σ Orionis

We have a programme to study the σ Orionis subgroup of the Orion OB1b association. Oliviera et al (2002) used 2MASS near-IR data to investigate discs around low-mass PMS stars and brown dwarfs in this region, finding that very few possess disks.

Kenyon et al (2005) use spectroscopy of LiI, H-alpha and NaI lines to assess membership, binarity and accretion rates across a photometrically selected sample of 76 objects. They confirm many objects as members, and conclude that contamination of photometric samples by non-members runs at less than 30%. Here we have the tables of all stars in our FoV, members, non-members and uncertain membership. These catalogues are in cluster format, and give I, R-I, radial velocities and LiI and NaI equivalent widths. Note the reservations about our photometry in the paper.

Burningham et al (2005a) extend the assessment of the validity of photometric selections by investigating how many members would be excluded by such methods, using radial velocities to assess membership. They confirm the result of Kenyon et al (2004) with respect to contamination, and further find that photometric selection techniques are efficient, excluding very few bona-fide group members. The spectra used for this investigation are available here.

Ben Burningham used data on this group to assess the effect of variability on pre-main-sequence CMDs. His paper is available either in its full form, or that presented at PPV. These last two papers also formed part of Ben's thesis.

In Mayne et al (2007) we create an empirical isochrone for this cluster. We derive an age using the empirical isochrone with a relative age ladder of 4-5 Myrs.
We present photometry of:
all stars in the FOV;
X-ray sources from Sanz-Forcada et al (2004) and
spectroscopic members from Burningham et al (2005b) and Kenyon et al. (2005).

In Bell et al (2013) we present UgriZ photometry and decduce an age of approximately 6Myr for this cluster.