SCYON Abstract

Received on February 18 2008

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

Authors	Nathan J. Mayne and Tim Naylor
Affiliation	School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL
Accepted by	Monthly Notices of the Royal Astronomical Society
Contact	nathan@astro.ex.ac.uk
URL	http://www.astro.ex.ac.uk/people/nathan/publications.html
Links	NGC 2244 / IC 5146 / ONC / NGC 6530 / Sigma Ori / NGC 2264 / NGC 2362 / IC 348 / NGC 7160 / h Per / chi Per / NGC 1960 / NGC 2547

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 nominal ages are: ≈2 Myrs for NGC 6530 and the ONC, ≈3 Myrs for λ Orionis, NGC 2264 and σ Orionis, ≈4-5 Myrs for NGC 2362, ≈13 Myrs for h and χ Per, ≈20 Myrs for NGC 1960 and ≈40 Myrs for NGC 2547. 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.