A&A 549, A2 (2013) Astronomy DOI: 10.1051/0004-6361/201220301 & c ESO 2012 Astrophysics Planetary companions orbiting M giants HD 208527 and HD 220074 B.-C. Lee1,I.Han1,andM.-G.Park2 1 Korea Astronomy and Space Science Institute, 776 Daedeokdae-Ro, Youseong-Gu, Daejeon 305-348, Korea e-mail: [bclee;iwhan]@kasi.re.kr 2 Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 702-701, Korea e-mail: [email protected] Received 29 August 2012 / Accepted 31 October 2012 ABSTRACT Aims. The purpose of the present study is to research the origin of planetary companions by using a precise radial velocity (RV) survey. Methods. The high-resolution spectroscopy of the fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at Bohyunsan Optical Astronomy Observatory (BOAO) was used from September 2008 to June 2012. Results. We report the detection of two exoplanets in orbit around HD 208527 and HD 220074 exhibiting periodic variations in RV of 875.5 ± 5.8 and 672.1 ± 3.7 days. The RV variations are not apparently related to the surface inhomogeneities, and a Keplerian motion of the planetary companion is the most likely explanation. Assuming possible stellar masses of 1.6 ± 0.4 and 1.2 ± 0.3 M, we obtain the minimum masses for the exoplanets at 9.9 ± 1.7 and 11.1 ± 1.8 MJup around HD 208527 and HD 220074 with orbital semi-major axes of 2.1 ± 0.2 and 1.6 ± 0.1 AU and eccentricities of 0.08 and 0.14, respectively. We also find that the previously known spectral classification of HD 208527 and HD 220074 was in error. Our new estimation of stellar parameters suggests that both HD 208527 and HD 220074 are M giants. Therefore, HD 208527 and HD 220074 are so far the first candidate M giants found to harbor a planetary companion. Key words. planetary systems – stars: individual: HD 208527 – techniques: radial velocities – stars: individual: HD 220074 1. Introduction of giant stars are also interesting because we can detect plan- ets of stars that are considerably more massive than the Sun. Most exoplanets have so far been detected by using spectro- For MS stars of the same mass this would be difficult because scopic radial velocity (RV) and photometric transit methods on a they rotate fast. Compared to MS stars, however, giants show ratio of two to one. While the transit method is limited by com- blended RV variations owing to various surface variations in the panions that are strongly biased toward short-periods with peri- line profiles, such as chromospheric activities and stellar pulsa- ods as short as a few days, the RV technique is relatively less tions. It thus makes it difficult for the giant star to purify the restricted, which can detect planetary systems with periods ap- ∼ RV origin through the examination process. Also, it is not so proximately from a day to 38 years (i.e. 47 Uma described in easy to determine the masses of the host stars, because stars of Gregory & Fischer 2010). different masses are located in roughly the same region of the It has been demonstrated that the RV method has a lim- H-R Diagram. And, interpreting the data is not easy, because ited range of stellar spectral types of the stars chosen as targets. only massive planets with relatively long orbital periods have Planet searches generally focus on late F, G, and K dwarfs be- been detected for giant stars, whereas surveys of MS stars often cause they are bright enough to have a high signal-to-noise ratio find low-mass planets with short orbital periods. (S/N) in the high-resolution spectra and have an ample num- Of the exoplanets detected by the RV method, just over 12% berofspectrallinesfortheRVmeasurements. Furthermore, are discovered around giants. However, no planet has been dis- finding planets around the Sun-like stars is a big issue in ev- covered around M giant stars because they are rare or maybe ery aspect. Unfortunately, there are no more candidate stars in because planet formation is weak around M giants, unlike for the main-sequence (MS) stage for ground-based observations. M dwarfs, which may have a significant probability (up to 13%) Recognizing the importance of exoplanet formation and its evo- of harboring short-period giant planets (Endl et al. 2003). Short- lution, several exoplanet survey groups have begun exploring term oscillations on M giants are rarely observed, yet they show evolved G, K giants (Frink et al. 2002; Setiawan et al. 2003;Sato periods in excess of one day (Koen & Laney 2000). Three- et al. 2003; Hatzes et al. 2005; Döllinger et al. 2007; Han et al. dimensional simulations (Freytag & Höfner 2008) predict typ- 2010) and low-mass (M dwarf) stars (Endl et al. 2003; Bonfils ical photometric periods of about several tens to a few hundred et al. 2012). days, similar to those of RV variations in K giants. A giant star has sharper spectral lines than a dwarf star, For the past four years, we have conducted precise RV mea- which allows more precise measurement of RV shift. RV surveys surements of 40 K dwarfs. Here, we present two stars with Based on observations made with the BOES instrument on the long-period and low-amplitude RV variations. In Sect. 2, we de- 1.8-m telescope at the Bohyunsan Optical Astronomy Observatory in scribe the observations and data reduction. In Sect. 3, the stel- Korea. lar characteristics of the host stars are reinterpreted, including Article published by EDP Sciences A2, page 1 of 7 A&A 549, A2 (2013) Table 1. Stellar parameters for the stars analyzed in the present paper. Parameters HD 208527 HD 220074 Reference Spectral type K5 V K1 V Hipparcos (ESA 1997) – M2 III Kidger et al. (2003); Tsvetkov et al. (2008) M1 III M2 III Deriveda mv [mag] 6.4 6.4 Hipparcos (ESA 1997) Mv [mag] –1.24 –1.52 Famaey et al. (2005) B − V [mag] 1.68 1.66 Hipparcos (ESA 1997) 1.70 ± 0.02 1.68 ± 0.01 van Leeuwen (2007) V − K [mag] 4.14 4.39 Cutri et al. (2003) age [Gyr] 2.0 ± 1.3 4.5 ± 2.8 Derivedb Distance [pc] 320.2 ± 61.1 290.2 ± 45.5 Famaey et al. (2005) RV [km s−1]4.79± 0.06 –36.89 ± 0.21 Famaey et al. (2005) Parallax [mas] 2.48 ± 0.38 3.08 ± 0.43 van Leeuwen (2007) Teff [K] 3950 5080 Wright et al. (2003) +598 +741 4643 −501 4971 −643 Ammons et al. (2006) 4035 ± 65 3935 ± 110 This work [Fe/H] –0.09 ± 0.16 –0.25 ± 0.25 This work log g 1.6 ± 0.3 1.3 ± 0.5 This work 1.4 ± 0.2 1.1 ± 0.2 Derivedb b R [R] 51.1 ± 8.3 49.7 ± 9.5 Derived b M [M]1.6± 0.4 1.2 ± 0.3 Derived −1 vrot sin i [km s ] 3.6 3.0 This work a Prot/sin i [days] 660.0 ± 116.6 832.4 ± 156.7 Derived −1 vmicro [km s ]1.7± 0.3 1.6 ± 0.3 This work Notes. (a) See text. (b) Derived using the online tool (http://stevoapd.inaf.it/cgi-bin/param). chromospheric activities and Hipparcos photometric varia- 3. Stellar characteristics tions. The RV measurements and orbital solutions are presented in Sect. 4. Finally, in Sect. 5, we discuss our results from our 3.1. Fundamental parameters study. The atmospheric parameters were determined based on the TGVIT (Takeda et al. 2005) code. To estimate the parame- 2. Observations and reduction ters, we used 169 (HD 208527) and 145 (HD 220074) equiv- alent width (EW) measurements of Fe I and Fe II lines. They = ± = . ± v = Since 2003, we have conducted a precise RV survey resulted in Teff 4035 65 K, log g 1 6 0.3, micro −1 for ∼300 stars, including 55 K giants (Han et al. 2010), ∼200 G 1.7 ± 0.3 km s ,[Fe/H] = −0.09 ± 0.16 for HD 208527, = ± = . ± . v = . ± giants (Omiya et al. 2008), 10 M giants, and 40 K dwarfs. and Teff 3935 110 K, log g 1 3 0 5, micro 1 6 −1 The observations were carried out as part of the study on 0.3kms ,and[Fe/H] = −0.25 ± 0.25 for HD 220074. The v = . the 40 K dwarfs. We used a fiber-fed, high-resolution (R = projected rotational velocities were derived, rot sin i 3 6 −1 45 000) Bohyunsan Observatory Echelle Spectrograph (BOES; and 3.0 km s , from the widths of the spectral lines regard- Kim et al. 2007). The BOES spectra covered a wavelength re- ing the line-broadening functions. Based on the rotational ve- . ± . ± . gion from 3500 to 10 500 Å. To provide precise RV measure- locities and the stellar radius of 51 1 8 3 and 49 7 9 5 R, ments of the K dwarfs, an iodine absorption (I ) cell was used we derived the range of the upper limit of the rotational period 2 . ± . ± with a wavelength region of 4900–6000 Å. Each estimated S/N of 660 0 116 6 (HD 208527) and 832 4 156.7 (HD 220074) days. at an I2 region was about 200 with a typical exposure time rang- ing from 10 to 20 min. The long-term stability of the BOES was Stellar masses were estimated from the theoretical stellar demonstrated by observing the RV standard star τ Ceti, which isochrones by using their position in the color–magnitude di- was constant with an rms scatter of 6.8 m s−1 over the time span agram based on the ones by Bertelli et al.