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Session 44 - Double Stars.
Display session, Thursday, January 08
Exhibit Hall,
We have begun a near-IR search for very-low-mass (VLM) companions to stars within 10 pc of the Sun, via a SNAP survey using NICMOS Camera 2 (NIC2). With a 10 \sigma detection limit of M_J \sim 22 at 10 pc, we can detect companions at separations of 0\farcs5 to 10\arcsec that are at least 11 mag fainter than the empirical end of the main sequence and at least 6.5 mag fainter than brown dwarf Gl 229B. By directly imaging in four bandpasses (F110W, F180M, F207M, F222M), we may readily distinguish VLM stars and cool brown dwarfs from white dwarfs and more distant main-sequence stars, thereby identifying companion candidates for follow-up observation. When completed, this survey will be the largest, most sensitive, volume-limited search for VLM companions ever undertaken. Together with existing IR speckle and wide-field surveys, our program will firmly establish the luminosity function and multiplicity fraction for stars within 10 pc. We present here the first results of our survey.
To properly subtract the scattered light of the target star from the field of view, the PSF's from each NIC2 image are examined and grouped according to brightness and diffraction variations. (Our data reveal asymmetric and time-varying artifacts in NIC2's PSF attributable to ghosts and misalignment of the camera's cold stop.) For each group of images, reference PSF's are selected for subtraction of other star images within that group. Examples of these subtractions from NIC2 images of the Gl 229 system are presented, along with radially varying sensitivity limits for each survey bandpass. These limits may be greatly degraded by miscalibration of the raw NIC2 images. Methods of validating the STScI pipeline calibration are presented.
