The Spitzer Space Telescope Survey of the Orion a and B Molecular Clouds. Ii. The Spatial Distribution and Demographics of Dusty Young Stellar Objects

Abstract

We analyze the spatial distribution of dusty young stellar objects (YSOs) identified in the Spitzer Survey of the Orion Molecular clouds, augmenting these data with Chandra X-ray observations to correct for incompleteness in dense clustered regions. We also devise a scheme to correct for spatially varying incompleteness when X-ray data are not available. The local surface densities of the YSOs range from 1 pc −2 to over 10,000 pc −2 , with protostars tending to be in higher density regions. This range of densities is similar to other surveyed molecular clouds with clusters, but broader than clouds without clusters. By identifying clusters and groups as continuous regions with surface densities ≥ 10 pc −2 , we find that 59% of the YSOs are in the largest cluster, the Orion Nebular Cluster (ONC), while 13% of the YSOs are found in a distributed population. A lower fraction of protostars in the distributed population is evidence that it is somewhat older than the groups and clusters. An examination of the structural properties of the clusters and groups show that the peak surface densities of the clusters increase approximately linearly with the number of members. Furthermore, all clusters with more than 70 members exhibit asymmetric and/or highly elongated structures. The ONC becomes azimuthally symmetric in the inner 0.1 pc, suggesting that the cluster is only ∼ 2 Myr in age. We find the star formation efficiency (SFE) of the Orion Germany-2 -B cloud is unusually low, and that the SFEs of individual groups and clusters are an order of magnitude higher than those of the clouds. Finally, we discuss the relationship between the young low mass stars in the Orion clouds and the Orion OB 1 association, and we determine upper limits to the fraction of disks that may be affected by UV radiation from OB stars or by dynamical interactions in dense, clustered regions.

How Do Stars Gain Their Mass? A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star-forming Regions

“…The fields include a total of 182 Class 0/I objects, 132 flat-spectrum objects, and 670 disks (see Table 1), as previously classified from Spitzer SEDs by Dunham et al (2010), Stutz et al (2013), and Megeath et al (2016). Each region includes 3-41 peaks with 850 μm fluxes above 0.5 Jy/ beam and 12-120 peaks above 0.12 Jy/beam, and 3-14 protostars associated with those peaks.…”

“…Each region includes 3-41 peaks with 850 μm fluxes above 0.5 Jy/ beam and 12-120 peaks above 0.12 Jy/beam, and 3-14 protostars associated with those peaks. All requested fields have a past epoch from the JCMT Gould Belt Survey (Ward- ), along with complementary Spitzer mid-IR (Dunham et al 2015;Megeath et al 2016) and Herschel far-IR imaging (André et al 2014). …”

“…Figure 2 and Table 1 show the total number of sources with peak 850 μm flux brighter than 0.125 Jy/beam for all regions in our survey, sorted by brightness bin and by association with protostars and disks (see also the Appendix). To better understand the distribution of protostellar cores versus starless cores in our regions, we collated the submillimeter peaks against catalogs of known protostars (Class 0/I objects) and disks (Class II objects), as identified in past Spitzer photometry (Dunham et al 2015;Megeath et al 2016) as well as extensive submillimeter imaging (e.g., Johnstone & Bally 1999;Hatchell et al 2005;Enoch et al 2006Enoch et al , 2007Johnstone et al 2006;Kirk et al 2006). Of the 342 bright peaks, 73 are associated with known protostars (Class 0/I) and 20 are associated with known disks (Class II).…”

“…SCUBA-2 850 μm images of all eight regions in our survey, co-added over the first year of data (see also Mairs et al 2017a). The marks show the location of Class 0, Class I, and flat spectrum protostars, as identified and classified by Dunham et al (2015) and Megeath et al (2016).…”

Episodic accretion in binary protostars emerging from self-gravitating solar mass cores

“…Observations have also revealed a considerable diversity in the morphology of young and developed star systems (Chen et al 2015;Megeath et al 2015). When investigated using numerical models, this diversity in the morphology of the systems has been found to be highly dependent on the initial conditions prevailing in star forming gas (Vanajakshi and Jenkins 1984;Sigalotti and Klapp 2000;Jessop and Ward 2001;Khesali et al 2013;Bate et al 2014).…”