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Astron. Astrophys. 335, 522-532 (1998)

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1. Introduction

Intense theoretical and observational efforts have been devoted recently to identification of brown dwarfs and to study of their properties. This effort is justified by their intrinsic interest as transition objects between stars and planets, but also by their role in a wide range of astrophysical problems, such as their contribution to the dark matter content of galactic disks, or the insights they provide to the processes of accretion and fragmentation leading to star formation. Moreover, they can test models of pre-main sequence evolution at very low masses and probe the complex chemistry of cool photospheres. Until recently the search for these objects had produced only probable candidates (see summary in Trimble 1995). However, the situation has changed dramatically in the last two years, with the detection of several objects with clearly substellar and even planetary masses (Rebolo 1998).

The discovery of bona-fide brown dwarfs elsewhere heightens the interest in searches for such objects in embedded clusters. In addition to the important observational advantage of relatively high luminosity and temperature, these sources sample a region of the temperature-surface-gravity diagram already abandoned by the more evolved brown dwarfs. Their potential detectability in significant numbers enables derivation of the shape of the low mass and substellar initial mass function, which is not possible at present with the evolved brown dwarfs. However, the observational distinction between massive brown dwarfs and very low mass stars is subtle at the ages of embedded clusters, and furthermore it can be masked by age effects and by circumstellar material.

The [FORMULA] Ophiuchi clouds contain one of the most nearby and most thoroughly studied populations of young stellar objects. Surveys of the area have been conducted at a variety of wavelengths (Barsony et al. 1989; Rieke et al. 1989; Wilking et al. 1989; Leous et al. 1991; André et al. 1992; Bouvier & Appenzeller 1992; Greene & Young 1992; Mezger et al. 1992; Comerón et al. 1993; André & Montmerle 1994; Greene et al. 1994; Casanova et al. 1995; Greene & Meyer 1995; Strom et al. 1995; Greene & Lada 1996; Barsony et al. 1997; Kenyon et al. 1998). Photometric and spectroscopic surveys in the near infrared have provided insights to the stellar mass function and the evolutionary status of the aggregate.

Nevertheless, some uncertainty remains in the classification of the faintest embedded sources, such as the relatively blue objects which are very likely substellar (Rieke & Rieke 1990, Comerón et al. 1993, Williams et al. 1995). High quality spectroscopy is not feasible for these sources, and photometric techniques are subject to potentially distorting effects from circumstellar material and line-of-sight extinction. To account for these factors more accurately, it is necessary to have a wide wavelength coverage, as has been made possible by the imaging camera ISOCAM on the Infrared Space Observatory (ISO). In this paper, we present and discuss the results of ISOCAM observations of some of the candidate brown dwarfs in [FORMULA] Ophiuchi. Some of the fields selected for our ISOCAM observations also contain low mass stars, whose characteristics we analyze together with those of the candidate brown dwarfs. In addition, we present recent follow-up observations in the visible and near infrared of one of the fields imaged by ISO.

Our source selection criteria and observations are described in Sect. 2. Sect. 3 summarizes our method of analysis. The results, including an object-by-object discussion, are presented in Sect. 4. Our conclusions are summarized in Sect. 5. Technical details of the ISOCAM data analysis are given in Appendix A.

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© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998