A deep look into the cores of young clusters^*

I. σ-Orionis

¹ Instituto de Astrofísica de Canarias, C/ vía Láctea s/n, 38205 La Laguna, Tenerife, Spain e-mail: bouy@iac.es
² Astronomy Department, University of California, Berkeley, CA 94720, USA
³ Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF-INTA), PO Box 78, 28691, Villanueva de la Cañada, Madrid, Spain
⁴ University of Central Florida, Department of Physics, PO Box 162385, Orlando, FL 32816-2385, USA
⁵ European Southern Observatory, Karl Schwartzschild Str. 2, 85748 Garching bei München, Germany
⁶ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
⁷ ZAH, Landessternwarte, Königstuhl, 69117 Heidelberg, Germany

Received: 27 May 2008
Accepted: 26 September 2008

Abstract

Context. Nearby young clusters are privileged places to study the star formation history. Over the last decade, the σ-Orionis cluster has been a prime location for the study of young very low mass stars, substellar and isolated planetary mass objects and the determination of the initial mass function.

Aims. To extend previous studies of this association to its core, we searched for ultracool members and new multiple systems within the central region of the cluster.

Methods. We obtained deep multi-conjugate adaptive optics (MCAO) images of the core of the σ-Orionis cluster with the prototype MCAO facility MAD at the VLT using the H and K_s filters. These images allow us to detect companions fainter by  mag as close as 02 on a typical source with  mag. These images were complemented by archival SofI K_s-band images and Spitzer IRAC and MIPS mid-infrared images

Results. We report the detection of 2 new visual multiple systems, one being a candidate binary proplyd and the other one a low mass companion to the massive star σ Ori E. Of the 36 sources detected in the images, 25 have a H-band luminosity lower than the expected planetary mass limit for members, and color consistent with the latest theoretical isochrones. Nine objects have additional Spitzer photometry and spectral energy distribution consistent with them being cluster members. One of them has a spectral energy distribution from H to 3.6 μm consistent with that of a 5.5 M_Jup cluster member. Complementary NTT/SofI and Spitzer photometry allow us to confirm the nature and membership of two L-dwarf planetary mass candidates.