[Paper Review] Ages and Age Spreads in Young Stellar Clusters

R. D. Jeffries|arXiv (Cornell University)|Sep 6, 2017

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper reviews the use of Hertzsprung-Russell diagrams and lithium depletion as age indicators in young stellar clusters, highlighting Francesco Palla’s foundational contributions. It argues that apparent age spreads in young clusters may stem from magnetic inhibition of convection and starspot coverage, which inflate low-mass pre-main-sequence stars, leading to underestimated ages when using standard evolutionary models—implicating a need to revise absolute age estimates and age spreads in young clusters.

ABSTRACT

I review progress towards understanding the time-scales of star and cluster formation and of the absolute ages of young stars. I focus in particular on the areas in which Francesco Palla made highly significant contributions - interpretation of the Hertzsprung-Russell diagrams of young clusters and the role of photospheric lithium as an age diagnostic.

Motivation & Objective

• To assess the reliability of pre-main-sequence (PMS) evolutionary models in estimating absolute ages of young stars.
• To evaluate whether observed age spreads in young clusters are real or artifacts of observational uncertainties and model limitations.
• To investigate the role of magnetic activity and episodic accretion in distorting HRD positions and lithium depletion patterns.
• To reconcile discrepancies between ages derived from HRD/CMD positions, lithium abundance, and high-mass stellar evolution.
• To determine whether current models overestimate the age spread in young clusters due to incorrect treatment of convection and stellar activity.

Proposed method

• Uses Hertzsprung-Russell diagrams (HRDs) and colour-magnitude diagrams (CMDs) to infer ages and masses of low-mass pre-main-sequence stars in young clusters.
• Applies photospheric lithium abundance as an independent age diagnostic, leveraging its mass- and age-dependent depletion in stellar photospheres.
• Compares HRD-based ages with lithium depletion ages and high-mass stellar evolution to cross-validate age estimates.
• Incorporates recent models of magnetic inhibition of convection and starspot coverage to explain discrepancies in HRD positions and luminosities.
• Analyzes eclipsing binary systems in star-forming regions to test the accuracy of PMS evolutionary models against observed radii and temperatures.
• Uses simulations and observational data from surveys such as IN-SYNC APOGEE and the Orion Nebula Cluster to assess age dispersion and rotation-activity correlations.

Experimental results

Research questions

• RQ1 To what extent do observed age spreads in young stellar clusters reflect real star formation histories versus observational and modeling uncertainties?
• RQ2 How do magnetic activity and starspot coverage affect the inferred positions of pre-main-sequence stars in the Hertzsprung-Russell diagram?
• RQ3 Can magnetic inhibition of convection explain the discrepancy between HRD-based ages and lithium depletion-based ages in young low-mass stars?
• RQ4 Why do PMS binary components in star-forming regions deviate from standard evolutionary model predictions in luminosity and effective temperature?
• RQ5 How do systematic errors in bolometric corrections and extinction estimates impact the inferred age spreads in young clusters?

Key findings

• Apparent age spreads in young clusters, particularly in the Orion Nebula Cluster and IC 348, are partially explained by observational uncertainties and astrophysical effects such as accretion and variability.
• Magnetic inhibition of convection and starspot coverage can inflate low-mass pre-main-sequence stars by ~10%, leading to cooler effective temperatures and underestimated ages when using standard models.
• When magnetic models (e.g., Jackson & Jeffries 2014; Somers & Pinsonneault 2015b) are applied, HRD-based ages are doubled and brought into better agreement with lithium depletion ages.
• The inclusion of magnetic effects resolves discrepancies between low-mass and high-mass stellar ages in clusters, bringing them into broad agreement.
• Eclipsing binary systems in young clusters show that standard models fail to predict observed radii and temperatures, suggesting fundamental flaws in PMS evolution modeling.
• A significant fraction of the observed dispersion in HRD and lithium depletion patterns is likely due to correlated effects of rotation, magnetic activity, and age spread, rather than purely stochastic age dispersion.