Astrophys. J., 556, 830-836 (2001/August-1)
The substellar mass function in σ Orionis.
BEJAR V.J.S., MARTIN E.L., ZAPATERO OSORIO M.R., REBOLO R., BARRADO Y NAVASCUES D., BAILER-JONES C.A.L., MUNDT R., BARAFFE I., CHABRIER G. and ALLARD F.
Abstract (from CDS):
We combine results from imaging searches for substellar objects in the σ Orionis cluster and follow-up photometric and spectroscopic observations to derive a census of the brown dwarf population in a region of 847 arcmin2. We identify 64 very low mass cluster member candidates in this region. We have available three-color (I, Z, and J) photometry for all of them, spectra for 24 objects, and K photometry for 27% of our sample. These data provide a well-defined sequence in the I versus I-J and I versus I-K color-magnitude diagrams and indicate that the cluster exhibits little reddening despite its young age (∼5 Myr). Using state-of-the-art evolutionary models, we derive a mass function from the low-mass stars (0.2 M☉) across the complete brown dwarf domain (0.075 to 0.013 M☉) and into the realm of free-floating planetary-mass objects (≤0.013 M☉). We find that the mass spectrum (dN/dm)∝m–α increases toward lower masses, with an exponent α=0.8±0.4. Our results suggest that planetary-mass isolated objects could be as common as brown dwarfs; both kinds of objects together would be as numerous as stars in the cluster. If the distribution of stellar and substellar masses in σ Orionis is representative of the Galactic disk, older and much lower luminosity free-floating planetary-mass objects with masses down to about 0.005 M☉ should be abundant in the solar vicinity, with a density similar to M-type stars.
Galaxy: Open Clusters and Associations: Individual: Name: sigma Orionis - Stars: Low-Mass, Brown Dwarfs - Stars: Luminosity Function, Mass Function - Stars: Pre-Main-Sequence
Table 1: [BMZ2001] S Ori JHHMMSS.s+DDMMSS N=78
CHABRIER C. misprint for CHABRIER G.
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