Candidate young brown dwarfs observed by ISOCAM

F. Comeron ¹, G.H. Rieke ², P. Claes ³, J. Torra ⁴, & R.J. Laureijs ⁵

¹ European Southern Observatory, Garching, Germany

² Steward Observatory, University of Arizona, Tucson, Arizona, USA

³ European Space Agency, ESTEC, Noordwijk, Netherlands

⁴ Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Barcelona, Spain

⁵ European Space Agency, Villafranca Satellite Tracking Station, Madrid, Spain

In the early stages of their evolution, while they are still bright and hot, brown dwarfs should be easy to distinguish in nearby star forming regions by present-day near infrared detectors. Several candidates have been identified so far in ground-based searches of nearby embedded aggregates; the sensitivity of ISOCAM in the 3 to 8  region has also made these objects accessible to the observation at those wavelengths.

We obtained ISOCAM observations or upper limits for very low mass members of the Rho Ophiuchi embedded cluster, including the best brown dwarf candidates previously identified. The selection was based on their near infrared fluxes in existing ground-based surveys. The intrinsic spectral energy distributions of such young embedded objects can be severely distorted by the extinction due to the intervening dust associated to the parental molecular cloud, by the short wavelength light of the central object that is reprocessed by circumstellar material, and by the luminosity that can be produced in the circumstellar environment by energy dissipation of accreting gas and dust. All these factors produce an emergent spectral energy distribution redder than the original one. On the other hand, it is necessary to reconstruct this intrinsic spectral energy distribution in order to estimate the temperature and luminosity of the central object, which, by comparison to pre-main sequence evolutionary tracks, will ultimately yield important parameters such as age and mass. The latter parameter ultimately decides whether the object is destined to become a hydrogen-burning star, or rather will fade and cool down indefinitely as a brown dwarf.

We will show how our new ISOCAM observations have been crucial in allowing a reliable decomposition of these distorting factors, producing in this way a much more robust determination of the intrinsic parameters of the central objects than the ones available so far. Estimates of the source temperatures from fitting of the photometry agree closely with spectroscopy for seven sources where both techniques have been applied. Three of the sources are very likely to be young brown dwarfs, five are transitional, and three appear to be low mass stars.