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Abstract
We present a study of star formation in the Orion star-forming complex. From a cryogenic survey of the molecular clouds using the Spitzer telescope, the inspiration for two follow-up projects were manifest. First we extend the mid-infrared photometric survey of the molecular clouds by Megeath et al. (2012) into the older, off-cloud regions of Orion to investigate the earlier epochs of star formation. We present a map of the young stellar objects identified in the off-cloud region and discuss the 'archeology’ of the older epochs of star formation in Orion. In particular, we identify stars associated with two previously known aggregates, the OB1b sub-group and the σ Orionis cluster. We argue that the OB1b sub-association, with its ~ 3500 estimated members represents the later stages of evolution of an Orion Nebula Cluster (ONC)-like cluster that expelled its gas and expanded. We also find that that the OB1b sub-group, as defined by the distribution of stars, extends to the ONC. This is also evidence, that the current northern tip of the A cloud where the ONC is located, extended north some ~ 5 degrees in which the OB1b would have formed. We argue that the σ Orionis group, with its ~ 400 estimated members is a later-stage analogue of the NGC 2024 young cluster to its north east. Indeed, the massive members of the σ Orionis group likely interacting with the Orion B cloud. The molecular cloud containing NGC 2024 could have extended outward toward σ Orionis. In the second part, we present a spectroscopic survey of Spitzer-identified young stellar objects in Orion. These multiple systems were identified in an HST/WFC3 1.6 μm survey targeting known protostars. Using low-resolution spectra on SpeX on the IRTF, NIHTS on the DCT and the G141 grism of WFC3 on HST, we obtain spectral types of the primary and companions for 13 systems between 400-1000 AU. We construct the Hertzsprung-Russell diagram for these and find both the protostars and more evolved pre-main sequence stars with discs are concentrated in along a common loci in the Hertzsprung-Russell diagram, above the 1 Myr isochrone. This suggests that all of the targeted objects in these regions are young. Furthermore, we find a M9 companion with a mass approximately 10MJupiter, one of the lowest mass companions found to a YSO found to date. This object, at a separation of 1000 AU, may be a precursor to planetary mass objects. Using spectral-type-difference as proxy for the mass ratio, we find a flat to slightly declining mass ratio distribution (q-0.5) is favoured. In our data and others, there is a hint of a paucity of ΔSpT ~ 4 systems. We speculate that the flat mass ratio distribution may result from dual formation modes. In one case, the companion can arise from the fragmentation of a natal core, in the other case, the circumstellar disc present round young stellar objects may fragment after the formation of the central protostar and as the accretion onto the disc has diminished or ended.
