Beta Pic Moving Group: HR Diagram and Pre-Main Sequence Isochrones for A/F-type Members
HR diagram positions for A- and F-type members of the Beta Pic Moving Group, with pre-main sequence isochrones. HR diagram positions were calculated using effective temperatures and bolometric magnitudes from Pecaut & Mamajek (2013; Astrophysical Journal Supplement 208, 9; http://adsabs.harvard.edu/abs/2013ApJS..208....9P) and revised Hipparcos trigonometric parallaxes from van Leeuwen (2007; Astronomy & Astrophysics, 473, 653; http://adsabs.harvard.edu/abs/2007A%26A...474..653V). The pre-main sequence isochrones were created using the Dartmouth evolutionary tracks for assumed protosolar composition from Dotter et al. (2008; Astrophysical Journal Supplement, 178, 89; http://adsabs.harvard.edu/abs/2008ApJS..178...89D). Members were drawn from the review by Zuckerman & Song (2004; Annual Review of Astronomy & Astrophysics, 42, 685; http://adsabs.harvard.edu/abs/2004ARA%26A..42..685Z). The ellipses represent 1 and 2 sigma uncertainties in effective temperature and luminosity. The dashed red line shows a 12 million year isochrone, which corresponds to the oft-cited age of the Beta Pic Moving Group from Zuckerman et al. (2001; Astrophysical Journal 562, L87; http://adsabs.harvard.edu/abs/2001ApJ...562L..87Z) and Zuckerman & Song (2004).
A 20 Myr isochrone does a reasonable job fitting the HR diagram positions for the A- and F-type stars, with Beta Pic itself (A6V) and 51 Eri (F0V) lying essentially on the zero-age main sequence (ZAMS) corresponding to stable CNO and p-p core burning. The F-type stars HR 9 and HIP 88399 are near the ZAMS, but appear to be in a second phase of core contraction intermediate between the 1st and 2nd luminosity minima (the 2nd luminosity minimum corresponds to the ZAMS; see Iben (1965; Astrophysical Journal 141, 993; http://adsabs.harvard.edu/abs/1965ApJ...141..993I; Section IV). The stars HIP 25486 and HD 199143 are still pre-main sequence and have not reached their first luminosity minimum. The HR diagram position for HIP 10680 has a large luminosity uncertainty due to a relatively larger uncertainty in its trigonometric parallax. The stars clearly do not cluster near the 12 Myr isochrone.
A pre-main sequence "turn-on" age of ~20 Myr agrees very well with the recent Li depletion boundary age of 21+-4 Myr measured by Binks & Jeffries (2013; MNRAS, in press; http://adsabs.harvard.edu/abs/2013arXiv1310.2613B), an older Li depletion age of 21+-9 Myr estimated by Mentuch et al. (2008; Astrophysical Journal 2008, 689, 1127; http://adsabs.harvard.edu)/abs/2008ApJ...689.1127M), and a classic isochronal age for Beta Pic of 20+-10 Myr by Barrado y Navascues et al. (1999; Astrophysical Journal, 520, L123; http://adsabs.harvard.edu/abs/1999ApJ...520L.123B). Hence, neither the Li depletion ages, nor the isochronal ages for the A/F-type members of the Beta Pic Moving Group, are consistent with an age as young as 12 Myr.
