1. INTRODUCTION2. TARGETS3. OBSERVATIONS3.1. Coronagraphic Search Observations3.2. Common Proper Motion Observations4. DATA ANALYSIS4.1. Reducing Images4.2. Identifying Brown Dwarf Companions5. SURVEY SENSITIVITIES5.1. Determining Limiting Magnitudes5.2. Mass Sensitivities6. RESULTS7. DISCUSSIONREFERENCESThe Astronomical Journal, 137:218–225, 2009 January doi:10.1088/0004-6256/137/1/218c2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.A DISTANCE-LIMITED IMAGING SURVEY OF SUBSTELLAR COMPANIONS TO SOLARNEIGHBORHOOD STARSJoseph C. Carson1, Kyle D. Hiner2,4, Gregorio G. Villar III2, Michael G. Blaschak2, Alexander L. Rudolph2,andKarl R. Stapelfeldt31Max-Planck-Institut f¨ur Astronomie, K¨unigstuhl 17, 69117 Heidelberg, Germany2Physics Department, California State Polytechnic University, Pomona, 3801 W. Temple Ave., Pomona, CA 91768, USA3Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109, USAReceived 2008 September 9; accepted 2008 October 9; published 2008 December 10ABSTRACTWe report techniques and results of a Palomar 200 inch (5 m) adaptive optics imaging survey of substellar compan-ions to solar-type stars. The survey consists of Kscoronagraphic observations of 21 FGK dwarfs out to 20 pc (mediandistance ∼17 pc). At 1separation (17 projected AU) from a typical target system, the survey achieves mediansensitivities 7 mag fainter than the parent star. In terms of companion mass, this corresponds to sensitivities of 50 MJ(1 Gyr), 70 MJ(solar age), and 75 MJ(10 Gyr), using the evolutionary models of Baraffe and colleagues. Usingcommon proper motion to distinguish companions from field stars, we find that no system shows positive evidenceof a previously unknown substellar companion (searchable separation ∼20–250 projected AU at the median targetdistance).Key words: methods: data analysis – stars: low-mass, brown dwarfs – surveys – techniques: high angular resolution1. INTRODUCTIONThe discovery of the brown dwarf GJ 229B (Nakajima et al.1995) heralded a stream of direct detections of substellar objects.In particular, field surveys like the Sloan Digital Sky Survey(SDSS; Gunn & Weinberg 1995), the Two Micron All SkySurvey (2MASS; Skrutskie et al. 1997), and the Deep NearInfrared Survey (DENIS; Epchtein 1997) has helped to raise thenumber of brown dwarf (L- and T-type) identifications today toover 600 (Gelino et al. 2008). However, the number of browndwarfs identified as companions to main-sequence stars remainsfew. At the time of writing this paper, there are only a handful ofbrown dwarfs confirmed as companions to main-sequence stars.Brown dwarfs that are part of stellar systems are particularlyinteresting because they often yield insights into brown dwarfand planet formation around stars. For instance, statistics on thefrequency of brown dwarf companions may shed light on thedifferences among planet, brown dwarf, and star formation. Andunlike the case of discovered field brown dwarfs, the presenceof a central star often reveals additional information, such asdistance, metallicity, and age, on the presumably coevolvedbrown dwarf.A number of high-contrast surveys have attempted to improveour knowledge of the moderate-to-wide separation (40 AU to afew hundred AU) substellar companion population around stars.For example, Biller et al. (2007) and Metchev & Hillenbrand(2004) each surveyed samples (45 targets, 101 targets, respec-tively) of young (age  250 Myr,  400 Myrs), relatively nearby( 50 pc,  160 pc) stars using adaptive optics (AO) systemson the Very Large Telescope (VLT) and Palomar/Keck tele-scopes, respectively; Lowrance et al. (2005)usedtheHubbleSpace Telescope (HST) Near-Infrared Camera and Multi-ObjectSpectrometer (NICMOS) to survey 45 young (median age ∼150 Myr), nearby (average distance ∼ 30 pc) stars for substellarcompanions; Lafreni`ere et al. 2007 used Gemini AO to observe85 young (median age ∼ 150 Myr), nearby (average distance4Present address: Department of Physics and Astronomy, University ofCalifornia, Riverside, 900 University Ave., Riverside, CA 92507, USA.∼ 22 pc) stars; and Carson et al. (2005, 2006) used PalomarAO to survey 80 nearby (median distance ∼ 17 pc) stars withunknown ages. These references represent some of the largerdirect-imaging, high-contrast surveys, but there are a number ofother surveys as well.The observations described in this document largely providean extension to Carson et al. (2005), although the new targetlist is focused more strongly on solar-type stars (the Carsonet al. 2005, 2006 surveys looked mostly at K and M dwarfs).Most of the competing surveys (e.g., Biller et al. 2007; Metchev& Hillenbrand 2004; Lowrance et al. 2005; Lafreni`ere et al.2007, and others) have focused on observing the youngestnearby stars. While this allows for a maximal substellar-objectself-luminosity (Baraffe et al. 2003), it inherently requires thatone examines stars at somewhat larger distances in order toachieve a large enough original sample to glean the youngeststars. Surveying only younger stars also leads to selectionbiases, as certain types of stars lend themselves better to agedeterminations than others (Mamajek & Hillenbrand 2008). Forour survey, we avoid age requirements in an effort to achievea more uniform census of the substellar companion populationaround the nearest solar-type stars.While explorations of the substellar space around nearbyFGK stars are scientifically interesting in their own right, theyalso provide important reconnaissance observations for the nextgeneration of planet-search imaging surveys, like expected pro-grams with Subaru (HiCIAO; Tamura et al. 2006), Palomar(Project 1640; Hinkley et al. 2008), VLT (SPHERE; Beuzitet al. 2008), Gemini (GPI; Macintosh et al. 2006), and po-tential space missions, like Terrestrial Planet Finder Corona-graph (TPF-C)5and Terrestrial Planet Finder Interferometer(TPF-I)6/Darwin.7Information on the presence of brown dwarfsis important for these future planet surveys because the existenceof an orbiting brown dwarf may affect the likelihood of therebeing a planet in that system. Even for southern hemisphere5http://planetquest.jpl.nasa.gov/TPF-C6http://planetquest.jpl.nasa.gov/TPF-I7http://www.esa.int/science/darwin218No. 1, 2009 DISTANCE-LIMITED IMAGING SURVEY OF SUBSTELLAR COMPANIONS 219future planet searches, whose targets may not overlap with thisdocument’s survey, the statistical information (from our