Searches in young stellar clusters

sigma and epsilon Orionis

Recent deep surveys in very young stellar clusters have shown that the fragmentation of clouds extends well below the frontier between stars and brown dwarfs, reaching the planetary-mass domain (Zapatero Osorio et al. 2000; Lucas and Roche 2000). Very recently the detection of a 2-8 M_Jup object has been reported free-floating in the sigma Orionis star cluster (Zapatero Osorio et al. 2002). This is the lowest mass object directly imaged outside the Solar System. It seems that Jupiter-like objects also exist isolated from stars. Their origins are not yet well understood.

The classical picture of formation of jupiters in protoplanetary disks has been recently challenged by the detection of a rather numerous population of objects with masses below 10 M_Jup in very young stellar clusters and star forming regions. In the young (age 1-8 Myr) sigma Orionis star cluster, the correlation of deep optical and infrared images obtained by our group allowed identification of 18 objects with masses within the planetary-mass regime, i.e., below the deuterium-burning mass limit (M less than 0.013 M_Sun). Spectroscopy of these objects resulted in classification as types L0-L4 (T_eff in the range 2000-1500 K), extending significantly the substellar spectral sequence in the cluster (Zapatero Osorio et al. 2000; Barrado y Navascués et al. 2001; Martín et al. 2001).

The behavior of the mass function in the 10-1 M_Jup is crucial to delineating the formation mechanism that gives rise to IPMOs (Isolated Planetary Mass Objects). Are they formed in protoplanetary disks around stars and then ejected?. Or are they formed in the same fragmentation process as low mass stars and brown dwarfs?. What is the minimum mass of objects produced in such process?.

If the recent findings in sigma Orionis were representative of other star forming regions and, in general, of the Galactic disk, free-floating Jupiter-like objects would be as numerous as solar-type stars. Some very efficient mechanism for formation of this kind of objects may be present in nature. It could possibly be more efficient than the much more intensively studied formation processes in protoplanetary disks.

Our refereed publications

Optical and infrared photometry of new very low-mass stars and brown dwarfs in the sigma Orionis cluster
Béjar, V. J. S.; Osorio, M. R. Zapatero; Rebolo, R.
2004, Astron. Nachr., in press. (astro-ph/0404125)

The sigma Orionis substellar population. VLT/FORS spectroscopy and 2MASS photometry
Barrado y Navascués, D.; Béjar, V. J. S.; Mundt, R.; Martín, E. L.; Rebolo, R.; Zapatero Osorio, M. R.; Bailer-Jones, C. A. L.
2004, A&A, 404, 171

Discovery of a very cool object with extraordinarily strong H_alpha emission
Barrado y Navascués, D.; Zapatero Osorio, M. R.; Martín, E. L.; Béjar, V. J. S.; Rebolo, R.; Mundt, R.
2002, A&A, 393, L85

A Methane, Isolated, Planetary-Mass Object in Orion
Zapatero Osorio, M. R.; Béjar, V. J. S.; Martín, E. L.; Rebolo, R.; Navascués, D. Barrado y.; Mundt, R.; Eislöffel, J.; Caballero, J. A.
2002, ApJ, 578, 536

Activity at the Deuterium-burning Mass Limit in Orion
Zapatero Osorio, M. R.; Béjar, V. J. S.; Martín, E. L.; Barrado y Navascués, D.; Rebolo, R.
2002, ApJ, 569, L99

Lithium and Halpha in stars and brown dwarfs of sigma Orionis
Zapatero Osorio, M. R.; Béjar, V. J. S.; Pavlenko, Ya.; Rebolo, R.; Allende Prieto, C.; Martín, E. L.; García López, R. J.
2002, A&A, 384, 937

Optical spectroscopy of isolated planetary mass objects in the sigma Orionis cluster
Barrado y Navascués, D.; Zapatero Osorio, M. R.; Béjar, V. J. S.; Rebolo, R.; Martín, E. L.; Mundt, R.; Bailer-Jones, C. A. L.
2001, A&A, 377 ,L9

Keck NIRC Observations of Planetary-Mass Candidate Members in the sigma Orionis Open Cluster
Martín, E. L.; Zapatero Osorio, M. R.; Barrado y Navascués, D.; Béjar, V. J. S.; Rebolo, R.
2001, ApJ, 558, L117

The Substellar Mass Function in sigma Orionis
Béjar, V. J. S.; Martín, E. L.; Zapatero Osorio, M. R.; Rebolo, R.; Barrado y Navascués, D.; Bailer-Jones, C. A. L.; Mundt, R.; Baraffe, I.; Chabrier, C.; Allard, F.
2001, ApJ, 556, 830

Discovery of Young Isolated Planetary Mass Objects in the sigma Orionis Star Cluster
Zapatero Osorio, M. R.; Béjar, V. J. S.; Martín, E. L.; Rebolo, R.; Barrado y Navascués, D.; Bailer-Jones, C. A. L.; Mundt, R.
2000, Science, 290, 103

An L-Type Substellar Object in Orion: Reaching the Mass Boundary between Brown Dwarfs and Giant Planets
Zapatero Osorio, M. R.; Béjar, V. J. S.; Rebolo, R.; Martín, E. L.; Basri, G.
1999, ApJ, 524L, 115

A Search for Very Low Mass Stars and Brown Dwarfs in the Young sigma Orionis Cluster
Béjar, V. J. S.; Zapatero Osorio, M. R.; Rebolo, R.
1999, ApJ, 521, 671

Brown dwarfs and very low-mass stars in the Pleiades cluster: a deep wide-field imaging survey
Bouvier, J.; Stauffer, J. R.; Martin, E. L.; Barrado y Navascues, D.; Wallace, B.; Bejar, V. J. S.
1998, A&A, 336, 490

Brown dwarfs in the Pleiades cluster. III. A deep IZ survey
Zapatero Osorio, M. R.; Rebolo, R.; Martín, E. L.; Hodgkin, S. T.; Cossburn, M. R.; Magazzù, A.; Steele, I. A.; Jameson, R. F.
1999, A&AS, 134, 537

Discovery of a Very Low Mass Binary with the Hubble Space TelescopeNear-Infrared Camera and Multiobject Spectrometer
Martín, E. L.; Basri, G.; Brandner, W.; Bouvier, J.; Zapatero Osorio, M. R.; Rebolo, R.; Stauffer, J.; Allard, F.; Baraffe, I.; Hodgkin, S. T.
1998, ApJ, 509L, 113

The First L-Type Brown Dwarf in the Pleiades
Martín, E. L.; Basri, G.; Zapatero-Osorio, M. R.; Rebolo, R.; López, R. J. García
1998, ApJ, 507L, 41

A New Pleiades Member at the Lithium Substellar Boundary
Martin, E. L.; Basri, G.; Gallegos, J. E.; Rebolo, R.; Zapatero-Osorio, M. R.; Bejar, V. J. S.
1998, ApJ, 499L, 61

A Brown Dwarf Candidate in the Praesepe Open Cluster
Magazzu, A.; Rebolo, R.; Zapatero Osorio, M. R.; Martin, E. L.; Hodgkin, S. T.
1998, ApJ, 497L, 47M

New Brown Dwarfs in the Pleiades Cluster
Zapatero Osorio, M. R.; Rebolo, R.; Martin, E. L.; Basri, G.; Magazzu, A.; Hodgkin, S. T.; Jameson, R. F.; Cossburn, M. R.
1997, ApJ, 491L, 81

Brown dwarfs in the Pleiades cluster. II. J, H and K photometry.
Zapatero Osorio, M. R.; Martin, E. L.; Rebolo, R.
1997, A&A, 323, 105

Brown dwarfs in the Pleiades cluster: a CCD-based R, I survey.
Zapatero Osorio, M. R.; Rebolo, R.; Martin, E. L.
1997, A&A, 317, 164

Spectroscopy of New Substellar Candidates in the Pleiades: Toward a Spectral Sequence for Young Brown Dwarfs
Martin, Eduardo L.; Rebolo, Rafael; Zapatero-Osorio, Maria Rosa
1996, ApJ, 469, 706

Brown Dwarfs in the Pleiades Cluster Confirmed by the Lithium Test
Rebolo, R.; Martin, E. L.; Basri, G.; Marcy, G. W.; Zapatero-Osorio, M. R.
1996, ApJ, 469L, 53

Stars approaching the substellar limit in the {alpha} Persei open cluster
Zapatero Osorio, M. R.; Rebolo, R.; Martin, E. L.; Garcia Lopez, R. J.
1996, A&A, 305, 519

Discovery of a Brown Dwarf in the Pleiades Star Cluster
Rebolo, R.; Zapatero-Osorio, M. R.; Martin, E. L.
1995, Nature, 377, 129

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