An infrared coronagraphic search for substellar companions

June 2001 • 2001PhDT........18L

Authors • Lowrance, Patrick James

Abstract • We present the results of a survey for the direct detection of substellar companions. We have used an infrared space-based coronagraph on the Near-InfraRed Camera and MultiObject Spectrometer (NICMOS) on the Hubble Space Telescope to observe approximately 50 single stars with a median age of 0.15 Gyr, an average distance of 30 pc, and an average H-magnitude of 7 mag. The survey included stars ranging from spectral type A through M and concentrated on the cooler, lower-mass stars where the contrast ratio in luminosity would be lower. Included in the observations were several members of the very young (10-40 Myr) TW Hydrae and Tucanae Asoociations located 50 pc from the Earth. Substellar objects are predicted to be more luminous when younger, and therefore easier to detect. The F160W filter (1.4-1.8 μm) and the coronagraph were used to search the region 0.4''-4'' around the primary stars. Our detection limit was improved by subtracting two images from the same orbit that were both centered on the star, but had a 29.9 degree difference in telescope orientation. Using independently determined ages and distances for the primaries, we can ascertain the masses of detected secondaries from their infrared magnitudes and theoretical evolutionary tracks. The lower mass limit of a detectable companion depends on the brightness and age of the target star as well as the angular separation between star and companion. For the median age of 0.15 Gyr, we were able to detect a 30M_Jupiter companion at separations between 15 and 200 AU. A 5M_Jupiter high mass planet would have been detected at 30 AU around 36% of our primaries. For several of our targets that were less than 30 Myr old, the lower mass limit was as low as a Jupiter mass, well into the high mass planet region. We present results that include the detection of two low- mass stellar companions, two very likely brown dwarfs (located 100 AU and 180 AU from their primaries), and one possible brown dwarf binary companion (located 200 AU from its primary). We add these likely substellar companions to the very few already known. Our results are consistent with a flat (α = 1) mass function seen in field stars and the Pleaides. We have detected a higher substellar binary frequency than other companion searches, especially radial velocity searches that display an apparent lack of brown dwarf companions. Therefore, we suggest the distribution of orbital separations of brown dwarf companions might be distinctly different from that of stellar companions.

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• ELECTR http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:9999003