Name/Affiliation: Eric E. Mamajek & Cameron P. M. Bell (University of Rochester)
The Beta Pic Moving Group: Further Support for an Older Age
If has been 30 years since Beta Pic was discovered to host a remarkable resolved dusty debris disk. More recently, Beta Pic was found to not only host a giant exoplanet, but that it is part of an entourage of dozens of other young, co-moving stars in the solar neighborhood: the Beta Pic Moving Group (BPMG). For most of the past decade, a kinematic age of ~12 Myr for the BPMG (determined by multiple kinematic studies) has been adopted by most studies as the best available group age. Recently, Binks & Jeffries (2014) estimated a Li depletion boundary age for the BPMG of 21+-4 Myr. We conduct a new kinematic analysis of the classic BPMG membership from Zuckerman & Song (2004), using 3 different methods (measuring expansion coefficients, estimating the dispersion in past positions, times of minimum separation between BPMG members and the group centroid) on the best available astrometry and radial velocities. All three methods failed to yield a useful kinematic age with small uncertainties. We conclude that there is no longer strong support for a kinematic age of ~12 Myr for BPMG. Comparison of the color-magnitude diagram positions for A/F-type stars in BPMG reveal that the A-type stars are clearly on the ZAMS (with the F0 dwarf 51 Eri being the coolest ZAMS member, defining the "main sequence turn-on"), whereas the F-type members are consistent with a pre-MS ~20 Myr isochrone. We conclude that the Li depletion boundary age and the main sequence turn-on isochronal age are in good agreement, and that the previous kinematic ages for BPMG are likely to be unreliable.