Testing pre-main-sequence models: the power of a Bayesian approach

Abstract

Pre-main-sequence (PMS) models provide invaluable tools for the study of star-forming regions as they allow us to assign masses and ages to young stars. Thus, it is of primary importance to test the models against observations of PMS stars with dynamically determined masses. We developed a Bayesian method for testing the present generation of PMS models, which allows for a quantitative comparison with observations, largely superseding the widely used isochrones and tracks qualitative superposition. Using the available PMS data, we tested the newest PISA PMS models, establishing good agreement with the observations. The data cover a mass range from ˜0.3 to ˜3.1 Msun, temperatures from ˜3 × 103 to ˜1.2 × 104 K and luminosities from ˜3 × 10-2 to ˜60 Lsun. Masses are correctly predicted within 20 per cent of the observed values in most of the cases, and for some of them the difference is as small as 5 per cent. Nevertheless, some discrepancies are also observed and critically discussed. By means of simulations, using typical observational errors, we evaluated the spread of log τsim- log τrec, i.e. simulated - recovered age distribution of the single objects. We also found that stars in binary systems simulated as coeval might be recovered as non-coeval, due to observational errors. The actual fraction of fake non-coevality is a complex function of the simulated ages, masses and mass ratios. We demonstrated that it is possible to recover the systems' ages with better precision than for single stars using the composite age-probability distribution, i.e. the product of the components' age distributions. Using this valuable tool, we estimated the ages of the presently observed PMS binary systems

Hierarchical formation of Westerlund 1: a collapsing cluster with no primordial mass segregation?

“…This degeneracy corresponds to the PMS to MS transition, where the lower-mass PMS stars that are about to enter the zeroage MS are actually brighter than slightly more massive stars already in MS. The likelihood function for stars observed in this region can thus show multiple peaks and so the mode, which we adopt as a mass estimator, will correspond to point of the isochrone which is closer to the measured magnitudes (see Gennaro et al 2012, for a detailed description). Nevertheless, this mass ambiguity affects only the lowest mass stars and it is not important for our general analysis and conclusions, which are mostly focused on the behaviour of the massive stars.…”

High-Precision Radio and Infrared Astrometry of LSPM J1314+1320ab. Ii. Testing Pre-Main-Sequence Models at the Lithium Depletion Boundary With Dynamical Masses

“…Binary stars are perhaps the most useful empirical calibrators available for testing stellar models because their components share a common age and composition, and dynamical masses can be derived from their orbital motion. While dozens of mass measurements have been obtained for pre-main-sequence stars (e.g., see reviews from White 2004 andMathieu et al 2007, p. 411;Gennaro et al 2012;Stassun et al 2014), most of these are for stars more massive than 0.5 M ☉ . Steady progress has been made to push measurements to lower masses (e.g., Simon et al 2000;Stassun et al 2006;Kraus et al 2015;Lodieu et al 2015;David et al 2016).…”

Grid-based estimates of stellar ages in binary systems

“…Recently, different techniques have been established for single stars, based upon a maximum likelihood estimation over a grid of pre-computed models (e.g. Gai et al 2011;Basu et al 2012;Gennaro et al 2012;Mathur et al 2012;Prada Moroni et al 2012;Valle et al 2014Valle et al , 2015. Such methods are very fast and flexible since they can simultaneously take all the available observables into account, reaching high precision in the estimates and selfconsistently evaluating their errors.…”