Very Low Mass Stars and Brown Dwarfs in the Belt of Orion

Summary

As part of our ongoing research into low–mass star formation in Orion, we have obtained deep photometric and spectroscopic observations of PMS objects in the σ Orionis cluster and near the other O stars in the belt of Orion. The photometry indicates the existence of objects with masses as low as 0.01 M_⊙ (100 M_jup). Spectroscopic follow-up has confirmed the sub-stellar nature of the candidate object tested.

Introduction

The Orion OB associations are one of the richest star forming regions in the local galaxy. Recently, we have made a concentrated effort to study the stars near belt of Orion within the Orion OB1a and OB1b associations. ROSAT observations totaling 100 Ksec of this region have been supplemented with ground based spectroscopy and photometry (Wolk 1996). These data demonstrated the clear existence of a 1-5 Myr old pre–main sequence of stars with common space motion and density of sources reaching a maximum at the location of σ Orionis. The confluence of this data lead us to conclude that the stars near σ Orionis form a young stellar cluster with a central density of at least 50 stars/pc³ (Walter et al. 1997, Walter et al. 1998). Similar clusters of older stars seem to exist near the other O stars in the belt of Orion.