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Planetary Companions in K Giants Beta Cancri, Mu Leonis, and Beta Ursae

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Planetary Companions in K Giants Beta Cancri, Mu Leonis, and Beta Ursae Astronomy & Astrophysics manuscript no. aa201322608 c ESO 2018 August 29, 2018 Planetary companions in K giants β Cancri, µ Leonis, and β Ursae Minoris⋆ B.-C. Lee,1 I. Han,1 M.-G. Park,2 D. E. Mkrtichian,3,4 A. P. Hatzes,5 and K.-M. Kim1 1 Korea Astronomy and Space Science Institute, 776, Daedeokdae-Ro, Youseong-Gu, Daejeon 305-348, Korea e-mail: [bclee;iwhan;kmkim]@kasi.re.kr 2 Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 702-701, Korea e-mail: [email protected] 3 National Astronomical Research Institute of Thailand, Chiang Mai 50200, Thailand 4 Crimean Astrophysical Observatory, Taras Shevchenko National University of Kyiv, Nauchny, Crimea, 98409, Ukraine e-mail: [email protected] 5 Th¨uringer Landessternwarte Tautenburg (TLS), Sternwarte 5, 07778 Tautenburg, Germany e-mail: [email protected] Received 5 September 2013 / Accepted 1 May 2014 ABSTRACT Aims. The aim of our paper is to investigate the low-amplitude and long-period variations in evolved stars with a precise radial velocity (RV) survey. Methods. The high-resolution, the fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) was used from 2003 to 2013 for a radial velocity survey of giant stars as part of the exoplanet search program at Bohyunsan Optical Astronomy Observatory (BOAO). Results. We report the detection of three new planetary companions orbiting the K giants β Cnc, µ Leo, and β UMi. The planetary nature of the radial velocity variations is supported by analyzes of ancillary data. The HIPPARCOS photometry shows no variations with periods close to those in RV variations and there is no strong correlation between the bisector velocity span (BVS) and the radial velocities for each star. Furthermore, the stars show weak or no core reversal in Ca II H lines indicating that they are inactive stars. The companion to β Cnc has a minimum mass of 7.8 MJup in a 605-day orbit with an eccentricity of 0.08. The giant µ Leo is orbited by a companion of minimum mass of 2.4 MJup having a period of 357 days and an eccentricity of 0.09. The giant β UMi is a known barium star and is suspected of harboring a white dwarf or substellar mass companion. Its companion has a minimum mass of 6.1 MJup, a period of 522 days, and an eccentricity e = 0.19. Key words. stars: planetary systems – stars: individual: β Cancri (HD 69267), µ Leonis (HD 85503), β Ursae Minoris (HD 131873) – stars: giant – technique: radial velocity 1. Introduction sis and the nature of RV measurements for each star will be pre- sented in Sect. 4. Finally, in Sect. 5, we discuss and summarize To date there are over 900 confirmed exoplanets that have been our results from this study. discovered by various methods. More than 60% of these have been detected by the precise radial velocity (RV) technique. At first, the RV method concentrated on searching for exoplanets around F, G, and K main-sequence stars because these have a plethora of narrow stellar lines that are amenable to RV mea- 2. Observations surements. Furthermore, dwarfs display much smaller intrinsic RV variations compared to evolved stars. All of the observations were carried out using the fiber-fed high-resolution Bohyunsan Observatory Echelle Spectrograph arXiv:1405.2127v1 [astro-ph.EP] 9 May 2014 Evolved stars such as K giants also have suitable absorption lines for RV measurements and allow us to probe planets around (BOES) at Bohyunsan Optical Astronomy Observatory in Korea. stars with higher mass compared to the surveys around dwarf The BOES covers a wavelength region from 3500Å to 10 500Å stars. Since the discovery of the first exoplanets around K gi- with one exposure and the spectrum is distributed over ∼ 85 or- ant stars (Hatzes & Cochran 1993; Frink et al. 2002) about 50 ders. The attached camera is equipped with 2K × 4K CCD (pixel exoplanets have been discovered orbiting giant stars. Not only size of 15 µ × 15 µ). For this survey we used a fiber with a core do RV surveys of giant stars allow us to probe planet formation diameter of 80 µ. This projected to 1.1 arcsec on the sky and around more massive stars but they also aid in our understanding yielded a resolving power of R = 90 000. To provide precise of how stellar evolution affects planetary systems. RV measurements an iodine absorption (I2) cell was used. This In this paper, we present precise RV measurements of three provided absorption lines suitable for wavelength calibration in evolved stars. In Sect. 2, we describe the observations. In Sect. 3, the wavelength region of 4900−6000 Å. The estimated signal- the stellar characteristics are determined. Radial velocity analy- to-noise (S/N) in the I2 region was about 200 using a typical exposure time ranging from 6 minutes to 15 minutes. The ba- ⋆ Based on observations made with the BOES instrument on the 1.8m sic reduction of spectra was performed with the IRAF software telescope at Bohyunsan Optical Astronomy Observatory in Korea. package and DECH (Galazutdinov 1992) code. 1 B.-C. Lee et al.: Planetary companions in K giants β Cancri, µ Leonis, and β Ursae Minoris Table 1. Stellar parameters for the stars analyzed in the present paper. Parameter β Cnc µ Leo β UMi Reference Spectraltype K4III K2III K4IIIvar HIPPARCOS (ESA 1997) mv [mag] 3.52 3.88 2.08 HIPPARCOS (ESA 1997) B-V [mag] 1.481 ± 0.006 1.222 1.465 ± 0.005 van Leeuwen (2007) age [Gyr] 1.85 ± 0.34 3.35 ± 0.70 2.95 ± 1.03 Deriveda d [pc] 90.0 ± 7.7 40.9 ± 1.3 38.7 ± 0.8 Famaey (2005) 92.99 ± 1.7 38.05 ± 0.23 40.14 ± 0.2 Anderson & Francis (2012) RV [km s−1] 22.6 ± 0.2 13.6 ± 0.2 16.9 ± 0.3 Gontcharov (2006) π [mas] 10.75 ± 0.19 26.28 ± 0.16 24.91 ± 0.12 van Leeuwen (2007) Teff [K] 3990 ± 20 4436 ± 4 – Massarotti et al. (2008) 4150 4660 4150 Lafrasse et al. (2010) – 4453 ± 19 4067 ± 13 Wu et al. (2011) 4092.1 ± 17.5 4538.2 ± 27.5 4126.0 ± 25.0 This work [Fe/H] [dex] – 0.30 ± 0.04 – 0.13 ± 0.05 Wu et al. (2011) – 0.17 0.25 ± 0.02 – 0.17 Anderson & Francis (2012) – 0.29 ± 0.06 0.36 ± 0.05 – 0.27 ± 0.07 This work log g [cgs] 1.9 2.1 1.9 Lafrasse et al. (2010) – 2.61 ± 0.08 1.70 ± 0.11 Wu et al. (2011) 1.4 ± 0.1 2.4 ± 0.1 1.5 ± 0.1 This work 1.28 ± 0.04 2.47 ± 0.04 1.39 ± 0.06 Deriveda −1 vmicro [km s ] 1.8 ± 0.1 1.4 ± 0.1 1.7 ± 0.1 This work R⋆ [R⊙] – 16.2 – Rutten (1987) 48.96 ± 4.23 – 42.06 ± 0.91 Piau et al. (2011) 47.2 ± 1.3 11.4 ± 0.2 38.3 ± 1.1 Deriveda a M⋆ [M⊙] 1.7 ± 0.1 1.5 ± 0.1 1.4 ± 0.2 Derived L⋆ [L⊙] 785.7 62.62 537.07 Anderson & Francis (2012) −1 vrot sin i [km s ] 2.1 1.2 1.7 de Medeiros & Mayor (1999) 3.7 ± 0.8 2.4 ± 0.5 1.7 ± 1.4 Głe¸bocki & Gnaci´nski (2005) 4.88 5.06 – Hekker & Mel´endez (2007) 6.9 4.5 – Massarotti et al. (2008) ′ log RHK – – – 4.68 Cornide et al. (1992) Prot / sin i [days] – 268 – Rutten (1987) 346–1137 114–480 625–6457 Derivedb (a) Derived using the online tool (http://stev.oapd.inaf.it/cgi-bin/param). (b) See text. 3. Stellar characteristics the PADOVA group. We also adopted a version of the Bayesian estimation method (Jørgensen & Lindegren 2005; da Silva et al. The stellar characteristics are important in discerning the nature 2006) by using the determined values for Teff, [Fe/H], mv, and of any RV variations. In particular evolved stars themselves can π. Our estimated parameters yielded R⋆ = 47.2 ± 1.3 R⊙, M⋆ = exhibit low-amplitude and long-period RV variations produced 1.7 ± 0.1 M⊙ (β Cnc); R⋆ = 11.4 ± 0.2 R⊙, M⋆ = 1.5 ± 0.1 M⊙ by rotation and surface activity such as spots, plages, or fila- (µ Leo); and R⋆ = 38.3 ± 1.1 R⊙, M⋆ = 1.4 ± 0.2 M⊙ (β UMi). ments. The stellar rotational period is important in identifying The basic parameters were taken from the HIPPARCOS the nature of any long-period RV variations. Sometimes low- satellite catalog (ESA 1997) and improved results for the amplitude and long-period RV variations arise from rotational parallaxes from van Leeuwen (2007). The giant star β Cnc modulation of surface structure (Lee et al. 2008; 2012). Results (HD 69267, HIP 40526, HR 3249) is a K4 III with a visual mag- of rotational velocities depend strongly on the adopted templates nitude of 3.52 and a B-V color index of 1.481 mag. The giant and methods used for calibration of line broadening and thus it µ Leo (HD 85503,HIP 48455,HR 3905) is a K2 III with a visual would be inappropriate to choose any one value as the ‘correct’ magnitude of 3.88 and a B-V color index of 1.222 mag. Lastly, one. With these determinations we have a range of vrot sin i mea- the K4 II star β UMi (HD 131873, HIP 72607, HR 5563) has a surements of 2.1−6.9 (β Cnc), 1.2−5.06 (µ Leo), and 0.3−3.1 visual magnitude of 2.08 and a B-V color index of 1.465 mag.
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