A sub-Neptune and a non-transiting Neptune-mass companion unveiled by ESPRESSO around the bright late-F dwarf HD 5278 (TOI-130)
A. Sozzetti, M. Damasso, A. S. Bonomo, Y. Alibert, S. G. Sousa, V. Adibekyan, M. R. Zapatero Osorio, J. I. González Hernández, S. C. C. Barros, J. Lillo-Box, K. G. Stassun, J. Winn, S. Cristiani, F. Pepe, R. Rebolo, N. C. Santos, R. Allart, T. Barclay, F. Bouchy, A. Cabral, D. Ciardi, P. Di Marcantonio, V. D'Odorico, D. Ehrenreich, M. Fasnaugh, P. Figueira, J. Haldemann, J. M. Jenkins, D.W. Latham, B. Lavie, G. Lo Curto, C. Lovis, C. J . A. P. Martins, D. Mégevand, A. Mehner, G. Micela, P. Molaro, N. J. Nunes, M. Oshagh, J. Otegi, E. Pallé, E. Poretti, G. Ricker, D. Rodriguez, S. Seager, A. Suárez Mascareño, J. D. Twicken, S. Udry
AAstronomy & Astrophysics manuscript no. HD5278_ESPRESSO © ESO 2021February 1, 2021
A sub-Neptune and a non-transiting Neptune-mass companionunveiled by ESPRESSO around the bright late-F dwarf HD 5278(TOI-130) (cid:63)
A. Sozzetti , M. Damasso , A. S. Bonomo , Y. Alibert , S. G. Sousa , V. Adibekyan , , M. R. Zapatero Osorio ,J. I. González Hernández , , S. C. C. Barros , , J. Lillo-Box , K. G. Stassun , J. Winn , S. Cristiani , , F. Pepe ,R. Rebolo , , , N. C. Santos , , R. Allart , , T. Barclay , , F. Bouchy , A. Cabral , , D. Ciardi ,P. Di Marcantonio , V. D’Odorico , , D. Ehrenreich , M. Fasnaugh , P. Figueira , , J. Haldemann ,J. M. Jenkins , D. W. Latham , B. Lavie , G. Lo Curto , C. Lovis , C. J . A. P. Martins , , D. Mégevand ,A. Mehner , G. Micela , P. Molaro , , N. J. Nunes , , M. Oshagh , , J. Otegi , , E. Pallé , , E. Poretti ,G. Ricker , D. Rodriguez , S. Seager , , , A. Suárez Mascareño , , J. D. Twicken , and S. Udry (A ffi liations can be found after the references) Received ???; accepted ???
ABSTRACT
Context.
Transiting sub-Neptune-type planets, with radii approximately between 2 and 4 R ⊕ , are of particular interest, as their study allows us togain insight on the formation and evolution of a class of planets not found in our Solar System. Aims.
We exploit the extreme radial velocity (RV) precision of the ultra-stable echelle spectrograph ESPRESSO on the VLT to unveil the physicalproperties of the transiting sub-Neptune TOI-130 b, uncovered by the TESS mission orbiting the nearby, bright, late F-type star HD 5278 (TOI-130)with a period P b = . Methods.
We use 43 ESPRESSO high-resolution spectra and broad-band photometry information to derive accurate stellar atmospheric andphysical parameters of HD 5278. We exploit the TESS light curve and spectroscopic diagnostics to gauge the impact of stellar activity on theESPRESSO RVs. We perform separate as well as joint analyses of the TESS photometry and the ESPRESSO RVs using fully Bayesian frameworksto determine the system parameters.
Results.
Based on the ESPRESSO spectra, the updated stellar parameters of HD 5278 are T e ff = ±
64 K, log g = . ± .
11 dex,[Fe / H] = − . ± .
04 dex, M (cid:63) = . + . − . M (cid:12) and R (cid:63) = . + . − . R (cid:12) . We determine HD 5278 b’s mass and radius to be M b = . + . − . M ⊕ and R b = . ± .
05 R ⊕ . The derived mean density, (cid:37) b = . + . − . g cm − , is consistent with the bulk composition of a sub-Neptune with a substantial( ∼ ∼
17% of the measured radius. Given the host brightness and irradiation levels,HD 5278 b is one of the best targets orbiting G-F primaries for follow-up atmospheric characterization measurements with HST and JWST. Wediscover a second, non-transiting companion in the system, with a period P c = . + . − . days and a minimum mass M c sin i c = . + . − . M ⊕ . Westudy emerging trends in parameters space (mass, radius, stellar insolation, mean density) of the growing population of transiting sub-Neptunes,and provide statistical evidence for a low occurrence of close-in, 10 −
15 M ⊕ companions around G-F primaries with T e ff (cid:38) Key words. planetary systems — planets and satellites: composition — stars: individual (TOI-130; HD 5278) — techniques: radial velocities —Techniques: photometric — methods: miscellaneous
1. Introduction
About 27% of the current catalog of > . − . ⊕ , foundin both single and multiple configurations. These objects are usu-ally referred to as sub-Neptunes or mini-Neptunes . They pop-ulate the region at the high-end of the so-called radius valley,a prominent feature of the bimodal radius distribution of small-size (1 − ⊕ ), close-in ( P (cid:46)
100 days) planets that was recentlyunveiled based on the analysis of Kepler mission data (Fultonet al. 2017; Fulton & Petigura 2018). The class of sub-Neptune-type planets does not exist in our Solar System. Understanding (cid:63)
Based on Guaranteed Time Observations collected at the Euro-pean Southern Observatory under ESO programme 1102.C-0744 by theESPRESSO Consortium. In the remainder of the paper we will adopt the nomenclature sub-Neptunes to refer to this class of planets why they seem so abundant around other stars (of varied spectraltype) in terms of their formation scenarios, evolutionary paths,and range of structural properties is still an open question in ex-oplanetary science.Broadly speaking, there is agreement today on the fact thatclose-in sub-Neptunes cannot be composed of purely rocky ma-terial (mixtures of iron and silicates), but must retain more orless significant fractions of volatile elements (Lopez et al. 2012;Rogers 2015; Lozovsky et al. 2018; Jin & Mordasini 2018;Bitsch et al. 2019; Venturini et al. 2020). However, when itcomes to details for explaining the full extent of the radius distri-bution of these objects, substantially di ff erent predictions fromtheoretical modeling arise. For instance, they could be rockyworlds with small amounts of volatiles in their interiors andsubstantial, thick H / He gaseous envelopes of primordial ori-gin (e.g., Owen & Wu 2017; Van Eylen et al. 2018; Gupta &Schlichting 2019). These go sometimes by the definition of gasdwarfs (Buchhave et al. 2014). Alternatively, they might con-
Article number, page 1 of 26 a r X i v : . [ a s t r o - ph . E P ] J a n & A proofs: manuscript no. HD5278_ESPRESSO tain significant amounts of water in primarily solid (ices) form,with thin, H / He-dominated atmospheres (e.g., Léger et al. 2004;Zeng et al. 2019; Madhusudhan et al. 2020; Venturini et al.2020). In the literature, these are referred to as water worldsor ocean planets. However, irradiated water-rich rocky planetsmight also possess endogenic thick H O-dominated, steam at-mospheres, in which up to 100% of the planetary water contentappears in vaporized form (Dorn et al. 2018; Zeng et al. 2019;Turbet et al. 2020; Mousis et al. 2020). If interactions betweenthe (initially primordial, H -dominated) atmosphere and magmaoceans at the surface are considered, then close-in sub-Neptunescould have atmospheres with variable degrees of (exogenic) hy-drogen and (endogenic) water (Kite et al. 2020).In principle, density determination for transiting planets withmeasured radius and mass allows to directly infer their bulkcomposition. However, mass-radius relationships for small, low-mass planets from theoretical modeling (e.g., Bitsch et al. 2019;Turbet et al. 2020, and references therein) carry intrinsic degen-eracies, with planets of vastly di ff erent composition predicted tohave observationally indistinguishable bulk densities. This dif-ficulty is also seen in the case of sub-Neptunes: discriminatingbetween water worlds / ocean planets and rocky planets with aH –He atmosphere is not possible based on mass and radius de-termination alone, particularly for those objects with radii in the2 . − . ⊕ range (Adams et al. 2008; Miller-Ricci et al. 2009;Lozovsky et al. 2018). Follow-up atmospheric characterizationmeasurements are therefore necessary. However, density mea-surements are still of critical importance, as atmospheric analy-ses rely on knowledge of mass and radius. As a matter of fact, precise mass determination often remains the limiting factor inatmospheric characterization studies for objects with radii belowthat of Neptune. Recent work (Batalha et al. 2019) advocates formasses measured to better than 20% precision (i.e. at the > σ level) in order for constraints on atmospheric composition to belimited solely by the quality of the transmission / emission spec-troscopy datasets.Achieving statistically significant mass determinations forsub-Neptune-type planets is not an easy task. By design, theTESS mission transit candidates are found orbiting stars sig-nificantly brighter than those observed by the Kepler mission,somewhat alleviating the problem of insu ffi cient RV measure-ment precision due to high photon noise that prevented system-atic RV follow-up for the vast majority of the Kepler candi-dates. However, ground-based follow-up e ff orts with precisionRVs still need to deal with correlated noise sources of stellarorigin, typically with amplitudes of the RV variations compa-rable or exceeding those of the planetary signals. Furthermore,the planetary systems often contain more than one (not neces-sarily transiting) planet, increasing the complexity of the Kep-lerian signals and boosting the demands for investment in ob-serving time. Finally, the larger RV amplitudes of the planetarysignals make high statistical confidence detections easier to at-tain around lower-mass primaries. For instance, the masses ofsub-Neptunes orbiting stars of any spectral type are measuredwith median precision of ∼ (cid:46) T e ff (cid:46) g > . ∼ > σ level, or better . It is therefore desirable to increase the Data from the transiting exoplanet catalogue TEPCat: sample of transiting sub-Neptunes with well-determined densi-ties, particularly in the sparsely populated regime of such com-panions orbiting F-G-type primaries.In this paper we present RV measurements from the newultra-stable spectrograph ESPRESSO (Pepe et al. 2014, 2020)that allow us to obtain a high-precision mass determination for atransiting sub-Neptune candidate with a period of 14.3 days dis-covered by TESS orbiting the bright ( V = . ∼
41 days.In Sect. 2 we present the TESS photometric time-series andthe spectroscopic follow-up observations. Sect. 3 focuses on thepresentation and discussion of the properties of the host star HD5278. In Sect. 4 we present our data analysis and results. Sect.5 contains a discussion on the properties of the HD 5278 plan-etary system, particularly seen in the context of the sample ofpresently-known sub-Neptunes orbiting G-F-type primaries. Wesummarize our results and provide concluding remarks in Sect.6.
2. Observations
TESS observed TOI-130 (HD 5278) on its Camera 3 at 2-minutecadence in four non-consecutive Sectors: Sector 1 (UT July 25 -August 22 2018), Sector 12 (UT May 21 - June 18 2019), Sec-tor 13 (UT June 19 - July 17 2019), and Sector 27 (UT July 4- July 30 2020). The target will be re-observed again in Sector39 (UT May 26 - June 24 2021). The publicly available TESSphotometric data were downloaded from the Mikulski Archivefor Space Telescopes (MAST) portal . We used the light curveproduced by the Presearch Data Conditioning Simple AperturePhotometry (PDCSAP; Smith et al. 2012; Stumpe et al. 2012,2014) algorithm adopted for processing the TESS images bythe NASA Ames Science Processing Operations Center (SPOC;Jenkins et al. 2016). The initial identification of TOI-130.01 asplanet candidate by SPOC was obtained in several steps, includ-ing a transit search using the Transiting Planet Search Module(TPS; Jenkins 2002; Jenkins et al. 2010), and the evaluation ofa set of transiting planet model fits and internal validation tests(Twicken et al. 2018; Li et al. 2019). The reported period of theplanet candidate based on the Sectors 1-13 transit search was14.339 days, with a preliminary transit depth of 409 ±
17 ppm. Atotal of 8 transits were identified, two in each TESS sector, andthe full dataset will be used in the following analysis.
We started pursuing the confirmation of the planet candidate col-lecting spectroscopic observations with the CORALIE spectro-graph (Queloz et al. 2000) mounted on the 1.2 m Swiss tele-scope at La Silla Observatory. A total of 14 spectra were ob-tained between UT September 7 2018 and UT October 5 2018,spanning approximately two orbit cycles. Exposure time was setto 900 sec. Radial velocities were derived with the automatedCORALIE pipeline (Queloz et al. 2000; Ségransan et al. 2010).The rms of the dataset is 10.5 m s − , comparing favorably to themean internal error of the observations (7.5 m s − ) and conclu- mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html Article number, page 2 of 26. Sozzetti et al.: HD 5278 b and c unveiled by ESPRESSO sively ruling out the possibility of an eclipsing binary, that wouldotherwise produce a signal with a much higher RV amplitude.
We carried out the RV monitoring of HD 5278 with ESPRESSOwithin the context of the Guaranteed Time Observations (GTO)sub-program aimed at measuring precise masses of transitingplanet candidates uncovered by the TESS and K2 missions (Pro-gram ID 1102.C-744, PI: F.Pepe). HD 5278 was observed over atotal time span of 403 days between October 2018 and Decem-ber 2019. All observations were carried out using the Fabry Perot(FP) as simultaneous calibration source, enabling the monitor-ing of instrumental drifts down to the 10 cm s − precision levelfor which ESPRESSO has ultimately been designed. The com-plete time series encompasses 43 high-resolution spectra, gath-ered with the 1-UT (UT3) mode at R =
138 000 and acquiredwith a fixed exposure time of 900 sec. The spectra have a me-dian S / N (cid:39)
230 per pixel at λ = . The DRS(Lovis et al. in prep.) outputs RV measurements based on a Gaus-sian fit of the cross correlation function (CCF) of the spectrumusing a binary mask computed from a stellar template (Baranneet al. 1996; Pepe et al. 2000). RV information is extracted fromthe full wavelength range covered by the instrument (3800 − − . From theDRS-extracted ESPRESSO spectra we also obtained (throughthe DACE interface ) the time-series of a number of useful spec-troscopic diagnostics of stellar activity, i.e. full-width half maxi-mum (FWHM), line bisector span (BIS), Mount Wilson S -index( S MW ), log R (cid:48) HK , and H α index. The RVs and activity indicators(along side their formal uncertainties) used in this work are listedin Tables B.1 and B.2, respectively.
3. Stellar Properties
HD 5278 (TOI-130, TIC 263003176, HIP 3911) is a high-propermotion, bright ( V = . G = . ∼
56 pc from the Sun (Gaia Collaboration et al. 2018;Lindegren et al. 2018). The field around HD 5278 is shownin Figure 1, which reproduces a sample image from the TESStarget pixel files of Sector 12 obtained with the publicly avail-able tpfplotter tool (Aller et al. 2020). The number of de-tected Gaia sources is not very high, and they are all ratherfaint ( ∆ G > . G = . ∼ . = ff erences in parallax and propermotion ( ∆ (cid:36) < max[1 . , σ (cid:36) ], ∆ µ < . µ ; see Smart et al.2019). Both conditions are simultaneously satisfied, thereforethe two stars are recognized as a resolved double star system. The Data Analysis Center for Exoplanets (DACE) platform is avail-able at https://dace.unige.ch https://github.com/jlillo/tpfplotter Table 1.
Astrometry, photometry, and spectroscopically derived stellarproperties of HD 5278
HD 5278, TOI-130, TIC 263003176, HIP 3911, TYC 9490-786-1
Parameter Value Refs.
Astrometry: α (J2000) 00:50:09.90 [1,2] δ (J2000) − µ α [mas yr − ] 139 . ± .
05 [1,2] µ δ [mas yr − ] 30 . ± .
04 [1,2] (cid:36) [mas] 17 . ± .
03 [1,2] d [pc] 56 . + . − . [3] Photometry:FUV
GALEX . ± .
080 [4]
NUV
GALEX . ± .
002 [4] B T . ± .
016 [5] V T . ± .
011 [5] G . ± . J . ± .
027 [7] H . ± .
031 [7] K s . ± .
027 [7] W . ± .
069 [8] W . ± .
022 [8] W . ± .
015 [8] W . ± .
048 [8]
Stellar Parameters:T e ff [K] 6203 ±
64 [9]log g [dex] 4 . ± .
11 [9][Fe / H] [dex] − . ± .
04 [9][Mg / H] [dex] − . ± .
08 [9][Si / H] [dex] − . ± .
04 [9] ξ [km s − ] 1 . ± .
03 [9] M (cid:63) [M (cid:12) ] 1 . − . + . [9] R (cid:63) [R (cid:12) ] 1 . − . + . [9] (cid:37) (cid:63) [g cm − ] 0 . − . + . [9] L (cid:63) [L (cid:12) ] 1 . − . + . [9] v sin i [km s − ] 4 . ± . P rot [days] < . < log R (cid:48) HK > − . ± .
03 [9] t [Gyr] 3 . ± . U , V , W ] [km s − ] [ − . ± . . ± . . ± .
10] [9]
References. [1] Gaia Collaboration et al. 2018; [2] (Lindegren et al.2018); [3] (Bailer-Jones et al. 2018); [4] (Bianchi et al. 2017); [5] (Høget al. 2000); [6] (Evans et al. 2018); [7] (Cutri et al. 2003); [8] (Cutri &et al. 2014); [9] this work.
The Starhorse catalog (Anders et al. 2019) reports e ff ective tem-perature T e ff = ±
300 K and mass M = . + . − . M (cid:12) for thesecondary (Gaia DR2 Source Id 4617759518796452608).We summarize in Table 1 the main astrometric, photomet-ric, and physical stellar parameters for HD 5278. The stel-lar atmospheric parameters for the target (e ff ective temperature T e ff , surface gravity log g , microturbulence ξ , and iron abun-dance [Fe / H]) were obtained using a standard technique of ex-
Article number, page 3 of 26 & A proofs: manuscript no. HD5278_ESPRESSO
Pixel Column Number P i x e l R o w N u m b e r E N
TIC 263003176 - Sector 12 m = -2 m = 0 m = 2 m = 4 m = 6 m = 8 m = 10 m = 12 123 4 56 789 101112 13 14151617 1819 20 212223 242526 2728 0.00.51.01.52.02.53.03.5 F l u x × ( e ) Fig. 1.
Target Pixel File (TPF) of HD 5278 from the TESS observa-tions in Sector 12 (composed with tpfplotter , Aller et al. 2020). TheSPOC pipeline aperture is overplotted with a red shaded region and theGaia DR2 catalog is also overlaid with symbol sizes proportional to themagnitude contrast with the target, which is marked with a white cross. citation and ionization balance (see, e.g., Sozzetti et al. 2004,2007; Sousa et al. 2011, and references therein). To this end, weused the StarII workflow of the data analysis software (DAS)of ESPRESSO Di Marcantonio et al. 2018 to produce a high-quality 1D ESPRESSO spectrum of HD 5278. We initiallycoadded, and normalized order by order 43 blaze-corrected bi-dimensional ESPRESSO spectra at the barycentric referenceframe. Then these spectra were merged and corrected for RV.The final 1D ESPRESSO spectrum with a pixel size of 0.5 kms − (see Fig. A.1) has S / N of about 1000 and 2000 at 4200 and5000 Å, respectively, and higher than 2000 for longer wave-lengths. Next, the
ARES v2 code (Sousa et al. 2007, 2015) withthe Sousa et al. (2008) input linelist was used to consistentlymeasure the equivalent widths (EWs) for each line. Metal abun-dances were derived under the assumption of local thermody-namic equilibrium (LTE), using the 2014 version of the spec-tral synthesis code
MOOG (Sneden 1973) and a grid of Ku-rucz ATLAS plane-parallel model stellar atmospheres (Kurucz1993). We obtained T e ff = ±
64 K, log g = . ± . ξ = . ± .
03 km s − , and [Fe / H] = − . ± .
04. Thequoted uncertainties in the atmospheric parameters include con-tributions (added in quadrature) from possible systematic errorswhich amount to 60 K, 0.04 dex, and 0.1 dex for T e ff , [Fe / H],and log g , respectively (Sousa et al. 2011). Using on a classicalcurve-of-growth analysis method based on the same tools andmodel atmospheres used for the stellar parameter determination(e.g. Adibekyan et al. 2012, 2015), we also report in Table 1 ourmeasurements of the Mg and Si abundances in HD 5278, whichwill be input to the analysis carried out in Sect. 5.1.We derived stellar mass, radius, and age using a twofold ap-proach. We first fed the T e ff and [Fe / H] estimates to the optimiza-tion code
PARAM v1.3 (da Silva et al. 2006; Rodrigues et al.2014, 2017), with the additional information of the Gaia DR2parallax and V T -band magnitude. We obtained M (cid:63) = . ± .
033 M (cid:12) , R (cid:63) = . ± .
027 R (cid:12) , and t = . ± . EXOFASTv2 code (see Eastman 2017; Eastman et al.2019, for a full description) to perform a fit to the star’s broad- λ ( µ m)-12-11-10-9-8 l og λ F λ ( e r g s - c m - ) Fig. 2.
The spectral energy distribution of HD 5278. Red markers depictthe photometric measurements with vertical error bars corresponding tothe reported measurement uncertainties from the catalog photometry.Horizontal error bars depict the e ff ective width of each passband. Theblack curve corresponds to the most likely stellar atmosphere model.Blue circles depict the model fluxes over each passband. band spectral energy distribution (SED) from the near ultravi-olet to the mid-infrared (see Figure 2). The SED fit (see e.g.Stassun & Torres 2016) was carried out using retrieved broad-band NUV photometry from GALEX, Tycho-2 B- and V-bandmagnitudes, Strömgren uvby photometry, 2MASS JHKs near-IR magnitudes, and WISE W1-W4 IR magnitudes and invokingwithin EXOFASTv2 the YY-isochrones (Yi et al. 2001) to modelthe star. We obtained: M (cid:63) = . − . + . M (cid:12) , R (cid:63) = . − . + . R (cid:12) , and t = . ± . EXOFASTv2 analysis that makes use of the well-sampled stellarSED.We computed the Galactic space velocity vector [ U , V , W ]for HD 5278 using the radial velocity from Gaia DR2 (RV = − . ± .
18 km s − , Gaia Collaboration et al. 2018) and theparallax and proper motion information listed in Table 1. Wecalculated the heliocentric velocity components in the direc-tions of the Galactic center, Galactic rotation, and north Galacticpole, respectively, using the formulation developed by Johnson& Soderblom (1987). The right-handed system was employedand the Solar motion was not subtracted from our calculations.The uncertainties associated with each space velocity compo-nent were obtained from the observational quantities after theprescription of Johnson & Soderblom 1987.According to the kinematic criteria defined by e.g., Gagnéet al. 2018 and Reddy et al. 2006, HD 5278 is not a member ofany known young stellar moving group with ages below a fewhundred Myr, and it has a 99.9% probability of being a memberof the field. Its low V and W velocities indicate that HD 5278is kinematically a member of the thin disk of the Galaxy forwhich ages typically younger than 7–9 Gyr are expected (Kilicet al. 2017). This agrees with the intermediate age of a few Gyrderived for this star using the EXOFASTv2 tool.The average value of log R (cid:48) HK ( − . ± .
03) and the B − V colour for HD 5278 imply a predicted stellar rotation period P rot = . ± . P rot = . ± . . ± . Article number, page 4 of 26. Sozzetti et al.: HD 5278 b and c unveiled by ESPRESSO
Fig. 3.
The TESS light curve of HD 5278 from sectors 1, 12, 13, and 27. The eight SPOC-detected transits are highlighted with vertical greendashed-dotted lines throughout. with the estimate in Table 1. We further constrained the value of P rot through a direct measurement of the projected rotational ve-locity ( v sin i ) of HD 5278 from the ESPRESSO co-added spec-trum. The v sin i was derived based on the analysis of 16 isolatediron lines with the FASMA spectral synthesis package (Tsan-taki et al. 2018), fixing all the stellar atmospheric parameters(from Table 1), macroturbulent velocity, and limb-darkening co-e ffi cient. The limb-darkening coe ffi cient (0.62) was determinedusing the aforementioned stellar parameters as described in Es-pinoza & Jordán (2015) assuming a linear limb darkening law.The macroturbulent velocity (4.4 km / s) was determined by usingthe temperature- and gravity-dependent empirical formula fromDoyle et al. (2014). We obtained v sin i = . ± . − . Thisallows us to put a 1 − σ upper limit to the stellar rotation period P rot < . v sin i value obtained for HD 5278 ison the low side for a star of this spectral type. For instance, Winnet al. (2017) presented a collection of v sin i measurements ofKepler stars with transiting planets. There is an overall tendencyfor v sin i to rise with e ff ective temperature, as expected (see theirFig. 6). Within the range from 6150 to 6250 K, out of a sampleof 42 stars only 2 have v sin i values as low as 4.1 km s − (thevalue obtained for HD 5278). Thus, either HD 5278 is amongthe slowest 5-10% of rotators in its temperature / mass range, orit has a value of sin i significantly lower than unity.
4. Data Analysis and System Parameters
In order to maximize the robustness of the recovered system pa-rameters, we conduct a two-step analysis of our data. In Sec-tion 4.1 we first model separately the TESS light curve, with theresulting planet parameters being used as priors in the subse-quent analysis of the ESPRESSO RVs in Section 4.2. In Section4.3 we describe the global analysis of the combined TESS andESPRESSO datasets, which we ultimately adopt as fiducial.
The
PDCSAP light-curve for the four TESS Sectors is shown inFigure 3. No clear activity features with amplitude larger than ∼
300 ppm are seen in the TESS data, indicating that the hoststar shows rather low levels of magnetic activity. Indeed, in theKepler sample stars of similar T e ff to that of HD 5278 have am-plitudes of the detected periodic variability on the order of 1mmag, 300 ppm corresponding to the lowest-amplitude period- icities found in the Kepler light-curves (see Figure 3 in McQuil-lan et al. 2014). We performed an independent transit search us-ing a median filter and the BLS algorithm (Kovács et al. 2002;Bonomo et al. 2012). We could recover the eight transits of TOI-130 b with a period of 14.34 days, but no other potential transitsignal was identified.For the purpose of the analysis of the TESS PDCSAP light-curve, we extracted data sub-samples centred around the mid-transit times and extending 1.5 times the transit duration beforetheir ingress and after their egress, and normalized each transitwith a first-order polynomial to the out-of-transit data. The tran-sit parameters were measured based on a Bayesian analysis ofthe TESS data performed using a di ff erential evolution MarkovChain Monte Carlo (DE-MCMC) method (Ter Braak 2006; East-man et al. 2013). The free parameters of our model are (under theassumption of a circular orbit) the transit epoch T , the orbitalperiod P , the transit duration T , the ratio of the planet to stel-lar radii R p / R (cid:63) , and the inclination i between the orbital planeand the plane of the sky. As can be seen in Figure 3, the qualityof the photometric data appears to degrade somewhat in Sec-tors 12, 13, and 27, with larger uncorrected systematics of likelyinstrumental nature. We then included in our global model fourscalar jitter terms σ S i , one for each of the four TESS sectors. Thetwo limb-darkening coe ffi cients (LDC) u and u of the quadraticlimb-darkening law (e.g., Claret 2018) appropriate for the atmo-spheric parameters of HD 5278 reported in Table 1 were initiallykept fixed. Uninformative priors were used for all model param-eters. A Gaussian prior was imposed on the transit stellar density (cid:37) (cid:63) from the EXOFASTv2 stellar parameters, i.e. N (0.933,0.055)g cm − . This prior indirectly a ff ects R p / R (cid:63) , i , and T , giventhat these parameters are related to a / R (cid:63) , the semi-major axis tostellar radius ratio, through Eq. 12 in Giménez (2006), and that (cid:37) (cid:63) ∝ ( a / R (cid:63) ) .A DE-MCMC run with a number of chains equal to twicethe number of free parameters was then carried out. After re-moving the “burn-in” steps and achieving convergence and goodmixing of the chains following the same criteria as in Eastmanet al. (2013), the medians of the posterior distributions and their ± .
13% intervals were evaluated and were taken as the finalparameters and associated 1 σ uncertainties, respectively. A DE-MCMC run with the inclusion of the LDCs as free model pa-rameters produced indistinguishable results (well within the 1 σ uncertainties on the model parameters). This is not unexpected,partly because of the small number of transit events available,partly because we are in a configuration ( T e ff > Article number, page 5 of 26 & A proofs: manuscript no. HD5278_ESPRESSO
Fig. 4.
Top: time series of the ESPRESSO RVs for HD 5278. Center:GLS periodogram of the original RV time series. Bottom: GLS peri-odogram of the residuals after removal of a sinusoid with a 41-day pe-riod. The vertical dashed line in the central and lower panels showsthe period of the transiting planet candidate from TESS photometry. Inthe center and bottom panels the horizontal long-dashed, dashed-dotted,and dotted lines represent 10%, 1%, and 0.1% FAP levels, respectively. fected by the choice of fixed LDCs (e.g., Csizmadia et al. 2013).The results reported in Table 2 are those obtained with fixedLDCs.
The top panel of Figure 4 shows the ESPRESSO RV time-seriesof HD 5278. The rms of the dataset is 3.1 m / s, almost a factor of10 larger than the typical internal errors of the observations. AGeneralized Lomb-Scargle (GLS; Zechmeister & Kürster 2009)analysis returns a dominant peak at ∼
41 days (central panel ofFigure 4), with a false alarm probability (FAP) of 0 . = . P =
41 days and amplitude 3 . / s (bottom panel of Fig.4). Some structure with modest power at longer periods remains.The dominant period in the RV data does not seem to beconnected to the rotation period of the star, constrained to be onthe order of the orbital period of the transiting companion, orshorter. Further evidence for the Keplerian origin of the signal at ∼
41 days comes from investigation of the stellar activity indica-
Fig. 5.
GLS periodograms of various activity diagnostics measured inthe ESPRESSO spectra of HD 5278. From the top to bottom: H α -index,Mount Wilson S-index, FWHM, and bisector span. In each panel, thetwo vertical dashed lines show the position of the two periodic signalsidentified in the RV time series. In the top panel the horizontal lineshave the same meaning as in Figure 4. tors. The four panels of Figure 5 show the GLS periodograms forFWHM, BIS, S-index, and H α index. No peak with significantpower is seen in the vicinity of either P = .
34 days or P = > < . α index, peakingat around 180 days. A marginally significant Spearman’s rankcorrelation coe ffi cient (0.30) is only obtained between RVs andH α index. The results from this analysis reinforce the hypothesisthat the signal at 41 days is produced by another, possibly non-transiting, planet in the system, and we therefore proceed to fit atwo-Keplerian model to the ESPRESSO RV data .The free parameters of our two-planet model are the time oftransit center T c , b , the time of inferior conjunction T c , c the orbitalperiods P b and P c , two RV zero points γ and γ for ESPRESSOdata obtained before and after the fiber-link upgrade interventionin Summer 2019, the RV semi-amplitudes K b , K c , √ e b cos ω b , √ e b sin ω b , √ e c cos ω c , and √ e c sin ω c , where e b , e c are the ec-centricities and ω b , ω c the arguments of periastron. The lack of We did check that the CORALIE RV measurements are not incon-sistent with the orbit fitting results presented in this Section but, as ex-pected, they do not have su ffi cient precision to add useful constraints onthe model parameters.Article number, page 6 of 26. Sozzetti et al.: HD 5278 b and c unveiled by ESPRESSO Fig. 6.
Top: The phase-folded transit light curve of HD 5278 b from8 individual transit events. The best-fit transit model produced in thecombined ESPRESSO + TESS data analysis in Sect. 4.3 is depicted bythe red curve. The black dots represent data averaged over 13.8 minbins.
Bottom: the post-fit residuals.evidence in the photometry, RVs, and spectroscopic activity in-dicators of any relevant modulation around the expected rota-tion period of the star makes us opt for modeling any additionalsource of noise in the RVs only in terms of two uncorrelated jitterterms ( σ jit , , σ jit , ) added in quadrature to the formal uncertain-ties of the RVs before and after the intervention.The ESPRESSO RV time-series analysis was carried out us-ing the publicly available Monte Carlo sampler and Bayesianinference tool MultiNestv3.10 (e.g. Feroz & Skilling 2013),through the pyMultiNest wrapper (Buchner et al. 2014). Thismultimodal nested sampling algorithm was setup to run with 500live points and a sampling e ffi ciency of 0.3. The results of theanalysis are reported in Table 2.The full solution indicates that the possible o ff set betweenthe two pre- and post-upgrade ESPRESSO datasets is at thelevel of 1 m s − , similar to that shown by Suárez Mascareñoet al. (2020) for a star of later spectral type (Proxima Cen). Theuncorrelated stellar jitter appears marginally larger during thesecond season of ESPRESSO observations, but the di ff erence isonly at the 1 . σ level. While the lack of variability in the TESSphotometry and spectroscopic activity indicators prevented usfrom obtaining clear information on the possible rotation periodof the primary, we did attempt to employ a more sophisticatedmodeling framework based on Gaussian Process (GP) regres-sion to mitigate stellar activity e ff ects. We performed a fit tothe ESPRESSO RVs using a two-Keplerian + GP with quasi-periodic kernel (for details see e.g., Damasso et al. 2018, 2020),with the upper limit on the putative rotation period of the starconstrained by the value reported in Table 1. The results (notshown) were unsatisfactory (an outcome not unexpected alsoconsidering the relatively small size of the RV dataset), with therotation parameter unconstrained and a statistically disfavouredmodel (marginal likelihood ln Z = − . Z = − . v sin i such as that in Mann et al. (2018), as no variability in the photometry is detected (such a relationwould in any case predict RV jitter ∼
10 times larger than themeasured one). If we use the updated empirical relation betweenthe Ca ii H & K activity indicator and RV jitter presented by Ho-jjatpanah et al. (2020), we would infer RV variability of 3 . ± .
4m s − . This estimate is nominally higher than, but approximatelywithin 1 σ of, the measured values. Finally, the detailed simula-tions by Meunier & Lagrange (2019) indicate that amplitudes ofphotometric variability of (cid:46)
300 ppm would be compatible withRV jitter around 1 m s − for a star with the spectral type of HD5278. Interestingly enough, given the typical value of log R (cid:48) HK we measure for the target, such low amplitudes of photometricmodulation are expected, in a statistical sense, for quasi-pole-onconfigurations (see Figure 4 of Meunier & Lagrange 2019).No significant orbital eccentricity is detected. We attemptedorbital fits with either one or both companions on circular orbits,with no significant improvements in likelihood. Tidal circular-ization timescales for HD 5278 b are on the order of tens of Gyr(e.g., Matsumura et al. 2008), and larger still for the outer com-panion, indicating that there is no expectation for the orbits to befully circularized. We therefore adopted the Keplerian solutionas baseline, and report the corresponding 1 σ -level upper limitson e in Table 2.The physical companion identified in Gaia DR2 is locatedat a projected linear separation of 120 AU. Given the nominalmasses for primary and secondary, this corresponds to a periodin the neighborhood of 1100 years. It is then worthwhile inves-tigating whether evidence of orbital motion from the compan-ion is in fact already present in the high-precision ESPRESSORVs of HD 5278. Following Torres (1999), given the expectedcompanion mass, separation in the sky, and distance, the maxi-mum expected RV slope is ˙ γ (cid:39) . − yr − . This is in prin-ciple already detectable in the ESPRESSO data. However, theresiduals to the two-planet fit show no evidence for a long-termtrend. We explored nevertheless this scenario adding a slope tothe global model. We only derived a statistically non-significant˙ γ = . ± .
67 m s − yr − , with very marginal Bayesian evi-dence in favour of the more complex model (ln Z = − . Z = − . While the system parameters derived based on the separate anal-ysis of the TESS light curve and ESPRESSO RVs are robust, itis desirable to perform a joint analysis of the combined datasetsfor two reasons: 1) it is possible to place tighter constraints onorbit eccentricity and derive self-consistent uncertainties in themodel parameters taking into account correlations; 2) given theevidence for the second companion in the RV data, it is possi-ble to derive self-consistent ephemeris and verify that HD 5278c did not undergo an inferior conjunction during the timespan ofthe TESS observations.The joint analysis of the TESS light curve and ESPRESSORV time-series was carried out with two di ff erent Bayesian tech-niques: i) MultiNestv3.10 , through its pyMultiNest wrapper,coupled to the code batman (Kreidberg 2015), which was setupto run with 500 live points; ii) a DE-MCMC method as for thetransit fitting, following the same implementation as in Bonomoet al. (2014, 2015). The model parameters in the two approachesare the same, with the only exception that the former method fits
Article number, page 7 of 26 & A proofs: manuscript no. HD5278_ESPRESSO
Table 2.
HD 5278 system parameters from separate analysis of TESS and ESPRESSO data. 1 σ uncertainties and upper limits are reported. Parameter Prior Value
Transit light curve parameters
Orbital period P [days] U ( − inf,inf) 14 . + . − . Transit epoch T c [BJD TDB − U ( − inf,inf) 8680 . + . − . Transit duration T [days] U ( − inf,inf) 0 . + . − . Radius ratio R p / R (cid:63) U ( − inf,inf) 0 . ± . i [deg] U (0.0,inf 88 . + . − . Jitter Sector 1 σ S1 U (0.0,inf) 0 . + . − . Jitter Sector 12 σ S12 U ( − inf,inf) 0 . + . − . Jitter Sector 13 σ S13 U ( − inf,inf) 0 . + . − . Jitter Sector 27 σ S27 U ( − inf,inf) 0 . + . − . u [fixed] 0.24 u [fixed] 0.30 Spectroscopic orbit parametersK b [m s − ] U (0,5) 1.96 + . − . P b [days] N (14.3399,0.0032) 14.33902 ± T c , b [BJD-2 450 000] N (8551.645659,0.002205) 8551.645 ± √ e b sin ω b U (-1,1) 0.354 + . − . √ e b cos ω b U (-1,1) − . + . − . K c [m s − ] U (0,10) 3.13 + . − . P c [days] U (0,45) 40.897 + . − . T c , c [BJD-2 450 000] U (8420,8475) 8431.8 ± √ e c sin ω c U (-1,1) 0 . + . − . √ e c cos ω c U (-1,1) − . + . − . σ jit , [m s − ] U (0,10) 0.82 + . − . γ [m s − ] U (-30450,-30350) − . ± . σ jit , [m s − ] U (0,10) 1.49 + . − . γ [m s − ] U (-30450,-30350) − . ± . Derived planetary parameters
Impact parameter b b . ± . e b + . − . ( < . ω b [rad] 1.94 + . − . Orbital semi-major axis a b [au] 0.120 ± M b [M ⊕ ] 7 . + . − . Radius R b [R ⊕ ] 2 . ± . ρ b [g cm − ] 2 . ± . g b [m s − ] 12 . + , − . Equilibrium temperature T eq , b [K] a ± S b [S ⊕ ] 132 ± e c + . − . ( < . ω c [rad] 0.59 + . − . Minimum mass M c sin i c [M ⊕ ] 18.1 ± a c [au] 0.242 ± a Black-body equilibrium temperature assuming a null Bond albedo and uniform heat redistribution to the night side.
Article number, page 8 of 26. Sozzetti et al.: HD 5278 b and c unveiled by ESPRESSO
Table 3.
HD 5278 system parameters derived from a joint ESPRESSO RVs + TESS photometry analysis. Errors and upper limits as in Table 2.
Parameter Prior Value
Fitted system parameters
Radius ratio R p / R (cid:63) U (0.017,0.020) 0 . + . − Inclination i [deg] U (85,90) 89 . + . − . Stellar density (cid:37) (cid:63) [ (cid:37) (cid:12) ] N (0.66,0.04) 0 . + . − . Jitter Sector 1 σ S1 U (0,0.003) 0 . + . − . Jitter Sector 12 σ S12 U (0,0.003) 0 . + . − . Jitter Sector 13 σ S13 U (0,0.003) 0 . + . − . Jitter Sector 27 σ S27 U (0,0.003) 0 . + . − . u [fixed] 0.24 u [fixed] 0.30 K b [m s − ] U (0,5) 1.93 + . − . P b [days] U (14,14.5) 14.339156 + . − . T c , b [BJD-2 450 000] U (8681,8682) 8680.69932 + . − . √ e b sin ω b U (-1,1) 0.09 + . − . √ e b cos ω b U (-1,1) − . + . − . K c [m s − ] U (0,6) 3.17 + . − . P c [days] U (0,45) 40.87 + . − . T c , c [BJD-2 450 000] U (8420,8475) 8431.8 + . − . √ e c sin ω c U (-1,1) 0 . + . − . √ e c cos ω c U (-1,1) − . + . − . σ jit , [m s − ] U (0,10) 0.83 + . − . γ [m s − ] U (-30450,-30350) − . ± . σ jit , [m s − ] U (0,10) 1 . + . − . γ [m s − ] U (-30450,-30350) − . ± . Derived planetary parameters
Impact parameter b b . + . − . a / R (cid:63) . + . − . Transit duration T [days] 0 . + . − . e b + . − . ( < . ω b [rad] 2.36 + . − . Orbital semi-major axis a b [au] 0.1202 ± M b [M ⊕ ] 7 . + . − . Radius R b [R ⊕ ] 2 . ± . ρ b [g cm − ] 2 . + . − . Surface gravity log g b [m s − ] 12 . + . − . Equilibrium temperature T eq , b [K] a ± S b [S ⊕ ] 132 ± e c + . − . ( < . ω c [rad] 0.14 + . − . Minimum mass M c sin i c [M ⊕ ] 18.4 + . − . Orbital semi-major axis a c [au] 0.2416 ± Article number, page 9 of 26 & A proofs: manuscript no. HD5278_ESPRESSO
Fig. 7.
Top: the ESPRESSO RV times series for HD 5278 with over-plotted the best-fit two-planet solution (yellow curve) obtained in thejoint ESPRESSO RV + TESS photometry analysis. Black filled andgrey open circles correspond to ESPRESSO observations obtained be-fore and after the technical intervention. Center: the phase-folded best-fit orbital model for the transiting companion HD 5278 b (blue curve).Individual observations are colour-coded as in the top panel, while thelarge red filled dots show them binned in phase. Bottom: the same forthe non-transiting companion HD 5278 c. In all panels, formal uncer-tainties have been inflated adding in quadrature the RV jitter values re-ported in Table 3.
As a preliminary step, We first ran the
MultiNest -based al-gorithm adopting a 1-planet model (only the transiting planet).This allowed us to perform model selection in a straightfor-
Fig. 8.
Mass-radius diagram of sub-Neptunes with masses detected atthe 3 σ level (or better), including HD 5278 b (star). The objects arecolor-coded by their equilibrium temperature. The di ff erent curves de-pict internal structure models of variable composition from Zeng &Sasselov (2013) and Zeng et al. (2019) (as reported in the legend):Earth-like (32.5% Fe + ), pure rock (100% MgSiO ),50% Earth-like rocky core +
50% H O layer by mass, 100% H O mass,Earth-like rocky core (99.7%) + envelope by mass at a temper-ature of 1000 K). The location of Uranus and Neptune is also indicated.Data of transiting systems used for this and the following plots are takenfrom the TEPCat catalogue as of September 2020. Along with TOI-130b (T-130 for simplicity), Kepler-36 c and K2-292 b (K-36 and K2-292for simplicity), which are discussed in the text, are also indicated in theplot. ward manner by comparing the resulting ln Z in the joint anal-ysis with that obtained in the case of the 2-planet model. Thecorresponding natural logarithm of evidence ratio is ∆ ln Z = ln Z (2planet) − ln Z (1planet) = . − . = + .
8. Wecan therefore conclude that there is, as expected, strong evidencein favour of the 2-planet model (e.g., Nelson et al. 2020).For the two-planet model the parameter best values and un-certainties as determined by both techniques are fully consistentand, for this reason, we report in Table 3 the results of the analy-sis from the first technique only. Figure 6 shows the phase-foldedtransit light-curve along with the best-fit transit model. Figure7 shows the two-planet fit superposed to the time-series of theESPRESSO RVs and the phase-folded plots of the best-fit orbitsfor both planets (with error bars taking into account the fittedvalues of RV jitter). Finally, the corner plots of Figures B.1 andB.2 summarize the results of the posterior distributions of themodel parameters.Overall, the results are in excellent agreement with those de-rived with the individual analysis of ESPRESSO RVs and TESSphotometry. The transit of HD 5278 b is found to be slightlymore central than in the case of the separate analysis of theTESS light curve. Tighter constraints are placed on the eccen-tricity of HD 5278 b, which is now found to be e b < .
12. Basedon the updated ephemeris derived for HD 5278 c, if transitingthe planet should have been spotted by TESS once in each of thefour Sectors analyzed. We carefully inspected the TESS lightcurve, found no evidence of such events, and therefore concludethat HD 5278 c indeed does not transit. We elect to adopt thenumbers in Table 3 as our fiducial system parameters.
Article number, page 10 of 26. Sozzetti et al.: HD 5278 b and c unveiled by ESPRESSO
5. Discussion
The RV semi-amplitude of the transiting companion ( K b = . + . − . m s − ) is clearly detected with ESPRESSO at > σ level. Given the stellar properties derived in Sect. 3, we infer M b = . + . − . M ⊕ , R b = . ± .
05 R ⊕ , and a planetary meandensity ρ b = . + . − . g cm − . In the mass-radius diagram (Figure8) of sub-Neptunes with ≥ σ mass determinations it occupies aseemingly unremarkable location. Its density is not particularlylow, compared to an object with similar mass but much largerradius such as Kepler-36 c (Carter et al. 2012). Its density is notparticularly high either, compared to an object with almost iden-tical radius but much larger mass such as K2-292 b (Luque et al.2019). Its size makes HD 5278 b sit atop the peak of the radiusdistribution on the outer side of the radius valley (Fulton et al.2017; Fulton & Petigura 2018).As shown in Figure 8, HD 5278 b’s composition could bedescribed as corresponding to a ∼ H -rich atmosphere (Zeng et al. 2019), or a mas-sive rocky planet surrounded by a H -dominated envelope of <
1% in planet mass (Lopez & Fortney 2014; Zeng et al. 2019).The planet’s mass and radius also fit the definition of an ice-dominated planet with a thin or non-existent H / He atmosphereDorn et al. (2017); Zeng et al. (2019); Venturini et al. (2020), orthat of a water-rich world with a significant fraction (20-50%)of water in super-critical steam state, and thus with an H O-dominated atmosphere and a more or less relevant H / He enve-lope (Madhusudhan et al. 2020; Turbet et al. 2020; Mousis et al.2020; Kite et al. 2020).Using the measured mass, radius and density of HD 5278b, we performed our own internal structure retrieval calculations(see Mortier et al. 2020), assuming the Fe, Si, and Mg stellarabundance ratios from Table 1 as proxies for the disk composi-tion at the time of planet formation, and with the constraint thatthe planetary bulk abundance matches the stellar one. We ex-plored two cases: a) a fully di ff erentiated planet that includes awater layer, and in which the gas envelope is solely composed ofH and He; b) a ’dry’ model, where the planet interior structure iscomposed of only an iron core and a silicate mantle. In the firstcase, we find core, mantle, and water mass fractions of 0 . + . − . ,0 . + . − . , and 0 . + . − . , respectively, where the error bars corre-spond to the 5% and 95% quantiles (approximately 2 σ ). The gasmass is small (upper 95 % quantile equal to ∼ .
03 M ⊕ ), andthe envelope contributes approximately 17% of the observed ra-dius (0 . + . − . R ⊕ ). In the second case, we obtain 0 . + . − . and0 . + . − . for the core and mantle mass fractions, respectively.The gas envelope corresponds to around 4% of the planet mass,and contributes 30% of the observed planet radius (0 . + . − . R ⊕ ).The corner plots with the posterior distributions of the mostrelevant parameters adjusted in our internal structure retrievalanalysis are shown in Figures B.3 and B.4. As expected, theyshow how rather di ff erent compositions exactly reproduce theobserved mass and radius of HD 5278 b. However, from a the-oretical viewpoint, sub-Neptunes are primarily expected to haveformed beyond the water ice line (e.g. Raymond et al. 2018;Bitsch et al. 2019; Venturini et al. 2020). Therefore, model a)is expected to be a closer representation of the composition of aplanet in the second peak of the radius valley, such as HD 5278b. We get back to this point in Sect. 5.3.As mentioned in the Introduction, breaking the degener-acy between core mass and atmospheric metallicity for sub-Neptunes can only be attained via atmospheric characterization measurements (particularly in the case of mostly cloud-free at-mospheres). In this respect, HD 5278 b is a very interestingtarget. For the case of low resolution spectroscopy in space(with e.g. JWST), using simple metrics for estimating the S / Nin transmission and emission (Gillon et al. 2016; Niraula et al.2017; Kempton et al. 2018), the planet always ranks highestamongst sub-Neptunes with T eq < T e ff > / π Men-sae c (Huang et al. 2018) and HD 86226 c (Teske et al. 2020).HD 5278 b is high up in the ranking even when objects aroundcooler primaries are included (see Figure 9 for an example). Forthe case of F-G-type primaries, HD 5278 is more than 2 magfainter than HD 39091 at K -band, somewhat alleviating the prac-tical di ffi culties in achieving photon-limited observations of verybright stars with JWST. Even in the case of very e ffi cient pro-duction of photochemical hazes, HD 5278 b belongs to a classof sub-Neptunes (with T eq < ff ects in transmission spectra). Finally, with T eq (cid:46) to CO gas as the dominant car-rier of carbon (Moses et al. 2013; Hu & Seager 2014) and thehigh-temperature range where relatively few cloud species areexpected to condense and organic hazes are unstable, so that at-mospheres are more likely to have large spectral features (e.g.,Fraine et al. 2014; Wakeford et al. 2017). HD 5278 b is thereforeless likely to show the presence of clouds and hazes.In principle, evidence of ongoing atmospheric escape in HD5278 b’s could be searched for probing the planet’s upper atmo-sphere at high spectral resolution in transmission. The observingchannel of choice would be the 10,833 Å helium triplet feature,a better suited diagnostic than the hydrogen Lyman α line in theUV, given the distance of the target . However, while the escap-ing / extended atmospheres of several hot Jupiters have success-fully been probed in this way (e.g., Allart et al. 2018; Spake et al.2018; Nortmann et al. 2018; Guilluy et al. 2020), recent attemptsat detecting the upper atmospheres in sub-Neptunes using thehelium infrared triplet have shown how both observational andtheoretical challenges must first be met Kasper et al. (2020).If the low levels of photometric variability of the primaryare indeed indicative of a likely quasi-pole-on configuration (seeprevious discussion on the Meunier & Lagrange 2019 simula-tion results), a finding corroborated by the low value of v sin i for a star with the temperature of HD 5278 (pointing to a pos-sibly large obliquity), this raises the possibility of a significant At ∼
60 pc, HD 5278 sits approximately at the distance limit foruseful measurements of the stellar Lyman α line flux, due to its expo-nential decay caused by absorption by neutral hydrogen atoms in theinterstellar medium. Article number, page 11 of 26 & A proofs: manuscript no. HD5278_ESPRESSO
Fig. 9. S / N for transmission spectroscopy observations with JWST(TSM; Kempton et al. 2018) vs. equilibrium temperature for sub-Neptunes with a measured mass, including HD 5278 b (red star). Blackfilled dots and empty squares identify the sample of planets around starswith T e ff < T e ff > spin-orbit misalignment for HD 5278 b. Given the planetary andstellar radius and vsin i value, the expected amplitude of theRossiter-McLaughlin e ff ect (for a planetary orbital plane wellaligned with the stellar spin axis) is on the order of 1.5 m s − ,which appears within reach with ESPRESSO (aiming at a fullRV sequence during a transit window), given the brightness ofthe host. We have shown how Bayesian evidence strongly favours thetwo-planet model. This, alongside the lack of activity-inducedRV signals, helped us build a solid case for explaining the RVmodulation with a 41-day period seen in the ESPRESSO data interms of an outer, non-transiting companion. The 10 σ -level de-tection ( K c = . + . − . m s − ) of its RV semi-amplitude impliesa minimum mass M c sin i c = . + . − . M ⊕ , making it a Neptune-mass planet. We found no evidence of its transit in the TESS pho-tometry, and therefore we can infer an upper limit to its orbitalinclination ( i c < arccos( a / R (cid:63) ) − ) i c < .
68 deg. This in turn im-plies a lower limit to the true mass of HD 5278 c M p , c > . ⊕ .The HD 5278 two-planet system is Hill-stable, followingstandard analytical criteria (e.g., Giuppone et al. 2013), even as-suming the eccentricities of the two companions match the upperlimits derived in Table 2. Stability is violated only in the limit ofa true mass of HD 5278 c in excess of 200 M ⊕ , which would im-ply a true inclination angle of i c (cid:46) e < .
1. In this case,the evolution history of the system is expected to have been lessinfluenced by chaos and instabilities.The Kepler mission multi-planet sample has been shown tohave a distribution of period ratios with excesses of systems(“peaks”) just wide of first-order (3:2, 2:1) and second-order(5:3, 3:1) mean motion resonances (MMR), and correspondingdeficits (“troughs”) just short of these commensurabilities (e.g.,Lissauer et al. 2011; Fabrycky et al. 2014; Delisle & Laskar2014; Xu & Lai 2017; Choksi & Chiang 2020). The HD 5278system is unremarkable in this sense, with a period ratio devia-tion from the closest second-order MMR (3:1) of -0.148, typi-cally a factor of 10 larger than those corresponding to the peaksand troughs of near-resonance configurations identified in theoriginal Kepler sample of multiples. The formation of a low-mass system such as the one we have uncovered around HD 5278is still open for debate. As the two planets do not lie near a pe-riod commensurability and they are not piled up at very short pe-riods, formation (mostly) in-situ in gas-poor conditions is viable(e.g., Lithwick et al. 2012; Ogihara et al. 2018; Terquem & Pa-paloizou 2019; Choksi & Chiang 2020, and references therein).Alternatively, formation in the gas-rich regions of the protoplan-etary disk followed by more or less ’clean’ disc-driven migrationand resonance capture can also possibly describe the present ar-chitecture of the HD 5278 system, provided the two planets sub-sequently escaped resonance due to dynamical instability e ff ectsdriven by a variety of other physical processes (e.g., Goldreich& Schlichting 2014; Izidoro et al. 2017; Lambrechts et al. 2019;Raymond & Morbidelli 2020, and references therein).Recent studies (Bryan et al. 2019) have shown that plane-tary systems with inner super Earths and sub-Neptunes are oftenaccompanied by outer companions of unclear nature (be theymassive planets or brown dwarfs). If confirmed by future obser-vations, the fact that the marginal RV trend we measured in theESPRESSO data might be compatible with the gravitational in-fluence of the distant binary companion to HD 5278 would ruleout the existence of massive substellar objects at shorter sepa-rations. In the Kervella et al. (2019) catalog of proper motionanomalies the sensitivity curve for HD 5278 implies that com-panions with masses of 0.37 M J au − / are ruled out based on thecombination of Gaia and Hipparcos astrometry alone. Based onthe analytical formulation in Kervella et al. (2019), at the exactseparation of HD 5278 c the detectable mass from proper motionanomaly is found to be around 60 M J , placing uninteresting con-straints on the true mass of the outer planet in the system. Takinginto account the loss of e ffi ciency in companion mass determi-nation for orbital periods vastly exceeding the Hipparcos-Gaiatime baseline, this technique would be marginally sensitive tothe low-mass binary companion at ∼
120 au. Massive ( (cid:38)
10 M J )companions at ∼
10 au would however be clearly detectable,and the lack of a statistically significant Hipparcos-Gaia propermotion di ff erence qualitatively points towards their absence, inagreement with the evidence from the ESPRESSO RVs. As mentioned in the Introduction, high-precision mass determi-nations, at the 20% level or better, have recently been recognizedas mandatory in order to carry out detailed atmospheric charac-terization studies (Batalha et al. 2019). Inferences on both massand chemical composition of the atmosphere based on transmis-sion / emission spectra alone (de Wit & Seager 2013) might bedominated by strong degeneracies, particularly in the case of Article number, page 12 of 26. Sozzetti et al.: HD 5278 b and c unveiled by ESPRESSO
Fig. 10.
Top left: mass-radius diagram with contours built using the exoplanets sample from Fig. 8. Black dots represent planets with massmeasurements at the > σ level of statistical significance. HD 5278 b is indicated by a large dark magenta star. The contours are color-coded interms of relative occurrence. Top right: The same plot in the planetary mass - stellar irradiation plane. Bottom left: the same plot in the planetarydensity vs. stellar irradiation plane. Bottom right: the same plot in the planetary density vs. mass plane. Table 4.
Results of the K-S on di ff erent subsets of the mass distributionof transiting sub-Neptunes: cut-o ff s at T e ff = e ff = e ff = e ff = e ff = e ff = Sub-samples Pr ( D ) N low N high ≥ σ confidence, as we did in Figure 8, the sample size (62objects) is indeed not yet very large, so the findings we presentin the following are to be considered illustrative. Given the lim- ited statistics available, the preparation of the contour / densityplots shown in the remainder of this Section is based on a simplebinned histograms analysis rather than the adoption of more so-phisticated approaches based on e.g. (weighted) kernel densityestimators (Morton & Swift 2014; Fulton et al. 2017).The upper left panel of Figure 10 shows how close-in sub-Neptunes with 5 (cid:46) M p (cid:46)
10 M ⊕ and 2 (cid:46) R p (cid:46) . ⊕ appearintrinsically more common than planets with higher masses andradii. The possibly double peaked radius distribution in this massregime (at ∼ . − . ⊕ and ∼ . − . ⊕ ) might well be anartifact due to the binned histogram approach utilized to derivethe density plots, in the face of small-number statistics. Interest-ingly, however, a hint of the presence of this feature is also seenin Figures 6 and 8 of Fulton & Petigura (2018). We also notethat planets in the larger radius range R p (cid:38) . ⊕ appear uni-formly distributed in the full mass range, and similarly planets inthe higher mass range 10 (cid:46) M p (cid:46)
15 M ⊕ seem to be uniformlydistributed over the whole 2 < R p < ⊕ range.The upper right panel of Figure 10 shows instead that sub-Neptunes with 5 < M p <
10 M ⊕ are found over a large intervalof irradiation levels (10 (cid:46) S ⊕ (cid:46) M p ∼ ⊕ in the 30 (cid:46) S ⊕ (cid:46) < M p <
15 M ⊕ ap-pear to be slightly more abundant in conditions of higher irradi- Article number, page 13 of 26 & A proofs: manuscript no. HD5278_ESPRESSO
Fig. 11.
Top: Mass-radius diagram of sub-Neptunes with masses deter-mined at the 3 σ level (or better), and with T e ff < e ff > ation (S ⊕ (cid:38) M p >
20 M ⊕ ) receiving S ⊕ (cid:38) (cid:37) ∼ . − . − are dis-tributed over almsot three orders of magnitude in stellar irradia-tion levels (3 (cid:46) S ⊕ (cid:46) (cid:46) S ⊕ (cid:46) S ⊕ ∼ −
100 range. The highest-density objects ( (cid:37) (cid:38) . − ) mostly reside at S ⊕ (cid:38) < M p <
10 M ⊕ can span an importantrange of densities (1 (cid:46) (cid:37) (cid:46) − ), the peak in relative occur-rence encompassing objects with 6 (cid:46) M p (cid:46) ⊕ and 2 (cid:46) (cid:37) (cid:46) < M p <
15 M ⊕ ) sub-Neptunes with (cid:37) (cid:46) − appear intrinsically slightly more common, although thehighest-mass sub-Neptunes also populate the tail of densest sys-tems.While one should exert caution not to over-interpret the pat-terns of variations in relative occurrence in parameter space pre-sented in Figure 10, given the still relatively small-number statis-tics we are dealing with, it is certainly possible to provide a few(more or less speculative) comments in connection to planet for- Fig. 12.
Top: planet mass vs. stellar irradiation for the sample defined inFigure 11, including HD 5278 b (red star). Filled dots and open squaresindicate objects around primaries with T e ff < e ff > mation and evolution scenarios. For example, the overdensity inthe upper left panel of Figure 10 is not unexpected, given theknown behaviour of planet frequency as a function of radius andmass for small planets (Mayor et al. 2011; Howard et al. 2012).It is tempting to try to explain the observed double peak in theradius and mass ranges ∼ . − . ⊕ - ∼ . − . ⊕ and ∼ . − . ⊕ , ∼ . − . ⊕ , respectively, as evidence ofintrinsic compositional di ff erences: the former peak could corre-spond to the high-mass tail of a population of truly rocky planets(or evaporated cores), while the latter peak could be made upof objects with icy cores and / or small gaseous envelopes (e.g.,Venturini et al. 2020; Neil & Rogers 2020).Interestingly, objects with ∼ ⊕ appear intrinsically morecommon over an important range of irradiation levels and densi-ties (top right and bottom right panels of Figure 10, respectively).The shape of the distribution of measured masses for this sam-ple of sub-Neptunes closely resembles that realized by planetformation and evolution simulations in the recent work of Ven-turini et al. (2020) that produce icy cores ( ∼
50% in mass), al-though the peak in the distribution in their analysis is shifted toslightly higher masses (9-10 M ⊕ ). This might further corrobo- Article number, page 14 of 26. Sozzetti et al.: HD 5278 b and c unveiled by ESPRESSO rate the notion that an important fraction of sub-Neptunes pop-ulating the second peak in the radius valley might be composedof water-rich planets with thin H / He atmospheres, rather thantruly rocky planets surrounded by H / He envelopes with massfractions of a few percent, as predicted by pure photoevaporationmodels Owen & Wu (2017) and core-powered mass-loss modelsGinzburg et al. (2018).The mild trend of more massive / higher density sub-Neptunes being more common in conditions of higher incidentfluxes seen in the upper right and bottom left panels of Figure10 can be qualitatively understood within the known paradigmsof planet formation and migration when e ff ects due to evapo-ration are included. In particular, for low-mass sub-Neptunes,the dominant e ff ect of intense irradiation is atmospheric escape,which increases the planetary density such that (cid:37) and S ⊕ are anti-correlated (e.g., Jin & Mordasini 2018).High-mass (M p >
15 M ⊕ ), high-density sub-Neptunes arerare (see bottom left and right panels of Figure 10). They typ-ically receive irradiation levels significantly lower than thosecharacterizing the evaporation desert (as defined by Lundkvistet al. 2016), therefore they are not likely to have been strippedof their envelopes. This sparse population likely carries the im-print of evolutionary pathways that might include original high-density formation and subsequent inward migration via mecha-nisms involving multiple companions (e.g., Kozai-Lidov oscil-lations. See for example Dawson & Chiang 2014; Mustill et al.2017), processes inhibiting gas accretion or promoting gas re-moval after formation (e.g., Ormel et al. 2015; Guilera et al.2020), or a possible history of impacts in the early stages of theformation process (Ogihara & Hori 2020; Venturini et al. 2020),for which only recently direct observational evidence has beenproduced (Santerne et al. 2018; Bonomo et al. 2019).The mass-radius diagram of sub-Neptunes shows another in-triguing feature. The two panels of Figure 11 highlight a dearthof M p >
10 M ⊕ companions around G- and F-type stars withT e ff > R p (cid:38) . ⊕ . AKolmogorov-Smirnov (K-S) test returned a 2.4% probability thatthe mass distributions of sub-Neptunes around primaries withT e ff < e ff > (cid:37) -M p diagram (bottom panel of Figure 12). The feature doesnot seem to be produced by an observational bias, as at a givenvalue of orbital period it is the less massive companion the onethat is harder to detect around an increasingly more massive pri-mary. The di ff erence in mass distribution vanishes if a cuto ff atslightly cooler temperatures is applied (T e ff > ff erences in the subsets of the distributionswith cuto ff s at T e ff > ∼
10) forwhich the K-S test provides reliable results.The hint of a lack of close-in, higher-mass (M (cid:38)
10 M ⊕ ) sub-Neptunes around G-F primaries can be interpreted as fossil ev-idence of planet formation around stars of varied mass. Moremassive primaries typically have higher-mass disks. Upon reach-ing the critical mass (10 M ⊕ or so), newly formed cores have ahigher chance of quickly accreting large amount of gas, ending Interestingly, the only companion with M p >
10 M ⊕ and a radiuscomparable to that of Uranus and Neptune is K2-100b, orbiting on avery short period a young late-F-type star. Its radius is expected toshrink even below 3 R ⊕ over the main-sequence lifetime of the star (Bar-ragán et al. 2019) up their formation process as giant planets. The threshold ap-pears to be for planets around primaries with T e ff (cid:38)
6. Summary and Conclusions
We performed high-precision RV follow-up with ESPRESSOfor mass confirmation of the transiting sub-Neptune candidateTOI-130 b, uncovered by TESS orbiting the bright, nearby, late-F dwarf HD 5278 with a period P b = .
34 days. Our mainfindings can be summarized as follows: a ) The combination of TESS photometry and ESPRESSORVs allowed us to determine a planetary mass and radius M b = . + . − . M ⊕ and R b = . ± .
05 R ⊕ , respectively. The resultingplanetary bulk density is (cid:37) b = . + . − . g cm − . Our ESPRESSORVs unveiled the presence of a second, non-transiting com-panion in the system, for which we measured a period P c = . + . − . days and determined a minimum mass M c sin i c = . + . − . M ⊕ . b ) HD 5278 b can be described as a water-rich ( ∼ ∼ .
4% and ∼
17% of its total mass and radius, respectively.A ’dry’ model with small rocky core, a large silicate mantle,and a sizable H / He envelope is also compatible with the mea-sured physical properties of the planet. Given the system param-eters, HD 5278 b represents one of the best targets orbiting G-Fprimaries for follow-up atmospheric characterization measure-ments with HST and JWST, which could be instrumental in re-solving compositional degeneracies. c ) The HD 5278 b,c planetary system, with a period ratio notparticularly close to a resonant configuration and low eccentrici-ties, is Hill-stable even in the case of highly non-coplanar orbits.On the one hand, such a scenario is possible in light of the verylow levels of stellar activity recorded, lack of detectable rotationin both RVs and photometry and low v sin i value, conditionsthat, given the spectral type of the primary, are statistically morelikely if the star is viewed not far from pole-on. On the otherhand, the lack of detectable eccentricity for both planets hintsat the fact that their orbits might not be particularly misaligned.Measurements of the spin-orbit (mis)alignment for HD 5278 bwould be telling. d ) We placed the properties of HD 5278 b in the context ofthe (still small) presently-known sample of sub-Neptunes withmeasured mass and radius. We showed that the lower-mass,smaller-radius component of the population is intrinsically morecommon, with a possibly double-peak radius distribution brack-eting 2.5 R ⊕ and a mass distribution clearly peaking at ∼ ⊕ .We highlighted a possible excess of higher-mass, higher-densitysub-Neptunes in conditions of stronger irradiation. We providedevidence that very high-density, higher-mass sub-Neptunes areintrinsically rare. These features can be understood in broadterms within the context of modeling e ff orts of planetary sys-tems formation processes and the subsequent orbital evolution. e ) We identified a marginally significant lack of more mas-sive sub-Neptunes around G-F primaries with T e ff (cid:38) ff erent mass distribution of close-insub-Neptunes around stars of varied spectral type.In conclusion, the results of our combinedESPRESSO + TESS analysis of the HD 5278 planetarysystem reinforce the importance of the detailed, high-precision
Article number, page 15 of 26 & A proofs: manuscript no. HD5278_ESPRESSO determination of the fundamental physical parameters oftransiting sub-Neptunes, particularly those orbiting solar-typeprimaries. With increasing sample sizes of the population withwell-determined properties, the preliminary evidence for trendsin parameter space (planet mass and radius, bulk density, stellarinsolation and e ff ective temperature) reported here will be puton firmer statistical grounds. It will then become possible tocompare not only qualitatively but also quantitatively theoreticalpredictions of planet formation and evolution models with theobserved population of sub-Neptunes, performing at higherresolution key diagnostic studies of e.g. the detailed shape of thecombined mass and radius distribution, or trends of occurrencerates and bulk composition with spectral type of the primary andirradiation levels / orbital separation. Acknowledgements.
The authors acknowledge the ESPRESSO projectteam for its e ff ort and dedication in building the ESPRESSO instrument.This work has received financial support from the ASI-INAF agreementn.2018-16-HH.0. M.D. acknowledges financial support from the FP7-SPACEProject ETAEARTH (GA No. 313014). The INAF authors acknowledgefinancial support of the Italian Ministry of Education, University, andResearch with PRIN 201278X4FL and the "Progetti Premiali" fundingscheme.The ESPRESSO Instrument Project was partially funded throughSNSF’s FLARE Programme for large infrastructures. This work has beencarried out in part within the framework of the NCCR PlanetS supportedby the Swiss National Science Foundation. This work was supported byFCT - Funda cão para a Ciência e Tecnologia through national funds and byFEDER through COMPETE2020 - Programa Operacional Competitividade eInternacionaliza cão by these grants: UID / FIS / / / / / / / FIS-AST / / / FIS-AST / / / FIS-AST / / / FIS-OUT / / / / / / / CP1215 / CT0004. S.G.S acknowledges thesupport from FCT through Investigador FCT contract nr. CEECIND / / / FSE (EC). This project has received funding from the EuropeanResearch Council (ERC) under the European Union’s Horizon 2020 researchand innovation programme (project Four Aces, grant agreement No 724427).V.A. acknowledges the support from FCT through Investigador FCT contractnr. IF / / / CP1273 / CT0001. Y.A. and J.H. acknowledge the SwissNational Science Foundation (SNSF) for supporting research through theSNSF grant 200020_192038. J.I.G.H. acknowledges financial support fromSpanish Ministry of Science and Innovation (MICINN) under the 2013 Ramóny Cajal programme RYC-2013-14875. J.I.G.H., A.S.M., R.R., and C.A.P.acknowledge financial support from the Spanish MICINN AYA2017-86389-P.A.S.M. acknowledges financial support from the Spanish Ministry of Scienceand Innovation (MICINN) under the 2019 Juan de la Cierva Programme.R. A. is a Trottier Postdoctoral Fellow and acknowledges support from theTrottier Family Foundation.This work was supported in part through a grantfrom FRQNT.This work has made use of data from the European SpaceAgency (ESA) mission
Gaia ( ),processed by the Gaia
Data Processing and Analysis Consortium (DPAC, ). Fundingfor the DPAC has been provided by national institutions, in particular theinstitutions participating in the
Gaia
Multilateral Agreement. This publicationmakes use of The Data & Analysis Center for Exoplanets (DACE), whichis a facility based at the University of Geneva (CH) dedicated to extrasolarplanets data visualisation, exchange and analysis. DACE is a platform ofthe Swiss National Centre of Competence in Research (NCCR) PlanetS,federating the expertise in Exoplanet research. The DACE platform is availableat https://dace.unige.ch . This paper includes data collected by the TESSmission, which are publicly available from the Mikulski Archive for SpaceTelescopes (MAST). We acknowledge the use of public TESS Alert data frompipelines at the TESS Science O ffi ce and at the TESS Science ProcessingOperations Center. Resources supporting this work were provided by theNASA High-End Computing (HEC) Program through the NASA AdvancedSupercomputing (NAS) Division at Ames Research Center for the production ofthe SPOC data products. References
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HD5278_ESPRESSO INAF - Osservatorio Astrofisico di Torino, Via Osservaorio 20, I-10025 Pino Torinese, Italye-mail: [email protected] Physics Institute of University of Bern, Gesellschaftsstrasse 6, CH-3012 Bern, Switzerland Instituto de Astrofísica e Ciências do Espaço, Universidade doPorto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal Departamento de Física e Astronomia, Faculdade de Ciências, Uni-versidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4,E-28850 Torrejón de Ardoz, Madrid, Spain Instituto de Astrofisica de Canarias, Via Lactea, E-38200 La Laguna,Tenerife, Spain Universidad de La Laguna, Departamento de Astrofísica, E- 38206La Laguna, Tenerife, Spain Centro de Astrobiología (CSIC-INTA), ESAC campus, E-28692Villanueva de la Cañada, Madrid, Spain Vanderbilt University, Department of Physics & Astronomy, 6301Stevenson Center Lane, Nashville, TN 37235, USA Department of Astrophysical Sciences, Princeton University,Princeton, NJ 08544, USA INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, I-34143 Trieste, Italy Institute for Fundamental Physics of the Universe, IFPU, Via Beirut2, 34151 Grignano, Trieste, Italy Département d’astronomie, Université de Genève, Chemin Pegasi51, 1290 Versoix, Switzerland Consejo Superior de Investigaciones Científicas, E-28006 Madrid,Spain Department of Physics, and Institute for Research on Exoplanets,Université de Montréal, Montréal, H3T 1J4, Canada NASA Goddard Space Flight Center, 8800 Greenbelt Road, Green-belt, MD 20771, USA University of Maryland, Baltimore County, 1000 Hilltop Circle, Bal-timore, MD 21250, USA Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciên-cias da Universidade de Lisboa, Campo Grande, PT1749-016 Lis-boa, Portugal Departamento de Física da Faculdade de Ciências da Univeridadede Lisboa, Edifício C8, 1749-016 Lisboa, Portugal Caltech / IPAC, 1200 E. California Blvd., Pasadena, CA 91125, USA Department of Physics and Kavli Institute for Astrophysics andSpace Research, Massachusetts Institute of Technology, Cambridge,MA 02139, USA ESO, European Southern Observatory, Alonso de Cordova 3107, Vi-tacura, Santiago NASA Ames Research Center, Mo ff ett Field, CA 94035, USA Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street,Cambridge, MA 02138, USA Centro de Astrofísica da Universidade do Porto, Rua das Estrelas,4150-762 Porto, Portugal INAF – Osservatorio Astronomico di Palermo, Piazza del Parla-mento 1, 90134 Palermo, Italy Institute for Computational Science, University of Zurich, Win-terthurerstr. 190, CH-8057 Zurich, Switzerland INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807Merate, Italy Kavli Institute for Astrophysics and Space Research, MassachusettsInstitute of Technology, Cambridge, MA 02139, USA Space Telescope Science Institute, 3700 San Martin Drive, Balti-more, MD 21218, USA Department of Physics and Kavli Institute for Astrophysics andSpace Research, Massachusetts Institute of Technology, Cambridge,MA 02139, USA Department of Earth, Atmospheric and Planetary Sciences, Mas-sachusetts Institute of Technology, Cambridge, MA 02139, USA Department of Aeronautics and Astronautics, MIT, 77 Mas-sachusetts Avenue, Cambridge, MA 02139, USA SETI Institute, Mountain View, CA 94043, USAArticle number, page 18 of 26. Sozzetti et al.: HD 5278 b and c unveiled by ESPRESSO
Appendix A: 1D Coadded ESPRESSO Spectrum ofHD 5278 Appendix B: Radial velocities, activity indicators,and posterior distributions
Table B.1 contains the ESPRESSO RV data and their uncertain-ties, Table B.2 lists the time-series of spectroscopic activity indi-cators. Figures B.1 and B.2 show the posterior distributions of allthe model parameters in the joint fit to the ESPRESSO RV dataand TESS light curve. Figures B.3 and B.4 show corner plots ofthe posterior distributions of the relevant parameters of the twointerior structure models.
Article number, page 19 of 26 & A proofs: manuscript no. HD5278_ESPRESSO
Fig. A.1.
Co-added, normalized, merged, and RV-corrected ESPRESSO spectrum of HD 5278 produced using the StarII DAS workflow on theavailable set of 43 2D ESPRESSO spectra of the target.Article number, page 20 of 26. Sozzetti et al.: HD 5278 b and c unveiled by ESPRESSO
Table B.1.
ESPRESSO radial velocities of HD 5278. In the last column, PRE and POST indicate spectra gathered before and after the intervention(see text).
BJD
UTC RV ± σ Case − − ) ( m s − )58416.564987 − .
87 0.61 PRE58420.623837 − .
01 0.98 PRE58421.615745 − .
98 0.37 PRE58426.662776 − .
76 0.36 PRE58432.512969 − .
95 0.42 PRE58459.511927 − .
04 0.39 PRE58466.533613 − .
95 0.47 PRE58477.521619 − .
33 0.58 PRE58488.535606 − .
38 0.36 PRE58490.544982 − .
07 0.51 PRE58686.921063 − .
91 0.37 POST58688.776700 − .
79 0.66 POST58688.788018 − .
99 0.66 POST58706.867055 − .
82 0.32 POST58719.844569 − .
52 0.28 POST58721.863285 − .
29 0.30 POST58725.809120 − .
87 0.53 POST58731.758304 − .
86 0.45 POST58737.814370 − .
17 0.38 POST58741.624983 − .
79 0.33 POST58753.617398 − .
60 0.28 POST58759.665785 − .
48 0.44 POST58761.674737 − .
95 0.44 POST58766.630575 − .
10 0.36 POST58768.626690 − .
71 0.33 POST58771.609041 − .
54 0.46 POST58773.688306 − .
31 0.38 POST58776.572183 − .
68 0.38 POST58776.694342 − .
16 0.51 POST58778.631061 − .
20 0.32 POST58780.645934 − .
84 0.26 POST58782.560951 − .
76 0.39 POST58784.531509 − .
10 0.57 POST58786.600937 − .
26 0.30 POST58788.597405 − .
11 0.34 POST58792.609247 − .
27 0.38 POST58801.619078 − .
47 0.36 POST58811.617055 − .
00 0.49 POST58815.611406 − .
80 0.33 POST58817.547039 − .
56 0.33 POST58819.554583 − .
31 0.65 POST
Article number, page 21 of 26 & A proofs: manuscript no. HD5278_ESPRESSO
Table B.2.
Spectroscopic activity indicators for HD 5278 (see main text).
BJD
UTC
FWHM ± σ BIS ± σ S MW ± σ H α ± σ log R (cid:48) HK ± σ Case − − ) ( m s − ) ( m s − ) ( m s − )58416.564987 9217.38 1.22 − .
72 1.22 0.15276 0.00007 0.19409 0.00003 − . − .
61 1.96 0.14156 0.00015 0.19536 0.00005 − . − .
07 0.73 0.15821 0.00003 0.19463 0.00002 − . − .
60 0.71 0.15796 0.00003 0.19356 0.00002 − . − .
20 0.84 0.15594 0.00004 0.19424 0.00002 − . − .
31 0.77 0.15720 0.00003 0.19401 0.00002 − . − .
07 0.94 0.15445 0.00005 0.19307 0.00002 − . − .
32 1.15 0.15084 0.00006 0.19498 0.00003 − . − .
49 0.72 0.15771 0.00003 0.19420 0.00002 − . − .
12 1.02 0.15389 0.00005 0.19423 0.00002 − . − .
53 0.73 0.15562 0.00003 0.19371 0.00001 − . − .
95 1.32 0.14594 0.00009 0.19404 0.00003 − . − .
75 1.33 0.14713 0.00009 0.19414 0.00003 − . − .
97 0.64 0.15624 0.00003 0.19359 0.00001 − . − .
03 0.56 0.15645 0.00002 0.19260 0.00001 − . − .
67 0.59 0.15586 0.00003 0.19183 0.00001 − . − .
90 1.07 0.15119 0.00006 0.19240 0.00002 − . − .
07 0.89 0.15447 0.00005 0.19196 0.00002 − . − .
81 0.76 0.15532 0.00004 0.19181 0.00002 − . − .
56 0.67 0.15650 0.00003 0.19167 0.00001 − . − .
43 0.57 0.15774 0.00002 0.19201 0.00001 − . − .
22 0.87 0.15529 0.00005 0.19140 0.00002 − . − .
14 0.89 0.15507 0.00005 0.19134 0.00002 − . − .
08 0.73 0.15627 0.00003 0.19349 0.00001 − . − .
20 0.67 0.15632 0.00003 0.19225 0.00001 − . − .
52 0.92 0.15308 0.00005 0.19140 0.00002 − . − .
84 0.76 0.15537 0.00004 0.19123 0.00002 − . − .
01 0.77 0.15689 0.00004 0.19341 0.00002 − . − .
14 1.02 0.15571 0.00006 0.19374 0.00002 − . − .
75 0.63 0.15701 0.00003 0.19376 0.00001 − . − .
98 0.53 0.15806 0.00002 0.19356 0.00001 − . − .
72 0.78 0.15610 0.00004 0.19400 0.00002 − . − .
46 1.13 0.15248 0.00007 0.19473 0.00003 − . − .
13 0.60 0.15859 0.00003 0.19447 0.00001 − . − .
94 0.67 0.15782 0.00003 0.19385 0.00001 − . − .
67 0.75 0.15683 0.00004 0.19428 0.00002 − . − .
68 0.72 0.15837 0.00003 0.19497 0.00001 − . − .
98 0.98 0.15492 0.00006 0.19406 0.00002 − . − .
22 0.67 0.15829 0.00003 0.19547 0.00001 − . − .
56 0.66 0.15879 0.00003 0.19577 0.00001 − . − .
66 1.30 0.15002 0.00009 0.19524 0.00003 − . Article number, page 22 of 26. Sozzetti et al.: HD 5278 b and c unveiled by ESPRESSO . . . . . p r e [ m / s ] . . . . K b [ m / s ] . . . . P b [ d a y s ] +1.433e1 . . . . T b , c o n j [ d a y s ] +8.55164e3 . . . . e b . . . . . b [ r a d ] K c [ m / s ] P c [ d a y s ] T c , c o n j [ d a y s ] . . . e c . . . . . c [ r a d ] . . . . . ji t , p o s t [ m / s ] jit, pre [m/s] . . . . p o s t [ m / s ] . . . . . pre [m/s] . . . . K b [m/s] . . . . P b [days] . . . . T b , conj [days] .
08 0 .
16 0 .
24 0 . e b . . . . . b [rad] K c [m/s]
25 30 35 40 P c [days] T c , conj [days] . . . e c . . . . . c [rad] . . . . . jit, post [m/s] . . . . post [m/s] Fig. B.1.
Posterior distributions of the photometry-related system parameters in the joint ESPRESSO RV + TESS photometry analysis.Article number, page 23 of 26 & A proofs: manuscript no. HD5278_ESPRESSO . . . . . [ s o l a r un i t ] . . . . . o r b . i n c l . [ d e g ] . . . . ji t , T E SS , s e c t . . . . ji t , T E SS , s e c t . . . . ji t , T E SS , s e c t . . . . R b /R * . . . . ji t , T E SS , s e c t .
54 0 .
60 0 .
66 0 .
72 0 . [solar unit] . . . . . orb. incl. [deg] . . . . jit, TESS, sect 1 . . . . jit, TESS, sect 12 . . . . jit, TESS, sect 13 . . . . jit, TESS, sect 27 Fig. B.2.
Posterior distributions of the spectroscopy-related system parameters in the joint ESPRESSO RV + TESS photometry analysis.Article number, page 24 of 26. Sozzetti et al.: HD 5278 b and c unveiled by ESPRESSO
Fig. B.3.
Corner plot showing the results of the internal structure retrieval for HD 5278 b in the case of a fully di ff erentiated planet model. Thedi ff erent columns refer to, respectively, the core (iron + sulfur) mass fraction, the silicate mantle mass fraction, the water mass fraction, the molarfraction of Fe, Si and Mg in the mantle, the molar fraction of Fe in the core, the log of the gas mass (in Earth masses), the log of the atmosphericthickness (Earth radii), the total radius and mass. In the two last columns, the blue lines show the observed values. For each column, the mean ofthe distribution as well as the 5% and 95% quantile are indicated on the top, and the 5% and 95 % quantiles are also indicated with dashed verticallines. Article number, page 25 of 26 & A proofs: manuscript no. HD5278_ESPRESSO