HIP 102152b: A low-mass planet candidate around an old solar twin Thiago Ferreira & Jorge Meléndez Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo – Brazil Contact: tfsantos@usp.br ABOUT THE STAR HIP 102152 is a solar twin (Teﬀ= 5718 K, [Fe/H] = -0.016, and log g = 4.325; [1]), placed near the end of the main sequence (t = 8.2 Gyr), and presents a rotation period of 35.7 days [2]. Due to its severe lithium depletion [3], and low activity levels (log R′ HK = −5.12) but high radial velocity (RV) RMS, it was identiﬁed as a promising planet-hosting system [4]. DATA ANALYSIS HIP 102152 was observed with the ESO/HARPS spectrograph mounted at the 3.6–m telescope in La Silla Observatory between Oct.–2011 and May–2019. From a Lomb-Scargle analysis, we detected a periodic signal of ∼23–d in the RV time-series, and besides known stellar activity, no modulation of the indicators was observed at this period, nor correlations with the RVs (see Figs. 1 and 2). We employed a joint Keplerian plus Gaussian Process (GP) model and estimated the signiﬁcance of the results via Markov chain sampling, where observations at two different epochs are correlated via the kernel Σij = η2 1·exp  −|ti −tj| 2 · η22 − 2 · sin2 ( π·|ti−tj|) η3 ) η24  , a function of the Doppler amplitude of the signal (η1), the time-scale for growth and decay of active regions (which is often comparable to the star’s rotation period, η2), the recurrence time-scale for active regions (η3), and a smoothing parameter (η4) [5]. For the Keplerian orbits, both the eccentricity (in the orthogonal basis with the periastron argument: √e sin(ω) and √e cos(ω)) and the radial velocity slope ( ˙γ) were set to freely vary, and we included separated jitter parameters for data taken before and after the ESO/HARPS upgrade in mid-2015. The other model variables include the GLS orbital period (P), its Doppler amplitude (K) and a reference time (Tc). 0.0 0.1 0.2 RV RV Prot = 35.7 d Porb = 22.9 d 0.0 0.1 0.2 BIS CCF BIS 0.0 0.1 0.2 FWHM CCF FWHM 0.0 0.1 0.2 S Ca II S (H+K) 0.0 0.1 0.2 H H 0.0 0.1 0.2 H H 0.0 0.1 0.2 0.3 H H 0.0 0.1 0.2 H H 0.0 0.1 0.2 H H 10 4 10 3 10 2 10 1 100 Frequencies / d 1 0.0 0.5 1.0 S. W. Spectral Window Figure 1. GLS periodogram of the RV and activity indicators of HIP 102151. The central blue line marks the star’s rotational period and its harmonics. The green line marks the planet’s orbital period, and the red line is the periodogram peak for each variable. The power level for which FAP levels are less than 1% and 5% are indicated as grey horizontal lines. 20 15 10 5 0 5 Bisector span / m s 1 p = -0.27 50 40 30 20 10 0 10 20 30 FWHM / m s 1 p = 0.13 0.10 0.11 0.12 0.13 0.14 0.15 0.16 Ca II S (H+K) p = 0.06 0.092 0.094 0.096 0.098 H p = 0.13 0.102 0.104 0.106 0.108 0.110 H p = -0.11 0.110 0.115 0.120 0.125 0.130 H p = -0.11 7.5 5.0 2.5 0.0 2.5 5.0 7.5 Radial Velocity / m s 1 0.125 0.130 0.135 0.140 0.145 0.150 H p = 0.17 7.5 5.0 2.5 0.0 2.5 5.0 7.5 Radial Velocity / m s 1 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 H p = 0.11 Figure 2. The blue line represents the best-ﬁt linear correlation between the RVs and several activity indicators. The Pearson correlation test p is indicated to quantify the signiﬁcance of the correlations. RESULTS HIP 102152b is consistent with a mini- Neptune (m sin(i) = 8 ± 1M⊕) on a circular orbit with period P = 22.9 ± 0.01– d, and no RV slope was observed due to acceleration from an outer massive companion in this system (see Fig. 3). The best-ﬁt solution (by combining Normal and Jeffrey priors) was obtained with the RadVel routine [6], implementing a maximum a posteriori optimisation and computing conﬁdence intervals from 1000 independent draws plus requiring a Gelman-Rubin statistics ˆR < 1.01. π(Vr|Θ) ∝N(P|22.9, 0.01) × N(K|2, 0.2) × N(T0|x(max y), 0.01) × J (η1|0.01, 100) × N(η2|35.7, 1.4) × N(η3|30, 0.7) × N(η4|0.46, 0.01) × N(σA,B|0, 0.1) Four models (circular and eccentric, with and w/ sl