The Sizes of the Nearest Young Stars

TL;DR

This study measures the sizes and youth indicators of stars in the AB Doradus and β Pictoris groups to identify candidates suitable for interferometric observations, aiding in age and evolutionary model constraints.

Contribution

It provides updated membership lists, effective temperature estimates, and predicts angular diameters for stars, highlighting those resolvable with current interferometers for improved stellar age studies.

Findings

Six stars in the northern hemisphere are spatially resolvable with the CHARA Array.

Eighteen stars in the southern hemisphere have diameters > 0.4 mas, including AU Mic.

Some stars, like GJ 393, are likely still pre-main sequence.

Abstract

We present moderate resolution (R $\sim$ 3575) optical spectra of 19 known or suspected members of the AB Doradus and $\beta$ Pictoris Moving Groups, obtained with the DeVeny Spectrograph on the 72-inch Perkins telescope at Lowell Observatory. For 4 of 5 recently proposed members, signatures of youth such as Li\,I 6708 \AA\, absorption and H$\alpha$ emission further strengthen the case for youth and membership. Effective temperatures are determined via line ratio analyses for the 11 F, G and early K stars observed, and via spectral comparisons for the 8 late-K and M stars observed. We assemble updated candidate membership lists for these Moving Groups that account for known binarity. We then use temperature, luminosity, and distance estimates to predict angular diameters for these stars; the motivation is to identify stars that can be spatially resolved with long-baseline optical/infrared interferometers in order to improve age estimates for these Groups and to constrain evolutionary models at young ages. Considering the portion of the sky accessible to northern hemisphere facilities (DEC $> -30$), 6 stars have diameters large enough to be spatially resolved ($\theta > 0.4$ mas) with the CHARA Array; this subsample includes the low mass M2.5 member of AB Dor, GJ 393, which is likely to still be pre-main sequence. For southern hemisphere facilities (DEC $< +30$), 18 stars have diameters larger than this limiting size, including the low mass debris disk star AU Mic (0.72 mas). However, the longest baselines of southern hemisphere interferometers (160-m) are only able to resolve the largest of these, the B6 star $\alpha$ Gru (1.17 mas); proposed long-baseline stations may alleviate the current limitations.