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A SUBSTELLAR COMPANION in a 1.3 Yr NEARLY CIRCULAR ORBIT of HD 16760∗,†

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A SUBSTELLAR COMPANION in a 1.3 Yr NEARLY CIRCULAR ORBIT of HD 16760∗,† The Astrophysical Journal, 703:671–674, 2009 September 20 doi:10.1088/0004-637X/703/1/671 C 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A. A SUBSTELLAR COMPANION IN A 1.3 yr NEARLY CIRCULAR ORBIT OF HD 16760∗,† Bun’ei Sato1, Debra A. Fischer2, Shigeru Ida3, Hiroki Harakawa3, Masashi Omiya4, John A. Johnson5, Geoffrey W. Marcy6, Eri Toyota7, Yasunori Hori3, Howard Isaacson2, Andrew W. Howard6, and Kathryn M. G. Peek6 1 Global Edge Institute, Tokyo Institute of Technology, 2-12-1-S6-6 Ookayama, Meguro-ku, Tokyo 152-8550, Japan; [email protected] 2 Department of Physics & Astronomy, San Francisco State University, San Francisco, CA 94132, USA 3 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan 4 Department of Physics, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan 5 Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA 6 Department of Astronomy, University of California, Berkeley, CA, USA 7 Kobe Science Museum, 7-7-6 Minatoshima-Nakamachi, Chuo-ku, Kobe, Hyogo 650-0046, Japan Received 2009 March 2; accepted 2009 July 31; published 2009 August 31 ABSTRACT We report the detection of a substellar companion orbiting the G5 dwarf HD 16760 from the N2K sample. Precise Doppler measurements of the star from Subaru and Keck revealed a Keplerian velocity variation with a period of 466.47 ± 0.35 d, a semiamplitude of 407.71 ± 0.84 m s−1, and an eccentricity of 0.084 ± 0.003. Adopting a stellar mass of 0.78 ± 0.05 M, we obtain a minimum mass for the companion of 13.13 ± 0.56 MJUP, which is close to the planet/brown-dwarf transition, and the semimajor axis of 1.084 ± 0.023 AU. The nearly circular orbit despite the large mass and intermediate orbital period makes this companion unique among known substellar companions. Key words: planetary systems – stars: individual (HD 16760) 1. INTRODUCTION by core accretion. Massive companions with 10 Jupiter-mass (MJUP) could be formed by core-accretion depending on the During the past decade, precise Doppler surveys have ob- assumed truncation condition for gas accretion (Ida & Lin served 3000 of the closest and brightest main-sequence stars 2004b;Alibertetal.2005; Mordasini et al. 2007). Or they could and have detected more than 250 extrasolar planets so far (e.g., be formed by gravitational disk instability as has been suggested Butler et al. 2006; Udry & Santos 2007). The planets show a for the formation of brown-dwarf companions. In fact, an upper surprising diversity in their masses, orbital periods, and eccen- limit of planet mass has not been well established theoretically tricities, and the statistical distribution and correlation of these or observationally. In core accretion, the limit is regulated by a parameters now begin to serve as the test cases for theories of balance between gap opening (e.g., Ida & Lin 2004a; Crida et al. planet formation and evolution (e.g., Ida & Lin 2004a, 2004b, 2006), the inflow rate due to viscous diffusion and dissipation 2005). timescales of protoplanetary disks (e.g., D’Angelo et al. 2003; Stellar metallicity is one of the key parameters that controls Dobbs-Dixon et al. 2007; Tanigawa & Ikoma 2007). Planet planet formation. It is well known that frequency of giant planets mass might also depend on subsequent evolution of planets by becomes higher as stellar metallicity increases (e.g., Santos et al. mechanisms such as giant impacts (Baraffe et al. 2008). In disk 2003; Fischer & Valenti 2005, and references therein). Such a instability, planet mass depends primarily on the mass ratio trend favors a core-accretion model as the main mechanism of between the disk and central star. Observational properties of giant planet formation, because high metallicity likely increases planets help to constrain the planet formation models. Currently, the surface density at the midplane of the protoplanetary disk, no significant difference in stellar metallicity has been found making it easier to build metal cores massive enough to accrete for planets above and below ∼ 10 MJUP (e.g., Santos et al. gas envelope (e.g., Ida & Lin 2004b;Alibertetal.2005). It 2003; Fischer & Valenti 2005). Indeed, very high-mass planets has also been noted that there is a possible flat tail in the (i.e., greater than 10 MJUP) are rare; a larger sample would low-metallicity regime of the frequency distribution (Santos improve statistics for extracting information regarding the upper et al. 2004; Sozzetti et al. 2009). This might suggest the limit of planet mass and correlations between planet mass and existence of a distinct formation mechanism, disk instability, metallicity. which is not dependent on metallicity (Boss 2002). Correlations The N2K consortium began precise Doppler surveys at Keck, between metallicity and orbital parameters are inconclusive. Magellan, and Subaru in 2004, targeting a new set of 2000 No significant trends have been found in period–metallicity metal-rich solar-type stars (Fischer et al. 2005). The main and eccentricity–metallicity distribution, although hints of some purpose of the survey was to search for short-period planets weak correlations have been pointed out (e.g., Santos et al. 2003; with a high-cadence observational strategy in order to find Fischer & Valenti 2005). prospective transiting planets. The sample was biased toward It has also been pointed out that in addition to the presence of high-metallicity stars in order to increase the detection rate of the gas giant planets, the mass of the detected planet or the total mass planets. From the collective N2K surveys, we have discovered of all planets in a system are correlated with high metallicity seven short-period (P 5 days) planets so far including: HD (e.g., Santos et al. 2003; Fischer & Valenti 2005), as predicted 88133 (Fischer et al. 2005), the transiting planet HD 149026 (Sato et al. 2005), HD 149143 and HD 109749 (Fischer et al. ∗ Based on data collected by the Subaru Telescope, which is operated by the 2006), HD 86081, HD 224693, and HD 33283 (Johnson et al. National Astronomical Observatory of Japan. 2006). † Based on observations obtained at the W. M. Keck Observatory, which is operated by the University of California and the California Institute of The metallicity-biased sample is expected to contain many Technology. Keck time has been granted by NOAO and NASA. long-period planets as well as short-period ones. Therefore, we 671 672 SATO ET AL. Vol. 703 have continued to observe stars with significant radial velocity Table 1 variations and we have also detected 13 intermediate-period Stellar Parameters for HD 16760 (18–1405 days) planets: HD 5319 and HD 75898 (Robinson Parameter Value et al. 2007), HD 11506, HD 125612, HD 170469, HD 231701 V 8.7 and the transiting planet HD 17156 (Fischer et al. 2007), the MV 5.41 double planet system HIP 14810 (Wright et al. 2007), HD B−V 0.715 205732 and HD 154672 (Lopez-Morales et al. 2008), HD B.C. −0.108 179079 and HD 73534 (Valenti et al. 2009). These planets Spectral type G5V help to investigate correlations between properties of planets Parallax (mas) 22.00 (2.35) and stellar metallicity in more detail, especially in the high- Teff (K) 5629 (44) metallicity regime. log g 4.47 (0.06) [Fe/H] +0.067 (0.05) Here, we report the discovery of a substellar companion to − v sin i (km s 1) 0.5 (0.5) the G5 dwarf star HD 16760 in a 1.3 yr nearly circular orbit M (M) 0.78 (0.05) from the N2K sample. The companion has a minimum mass of Star RStar (R) 0.81 (0.27) 13.13 MJUP, which is just above the deuterium-burning threshold LStar (L) 0.72 (0.43) that is often used to distinguish brown dwarfs from planets. SHK 0.176 − log RHK 4.93 2. STELLAR PARAMETERS HD 16760 (HIP 12638) is listed in the Hipparcos catalog et al. 2002) to provide a fiducial wavelength reference for precise (ESA 1997) as a G5V star with a visual magnitude V = 8.7 and radial velocity measurements. We adopted the setup of StdI2b a color index B − V = 0.715. The revised Hipparcos parallax for all the data, which simultaneously covers a wavelength = ± region of 3500–6100 Å by a mosaic of two CCDs. The slit width of π 22.00 2.35 mas (van Leeuwen 2007) corresponds to a distance of 45.5 pc and yields the absolute visual magnitude of was set to 0.8 for the first four data (2004–2005) and 0.6forthe last six ones (2006–2008), giving a reciprocal resolution (λ/Δλ) MV = 5.41. The star is probably a visual binary system having a Hipparcos double star catalog entry. The secondary is separated of 45,000 and 60,000, respectively. Typical signal-to-noise ratio −1 by 14.6 arcsec which corresponds to a projected separation of (S/N) was 140–200 pix with exposure time of 110–300 s about 660 AU. A high-resolution spectroscopic analysis with depending on the observing conditions. Radial velocity analysis Spectroscopy Made Easy (SME; Valenti & Piskunov 1996) for an I2-superposed stellar spectrum was carried out with a code described in Valenti & Fischer (2005) derives an effective developed by Sato et al. (2002), which is based on a technique temperature T = 5629 ± 44 K, a surface gravity log g = by Butler et al. (1996) and Valenti et al. (1995), giving a Doppler eff −1 4.47 ± 0.06, rotational velocity v sin i = 0.5 ± 0.5 km s−1, precision of about 4–5 m s .
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