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

Creators: Megeath, S. T.; Gutermuth, R.; Muzerolle, J.; Kryukova, E.; Hora, J. L.; Allen, L. E.; Flaherty, K.; Hartmann, L.; Myers, P. C.; Pipher, J. L.; Stauffer, J.; Young, E. T.; Fazio, G. G.

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 Nebula 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 shows 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 that the star formation efficiency (SFE) of the Orion 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 dynamical interactions in dense, clustered regions.

Additional Information

© 2016. The American Astronomical Society. Received 2014 July 5; accepted 2015 September 8; published 2015 December 21. This work benefited immeasurably from discussions with Fred Adams, John Bally, Cesar Briceno, Neal Evans, Gabor Furesz, Charlie Lada, Pavel Kroupa, Thomas Henning, Amy Stutz and Scott Wolk. The analysis of the data would not be possible without the superb support we received from the staff of the Spitzer Science Center. This publication makes use of data products from the 2MASS, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis. Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. It received support through that provided to the IRAC and MIPS instruments by NASA through contracts 960541 and 960785, respectively, issued by JPL. Support for this work was also provided by NASA through awards issued to S. T. M. and JLP by JPL/Caltech. This paper was mostly completed while S. T. M. was on sabbatical at the Max-Planck-Institut für Radioastronomie and further revised while continuing his sabbatical at the Max-Planck-Institut für Astronomie. S. T. M. thanks those institutes for their support.

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