Empirical Isochrones for Low Mass Stars in Nearby Young Associations

Abstract

Absolute ages of young stars are important for many issues in pre-main-sequence stellar and circumstellar evolution but long have been recognized as difficult to derive and calibrate. In this paper, we use literature spectral types and photometry to construct empirical isochrones in Hertzsprung–Russell diagrams for low mass stars and brown dwarfs in the η Cha, epsilon Cha, and TW Hya Associations and the β Pic and Tuc-Hor Moving Groups. A successful theory of pre-main-sequence evolution should match the shapes of the stellar loci for these groups of young stars. However, when comparing the combined empirical isochrones to isochrones predicted from evolutionary models, discrepancies lead to a spectral type (mass) dependence in stellar age estimates. Improved prescriptions for convection and boundary conditions in the latest models of pre-main-sequence evolution lead to a significantly improved correspondence between empirical and model isochrones, with small offsets at low temperatures that may be explained by observational uncertainties or by model limitations. Independent of model predictions, linear fits to combined stellar loci of these regions provide a simple empirical method to order clusters by luminosity with a reduced dependence on spectral type. Age estimates calculated from various sets of modern models that reproduce Li depletion boundary ages of the β Pic Moving Group also imply a ~4 Myr age for the low mass members of the Upper Sco OB Association, which is younger than the 11 Myr age that has been recently estimated for intermediate and high mass members.

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© 2015 The American Astronomical Society. Received 2014 November 12; accepted 2015 May 21; published 2015 July 15. We thank the referee, Cameron Bell, for a careful read of the manuscript, which led to substantial improvements in the robustness and clarity of results and in the self-consistency within the paper. G.J.H. thanks Adam Kraus for discussions on preparing the paper and Jessy Jose for valuable comments on the paper. G.J.H. is also grateful for the role of the 2014 Oort Workshop on Episodic Accretion, jointly organized by Ewine van Dishoeck and Neal Evans, in motivating this analysis, and discussions with Lee Hartmann and Isabelle Baraffe at the workshop on early ideas for this paper. We appreciate Gregory Feiden sharing the new Dartmouth tracks prior to their publication and for discussion of those tracks and magnetic fields. G.J.H. is supported by a Youth Qianren grant and general grant # 11473005 awarded by the National Science Foundation of China. This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France.

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Published - 0004-637X_808_1_23.pdf

Submitted - 1505.06518v1.pdf

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