1. Searching for Brown Dwarf Companions to Nearby Stars Michael W. McElwain, James E. Larkin & Adam J. Burgasser (UC Los Angeles) Background on Brown Dwarfs Simulated OSIRIS Observations Brown dwarfs are intrinsically faint objects with spectral features similar to the planets Jupiter and Saturn. Until recently, these objects have eluded detection due to their faint luminosities that almost entirely emit at near infrared wavelengths (~1μm to 3 μm). Results from studies show that the distribution of brown dwarf separations from main sequence stars peaks at small distances, where the brown dwarf is located within the halo of the host star for even the best ground based telescopes. When a brown dwarf is located in the halo of the host star, the brown dwarf spectrum is added to the host spectrum. A characterization of the host and brown dwarf spectrum can help construct a digital filter that will exploit the differences in the spectra. The ideal filter will optimally amplify the brown dwarf spectrum relative to the host spectrum, thus extracting the otherwise hidden companion from the halo. The OSIRIS instrument is an infrared imaging spectrograph expected to operate with the Keck Adaptive Optics System in the summer of 2004. Its data product is a digital image that contains a spectrum at each pixel, which makes it a perfect match to implement this new technique. OSIRIS is currently being assembled at UCLA. Using Digital Filters to Increase Companion Contrast at Unresolved Separations x y OSIRIS Data Cube Cartoon  Normalized Input SpectraWeighted Digital Filter Brown dwarf spectrum Sun-like spectrum (Brown dwarf spectrum / Sun-like spectrum) -1 Brown dwarfs have an insufficient mass to obtain the core temperatures necessary for sustained hydrogen fusion. Brown dwarfs have masses between stars and planets. *When the companion is separated from the host star, there is no need to use a digital filter. The digital filtering technique is only used to probe the regions where the companion is in the halo of the host star. At unresolved separations, the light emitted from the companion is overwhelmed by the light from the host star. -The signal from the companion is overlaid on the signal from the host star. Brown dwarfs have different spectral characteristics than typical stars. - Knowledge about the spectral information can be used to design a weighted filter which maximizes the contrast between the brown dwarf and host. - The objective is to effectively null the signal from the host star while amplifying the signal of the companion. In order to do this, the spectrum contained within each pixel of the image is multiplied by a filter. This assigns the appropriate weights to each spectral channel. In practice, a suite of filters will be applied to see if any will reveal a companion. OSIRIS – OH Suppressing Infra-Red Imaging Spectrograph, to be delivered to Keck in the summer of 2004. Advantages of Using OSIRIS - OSIRIS obtains high angular resolution by using the Keck Adaptive Optics System. - OSIRIS has moderate spectral resolution. - When the companions are separated, a simultaneous direct image and spectrum is taken of both the host star and the companion. The positions of both the host and brown dwarf (astrometry) are used to determine separations, and the spectrum (spectroscopy) is critical in classifying the companion. - Keck is the largest telescope available, which means that it collects the most light and has the smallest diffraction limit. Companion located in the halo of a solar type star. In this particular case, the companion is visible from direct imaging alone. Liu et al. 2002 Spectrum obtained at each pixel in the image. This adds up to over 4,000 simultaneous spectra! Before filterAfter filter Brown dwarf Sun-like star Why Study Brown Dwarfs? Brown dwarfs have recently been discovered and classified into new spectral types. - Fundamental property of star formation. - Characterize the Solar Neighborhood. - Brown dwarf atmospheres are similar to the gas giants’ in our own Solar System. Identification of the complicated molecular behavior in cool atmospheres is critical for extrasolar planet studies. - Discover even lower mass objects than currently known. - Possible solution to the dark matter problem. *Unlikey Why Study Brown Dwarfs Companions? - Orbital measurements can be used to determine brown dwarf masses and radii, which would then constrain their temperatures and luminosities. - Characterize star formation and the frequency of brown dwarf companions. - These observations approach the limit for making direct detections of extrasolar planets. *Increases companion contrast by at least a factor of 10.