Using rotation, magnetic activity and lithium to estimate the ages of low mass stars

Abstract

Abstract. The rotation rate, level of magnetic activity and surface lithium abundance are age-dependent quantities in stars of about a solar mass and below. The physical reasons for the evolution of these phenomena are qualitatively understood, but accurate quantitative models remain dependent on empirical calibration using the Sun and stars of known age, chiefly in clusters. In this work I review the status of these "empirical age indicators", outlining the astrophysics of their time dependence, describing the measurements, assessing the precision (and accuracy) of age estimates when applied to individual stars, and identifying their principle limitations in terms of the mass and age ranges over which they are useful. Finally, I discuss the "lithium depletion boundary" technique which, in contrast to the empirical methods, appears to provide robust, almost model-independent ages that are both precise and accurate, but which is only applicable to coeval groups of stars.

TheGaia-ESO Survey: A lithium-rotation connection at 5 Myr?

“…Hence the maximum abundances measured in NGC 2264 would be consistent with the meteoritic abundance. els, is highly sensitive to the physics included in the model, most notably the treatment of convection (e.g., Jeffries 2014). Hence, any model-based prediction of lithium abundances as a function of mass and age in the PMS is subject to those uncertainties, and they are very sensitive to the assumed cluster age.…”

“…Lithium, a fragile element, is burnt at a temperature of 2.5 MK, which corresponds to the temperature at the base of the convective zone of a solar-mass star on the zero-age main sequence (ZAMS; Siess et al 2000). Hence, lithium is slowly depleted and its surface abundance steadily decreases over time in solar-type and lower mass stars (e.g., Basri et al 1991;Martin et al 1994;Sestito & Randich 2005;Randich 2010;Jeffries 2014). The rate of this secular evolution may be modified, either increased or reduced, by so-called nonstandard transport processes, such as rotational mixing, internal magnetic fields, gravity waves, or tidal interactions and by structural changes induced by, for instance, rotation, magnetic activity, metallicity, or accretion (e.g., Pinsonneault et al 1990;Zahn 1992;Ventura et al 1998;Piau & Turck-Chièze 2002;Talon & Charbonnel 2005;Denissenkov 2010;Eggenberger et al 2012;Théado & Vauclair 2012).…”

On the uncertain nature of the core of α Cen A

“…On the one hand, one might argue that there exist priors from gyrochronology. On the other hand, empirical age estimates are usually given on a modelindependent scale (see for instance Jeffries 2014). This means that a meaningful comparison with the output from ASTEC would require a calibration of that age scale based on our theoretical models.…”

Clusters: Age Scales for Stellar Physics

“…Eventually, we should be able to construct different ages scales which should be consistent with each other, and should produce absolute as well as relative ages". Figure 26, based on the results discussed here and on previous reviews (Mermilliod 2000;Soderblom 2010;Soderblom et al 2014;Jeffries 2014), displays one possible age stairway, based on different techniques and the degree of confidence. The first level, the true anchors we have at our disposal, does not provide a reference for ages younger than the Sun, so we have to rely on the most trustable.…”

“…et al (2008), Dobbie et al (2010), Cargile et al (2010), Jeffries et al (2013), Binks & Jeffries (2014), Juarez et al (2014), Malo et al (2014), Dahm (2015), and Murphy & Lawson (2015). Recent overviews have been presented in Barrado y Navascués et al (2011), Soderblom et al (2014 and Jeffries (2014). From the theoretical point of view, the LDB technique and its limitations are dealt in Burke et al (2004) and Tognelli et al (2015).…”