other young clusters.

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

“…These discrepancies reflect discordant HR-diagram ages for the lower-and high-mass populations in the cluster (Herczeg & Hillenbrand 2015). 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.…”

The Gaia-ESO survey: a lithium depletion boundary age for NGC 2232

“…There are also direct and indirect indications that magnetically active stars, whether they are fast-rotating and young or members of close, tidally-locked binary systems, have larger radii than predicted by the most commonly used stellar models (Morales et al 2009;Torres 2013;Malo et al 2014b;Kraus et al 2015Kraus et al , 2017Rizzuto et al 2020). This has led to suggestions that rotation, magnetic fields and high surface coverage of starspots may significantly alter the evolutionary tracks and isochrones in young clusters (Feiden & Chaboyer 2013;Jackson & Jeffries 2014a;Somers & Pinsonneault 2015;MacDonald & Mullan 2017). If so, this would lead to an underestimate of young cluster ages by factors of ∼ 2 and a significant underestimate of stellar masses, particularly at low masses, when models that neglect these effects are adopted (e.g.…”

Magnetic Field of the Eclipsing M-dwarf Binary YY Gem

“…On the one hand, formation of dark cool spots in the magnetised areas at the stellar surface suppresses radiative losses resulting in a radius increase compared to unspotted star of the same luminosity (Chabrier et al 2007;Morales et al 2010;Jackson & Jeffries 2014). On the other hand, inflated radii can be explained by the modification of stellar structure by the magnetic suppression or stabilisation of the interior convection (Mullan & MacDonald 2001;Feiden & Chaboyer 2012, 2013MacDonald & Mullan 2013, 2017a. Relative importance of these two effects as a function of stellar mass as well as validity of alternative theoretical approaches to treating magnetoconvection in one-dimensional stellar structure models are hotly debated topics.…”

“…Torres et al 2010;Eker et al 2015;Moya et al 2018). A number of theoretical studies attempted to explain the inflated radii of the components of YY Gem by developing different versions of non-conventional stellar interior structure models which incorporated effects of magnetic field and surface inhomogeneities (Feiden & Chaboyer 2013;MacDonald & Mullan 2014, 2017aJackson & Jeffries 2014).…”