Abstract
Low-mass stars exhibit substantial pre-main sequence evolution during the first ∼100 Myr of their lives. Thus, young M-type stars are prime targets for isochronal dating, especially in young moving groups (YMGs), which contain large amounts of stars in this mass and age range. If the mass and luminosity of a star can both be directly determined, this allows for a particularly robust isochronal analysis. This motivates in-depth studies of low-mass binaries with spatially resolvable orbits, where dynamical masses can be derived. Here we present the results of an observing campaign dedicated to orbital monitoring of AB Dor Ba/Bb, which is a close M-dwarf pair within the quadruple AB Dor system. We have acquired eight astrometric epochs with the SPHERE/ZIMPOL and NACO instruments, which we combine with literature data to improve the robustness and precision for the orbital characterization of the pair. We find a system mass 0.66−0.12+0.12 M⊙ and bolometric luminosities in logL/L⊙ of −2.02 ± 0.02 and −2.11 ± 0.02 for AB Dor Ba and Bb, respectively. These measurements are combined with other YMG pairs in the literature to start building a framework of empirical isochrones in mass–luminosity space. This can be used to calibrate theoretical isochrones and to provide a model-free basis for assessing relative stellar ages. We note a tentative emerging trend where the youngest moving group members are largely consistent with theoretical expectations, while stars in older associations such as the AB Dor moving group appear to be systematically underluminous relative to isochronal expectations.
Highlights
Stellar systems that are both young and nearby are of importance for a range of present-day scientific topics, not least for the purpose of direct imaging of exoplanets (e.g. Marois et al 2008; Macintosh et al 2015; Chauvin et al 2017) and disks (e.g. Schneider et al 2009; Thalmann et al 2013; Boccaletti et al 2015)
While AB Dor moving group (ABMG) is clearly older than 5–20 Myr, which is the approximate age of the youngest young moving groups (YMGs) such as the TW Hya or β Pic associations (e.g. Bell et al 2015), its specific age has remained uncertain, with different studies suggesting age ranges from a lower limit of 30 Myr (Close et al 2005) all the way to an upper limit of 200 Myr (Bell et al 2015)
The point spread function (PSF) fitting procedure outputs differential coordinates δx and δy, and the differential brightness δm between AB Dor Ba and AB Dor Bb, with uncertainties represented by the standard errors from frame-to-frame scatter and PSF-to-PSF reference scatter added in quadrature
Summary
Stellar systems that are both young and nearby are of importance for a range of present-day scientific topics, not least for the purpose of direct imaging of exoplanets (e.g. Marois et al 2008; Macintosh et al 2015; Chauvin et al 2017) and disks (e.g. Schneider et al 2009; Thalmann et al 2013; Boccaletti et al 2015). M dwarfs remain for a long time in the pre-main sequence phase, and dynamical masses allow for direct comparison between observational data and isochronal models, which make M-dwarf binaries in YMGs an important sample for astrophysical calibrations (Janson et al 2017). In this context, AB Dor Ba and Bb are of high priority, given their short orbital period, which makes them promising for calibrating theoretical models of young stars, as well as for potentially constraining the age of the system if well matching isochrones are found.
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