Orion revisited. II. The foreground population to Orion A

Aims: Following the recent discovery of a large population of young stars in front of the Orion nebula, we carried out an observational campaign with the DECam wide-field camera covering ≈10 deg² centered on NGC 1980 to confirm, probe the extent of, and characterize this foreground population of pre-main-sequence stars.
Methods: We used multiwavelength wide-field images and catalogs to identify potential foreground pre-main-sequence stars using a novel probabilistic technique based on a careful selection of colors and luminosities.
Results: We confirm the presence of a large foreground population towards the Orion A cloud. This population contains several distinct subgroups, including NGC 1980 and NGC 1981, and stretches across several degrees in front of the Orion A cloud. By comparing the location of their sequence in various color-magnitude diagrams with other clusters, we found a distance and an age of 380 pc and 5 ~ 10 Myr, in good agreement with previous estimates. Our final sample includes 2123 candidate members and is complete from below the hydrogen-burning limit to about 0.3 M_⊙, where the data start to be limited by saturation. Extrapolating the mass function to the high masses, we estimate a total number of ≈2600 members in the surveyed region.
Conclusions: We confirm the presence of a rich, contiguous, and essentially coeval population of about 2600 foreground stars in front of the Orion A cloud, loosely clustered around NGC 1980, NGC 1981, and a new group in the foreground of the OMC-2/3. For the area of the cloud surveyed, this result implies that there are more young stars in the foreground population than young stars inside the cloud. Assuming a normal initial mass function, we estimate that between one to a few supernovae must have exploded in the foreground population in the past few million years, close to the surface of Orion A, which might be responsible, together with stellar winds, for the structure and star formation activity in these clouds. This long-overlooked foreground stellar population is of great significance, calling for a revision of the star formation history in this region of the Galaxy.

Appendix A is available in electronic form at http://www.aanda.orgFull Table A.1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/564/A29