ISOCAM based search for Brown Dwarfs in the Hyades cluster

Babar Ali ¹, William Forrest ², John Stauffer ³ & Sandy Leggett ⁴,

¹ University of Rochester Dept. of Physics & Astronomy B&L Hall, Wilson Blvd. / River Campus Rochester, NY 14627 USA

² Harvard-Smithsonian Center for Astrophysics 60 Garden Street Cambridge, MA 02138 USA

³ United Kingdom InfraRed Telescope Joint Astronomy Centre 660 N. A'ohoku Place Hilo, Hawaii 96720 USA

We report on an ISOCAM based program to search for brown dwarfs which are members of the Hyades cluster. We use the LW1 (4.5 micron) and the LW2 (6.75 micron) filters with the 6 arc-sec. PFOV lens to image 24 fields near known low mass Hyads (with no known companions) to maximize the probability that another, even lower mass, brown dwarf may lurk nearby. At the age of Hyades (620 Myr; Perryman et al. 1998, AA, 331, 81), the brown dwarf members of the cluster are easily identified because their surface temperatures have cooled to below 1500 K degrees and, hence, lie below the bottom ``edge'' of the main-sequence for solar metallicity stars. Such low effective temperatures, however, require flux sensitivity of <50 micro-Jy to detect brown dwarfs with masses > 0.02 solar masses (Burrows et al. 1996, ApJ, 491, 856) at the distance of Hyades (Perryman et al. 1998, AA, 331, 81). We have devised an observing strategy and an analysis procedure that is capable of reaching this flux sensitivity. This procedure bypasses many of the common problems associated with the ISOCAM data such as charged particle hits by: (1) treating each pixel as an individual detector, and (2) by fitting the flux history of the pixel (referred to as the Pixel Response Function - PRF) with a model PRF obtained from the program Hyades member present in the field. The observed PRF is similar to a ``square-wave'' pattern. The fit comprises a multiplicative component of the PRF (the signal) and an additive offset (dark current and background). The charged particle hits are identified as deviations from the PRF and rejected. The preliminary calibration results suggest that we detect sources with fluxes as faint as 150 micro-Jy at a few sigma level. Further refinements in the procedures (such as accounting for pixel-to-pixel variations) are expected to reach even fainter flux levels. The flux sensitivity thus achieved allows us to carry out our main scientific objective. A preliminary examination of the resulting images shows a number of sources of interest in the ISOCAM fields. We categorize these ISOCAM sources into the following groups: (A) point sources undetected on the Palomar Observatory Sky Survey (POSS) plates - 2 sources. (B) point sources near the detection limit of the POSS - 2 sources. (C) Extended sources undetected on the POSS - 2 sources, and (D) extended sources detected on the POSS - 3 sources. Roughly 25% of the data have been fully reduced. We expect the the reduction and source count to be complete by September 1998.