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Published September 1, 2000 | Submitted + Published
Journal Article Open

Constraints on the Stellar/Substellar Mass Function in the Inner Orion Nebula Cluster


We present the results of a 0."5-0."9 FWHM imaging survey at K (2.2 μm) and H (1.6 μm) covering ~5.'1 × 5.'1 centered on θ^1C Ori, the most massive star in the Orion Nebula Cluster (ONC). At the age and distance of this cluster, and in the absence of extinction, the hydrogen-burning limit (0.08 M_☉) occurs at K ≈ 13.5 mag, while an object of mass 0.02 M_☉ has K ≈ 16.2 mag. Our photometry is complete for source detection at the 7 σ level to K ≈ 17.5 mag and thus is sensitive to objects as low-mass as 0.02 M_☉ seen through visual extinction values as high as 10 mag. We use the observed magnitudes, colors, and star counts to constrain the shape of the inner ONC stellar mass function across the hydrogen-burning limit. After determining the stellar age and near-infrared excess properties of the optically visible stars in this same inner ONC region, we present a new technique that incorporates these distributions when extracting the mass function from the observed density of stars in the K-(H-K) diagram. We find that our data are inconsistent with a mass function that rises across the stellar/substellar boundary. Instead, we find that the most likely form of the inner ONC mass function is one that rises to a peak around 0.15 M_☉, and then declines across the hydrogen-burning limit with slope N(log M) ∝ M^(0.57). We emphasize that our conclusions apply to the inner 0.71 pc × 0.71 pc of the ONC only; they may not apply to the ONC as a whole where some evidence for general mass segregation has been found.

Additional Information

© 2000 American Astronomical Society. Received 1999 November 22. Accepted 2000 March 30. We thank Mike Liu and James Graham for sharing their method and source code for "debleeding" of NIRC images. We thank Andrea Ghez for providing her image distortion coefficients. We thank Keith Matthews for consultation regarding these and other NIRC features. Shri Kulkarni and Ben Oppenheimer suggested a mutually advantageous exchange of telescope time, which enabled us to obtain the Z-band observations. Ted Bergin kindly provided his C^(18)O data to us and Richard Wainscoat gave us a base code for his star count model. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation. L. A. H. acknowledges support from NASA Origins of Solar Systems grant NAG 5-7501. J. M. C. acknowledges support from NASA Long Term Space Astrophysics grant NAG 5-8217, and from the Owens Valley Radio Observatory, which is operated by the California Institute of Technology through NSF grant 96-13717.

Attached Files

Published - Hillenbrand_2000_ApJ_540_236.pdf

Submitted - 0003293.pdf


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August 21, 2023
October 26, 2023