Astrophys. J., 754, 30 (2012/July-3)
New isolated planetary-mass objects and the stellar and substellar mass function of the σ Orionis cluster.
PENA RAMIREZ K., BEJAR V.J.S., ZAPATERO OSORIO M.R., PETR-GOTZENS M.G. and MARTIN E.L.
Abstract (from CDS):
We report on our analysis of the VISTA Orion ZY JHKs photometric data (completeness magnitudes of Z = 22.6 and J = 21.0 mag) focusing on a circular area of 2798.4 arcmin2 around the young σ Orionis star cluster (∼3 Myr, ∼352 pc, and solar metallicity). The combination of the VISTA photometry with optical, WISE and Spitzer data allows us to identify a total of 210 σ Orionis member candidates with masses in the interval 0.25-0.004 M☉, 23 of which are new planetary-mass object findings. These discoveries double the number of cluster planetary-mass candidates known so far. One object has colors compatible with a T spectral type. The σ Orionis cluster harbors about as many brown dwarfs (69, 0.072-0.012 M☉) and planetary-mass objects (37, 0.012-0.004 M☉) as very low mass stars (104, 0.25-0.072 M☉). Based on Spitzer data, we derive a disk frequency of ∼40% for very low mass stars, brown dwarfs, and planetary-mass objects in σ Orionis. The radial density distributions of these three mass intervals are alike: all are spatially concentrated within an effective radius of 12' (1.2 pc) around the multiple star σ Ori, and no obvious segregation between disk-bearing and diskless objects is observed. Using the VISTA data and the Mayrit catalog, we derive the cluster mass spectrum (ΔN/ΔM ∼ M –α) from ∼19 to 0.006 M☉(VISTA ZJ completeness), which is reasonably described by two power-law expressions with indices of α = 1.7±0.2 for M > 0.35 M☉, and α = 0.6±0.2 for M < 0.35 M☉. The σ Orionis mass spectrum smoothly extends into the planetary-mass regime down to 0.004 M☉. Our findings of T-type sources (<0.004 M☉) in the VISTA σ Orionis exploration appear to be smaller than what is predicted by the extrapolation of the cluster mass spectrum down to the survey J-band completeness.
brown dwarfs - circumstellar matter - infrared: stars - open clusters and associations: individual: σ Orionis - stars: low-mass - stars: luminosity function, mass function
VizieR on-line data:
<Available at CDS (J/ApJ/754/30): table3.dat table5.dat table6.dat table7.dat table8.dat refs.dat>
Table 8: [BMZ2001] S Ori JHHMMSS.s+DDMMSS N=20. Tables 4+9+11: [PBZ2012] JHHMMSS.ss+DDMMSS.s N=30+14+4.
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