A population of very young brown dwarfs and free‐floating planets in Orion

We describe the results of a very deep imaging survey of the Trapezium cluster in the IJH bands, using the UKIRT high-resolution camera UFTI. Approximately 32 per cent of the 515 point sources detected are brown dwarf candidates, including several free-floating objects with masses below the deuterium-burning (planetary) threshold at 0.013 M⊙, which are detectable because of their extreme youth. We have confidence that almost all the sources detected are cluster members, since foreground contamination is minimal in the 33-arcmin2 area surveyed, and the dense backdrop of OMC-1 obscures all background stars at these wavelengths. Extinction is calculated from the (J−H) colours, permitting accurate luminosity estimates, and temperatures are derived from the dereddened (I−J) colours. There is some evidence for a cut-off in the luminosity function below the level corresponding to several Jupiter masses, which may represent the bottom end of the initial mass function. Since star formation is complete in the Trapezium, this limit could have wide significance, if confirmed. However, it could well be an effect of the dispersal of the molecular cloud by the central O-type stars, a process for which the time-scale will vary between star formation regions.

[1]  S. Leggett.  Infrared Colors of Low-Mass Stars , 1992 .

[2]  G. Rieke,et al.  The Low-Mass IMF in the rho Ophiuchi Cluster , 1995 .

[3]  M. Bessell,et al.  JHKLM PHOTOMETRY: STANDARD SYSTEMS, PASSBANDS, AND INTRINSIC COLORS , 1988 .

[4]  J. Mathis,et al.  The relationship between infrared, optical, and ultraviolet extinction , 1989 .

[5]  Oasa,et al.  Isolated and companion young brown dwarfs in the taurus and chamaeleon molecular clouds , 1998, Science.

[6]  D. Depoy,et al.  A 2.2 micrometer imaging survey of the Orion a molecular cloud , 1995 .

[7]  C. Tinney.  The faintest stars - The luminosity and mass functions at the bottom of the main sequence , 1993 .

[8]  C. Tinney.  The Bottom of the Main Sequence — And Beyond , 1995 .

[9]  C. Lada,et al.  Low-Mass Star Formation and the Initial Mass Function in IC 348 , 1998 .

[10]  M. Tamura,et al.  A Deep Near-Infrared Survey of the Chamaeleon I Dark Cloud Core , 1999 .

[11]  G. Rieke,et al.  The Stellar Population in the rho Ophiuchi Cluster , 1993 .

[12]  M. Rieke,et al.  Properties of Low-Mass Objects in NGC 2024 , 1996 .

[13]  E. Young,et al.  IRAS Observations of the Rho Ophiuchi Infrared Cluster: Spectral Energy Distributions and Luminosity Function , 1989 .

[14]  J. Lunine,et al.  Protostars and planets III , 1993 .

[15]  A Preliminary Study of the Orion Nebula Cluster Structure and Dynamics , 1998 .

[16]  S. Kulkarni,et al.  Discovery of a cool brown dwarf , 1995, Nature.

[17]  T. Miller.  Dust in the galactic environment , 1993 .

[18]  G. Rieke,et al.  The Low-Mass Initial Mass Function in Young Clusters: L1495E , 1998 .

[19]  L. Hillenbrand.  On the Stellar Population and Star-Forming History of the Orion Nebula Cluster , 1997 .

[20]  F. Allard,et al.  Infrared Colors at the Stellar/Substellar Boundary , 1998, astro-ph/9807255.

[21]  Spectroscopy of Brown Dwarf Candidates in the ρ Ophiuchi Molecular Core , 1998, astro-ph/9810508.

[22]  C. O’Dell,et al.  Hubble Space Telescope Mapping of the Orion Nebula. I. A Survey of Stars and Compact Objects , 1996 .

[23]  T. Guillot,et al.  A Nongray Theory of Extrasolar Giant Planets and Brown Dwarfs , 1997, astro-ph/9705201.

[24]  Z. Wen,et al.  Postrefurbishment mission Hubble Space Telescope images of the core of the Orion Nebula: Proplyds, Herbig-Haro objects, and measurements of a circumstellar disk , 1994 .

[25]  M. Skrutskie,et al.  in Protostars and Planets III , 1993 .

[26]  M. Osorio,et al.  Discovery of a brown dwarf in the Pleiades star cluster , 1995, Nature.

[27]  L. Hartmann,et al.  The Embedded Young Stars in the Taurus-Auriga Molecular Cloud. II. Models for Scattered Light Images , 1993 .

[28]  S. Strom,et al.  The nature of the objects of Joy: A study of the T Tauri phenomenon , 1976 .