Are pre-main-sequence stars older than we thought?

TL;DR

This study suggests that the ages of young star clusters are underestimated by traditional methods, proposing they are 1.5 to 2 times older, which impacts theories of planet formation and cluster evolution.

Contribution

The paper introduces a revised age scale for young clusters based on fitting colour-magnitude diagrams, and improves the tau-squared fitting method for sequences with vertical segments.

Findings

Derived ages are 1.5 to 2 times longer than previous estimates.

Revised ages resolve discrepancies in planet formation timescales.

Modified tau-squared method better fits vertical sequences in diagrams.

Abstract

We fit the colour-magnitude diagrams of stars between the zero-age main-sequence and terminal-age main sequence in young clusters and associations. The ages we derive are a factor 1.5 to 2 longer than the commonly used ages for these regions, which are derived from the positions of pre-main-sequence stars in colour-magnitude diagrams. From an examination of the uncertainties in the main-sequence and pre-main-sequence models, we conclude that the longer age scale is probably the correct one, which implies we must revise upwards the commonly used ages for young clusters and associations. Such a revision would explain the discrepancy between the observational lifetimes of proto-planetary discs and theoretical calculations of the time to form planets. It would also explain the absence of clusters with ages between 5 and 30Myr. We use the $\tau^2$ statistic to fit the main-sequence data, but find that we must make significant modifications if we are to fit sequences which have vertical segments in the colour-magnitude diagram. We present this modification along with improvements to methods of calculating the goodness-of-fit statistic and parameter uncertainties. Software implementing the methods described in this paper is available from http://www.astro.ex.ac.uk/people/timn/tau-squared/