Journal of the Korean Astronomical Society https://doi.org/10.5303/JKAS.2020.53.1.27 53: 27 ∼ 34, 2020 February pISSN: 1225-4614 · eISSN: 2288-890X Published under Creative Commons license CC BY-SA 4.0 http://jkas.kas.org ASEARCH FOR EXOPLANETS AROUND NORTHERN CIRCUMPOLAR STARS VI. DETECTION OF PLANETARY COMPANIONS ORBITING THE GIANTS HD 60292 AND HD 112640 Byeong-Cheol Lee1,2, Myeong-Gu Park3, Inwoo Han1,2, Tae-Yang Bang3, Hyeong-Il Oh1,3, and Yeon-Ho Choi3 1Korea Astronomy and Space Science Institute 776, Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Korea; [email protected], [email protected], [email protected] 2Astronomy and Space Science Major, University of Science and Technology, Gajeong-ro Yuseong-gu, Daejeon 34113, Korea 3Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 41566, Korea; [email protected], [email protected], [email protected] Received November 12, 2019; accepted January 28, 2020 Abstract: We report the detection of exoplanet candidates in orbits around HD 60292 and HD 112640 from a radial velocity (RV) survey. The stars exhibit RV variations with periods of 495 ± 3 days and 613 ± 6 days, respectively. These detections are part of the Search for Exoplanets around Northern Circumpolar Stars (SENS) survey using the fiber-fed Bohyunsan Observatory Echelle Spectrograph installed at the 1.8-m telescope of the Bohyunsan Optical Astronomy Observatory in Korea. The aim of the survey is to search for planetary or substellar companions. We argue that the periodic RV variations are not related to surface inhomogeneities; rather, Keplerian motions of planetary companions are the most likely interpretation. Assuming stellar masses of 1:7 ± 0:2M (HD 60292) and 1:8 ± 0:2M (HD 112640), we obtain minimum planetary companion masses of 6:5 ± 1:0MJup and 5:0 ± 1:0MJup, and periods of 495:4 ± 3:0 days and 613:2 ± 5:8 days, respectively. Key words: planetary systems | stars: individual: HD 60292; HD 112640 | techniques: radial velocities 1.I NTRODUCTION amplitude and long-period RV variations in HD 60292 and HD 112640, possibly caused by a planetary com- Since the discovery of the first exoplanet around a main- panion. In Section2, we describe the observations and sequence (MS) star (Mayor & Queloz 1995) using the data reduction. In Section3, the stellar characteristics radial velocity (RV) method, approximately 300 exo- of the host stars are derived. The measurements of RV planets were found in the same way until 2010. During variations and their possible origins are presented in the 15 years of this early stage of the RV method, Sun- Sections4 and5. Finally, in Section6, we discuss our like dwarf stars were the major targets because of the results. benefits of analysis they provide compared with giant stars. Giants may present more complex RV variations 2.O BSERVATIONS AND DATA REDUCTION because of various surface processes that affect spectral line profiles: stellar pulsations, chromospheric activities, The observations were obtained as a part of the ex- spots, and large convection cells. oplanet survey of the SENS program. The fiber-fed, In 2010, a new program, the Search for Exoplanet high-resolution (R = 45 000) Bohyunsan Observatory around Northern circumpolar Stars (SENS; Lee et al. Echelle Spectrograph (BOES; Kim et al. 2007) installed 2015), was started. The main goal of SENS is to observe at the 1.8-m telescope of BOAO, Korea, was used. In stars that are accessible around the year in order to order to provide precise RV measurements, an iodine have better sampling for our targets and thus increase absorption (I2) cell was used with a wavelength region the planet detection efficiency. Almost all of the targets of 4900{6000 A.˚ The average signal-to-noise (S/N) for are giant stars (Section 2 in Lee et al. 2015). From the the I2 region was about 150 at typical exposure times SENS survey, we detected 20 planetary companions (Lee ranging from 15 to 20 minutes. et al. 2015; Lee et al. 2017; Bang et al. 2018; Jeong et We obtained 30 (HD 60292) and 28 (HD 112640) al. 2018) and several periodic RV variations probably spectra from 2009 to 2019. The basic reduction of spec- due to processes other than orbital motions around G-, tral data was done using the IRAF software package K-, and M-giant stars. Lack of knowledge about planet and the precise RV measurements related to the I2 anal- formation and evolution makes RV surveys of giant stars ysis were implemented using the RVI2CELL (Han et al. an important endeavor. 2007) code, which is based on the method by Butler et In this paper, we report the detection of low- al.(1996) and Valenti et al.(1995). The long-term stability of the BOES was demon- Corresponding author: B.-C. Lee strated with observations of the RV standard star τ Ceti. 27 28 Lee et al. Table 1 RV measurements of HD 60292 from February 2010 to April 2019. JD−2450000 ∆RV ±σ JD−2450000 ∆RV ±σ JD−2450000 ∆RV ±σ (Days) (m s−1) (m s−1) (Days) (m s−1) (m s−1) (Days) (m s−1) (m s−1) 5248.145727 −127.7 15.0 7527.992273 108.5 15.8 8148.066892 −35.7 14.9 5277.085340 −147.6 17.1 7676.359815 −44.0 15.6 8151.161235 −15.1 24.9 5844.338471 26.0 12.2 7705.130424 −75.5 14.8 8277.997266 −119.4 34.3 6288.274108 −90.4 12.1 7758.056809 −99.4 17.3 8290.984081 −19.0 19.3 6620.140717 61.2 13.2 7820.021457 −40.1 13.4 8370.312680 67.1 19.9 6740.035182 −145.2 13.8 7856.096216 4.1 15.9 8422.175121 66.6 17.6 6823.996642 −78.8 16.3 7896.007832 75.7 16.6 8451.303967 87.2 16.2 6972.300781 173.9 14.9 8052.184820 44.9 13.2 8473.010321 82.8 17.0 7066.236207 191.4 17.3 8052.301831 30.4 13.3 8516.003745 6.5 17.6 7302.298077 −109.4 12.4 8092.172561 30.6 16.2 8577.012946 89.0 15.1 Table 2 RV measurements of HD 112640 from December 2009 to May 2019. JD−2450000 ∆RV ±σ JD−2450000 ∆RV ±σ JD−2450000 ∆RV ±σ (Days) (m s−1) (m s−1) (Days) (m s−1) (m s−1) (Days) (m s−1) (m s−1) 5171.293230 115.4 14.6 7528.017873 119.0 9.4 8291.035273 16.9 12.3 5225.249646 153.3 16.4 7820.041391 −100.5 10.8 8369.980304 −98.4 13.8 5277.154579 73.6 14.3 7896.074337 −82.8 9.9 8423.327329 −93.2 12.0 6426.057858 98.5 8.9 8052.317284 −23.2 10.7 8451.326479 −141.6 15.4 6740.149723 13.9 12.4 8093.221608 −40.8 10.7 8473.212945 −171.5 12.1 6824.015415 34.6 8.3 8151.265260 25.6 16.3 8531.133694 −42.5 10.5 6972.315804 121.4 10.0 8234.268721 7.0 10.8 8532.142302 −99.5 10.9 7093.246045 −12.2 13.4 8278.016400 36.4 11.8 8627.033024 −11.7 11.0 7093.986619 56.0 14.5 8287.982494 47.0 10.5 7148.243269 −32.9 11.6 8290.066408 32.2 10.9 RVs measured by the BOES are constant with an rms different from ours. They assumed luminosity class V scatter of ∼ 7 m s−1 (Lee et al. 2013). The RV mea- for some stars with unknown luminosity class; thus, the surements for HD 60292 and HD 112640 are listed in K giant HD 60292 was misidentified as a K dwarf star. Tables1 and2. HD 60292 has been known to be of spectral type K0 (ESA 1997) and an apparent magnitude of 6.95. Its 3.S TELLAR CHARACTERISTICS absolute magnitude of −0:6 suggests that it is a giant Generally, long-period, low-amplitude RV variations star. The values of the stellar gravity and radius are caused by surface activities can occur in evolved stars. consistent with those of a giant star. Likewise, the Thus, investigation of the stellar characteristics is cru- surface gravity of HD 112640 is consistent with that of a cial for identifying the origin of any RV variations. The giant but inconsistent with a dwarf. Our new estimations main photometric parameters for HD 60292 (HIP 37196) of stellar parameters suggest that both HD 60292 and and HD 112640 (HIP 63203) were acquired from the HD 112640 are K giant stars. HIPPARCOS catalog (ESA 1997). The astrometric In evolved stars, estimating rotational periods is im- parallax (π) was taken from the Gaia database (Gaia portant for distinguishing RV variations from rotational Collaboration et al. 2018). We determined stellar param- modulation because long-period RV variations with low eters directly from our spectra by measuring 170 (HD amplitude may stem from stellar rotation modulation 60292) and 154 (HD 112640) equivalent widths (EW) (Lee et al. 2008; Lee et al. 2012a). In order to calculate of Fe i and Fe ii lines. We found Teff = 4348 ± 28 K, the stellar rotational velocity, a line-broadening model [Fe=H] = −0:17 ± 0:05, log g = 1:9 ± 0:1, and vmicro = (Takeda et al. 2008) was used. For the determination −1 1:7 ± 0:1 km s for HD 60292, and Teff = 4155 ± 15 K, of line broadening, we used the automatic spectrum- [Fe=H] = −0:09 ± 0:07, log g = 1:7 ± 0:1, and vmicro = fitting technique (Takeda 1995).