Fitting the young main-sequence; distances, ages and age spreads

TL;DR

This study derives distances and ages for young star clusters using main-sequence models, showing model robustness, revising age estimates, and discussing uncertainties like metallicity and age spreads.

Contribution

It provides a new, precise distance to the ONC and revises cluster ages, highlighting the impact of metallicity and introducing the R-C gap overlap as an age diagnostic.

Findings

Distances are robust across models.

Revised cluster ages ranging from 2 to 40 Myrs.

New distance to ONC: 391 pc, rejecting larger estimates.

Abstract

We use several main-sequence models to derive distances (and extinctions), with statistically meaningful uncertainties for 11 star-forming-regions and young clusters. The model dependency is shown to be small, allowing us to adopt the distances derived using one model. Using these distances we have revised the age order for some of the clusters of Mayne et al (2007). The new (approximate) nominal ages are: 2 Myrs for NGC6530 and the ONC, 3 Myrs for Lambda Orionis, NGC2264 and Sigma Orionis, 4-5 Myrs for NGC2362, 13 Myrs for h and chi Per, 20 Myrs for NGC1960 and 40 Myrs for NGC2547. In cases of significantly variable extinction we have derived individual extinctions using a revised Q-method (Johnson and Morgan, 1953). These new data show that the largest remaining uncertainty in deriving an age ordering (and necessarily ages) is metallicity. We also discuss the use of a feature we term the R-C gap overlap to provide a diagnostic of isochronal age spreads or varying accretion histories within a given star-formation-region. Finally, recent derivations of the distance to the ONC lie in two groups. Our new more precise distance of 391(+12,-9) pc allows us to decisively reject the further distance, we adopt 400 pc as a convenient value.