Hα emission-line stars in molecular clouds^⋆

II. The M 42 region

¹ Observational Astronomy, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
e-mail: Bertil.Pettersson@physics.uu.se
² Universidade Federal de Sergipe, Departamento de Física, Av. Marechal Rondon s/n, 49100-000 São Cristóvão, SE, Brazil
e-mail: tina@ufs.br
³ Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii at Manoa, 640 North Aohoku Place, Hilo HI 96720, USA
e-mail: reipurth@ifa.hawaii.edu

Received: 7 February 2014
Accepted: 4 June 2014

Abstract

We present a deep survey of Hα emission-line stars in the M 42 region using wide-field objective prism films. A total of 1699 Hα emission-line stars were identified, of which 1025 were previously unknown, within an area of 5.̊5 × 5.̊5 centred on the Trapezium Cluster. We present Hα strength estimates, positions, and JHK_s photometry extracted from 2MASS, and comparisons to previous surveys. The spatial distribution of the bulk of the stars follows the molecular cloud as seen in CO and these stars are likely to belong to the very young population of stars associated with the Orion Nebula Cluster. Additionally, there is a scattered population of Hα emission-line stars distributed all over the region surveyed, which may consist partly of foreground stars associated with the young NGC 1980 cluster, as well as some foreground and background dMe or Be stars. The present catalogue adds a large number of candidate low-mass young stars belonging to the Orion population, selected independently of their infrared excess or X-ray emission.