Orion revisited

III. The Orion Belt population^⋆

¹ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
e-mail: karolina.kubiak@gmail.com
² Center for Astrobiology (INTA-CSIC), Camino Bajo del Castillo S/N, 28692 Villanueva de la Cañada, Madrid, Spain
³ University of Milan, Department of Physics, via Celoria 16, 20133 Milan, Italy
⁴ CENTRA, Instituto Superior Tecnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
⁵ Universidade do Porto, Departamento de Engenharia Fisica da Faculdade de Engenharia, rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal
⁶ Dpto. de Inteligencia Artificial, ETSI Informatica, UNED, Juan del Rosal, 16, 28040 Madrid, Spain
⁷ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁸ Universitäts-Sternwarte Ludwig-Maximilians-Universität (USM), Scheinerstr. 1, 81679 München, Germany
⁹ Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstr.1, 85748 Garching, Germany

Received: 13 May 2016
Accepted: 30 July 2016

Abstract

Aims. This paper continues our study of the foreground population to the Orion molecular clouds. The goal is to characterize the foreground population north of NGC 1981 and to investigate the star formation history in the large Orion star-forming region. We focus on a region covering about 25 square degrees, centered on the ϵ Orionis supergiant (HD 37128, B0 Ia) and covering the Orion Belt asterism.

Methods. We used a combination of optical (SDSS) and near-infrared (2MASS) data, informed by X-ray (XMM-Newton) and mid-infrared (WISE) data, to construct a suite of color–color and color–magnitude diagrams for all available sources. We then applied a new statistical multiband technique to isolate a previously unknown stellar population in this region.

Results. We identify a rich and well-defined stellar population in the surveyed region that has about 2000 objects that are mostly M stars. We infer the age for this new population to be at least 5 Myr and likely ~10 Myr and estimate a total of about 2500 members, assuming a normal IMF. This new population, which we call the Orion Belt population, is essentially extinction-free, disk-free, and its spatial distribution is roughly centered near ϵ Ori, although substructure is clearly present.

Conclusions. The Orion Belt population is likely the low-mass counterpart to the Ori OB Ib subgroup. Although our results do not rule out Blaauw’s sequential star formation scenario for Orion, we argue that the recently proposed blue streams scenario provides a better framework on which one can explain the Orion star formation region as a whole. We speculate that the Orion Belt population could represent the evolved counterpart of an Orion nebula-like cluster.