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CALTECH GE 133 - A LOW-MASS PLANET WITH A POSSIBLE SUB-STELLAR-MASS

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A LOW-MASS PLANET WITH A POSSIBLE SUB-STELLAR-MASS HOSTIN MICROLENSING EVENT MOA-2007-BLG-192D. P. Bennett,1,2I. A. Bond,1,3A. Udalski,4,5T. Sumi,1,6F. Abe,1,6A. Fukui,1,6K. Furusawa,1,6J. B. Hearnshaw,1,7S. Holderness,1,8Y. Itow,1,6K. Kamiya,1,6A. V. Korpela,1,9P. M. Kilmartin,1,10W. Lin,1,3C. H. Ling,1,3K. Masuda,1,6Y. Matsubara,1,6N. Miyake,1,6Y. Muraki,1,11M. Nagaya,1,6T. Okumura,1,6K. Ohnishi,1,12Y. C. Perrott,1,13N. J. Rattenbury,1,14T. Sako,1,6To. Saito,1,15S. Sato,1,16L. Skuljan,1,3D. J. Sullivan,1,9W. L. Sweatman,1,3P. J. Tristram,1,10P. C. M. Yock,1,13M. Kubiak,4,5M. K. Szyman´ski,4,5G. Pietrzyn´ski,4,5,17I. Soszyn´ski,4,5O. Szewczyk,4,5,17x. Wyrzykowski,4,5,18K. Ulaczyk,4,5V. Batista,19J. P. Beaulieu,19S. Brillant,20A. Cassan,21P. Fouque´,22P. Kervella,23D. Kubas,20and J. B. Marquette19Received 2008 January 16; accepted 2008 May 6ABSTRACTWe report the detection of an extrasolar planet of mass ratio q  2 ; 104in microlensing event MOA-2007-BLG-192.The best-fit microlensing model shows both the microlensing parallax and finite source ef fects, and these can be combinedto obtain the lens masses of M ¼ 0:060þ0:0280:021M for the primary and m ¼ 3:3þ4:91:6Mfor the planet. However, theobservational coverage of the planetary deviation is sparse and incomplete, and the radius of the source was estimatedwithout the benefit of a source star color measurement. As a result, the 2  limits on the mass ratio and finite sourcemeasurements are weak. Nevertheless, the microlensing parallax signal clearly favors a substellar mass planetary host,and the measurement of finite source effects in the light curve supports this conclusion. Adaptive optics images takenwith the Very Large Telescope (VLT) NACO instrument are consistent with a lens star that is either a brown dwarf or astar at the bottom of the main sequence. Follow-up VLT and/or Hubble Space Telescope (HST ) observations will eitherconfirm that the primary is a brown dwarf or detect the low-mass lens star and enable a precise determination of itsmass. In either case, the lens star, MOA-2007-BLG-192L, is the lowest mass primary known to have a companionwith a planetary mass ratio, and the planet, MOA-2007-BLG-192Lb, is probably the lowest mass exoplanet found todate, aside from the lowest mass pulsar planet.Subject headinggs: gravitational lensing — planetary systems1. INTRODUCTIONWhen the first extrasolar planets were discovered orbiting main-sequence stars more than a decade ago ( Mayor & Queloz 1995;Marcy & Butler 1996; Butler & Marcy 1996), the radial velocitysurveys responsible for the discoveries focused their observationson stars of spectral type F, G, and K, because such stars offered thegreatest planet detection sensitivity. However, observations of star-forming regions indicate that stars of virtually all types have evi-dence of protoplanetary disks, suggesting that the first stages ofthe planet formation are practically independent of star type. Theradial velocity surveys have since expanded their target lists to in-clude stars ranging from a spectral type of mid-M to stars that arethought to have spectral type A when they were on the main se-quence. Gas giant planets have been found in orbit around starsof all these types (Butler et al. 2006; Sato et al. 2007; Johnson et al.2007; Niedzielski et al. 2007; Lovis & Mayor 2007), but they ap-pear to be increasingly rare in orbit around low-mass stars.In the past few years, microlensing surveys (Bennett 2008)have extended the range of sensitivity to cool, ‘‘super-Earth’’ planets(Beaulieu et al. 2006; Gould et al. 2006) with masses of 10 Min orbits beyond the ‘‘snow-line’’ (Ida & Lin 2004; Laughlin et al.2004; Kennedy et al. 2006), where the core accretion theory pre-dicts that the most massive planets should form. These discoveriesindicate that such low-mass planets are significantly more com-mon than gas giants in orbit around the stars of P1 M that are1MOA Collaboration.2Department of Physics, University of Notre Dame, IN 46556; [email protected] of Information and Mathematical Sciences, Massey University,Auckland, New Zealand; [email protected] Collaboration.5Warsaw University Observatory, Al. Ujazdowskie 4 00-478 Warszawa,Poland; [email protected] Environment Laboratory, Nagoya University, Nagoya464-8601, Japan; [email protected] of Physics and Astronomy, University of Canterbury, PrivateBag 4800, Christchurch, New Zealand.8Computer Science Department, University of Auckland, Auckland, New Zealand.9School of Chemical and Physical Sciences, Victoria University, Wellington,New Zealand.10Mt. John Observatory, P.O. Box 56, Lake Tekapo 8770, New Zealand.11Konan University, Kobe, Japan.12Nagano National College of Technology, Nagano 381-8550, Japan.13Department of Physics, University of Auckland, Auckland, New Zealand.14Jodrell Bank Observatory, The University of Manchester, Macclesfield,Cheshire SK11 9DL, UK.15Tokyo Metropolitan College of Aeronautics, Tokyo 116-8523, Japan.16Department of Physics and Astrophysics, Faculty of Science, NagoyaUniversity, Nagoya 464-8602, Japan.17Departamento de Fisica, Astronomy Group, Universidad de Concepcio´n,Casilla 160-C, Concepcio´n, Chile.18Institute of Astronomy , University of Cambridge, Madingley Road, CambridgeCB3 0HA, UK.19Institut d’Astrophysique de Paris, UMR7095 CNRS, Universite´Pierre &Marie Curie, 98 bis Boulevard Arago, 75014 Paris, France.20European Southern Observatory , Casilla 19001, Vitacura 19, Santiago, Chile.21Astronomisches Rechen-Institut, Zentrum f u¨r Astronomie, HeidelbergUniversity, Mo¨nchhofstr. 12Y14,69120 Heidelberg, Germany.22Observatoire Midi-Pyre´ne´es, UMR 5572, 14, Avenue Edouard Belin,31400 Toulouse, France.23LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Universite´Paris Diderot, 5 Place Jules Janssen, F-92195 Meudon, France.663The Astrophysical Journal, 684:663Y683, 2008 September 1# 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.probed by the microlensing method. Because microlensing doesnot rely on light from the planetary host star in order to detect theplanet, its sensitivity extends to host star masses well below the0.25 M lower limit for current radial velocity surveys.In this paper, we present the analysis of microlensing eventMOA-2007-BLG-192, and show that the lens system is likely tobe a low-mass planet orbiting a primary that is


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CALTECH GE 133 - A LOW-MASS PLANET WITH A POSSIBLE SUB-STELLAR-MASS

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