arXiv:2007.10995v2 [astro-ph.EP] 3 Aug 2020 32 4 21 19 [email protected] eateto hsc n al nttt o Astrophysics for Institute Kavli and Physics of Department atiH Kristiansen H. Martti eateto srnm srpyisadCne o Exopl for Center and Astrophysics & Astronomy of Department 31 nentoa etrfrAvne tde IA)adICIFI and (ICAS) Studies Advanced for Center International ai .Latham W. David eatmnod ´sc srnma audd eCiˆenci de Faculdade Astronomia, F´ısica e de Departamento yee sn L using 2020 Typeset 5, August version Draft 11 2 EIIsiue alSgnCne o h td fLf nthe in Life of Study the for Center Sagan Carl Institute, SETI 28 naaUiest,Gaut colo aua n ple S Applied and Natural of School Graduate University, Ankara eateto at,AmshrcadPaeaySine,M Sciences, Planetary and Atmospheric Earth, of Department utainAtooia pis aut fSineadEng and Science of Faculty Optics, Astronomical Australian rnec Pepe, Francesco tpe .Kane R. Stephen 16 nrwVanderburg Andrew 26 ao .Bean L. Jacob 9 18 naaUiest,Fclyo cec,Dprmn fAstro of Department Science, of Faculty University, Ankara eateto srnm srpyis nvriyo Chic of University Astrophysics, & Astronomy of Department St T pc,Ntoa pc nttt,TcnclUniversity Technical Institute, Space National Space, DTU as Daylan Tansu nttt eAto´sc ieca oEspa¸co, Universi Ciˆencias Astrof´ısica do e de Instituto 12 17 paeUdry ephane ´ 29 eateto eoatc n srnuis I,7 Massac 77 MIT, Astronautics, and Aeronautics of Department at lnt aoaoy angeIsiuino Washin of Institution Carnegie Laboratory, Planets & Earth 13 oahnHorner Jonathan eateto hsc n srnm,N8 S,BihmYoun Brigham ESC, N283 Astronomy, and Physics of Department 10 A etr 2 ae aoaoy h enyvnaSaeUnive State Pennsylvania The Laboratory, Davey 525 Center, 1 6 eateto srpyia cecs rneo Universi Princeton Sciences, Astrophysical of Department T eateto at n lntr cecs nvriyo C of University Sciences, Planetary and Earth of Department 20 bevtied e`v,Uiested e`v,5 h des ch. Gen`eve, Gen`eve, 51 Universit´e de de Observatoire etrfrAstrophysics for Center 24 E 30 etefrAtohsc,Uiest fSuhr Queenslan Southern of University Astrophysics, for Centre X etefrAtohsc eerh nvriyo Hertfords of University Research, Astrophysics for Centre 22 , 3 xpaeaySinea SW colo hsc NWSydne UNSW , Physics of School USNW, at Science Exoplanetary 5 10 7 twocolumn i-asil nvri,CR,CE,Lbrtied’Astro Laboratoire CNES, CNRS, Universit, Aix-Marseille eateto srnm,Clfri nttt fTechnolo of Institute California Astronomy, of Department CRPaeS etefrSae&Hbtblt,University Habitability, & Space for Centre NCCR/PlanetS, eateto srnm,TeUiest fTxsa Austin, at Texas of University The Astronomy, of Department oetA Wittenmyer A. Robert 25 15 aaSeager Sara , rred bevtr,Osrao ydnensVj7 DK- 7, Vej Gyldenkernes Observator Observatory, Brorfelde 16 23 , 8 colo hsc,Uiest fNwSuhWls yny NS Sydney, Wales, South New of University Physics, of School , 6 , 5 26 25, , 4, otws eerhIsiue 20ClbaR,SnAntonio San Rd, Culebra 6220 Institute, Research Southwest eateto srnm,Uiest fClfri,Berkel California, of University Astronomy, of Department 1 6 rnosBouchy Francois 27 ai Ragozzine Darin ‡ Selc¸uk Yalc¸ınkaya aaFetherolf Tara rniso nw lnt riigaNkdEeStar Naked-Eye a Orbiting Planets Known of Transits utainAtooia bevtr,NrhRd,NW2113 NSW Ryde, North Observatory, Astronomical Australian aaiDeleuil Magali , hitpeLovis Christophe 7, 14 † tl nAASTeX63 in style AAAe eerhCne,Mfft il,C,905 USA 94035, CA, Field, Moffett Center, Research Ames NASA , e Mo Teo 20 , nrwW Howard W. Andrew ,1,12 11, 4, | Harvard cnik ˇ ohaN Winn N. Joshua , , , 29 6 , 15) unsArs Argentina Aires, Buenos (1650), 1 3 , , .Pu Butler Paul R. 1 20 Ozg ainS Damien ¨ , & org .Diaz F. Rodrigo , 2 aai .Hinkel R. Natalie 6 ucnJ Wright J. Duncan mtsna,6 adnSre,Cmrde A018 USA 02138, MA Cambridge, Street, Garden 60 Smithsonian, uhP Osborn P. Hugh efryW Marcy W. Geoffrey rBas¸tur ¨ R010 Turkey TR-06100, n pc eerh ascuet nttt fTechnology of Institute Massachusetts Research, Space and sd nvriaed ot,Rad ap lge /,4169-0 s/n, Alegre, Campo do Rua Porto, do Universidade da as 23,USA 02139, CNCT,Ey-NA,Cmu iult,2 eMy Franc y Mayo de 25 Miguelete, Campus ECyT-UNSAM, (CONICET), Portugal nt n aial olsadPn tt Extraterrestrial State Penn and Worlds Habitable and anets egransan, ´ neig aqai nvriy ot ye S 13 Aust 2113, NSW Ryde, North University, Macquarie ineering, ine,Dprmn fAtooyadSaeSine,Tando Sciences, Space and Astronomy of Department ciences, , aed ot,RadsEtea,45-6 ot,Portugal Porto, 4150-762 Estrelas, das Rua Porto, do dade nvre ffSt:21 ann a,Mdsn I573 USA 53713, WI Madison, Way, Waunona 2613 Off-Site: Universe, 22 sahstsIsiueo ehooy abig,M 02139 MA Cambridge, Technology, of Institute assachusetts urk ¨ , 13 oadIsaacson Howard oyadSaeSine,Tnoa,T-60,Turkey TR-06100, Tandogan, Sciences, Space and nomy fDnak lkrvj37 K20 ygy Denmark Lyngby, DK-2800 327, Elektrovej Denmark, of g,54 ot li vne hcg,I 03,USA 60637, IL Chicago, Avenue, Ellis South 5640 ago, o .Jenkins M. Jon , , 17 9 , 21 tn W ahntn C 01-35 USA 20015-1305, DC, Washington, NW, gton, ereR Ricker R. George ig .Campante L. Tiago y v ae rneo,N 84,USA 08544, NJ Princeton, Lane, Ivy 4 ty, , 6 , ,4 5 4, 3, uet vne abig,M 23,USA 02139, MA Cambridge, Avenue, husetts airDumusque Xavier 8 alets 20Suen,Switzerland Sauverny, 1290 Maillettes, , ..Tinney C.G. ie afil,HrsA1 A,UK 9AB, AL10 Herts Hatfield, hire, st,Uiest ak A182 USA 16802, PA Park, University rsity, 20 ,Tooma L 30 Australia 4350, QLD Toowoomba, d, ob Ostberg Colby lfri,Rvrie A951 USA 92521, CA Riverside, alifornia, and , Dalba A. Paul y aaea A915 USA 91125, CA Pasadena, gy, hsqed asil,France Marseille, de physique 23 nvriy rv,U 40,USA 84602, UT Provo, University, g fBr,Br,Switzerland Bern, Bern, of aieMarmier Maxime 30Tløe Denmark Tølløse, 4340 utn X772 USA 78712, TX Austin, ao .Wright T. Jason y A970 USA 94720, CA ey, ,Sde 02 Australia 2052, Sydney y, X728 USA 78238, TX , , , 14 02 Australia 2052, W 3 20 23, , Allart Romain 30 Australia , uhRA Jones R.A. Hugh agrtC Turnbull C. Margaret , 4 , , , 1 6 8 19 18, oadVanderspek Roland , 1, ke ua Esmer Murat Ekrem ai Ehrenreich David ∗ oieD Nielsen D. Louise rdD Carter D. Brad , 6 32 io .O’Toole J. Simon , 6 eeyBailey Jeremy , 24 abig,MA Cambridge, , , 6 , 4 , Intelligence 20 , 7Porto, 07 , 31 USA , , 9 6 ralia gan, , 7 28 27, , 15 ia 2 Stephen R. Kane et al.
ABSTRACT Some of the most scientifically valuable transiting planets are those that were already known from radial velocity (RV) surveys. This is primarily because their orbits are well characterized and they preferentially orbit bright stars that are the targets of RV surveys. The Transiting Exoplanet Sur- vey Satellite (TESS) provides an opportunity to survey most of the known exoplanet systems in a systematic fashion to detect possible transits of their planets. HD 136352 (Nu2 Lupi) is a naked-eye (V =5.78) G-type main-sequence star that was discovered to host three planets with orbital periods of 11.6, 27.6, and 108.1 days via RV monitoring with the HARPS spectrograph. We present the detection and characterization of transits for the two inner planets of the HD 136352 system, revealing radii of +0.058 +0.078 1.482−0.056 R⊕ and 2.608−0.077 R⊕ for planets b and c, respectively. We combine new HARPS observa- tions with RV data from Keck/HIRES and the AAT, along with TESS photometry from Sector 12, to perform a complete analysis of the system parameters. The combined data analysis results in extracted +1.2 −3 +0.41 −3 bulk density values of ρb =7.8−1.1 gcm and ρc =3.50−0.36 gcm for planets b and c, respectively, thus placing them on either side of the radius valley. The combination of the multi-transiting planet system, the bright host star, and the diversity of planetary interiors and atmospheres means this will likely become a cornerstone system for atmospheric and orbital characterization of small worlds.
Keywords: planetary systems – techniques: photometric – techniques: radial velocities – stars: indi- vidual (HD 136352)
1. INTRODUCTION and their host stars are relatively bright. Well- The discovery of transiting exoplanets has enabled known examples of RV planets later found to tran- a plethora of science not accessible through other ex- sit include HD 209458b (Charbonneau et al. 2000; oplanet detection techniques. Transiting planets or- Henry et al. 2000), HD 189733b (Bouchy et al. 2005), biting bright stars are especially important in fur- and HD 80606b (Naef et al. 2001; Fossey et al. 2009; thering our knowledge of planetary systems because Garcia-Melendo & McCullough 2009; Laughlin et al. they offer unique windows to comparative exoplane- 2009). Though the TERMS survey successfully dis- tology. First, they allow for a measurement of both covered new planets (Wang et al. 2012), destroyed the planetary mass and radius, and thereby to place old planets (Kane et al. 2016b), characterized numer- constraints on the planet interior structure. Sec- ous host stars (e.g., Dragomir et al. 2012; Hinkel et al. ond, they are amenable to atmospheric characteriza- 2015), and ruled out transits (e.g., Kane et al. 2011b,a; tion through transmission spectroscopy (e.g., Sing et al. Pilyavsky et al. 2011; Henry et al. 2013), the primary 2016; Kempton et al. 2018), to secondary eclipse mea- science goal was largely impeded by ground-based ob- surements (e.g., Kreidberg et al. 2019), and to or- servational window functions (von Braun et al. 2009). bital geometry characterization through the Rossiter- However, the Transiting Exoplanet Survey Satellite McLaughlin effect (e.g., Fabrycky & Winn 2009). In (TESS) has observed most of the sky during the pri- multi-planet systems, they also allow for a deep- mary mission (Ricker et al. 2015), including the known ened understanding of the system architecture through exoplanet hosts. The survey strategy of TESS thus pro- planet-planet dynamics accessible through the model- vides a space-based means to systematically examine all ing of transit timing variations (e.g., Jontof-Hutter et al. of the known RV systems for potential transits of their 2016). planets that pass through inferior conjunction during The Transit Ephemeris Refinement and Monitor- the TESS observing window (Kane & von Braun 2008; ing Survey (TERMS) has been operating since 2008 Dalba et al. 2019). This has been demonstrated through with the primary goal of detecting transits for the transit detection of an additional planet in the known radial velocity (RV) exoplanets (Kane et al. pi Mensae system (Huang et al. 2018) and the known 2009). The appeal of transits for known RV plan- RV planet in the HD 118203 system (Pepper et al. ets is that their orbits are already characterized 2020). HD 136352 (also known as Nu2 Lupi, LHS 395, GJ 582, and HIP 75181) is a G-type main-sequence star ∗ NSF Astronomy and Astrophysics Postdoctoral Fellow that was observed for nearly 11 years using the High Ac- † NASA Sagan Fellow curacy Radial velocity Planet Searcher (HARPS) spec- ‡ Kavli Fellow trograph (Pepe et al. 2000). Analysis of the HARPS Transits of Known Planets Orbiting a Naked-Eye Star 3 data by Udry et al. (2019) uncovered the signatures of be transiting in line with the Dalba et al. (2019) predic- three planets orbiting the star with periods and mini- tions. Note that the TESS extended mission will likely mum masses in the range of 11–110 days and 5–10 M⊕ lead to further transit detections of known RV planets respectively. The host star was observed by TESS whose orbital periods are longer than TESS observations during Sector 12 of the primary mission. A TESS of their host star during the primary mission. Guest Investigator (GI) program designed to monitor The host star HD 136352 is a G3/5V star that has the known hosts (PI: Kane) immediately detected tran- been spectroscopically observed dozens of times over sits of the two inner planets (b and c). As of 2020 the last three decades (Hinkel et al. 2014). Compil- June 9, the HD 136352 system is one of less than 100 ing the observed stellar parameters, to be used as pri- naked-eye exoplanet host stars in the sky, according to ors in Section 4.1, we found that the median stellar data from the NASA Exoplanet Archive (Akeson et al. radius R∗ = 1.02 ± 0.02 R⊙, Teff = 5692 ± 218 K, 2013). HD 136352 is now also one of only three naked- and log g = 4.39 ± 0.33, such that the uncertainties eye stars that host more than one transiting planet, reflect the spread or range in all of the measured val- the other two systems being HD 219134 (Motalebi et al. ues. The iron content, or [Fe/H], ranges from -0.16 dex 2015; Vogt et al. 2015; Gillon et al. 2017b) and HR 858 (Carretta et al. 2000) to -0.46 dex (Francois 1986), with (Vanderburg et al. 2019). In all cases, the measure- a median value of −0.29 ± 0.15 dex, where all observa- ment accuracy correlates with the stellar magnitude, tions were solar renormalized to Lodders et al. (2009). and therefore the search for transiting planets around Based on the 32 other elemental abundances reported bright stars is of paramount importance. in the Hypatia Catalog (Hinkel et al. 2014), it is clear Here we present the detection of transits for the two that HD 136352 is a relatively metal-poor star, consis- inner planets of the HD 136352 system from TESS pho- tent with the star’s thick-disk kinematics. Some of the tometry, and provide a combined analysis of all available elements, particularly those in the iron peak and be- RV and photometric data. In Section 2 we describe the yond the iron peak nucleosynthetic groups, have abun- transit probabilities of the planets and the properties dances that are more dramatically sub-solar, such as of the host star. Section 3 provides details regarding [Cr/H] = −0.31 ± 0.14 dex, [Mn/H] = −0.48 ± 0.25 dex, the RV observations of the star and the detrending of and [Y/H] = −0.34 ± 0.07 dex. On the other hand, the TESS photometry. A description of the data anal- the α-elements are closer to solar, such as [O/H] = ysis is provided in Section 4, including a combined fit −0.02 ± 0.15 dex and [Mg/H] = −0.04 ± 0.15 dex. The for all available data and a discussion of the location of average of the α-elements, particularly C, O, Mg, Si, the planets with respect to the overall demographics of S, Ca, and Ti, is [α/H] = -0.12 dex. The C/O molar exoplanets. In Section 5 we present a discussion of the fraction for HD 136352 is 0.35, where stars with a C/O potential for atmospheric characterization of the transit- ratio ∼0.8–1.0 are likely to produce geodynamically in- ing planets. Suggestions for future work and concluding active planets (Bond et al. 2010; Unterborn et al. 2014; remarks are provided in Section 6. Hinkel & Unterborn 2018).
2. SYSTEM PROPERTIES 3. OBSERVATIONS The HD 136352 system is known to host three planets The observational data of the system considered here with orbital periods of 11.6, 27.6, and 108.1 days. The include almost 20 years of precision RV measurements planetary orbits are near-circular in nature, and the de- and one sector of TESS photometry during Cycle 1. The tailed properties of the new parameters provided by this star is identified in the TESS Input Catalog (TIC) as work may be found in Section 4. Since the planets have TIC 136916387 (Stassun et al. 2018, 2019). short orbital periods, they also have relatively high ge- ometric transit probabilities (Kane & von Braun 2008; 3.1. Radial Velocities Stevens & Gaudi 2013). We calculate the a priori tran- The RV data used for this analysis were acquired from sit probabilities as 4.88%, 2.77%, and 1.19% for the b, c, three different observing facilities. The first dataset and d planets, respectively. A detailed analysis of tran- consists of 246 RV measurements obtained over a pe- sit probabilities for known RV exoplanets by Dalba et al. riod of 13.2 years using the HARPS spectrograph, of (2019) resulted in a prediction of ∼3 transit detections which 240 measurements were previously published by during the TESS primary mission. Our reporting of Udry et al. (2019) when announcing the discovery of the transits for two of the HD 136352 planets, combined HD 136352 system. The full details of the instrument with the detected transit for HD 118203b (Pepper et al. and observations may be found in Udry et al. (2019) 2020), brings the total number of RV planets revealed to and references therein. Note that the most recent (6) 4 Stephen R. Kane et al.
Figure 1. Primary data sources used in this analysis. Top: The combined RV data spanning a period of 21 years, acquired using the HARPS (orange circles), AAT/UCLES (green triangles), and Keck/HIRES (blue diamonds) instruments. Bottom: The Sector 12 TESS photometry, with vertical arrows indicating the location of the two transits for planet b and the single transit for planet c. HARPS measurements were acquired after an instru- shown in the top panel of Figure 1 and a subset of 10 ment upgrade and so were treated as an independent RVs from each instrument are provided in Table 1. The dataset in the combined fit to the data described in Sec- mean measurement uncertainties are 0.42, 1.27, and 1.17 tion 4.1. The second dataset consists of 169 RV mea- m/s for the HARPS, AAT, and HIRES data sets respec- surements obtained over a period of 17.3 years using tively. Although the highest of these mean uncertainties the UCLES high-resolution spectrograph (Diego et al. is associated with the AAT data, the AAT dataset also 1990) on the 3.9m Anglo-Australian Telescope (AAT). has the longest time baseline, making it a valuable ad- The instrument uses an iodine absorption cell to pro- dition to the analysis. vide wavelength calibration from 5000 to 6200 A,˚ by embedding iodine absorption lines on the stellar spec- 3.2. TESS Photometry trum (Valenti et al. 1995; Butler et al. 1996). The AAT The TESS spacecraft observed HD 136352 during Sec- RV observations were conducted as part of the Anglo- tor 12 of its primary mission between May 21, 2019 and Australian Planet Search, described in more detail by June 18, 2019. Because HD 136352 is a bright, nearby Wittenmyer et al. (2020) and references therein. The dwarf star, images from pixels surrounding the star were third dataset consists of 43 RV measurements obtained saved and downloaded every two minutes, compared to over a period of 12.0 years using the HIRES echelle spec- 30 minute sampling for most of the sky. These im- trograph on the Keck I telescope (Vogt et al. 1994), of ages were downlinked from the spacecraft, processed which 23 measurements were previously published by by the Science Processing Operations Center (SPOC) Howard & Fulton (2016). The combined RV dataset are pipeline (based at NASA Ames Research Center) and Transits of Known Planets Orbiting a Naked-Eye Star 5
Table 1. HD 136352 radial velocities. The light curve of HD 136352 produced by the SPOC pipeline contains residual systematic errors, so we ex- Instrument Date RV σ tracted our own custom light curve from the TESS pixel (BJD – 2450000) (m/s) (m/s) data. Our approach is very similar to the one used HARPS 3152.7661 6.188 0.770 by Vanderburg et al. (2019) to produce a light curve of HARPS 3154.6851 1.984 0.730 another bright star, HR 858. We first extracted light curves of HD 136352 from twenty different photomet- HARPS 3204.5378 -2.172 0.520 ric apertures. We then removed instrumental system- HARPS 3816.8345 0.609 0.280 atics by decorrelating each of the 20 light curves with HARPS 3836.7974 3.438 0.270 other time series via matrix inversion (while exclud- HARPS 3950.5459 3.422 0.460 ing points in-transit from the fit). In particular, we HARPS 3980.4880 0.281 0.380 decorrelated against the first and second order time se- HARPS 4230.8179 3.180 0.300 ries of the means and standard deviations of the en- HARPS 4231.7510 2.023 0.360 gineering quaternion measurements within each expo- HARPS 4234.7314 -1.242 0.300 sure. We also decorrelated against the high-frequency AAT 915.1653 -8.280 1.800 (band 3) common mode systematics in the cotrend- AAT 1237.2321 -6.310 3.380 ing basis vectors calculated by the SPOC Pre-search AAT 1274.2877 -7.850 2.470 Data Conditioning module (PDC) and the time series AAT 1276.1555 -12.310 3.210 of background flux measurements (Smith et al. 2012; AAT 1384.0170 -2.360 1.840 Stumpe et al. 2012, 2014). In total, we we fit a model AAT 1683.0382 13.650 2.100 with 46 free parameters to the 16,865 out-of-transit dat- AAT 1684.1084 3.790 2.000 apoints. Finally, we calculated the point-to-point pho- AAT 1718.0880 2.790 2.100 tometric scatter for each of the 20 light curves and chose AAT 1743.9812 1.120 1.840 the one with the highest precision. This procedure is de- AAT 1766.8840 -6.080 2.040 scribed in more detail in Section 2.1 of Vanderburg et al. HIRES 6145.7617 -8.060 1.288 (2019). HIRES 6880.7524 -4.906 1.400 The resulting decorrelated light curve still showed low- HIRES 5024.8408 5.643 1.286 frequency variability (likely a combination of both slow HIRES 5024.8418 8.699 1.252 instrumental drifts and astrophysical variability). We HIRES 5024.8428 8.706 1.286 modeled these low-frequency trends with a basis spline HIRES 5052.8169 -3.129 1.219 and simultaneously determined the spline function along HIRES 5052.8184 -4.245 1.199 with the transit model parameters. We introduced dis- continuities to the basis spline at the times of space- HIRES 5052.8198 -3.695 1.236 craft momentum dumps. We removed the variability by HIRES 4246.9380 1.427 1.239 dividing the best-fit spline from the light curve. Note HIRES 4246.9390 2.457 1.268 that the amplitude of the low-frequency variability is The full data set is available online. less than a few hundred ppm with timescales greater than 1 day. Though the variability amplitude is a large fraction of the transit depths described in Section 4, the timescales of the transits are much shorter, and we were searched for transits (Jenkins et al. 2016, 2020). The thus able to effectively remove the variability with neg- SPOC transiting planet search algorithm detected a pos- ligible effect on the subsequent data analysis. The light sible transit-like signal when it lined up one transit of curve resulting from our detrending, with both instru- HD 136352 b with the single transit of HD 136352 c, mental systematics and stellar variability removed, is but the signal was rejected by an automated classifi- shown in Figure 1. cation algorithm because the two transits have signifi- cantly different depths. HD 136352 was therefore not 4. DATA ANALYSIS alerted as a TESS planet candidate host star. We sub- Here, we describe the extracted properties of the star sequently identified the transits of HD 136352 b and c and planets, as well as their location within the context in a visual inspection of the light curve, which resulted of the known exoplanet population. in an allocated TESS Object of Interest (TOI) number of 2011. 4.1. Extraction of System Parameters 6 Stephen R. Kane et al.
1.0006 Using the data described in Section 3, we performed b our analysis using the EXOFASTv2 tool1, described in 1.0004 detail by Eastman et al. (2013, 2019). Following pre- 1.0002 Flux vious applications of EXOFASTv2 (e.g., Dalba et al. 1.0000 2020), we conducted two fits to extract the system parameters. In the first, we fit archival photometry 0.9998 of HD 136352 to modeled spectral energy distribu- Normalized 0.9996 Best-Fit Model TESS Data tions (SED). We applied normal priors on the param- 0.9994 30-min bin eters R⋆, M⋆, Teff , and [Fe/H] using the values pro- vided in Section 2. We also included a normal prior on parallax (68.164 ± 0.097 mas) based on measure-