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Journal of the Korean Astronomical Society - Vol. 53 , No. 2

[ Article ]
Journal of the Korean Astronomical Society - Vol. 53, No. 1, pp.27-34
Abbreviation: JKAS
ISSN: 1225-4614 (Print) 2288-890X (Online)
Print publication date 29 Feb 2020
Received 12 Nov 2019 Accepted 28 Jan 2020
DOI: https://doi.org/10.5303/JKAS.2020.53.1.27

A SEARCH 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 ; YEON-HO CHOI3
1Korea Astronomy and Space Science Institute 776, Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Korea (bclee@kasi.re.kr)(iwhan@kasi.re.kr)(ymy501@kasi.re.kr)
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 (mgp@knu.ac.kr)(qkdxodid1230@knu.ac.kr)(apollo.choe@gmail.com)

Correspondence to : B.-C. Lee


Published under Creative Commons license CC BY-SA 4.0
Funding Information ▼

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.


Keywords: planetary systems, stars: individual: HD 60292; HD 112640, techniques: radial velocities

1. INTRODUCTION

Since the discovery of the first exoplanet around a mainsequence (MS) star (Mayor & Queloz 1995) using the radial velocity (RV) method, approximately 300 exoplanets were found in the same way until 2010. During the 15 years of this early stage of the RV method, Sunlike dwarf stars were the major targets because of the benefits of analysis they provide compared with giant stars. Giants may present more complex RV variations because of various surface processes that affect spectral line profiles: stellar pulsations, chromospheric activities, spots, and large convection cells.

In 2010, a new program, the Search for Exoplanet around Northern circumpolar Stars (SENS; Lee et al. 2015), was started. The main goal of SENS is to observe stars that are accessible around the year in order to have better sampling for our targets and thus increase the planet detection efficiency. Almost all of the targets are giant stars (Section 2 in Lee et al. 2015). From the SENS survey, we detected 20 planetary companions (Lee et al. 2015; Lee et al. 2017; Bang et al. 2018; Jeong et al. 2018) and several periodic RV variations probably due to processes other than orbital motions around G-, K-, and M-giant stars. Lack of knowledge about planet formation and evolution makes RV surveys of giant stars an important endeavor.

In this paper, we report the detection of low-amplitude and long-period RV variations in HD 60292 and HD 112640, possibly caused by a planetary companion. In Section 2, we describe the observations and data reduction. In Section 3, the stellar characteristics of the host stars are derived. The measurements of RV variations and their possible origins are presented in Sections 4 and 5. Finally, in Section 6, we discuss our results.


2. OBSERVATIONS AND DATA REDUCTION

The observations were obtained as a part of the exoplanet survey of the SENS program. The fiber-fed, high-resolution (R = 45 000) Bohyunsan Observatory Echelle Spectrograph (BOES; Kim et al. 2007) installed at the 1.8-m telescope of BOAO, Korea, was used. In order to provide precise RV measurements, an iodine absorption (I2) cell was used with a wavelength region of 4900–6000 Å. The average signal-to-noise (S/N) for the I2 region was about 150 at typical exposure times ranging from 15 to 20 minutes.

We obtained 30 (HD 60292) and 28 (HD 112640) spectra from 2009 to 2019. The basic reduction of spectral data was done using the IRAF software package and the precise RV measurements related to the I2 analysis were implemented using the RVI2CELL (Han et al. 2007) code, which is based on the method by Butler et al. (1996) and Valenti et al. (1995).

The long-term stability of the BOES was demonstrated with observations of the RV standard star τ Ceti. RVs measured by the BOES are constant with an rms scatter of ∼ 7 m s−1 (Lee et al. 2013). The RV measurements for HD 60292 and HD 112640 are listed in Tables 1 and 2.

Table 1 
RV measurements of HD 60292 from February 2010 to April 2019.
JD−2450000
(Days)
RV
(m s−1)
±σ
(m s−1)
JD−2450000
(Days)
RV
(m s−1)
±σ
(m s−1)
JD−2450000
(Days)
RV
(m s−1)
±σ
(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
(Days)
RV
(m s−1)
±σ
(m s−1)
JD−2450000
(Days)
RV
(m s−1)
±σ
(m s−1)
JD−2450000
(Days)
RV
(m s−1)
±σ
(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


3. STELLAR CHARACTERISTICS

Generally, long-period, low-amplitude RV variations caused by surface activities can occur in evolved stars. Thus, investigation of the stellar characteristics is crucial for identifying the origin of any RV variations. The main photometric parameters for HD 60292 (HIP 37196) and HD 112640 (HIP 63203) were acquired from the HIPPARCOS catalog (ESA 1997). The astrometric parallax (π) was taken from the Gaia database (Gaia Collaboration et al. 2018). We determined stellar parameters directly from our spectra by measuring 170 (HD 60292) and 154 (HD 112640) equivalent widths (EW) of Fe i and Fe ii lines. We found Teff = 4348 ± 28 K, [Fe/H] = −0.17 ± 0.05, log g = 1.9 ± 0.1, and vmicro = 1.7 ± 0.1 km s−1 for HD 60292, and Teff = 4155 ± 15 K, [Fe/H] = −0.09 ± 0.07, log g = 1.7 ± 0.1, and vmicro = 1.5 ± 0.1 km s−1 for HD 112640, respectively, using the software program TGVIT (Takeda et al. 2005).

The planet mass is rather uncertain due to differences in masses of the host stars as measured by different methods or authors. Tetzlaff et al. (2011) derived values for age and mass of HD 60292 which are significantly different from ours. They assumed luminosity class V for some stars with unknown luminosity class; thus, the K giant HD 60292 was misidentified as a K dwarf star.

HD 60292 has been known to be of spectral type K0 (ESA 1997) and an apparent magnitude of 6.95. Its absolute magnitude of −0.6 suggests that it is a giant star. The values of the stellar gravity and radius are consistent with those of a giant star. Likewise, the surface gravity of HD 112640 is consistent with that of a giant but inconsistent with a dwarf. Our new estimations of stellar parameters suggest that both HD 60292 and HD 112640 are K giant stars.

In evolved stars, estimating rotational periods is important for distinguishing RV variations from rotational modulation because long-period RV variations with low amplitude may stem from stellar rotation modulation (Lee et al. 2008; Lee et al. 2012a). In order to calculate the stellar rotational velocity, a line-broadening model (Takeda et al. 2008) was used. For the determination of line broadening, we used the automatic spectrum-fitting technique (Takeda 1995). We estimated rotational velocities vrot sin(i) = 3.4 km s−1 for HD 60292 and vrot sin(i) = 3.6 km s−1 for HD 112640. Using the rotational velocities and the stellar radii, we derived upper limits for the rotational periods Prot = 2πR*/[vrot sin(i)] of 400 days for HD 60292 and 545 days for HD 112640. Table 3 summarizes the basic stellar parameters.

Table 3 
Stellar parameters for HD 60292 and HD 112640 assumed in this work.
Parameter Unit HD 60292 HD 112640 Reference
Spectral type K0 K0 HIPPARCOS ESA (1997)
K0 III K0 III This work
mv mag 6.95 6.54 HIPPARCOS ESA (1997)
Mv mag −0.6 −0.8 This work
B-V mag 1.315 ± 0.010 1.408 ± 0.007 HIPPARCOS ESA (1997)
age Gyr 0.63 ± 0.24 Tetzlaff et al. (2011)
Gyr 1.8 ± 0.5 1.6 ± 0.3 This work
d pc 328.9 296.7 McDonald et al. (2017)
RV km s−1 −12.7 ± 0.1 −12.36 ± 0.15 Gaia Collaboration et al. (2018)
π mas 3.14 ± 0.03 3.06 ± 0.02 Gaia Collaboration et al. (2018)
Teff K 4348 ± 28 4155 ± 15 This work
[Fe/H] dex −0.17 ± 0.05 −0.09 ± 0.07 This work
log g cgs 1.9 ± 0.1 1.7 ± 0.1 This work
vmicro km s−1 1.7 ± 0.1 1.5 ± 0.1 This work
R* R 27.0 ± 1.5 39.0 ± 5.0 Gaia Collaboration et al. (2018)
M* M 1.7 ± 0.2 1.8 ± 0.2 This work
M 6.3 ± 0.5 Tetzlaff et al. (2011)
L* L 247.8 283.5 McDonald et al. (2017)
vrot sin(i) km s−1 3.4 ± 0.3 3.6 ± 0.2 This work
Prot / sin(i) days 400 545 This work


4. ORBITAL SOLUTIONS FROM RADIAL VELOCITIES

In order to find periodicities in the RV time series, we used the Lomb-Scargle (L-S) periodogram (Lomb 1976; Scargle 1982), which is appropriate for analyzing long-term variability in unequally spaced data. We determined the orbital elements by fitting the RV with Keplerian orbit models in the Systemic Console 2 software (Meschiari et al. 2009). We estimated the parameter uncertainties using the bootstrap routine within Systemic Console 2 with 10 000 synthetic data set realisations. The RV measurements and the RV phase diagrams for the orbit are shown in Figures 14. The best-fit Keplerian orbits have periods of 495.4 ± 3.0 days for HD 60292 and 613.2 ± 5.8 days for HD 112640. The corresponding semi-amplitudes K are 120.0 ± 15.3 m s−1 for HD 60292 and 83.0 ± 8.5 m s−1 for HD 112640. There are no significant periodic residuals left in the data after subtracting the main period (top panels in Figures 5 and 6). Assuming stellar masses of 1.7 ± 0.2 M (HD 60292) and 1.8±0.2 M (HD 112640), we find minimum masses of a planetary companions of 6.5 ± 1.0 MJup at a distance from the host star a = 1.5 au for HD 60292, and 5.0 ± 1.0 MJup at a = 1.7 au for HD 112640. All orbital elements are listed in Table 4.


Figure 1. 
RV measurements (top) and the residuals (bottom) of HD 60292 from 2010 to 2019. The solid line is the orbital solution with a period of 495.4 days.


Figure 2. 
Phase diagram for HD 60292. The solid line indicates the best Keplerian fit.


Figure 3. 
RV measurements (top) and the residuals (bottom) of HD 112640 from 2010 to 2019. The solid line is the orbital solution with a period of 613.2 days.


Figure 4. 
Phase diagram for HD 112640. The solid line indicates the best Keplerian fit.

Table 4 
Orbital parameters for HD 60292b and HD 112640b.
Parameter Unit HD 60292 HD 112640
Period days 495.4 ± 3.0 613.2 ± 5.8
Tperiastron JD 2455773.7 ± 19.4 2455300.4 ± 38.8
K m s−1 120.0 ± 15.3 83.0 ± 8.5
e 0.27 ± 0.1 0.24 ± 0.18
ω deg 83.5 ± 14.2 114.4 ± 22.8
slope m s−1 day−1 −1.6 × 10−6 2.9 × 10−3
Nobs 30 28
σ (O-C) m s−1 37.7 29.3
M* M 1.7 ± 0.2 1.8 ± 0.2
m sin(i) MJup 6.5 ± 1.0 5.0 ± 1.0
a au 1.5 ± 0.1 1.7 ± 0.1


5. ORIGIN OF RADIAL VELOCITY VARIATIONS

Evolved stars often exhibit pulsations as well as surface activity, which result in low-amplitude periodic RV variations. Generally, short-term RV variations with low amplitudes have been known to be the result of stellar pulsations (Hatzes & Cochran 1998), while longterm RV variations may be caused by three kinds of phenomena: stellar pulsation, rotational modulation of inhomogeneous surface features, or planetary (substellar) companions. In order to establish the origin of the RV variations for HD 60292 and HD 112640, we analyzed HIPPARCOS light curves, spectral line bisectors, and stellar chromospheric activity.

5.1. Light Curves

In order to search for brightness variations caused by rotational modulation of cool stellar spots or pulsations, we analyzed the HIPPARCOS photometry data. The available photometry database comprises 106 (HD 60292) and 140 (HD 112640) HIPPARCOS measurements from 1990 to 1993. The rms photometric errors were 0.013 mag for HD 60292 and 0.011 mag for HD 112640, corresponding to relative flux variations of 0.18% and 0.16% over the time span of the observations, respectively. The “HIPPARCHOS” panels of Figures 5 and 6 show the L-S periodograms of the light curves, showing no significant periodic variations.


Figure 5. 
L-S periodograms of the RV time series, the HIPPARCOS light curves, the bisector time series, and the hydrogen line EW time series (from top to bottom) for HD 60292. The vertical dashed line indicates the location of a period of 495 days. The periodogram of the RV measurements (top panel) shows significant power at a period of 495.4 days. The dashed line marks the periodogram of the residual. The horizontal dotted line indicates a false alarm probability threshold of 1%.


Figure 6. 
L-S periodograms of the RV time series, the HIPPARCOS light curves, the bisector time series, and the hydrogen line EW time series (from top to bottom) for HD 112640. The vertical dashed line indicates the location of a period of 613 days. The periodogram of the RV measurements (top panel) shows significant power at a period of 613.2 days. The dashed line marks the periodogram of the residual. The horizontal dotted line indicates a false alarm probability threshold of 1%.

5.2. Line Bisectors

RV variations due to planetary companions are not supposed to affect the shape of spectral lines. This distinguishes them from rotational modulations of stellar surface inhomogeneities which can cause variable asymmetries in spectral line profiles Queloz et al. (2001).

To investigate spectral line profile variations, we calculated (a) the RV difference of the central values at high and low flux levels of the line profiles (BVS; bisector velocity span), and (b) the difference of the velocity span of the upper half and lower half of the bisectors (BVC; velocity curvature)(Hatzes et al. 2005; Lee et al. 2013). For calculating bisectors, we selected two unblended strong lines, Fe i 6358.7 Å and Ni i 6767.8 Å, in the spectrum of HD 60292, and three unblended strong lines, Fe i 6065.5 Å, Fe i 6421.3 Å, and Ni i 6767.8 Å, in the spectrum of HD 112640 that are located beyond the I2 absorption region. To avoid the line core and wings, the BVS of a given line was measured using the line profiles at flux levels of 0.4 and 0.8 times the central depth as span points. The average BVS and BVC were computed using the bisector measurements of several spectral lines after subtracting the mean value for each spectral line. Table 5 shows the averaged BVS and BVC values for each star. Figures 5 and 6 (third panels from top) show the LS periodograms of the BVS and BVC time series. None of the parameters shows significant periodic variability.

Table 5 
Bisector measurements of HD 60292 and HD 112640.
HD 60292 HD 112640
JD−2450000
(Days)
BVS
(m s−1)
BVC
(m s−1)
JD−2450000
(Days)
BVS
(m s−1)
BVC
(m s−1)
5248.146 15.61 −37.97 5171.293 7.56 −37.52
5277.085 −34.50 84.45 5225.250 −44.37 −34.74
5844.338 −5.92 −21.57 5277.155 −52.7 24.88
6288.274 −86.76 49.27 6426.058 −58.6 −21.59
6620.141 −62.70 23.54 6740.150 5.89 −5.25
6740.035 −63.47 −24.52 6824.015 −26.0 38.66
6823.997 13.57 −36.48 6972.316 −8.42 −0.32
6972.301 33.22 56.91 7093.246 −7.84 −40.51
7066.236 204.64 −59.72 7093.987 49.15 19.06
7302.298 −51.39 −11.03 7148.243 0.713 46.35
7527.992 −45.75 22.03 7528.018 −7.97 −6.74
7676.360 −47.42 −44.45 7820.041 6.43 40.94
7705.130 −34.88 −28.5 7896.074 −30.7 20.68
7758.057 −11.75 43.1 8052.317 −17.1 21.91
7820.021 28.37 −16.78 8093.222 3.656 −72.59
7856.096 −49.62 39.375 8151.265 27.80 −14.31
7896.008 83.29 −11.53 8234.269 11.04 12.98
8052.185 −51.7 12.74 8278.016 16.58 −16.17
8052.302 −23.4 −12.64 8287.982 15.96 36.64
8092.173 −8.98 11.85 8290.066 54.76 14.26
8148.067 137.08 1.81 8291.035 −14.6 23.17
8151.161 115.91 87.43 8369.980 −20.05 −23.38
8277.997 17.58 −61.81 8423.327 5.163 18.41
8290.984 −66.05 35.125 8451.326 −13.2 11.81
8370.313 −30.22 −20.53 8473.213 −1.83 −5.841
8422.175 −74.02 −49.55 8531.134 58.27 25.81
8451.304 −84.29 9.71 8532.142 40.8 −76.49
8473.010 40.018 −49.49
8516.004 143.62 9.42

5.3. Chromospheric Activity

The EW variations of Ca ii H&K, Hα, Hβ, and the Ca ii triplet lines (Larson et al. 1993) are usually used as indicators of chromospheric activity. Such activity could have a significant effect on the RV variations. Ca ii H&K is emitted in the chromosphere; the core of the spectral line often exhibits central reversal in the presence of chromospheric activity (Saar & Donahue 1997). Unfortunately, the signal-to-noise ratio of our data is insufficient to resolve the emission features in the Ca ii H&K line cores for both stars. The Ca ii triplets are not suitable either because of fringing and saturation of our CCD spectra at wavelengths above 8000 Å.

Thus, we use the Hα and Hβ lines in this study. We measured the H line EWs using a band pass of ±1.0 Å centered on the line core to avoid nearby blending lines and weak telluric lines. We find mean Hα EWs of 1179.03 ± 1.76 mÅ for HD 60292 and 1213.15 ± 1.75 mÅ for HD 112640. The mean EWs of the Hβ lines are 892.07 ± 1.16 mÅ for HD 60292 and 981.55 ± 0.99 mÅ for HD 112640. The Hα and the Hβ EWs show rms variations with time of less than 0.2% for both stars. L-S periodograms of the H line EW time series are shown in the bottom panels of Figures 5 and 6. There are no significant excesses in spectral power around 495.4 days (HD 60292) and 613.2 (HD112640) days. Table 6 lists our H line EW measurements.

Table 6 
H line EW measurements of HD 60292 and HD 112640.
HD 60292 HD 112640
JD−2450000
(Days)
Hα EW
(mÅ)
Hβ
(mÅ)
JD−2450000
(Days)
Hα
(mÅ)
Hβ
(mÅ)
5248.146 1180.643 891.644 5171.293 1213.408 981.966
5277.085 1180.219 891.079 5225.250 1214.216 980.599
5844.338 1177.168 890.480 5277.155 1212.625 979.010
6288.274 1178.035 892.022 6426.058 1212.366 981.315
6620.141 1182.011 891.137 6740.150 1212.085 980.095
6740.035 1179.050 891.154 6824.015 1212.856 983.598
6823.997 1177.988 891.509 6972.316 1215.788 981.938
6972.301 1178.517 892.528 7093.246 1210.452 981.799
7066.236 1179.244 892.555 7093.987 1211.815 981.412
7302.298 1178.289 893.303 7148.243 1211.685 981.387
7527.992 1179.254 892.373 7528.018 1211.749 982.022
7676.360 1180.701 892.309 7820.041 1210.488 980.663
7705.130 1179.052 892.762 7896.074 1225.799 981.743
7758.057 1179.875 892.459 8052.317 1214.724 981.876
7820.021 1178.439 893.887 8093.222 1216.208 980.331
7856.096 1179.457 893.789 8151.265 1214.098 983.308
7896.008 1180.357 892.282 8234.269 1214.034 982.046
8052.185 1182.030 892.287 8278.016 1212.544 980.416
8052.302 1181.797 891.903 8287.982 1211.535 981.394
8092.173 1180.817 891.955 8290.066 1212.784 981.886
8148.067 1178.317 892.154 8291.035 1212.168 980.424
8151.161 1178.387 891.091 8369.980 1216.661 981.619
8277.997 1173.709 888.710 8423.327 1216.678 982.092
8290.984 1176.458 889.870 8451.326 1214.280 981.575
8370.313 1177.928 893.459 8473.213 1212.782 982.343
8422.175 1178.610 893.003 8531.134 1212.340 982.213
8451.304 1179.266 892.823 8532.142 1211.557 982.777
8473.010 1179.204 891.900
8516.004 1176.921 893.549


6. DISCUSSION

The analysis of the RV measurements for HD 60292 and HD 112640 revealed low-amplitude variations with periods of 495.4 days and 613.2 days, respectively, that persisted over six cycles between 2009 and 2019 in both cases. Low-amplitude and long-term – referring to periods longer than a few hundred days – RV variations in evolved stars, may be caused by stellar pulsations, rotational modulations of inhomogeneous surface features, or planetary (substellar) companions.

Of these, variations by pulsations are unlikely because the period of the fundamental radial mode is of the order of days. We calculated the expected pulsation periods and RV amplitudes using the relationships of Kjeldsen, & Bedding (1995). The periods are far too short: 1.2 days for HD 60292 and 2.5 days for HD 112640. Furthermore, we expect the semi-amplitudes of pulsations to be 34 m s−1 and 56 m s−1 , respectively, well below the dispersions of 120 m s−1 and 83 m s−1 measured for HD 60292 and HD 112640.

The H lines were used to monitor chromospheric activities, however do not reveal any significant variation. The HIPPARCOS light curves do not show any variability, the BVSs do not display any relation with the RV measurements. Eventually, we can exclude rotational modulation or stellar surface activity as the causes for the observed RV observations, leaving us with the planetary companion hypothesis. Both stars show rotation periods (upper limits) of 400 days (HD 60292) and 545 days (HD 112640) that are too short to explain the observed RV modulations.

The residuals left after subtracting the principal periodic signals from the RV time series show rms scatters of 37.7 m s−1 for HD 60292 and 29.3 m s−1 for HD 112640. These values are significantly higher than the typical RV precisions obtained from measurements of the RV standard star τ Ceti (7 m s−1 ) and the typical internal errors of individual RV measurements of K giants in our survey, expected to be ∼ 15 m s−1 for HD 60292 and ∼ 11 m s−1 for HD 112640. The variations appear to be caused by systematics in the RV measurements and are typical for giant stars (Setiawan et al. 2003; Hatzes et al. 2005; Döllinger et al. 2007; de Medeiros et al. 2009; Han et al. 2010; Lee et al. 2012b) and might also be due to pulsation/convection activity.

Eventually, we conclude that K giants HD 60292 and HD 112640 host planetary companions with minimum masses of 6.5 MJup and 5.0 MJup, respectively. These parameters are similar to those of companions around other K giant stars.


Acknowledgments

BCL acknowledges partial support by the KASI (Korea Astronomy and Space Science Institute) grant 2019-1-830-03. MGP and TYB acknowledge support by the KASI under the R&D program supervised by the Ministry of Science, ICT and Future Planning and by the National Research Foundation of Korea to the Center for Galaxy Evolution Research (No. 2017R1A5A1070354) and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2019R1I1A3A02062242). This research made use of the SIMBAD database, operated at the CDS, Strasbourg, France. We thank the developers of the Bohyunsan Observatory Echelle Spectrograph (BOES) and all staff of the Bohyunsan Optical Astronomy Observatory (BOAO).


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