Constraints on the Stellar/Substellar Mass Function
in the Inner Orion Nebula Cluster
Lynne A. Hillenbrand
John M. Carpenter
Department of Astronomy
California Institute of Technology
Pasadena, CA 91125
We present the results of a 0.5-0.9" FWHM imaging survey at K (2.2 micron)
and H (1.6 micron) covering ~ 5.1' × 5.1' centered on
Theta¹C 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 Mo) occurs at
K~13.5 mag while an object of mass 0.02 Mo
has K~16.2 mag. Our photometry is complete
for source detection at the 7 sigma level to K~17.5 mag and
thus is sensitive to objects as low-mass as 0.02o seen through
visual extinction values as high as 10 magnitudes. 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/sub-stellar 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 Mo, and then declines across the hydrogen-burning limit
with slope N(log M) ~ M0.57+/-0.05.
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.
Compressed postscript version of entire paper including figures