Substellar Companions to Evolved Intermediate-Mass Stars: HD 145457 and HD 180314
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PASJ: Publ. Astron. Soc. Japan 62, 1063–1069, 2010 August 25 c 2010. Astronomical Society of Japan. Substellar Companions to Evolved Intermediate-Mass Stars: HD 145457 and HD 180314 Bun’ei SATO,1 Masashi OMIYA,2 Yujuan L IU,3 Hiroki HARAKAWA,4 Hideyuki IZUMIURA,5,6 Eiji KAMBE,5 Eri TOYOTA,7 Daisuke MURATA,8 Byeong-Cheol LEE,2,9 Seiji MASUDA,10 Yoichi TAKEDA,6,11 Michitoshi YOSHIDA,12 Yoichi I TOH,8 Hiroyasu ANDO,6,11 Eiichiro KOKUBO,6,11 Shigeru IDA,4 Gang ZHAO,13 and Inwoo HAN2 1Global Edge Institute, Tokyo Institute of Technology, 2-12-1-S6-6 Ookayama, Meguro-ku, Tokyo 152-8550 [email protected] 2Korea Astronomy and Space Science Institute, 61-1 Hwaam-dong, Yuseong-gu, Daejeon 305-348, Korea 3Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100012, China 4Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551 5Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Kamogata, Okayama 719-0232 6The Graduate University for Advanced Studies, Shonan Village, Hayama, Kanagawa 240-0193 7Kobe Science Museum, 7-7-6 Minatoshima-Nakamachi, Chuo-ku, Kobe, Hyogo 650-0046 8Graduate School of Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501 9Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 702-701, Korea 10Tokushima Science Museum, Asutamu Land Tokushima, 45-22 Kibigadani, Nato, Itano-cho, Itano-gun, Tokushima 779-0111 11National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 12Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526 13National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100012, China (Received 2010 April 5; accepted 2010 May 17) Abstract We report on the detection of two substellar companions orbiting around evolved intermediate-mass stars by precise Doppler measurements at Subaru Telescope and Okayama Astrophysical Observatory. HD 145457 is a K0 giant with a mass of 1.9 Mˇ, and has a planet of minimum mass, m2 sin i = 2.9 MJ, orbiting with a period of P = 176 d and eccentricity of e = 0:11. HD 180314 is also a K0 giant with 2.6Mˇ, and hosts a substellar companion of m2 sin i = 22 MJ, which falls in the brown-dwarf mass regime, in an orbit with P = 396 d and e = 0.26. HD 145457 b is one of the innermost planets and HD 180314 b is the seventh candidate of a brown-dwarf-mass companion found around evolved intermediate-mass stars. Key words: stars: individual (HD 145457) — stars: individual (HD 180314) — stars: planetary systems — techniques: radial velocities 1. Introduction Sato et al. 2008a; Hatzes et al. 2005, 2006; Johnson et al. 2010; Lovis & Mayor 2007; Niedzielski et al. 2009a; D¨ollinger et al. Over 400 exoplanets have been discovered by various tech- 2009; de Medeiros et al. 2009; Liu et al. 2009; Omiya et al. niques during the past 15 years.1 Among the techniques, 2009). Intermediate-mass dwarfs, namely BA-type dwarfs, are a precise Doppler technique has been the most powerful more difficult for Doppler planet searches because of a paucity method for planet detection around various types of stars, of spectral features and their rotational broadening. It is thus including solar-type stars, evolved giants and subgiants, early- difficult to achieve a high measurement precision in radial type stars, and so on (e.g., Udry & Santos 2007 and refer- velocity (but see e.g., Galland et al. 2005, 2006). On the ences therein). Recently, precise Doppler measurements in other hand, those in evolved stages, namely GK-type giants the infrared wavelength region have started to explore planets and subgiants, have many sharp absorption lines in their spectra around very low-mass stars down to 0.1Mˇ (Bean et al. 2010). suitable for precise radial velocity measurements, which make These surveys will help us to understand the properties of them promising targets for Doppler planet searches. Actually, planets as a function of the stellar mass, age, evolutionary more than 30 substellar companions to such evolved stars have stage, and so on, and thus provide a more general picture of already been found, and they have shown remarkable proper- planet formation and evolution. ties: more than a twice higher occurrence rate of giant planets In particular, planets around giants and subgiants have been for intermediate-mass subgiants than that for lower-mass stars extensively surveyed over the past several years, mainly from (Johnson et al. 2007a; Bowler et al. 2010), a larger typical mass the viewpoint of planet searches around intermediate-mass of giant planets (Lovis et al. 2007; Omiya et al. 2009), a lack (1.5–5Mˇ) stars (e.g., Frink et al. 2002; Setiawan et al. 2005; of inner planets with semimajor axes <0.6 AU (Johnson et al. 2007b; Sato et al. 2008b; Niedzielski et al. 2009b), and the 1 See, e.g., table at hhttp://exoplanet.eu/i. lack of a metal-rich tendency in host stars (Pasquini et al. 2007; 1064 B. Sato et al. [Vol. 62, Takeda et al. 2008). All of these statistical properties should be completed initial screening for about 300 stars during the confirmed by collecting a larger number of samples. last 4 years. Since 2001, we have been carrying out a Doppler planet search program targeting 300 GK giants at Okayama 3. Observations and Radial-Velocity Analysis Astrophysical Observatory (OAO), and have discovered 9 planets and 1 brown dwarf so far from the program (Sato We obtained a total of 3 spectra for each of HD 145457 et al. 2003, 2007, 2008a, 2008b; Liu et al. 2008). In order and HD 180314 in 2006 April, May, and July using the High to further extend the survey, we have established an inter- Dispersion Spectrograph (HDS: Noguchi et al. 2002) equipped national consortium between Chinese, Korean, and Japanese with Subaru. We used an iodine absorption cell (I2 cell: researchers using 2 m class telescopes in three countries (East- Kambe et al. 2002) to provide a fiducial wavelength refer- Asian Planet Search Network: Izumiura 2005), which recently ence for precise radial-velocity measurements. We adopted announced discoveries of a planet (Liu et al. 2009) and a brown the setup of StdI2b in the first two runs and StdI2a in the dwarf (Omiya et al. 2009) around GK giants. A total of about third one, which covered a wavelength region of 3500–6200A˚ 600 GK giants are now being surveyed by the consortium. and 4900–7600A˚ , respectively, in order to inspect as many In this paper, we report on the detections of two absorption lines as possible for stellar abundance analysis. new substellar companions around intermediate-mass giants The slit width was set to 0:006, giving a wavelength resolu- (HD 145457 and HD 180314) from our newly started planet tion (=Δ) of 60,000. The typical signal-to-noise ratio (S=N) search program using the Subaru 8.2 m telescope and the was 150–230 pixel1 for HD 145457 and 100–170 pixel1 for above-mentioned 2 m class telescopes. The substellar compan- HD 180314 with an exposure time of 45–180 s, depending on ions presented here were uncovered by initial screening with the weather condition. Subaru, and followed up with the OAO 1.88 m telescope. We After observations at Subaru, we found that the stars showed describe an outline of the Subaru survey in section 2 and our large radial-velocity variations with >45 m s1,andthen observations in section 3. The stellar properties are presented started follow-up observations using the 1.88 m telescope with in section 4, and radial velocities, orbital solutions, and results the High Dispersion Echelle spectrograph (HIDES: Izumiura of line shape analysis are provided in section 5. Section 6 is 1999) at OAO. We collected a total of 23 and 21 data points for devoted to a summary and discussion. HD 145457 and HD 180314, respectively, from 2008 March to 2010 March. The wavelength region was set to cover 2. Subaru Survey for Planets around GK Giants 3750–7500A˚ using the RED cross-disperser, and the slit width was set to 0:0078, giving a wavelength resolution of 67000 by The main purpose of the Subaru program is to quickly iden- about 3.3 pixels sampling. We used an I2 cell for precise tify planet-hosting candidates from hundreds of sample stars radial-velocity measurements and also took pure stellar spectra by taking advantage of the large telescope aperture. Our basic without the I2 cell for abundance analysis. The typical S=Nwas strategy is thus to observe each star three times in a semester 170 pixel1 for both stars with an exposure time of 1500 s. The with an interval of about 1.5 months for the first screening to reduction of echelle data for HDS and HIDES was performed identify stars showing large radial-velocity variations. Since using the IRAF2 software package in the standard manner. it is known that planets with periods of less than 130 days For precise Doppler analysis, we basically adopted the seem to be rare around giants, we at first focus on the detec- modeling technique of an I2-superposed stellar spectrum tion of planets with longer periods. For stars that turn out (star + I2) detailed in Sato et al. (2002), which is based on to show large radial-velocity variations, we conduct follow-up a method by Butler et al. (2006). In this technique, a star + I2 observations using the 1.8 m telescope at Bohyunsan Optical spectrum is modeled as a product of a high-resolution I2 and Astronomy Observatory (BOAO; Korea), the 2.16 m telescope a stellar template spectrum convolved with a modeled instru- at Xinglong station (China), and the 1.88 m telescope at OAO mental profile (IP) of the spectrograph.