On the Reliability of Stellar Ages and Age Spreads Inferred From Pre-Main-Sequence Evolutionary Models

Abstract

We revisit the problem of low-mass pre-main-sequence stellar evolution and its observational consequences for where stars fall on the Hertzsprung-Russell diagram (HRD). In contrast to most previous work, our models follow stars as they grow from small masses via accretion, and we perform a systematic study of how the stars' HRD evolution is influenced by their initial radius, by the radiative properties of the accretion flow, and by the accretion history, using both simple idealized accretion histories and histories taken from numerical simulations of star cluster formation. We compare our numerical results to both non-accreting isochrones and to the positions of observed stars in the HRD, with a goal of determining whether both the absolute ages and the age dispersions inferred from non-accreting isochrones are reliable. We show that non-accreting isochrones can sometimes overestimate stellar ages for more massive stars (those with effective temperatures above ∼3500 K), thereby explaining why non-accreting isochrones often suggest a systematic age difference between more and less massive stars in the same cluster. However, we also find the only way to produce a similar overestimate for the ages of cooler stars is if these stars grow from ∼0.01 M seed protostars that are an order of magnitude smaller than predicted by current theoretical models, and if the size of the seed protostar correlates systematically with the final stellar mass at the end of accretion. We therefore conclude that, unless both of these conditions are met, inferred ages and age spreads for cool stars are reliable, at least to the extent that the observed bolometric luminosities and temperatures are accurate. Finally, we note that the time dependence of the mass accretion rate has remarkably little effect on low-mass stars' evolution on the HRD, and that such time dependence may be neglected for all stars except those with effective temperatures above ∼4000 K.

The Pisa pre-main sequence tracks and isochrones

“…This is an important point to emphasize because the inclusion of these processes affects the evolution of pre-MS models introducing additional sources of uncertainty (see e.g., Pinsonneault et al 1990;D'Antona et al 2000;Siess 2001;Stahler & Palla 2004;Baraffe et al 2009;Hosokawa et al 2011).…”

Hubble Tarantula Treasury Project. Ii. The Star-Formation History of the Starburst Region NGC 2070 in 30 Doradus

“…Among the drawbacks, the intrinsic faintness of TOns compared to the MSTOs makes TOns prone to photometric errors and incompleteness, issues that are exacerbated in the UV CMD because older TOns tend to be also redder. From the theoretical side, the TOn visibility is intimately connected with the PMS evolutionary times, which are still model dependent (see, e.g., Baraffe et al 2009;Hosokawa et al 2011;Soderblom et al 2014, p. 219 and references therein).…”

Simulations of protostellar collapse using multigroup radiation hydrodynamics

“…Impact of the mass of Larson's second core for early protostar evolution Baraffe et al (2009) and Baraffe & Chabrier (2010) showed that episodic accretion on a newborn protostar provides a plausible explanation for the observed luminosity spread in young stellar clusters and star forming regions without invoking any age spread and can also explain unexpectedly high depletion levels of lithium that have been observed in some young objects. This scenario was questioned by Hosokawa et al (2011) who argued that the scenario could not hold in the lower (T eff 3500 K) part of the Hertzsprung-Russell diagram. The issue has been addressed in detail in Baraffe et al (2012), where the authors have shown that the only reason why Hosokawa et al (2011) could not reproduce the observed luminosity spread in the aforementioned domain stems from their assumed value for the second Larson core mass (i.e.…”