Main-Sequence Fitting Distance to the σ Ori Cluster

Abstract

The σ Ori cluster is an unbound aggregate of a few hundred young, low-mass stars centered on the multiple system σ Ori. This cluster is of great interest because it is at an age when roughly half of the stars have lost their protoplanetary disks, and the cluster has a very large population of brown dwarfs. One of the largest sources of uncertainty in the properties of the cluster is that the distance is not well known. The directly measured Hipparcos distance to σ Ori AB is 350⁺¹²⁰ _-90 pc. On the other hand, the distance to the Orion OB1b subgroup (of which σ Ori is thought to be a member), 473 +/- 40 pc, is far better determined, but it is an indirect estimate of the cluster's distance. Also, Orion OB1b may have a depth of 40 pc along our line of sight. We use main-sequence fitting to nine main-sequence cluster members to estimate a best-fit distance of 420 +/- 30 pc, assuming a metallicity of -0.16 +/- 0.11 or 444 pc assuming solar metallicity. A distance as close as 350 pc is inconsistent with the observed brightnesses of the cluster members. At the best-fit distance, the age of the cluster is 2-3 Myr.