Experimental Astronomy manuscript No. (will be inserted by the editor) Determination of stellar parameters for Ariel targets: a comparison analysis between different spectroscopic methods. Anna Brucalassi · Maria Tsantaki · Laura Magrini · Sergio Sousa · Camilla Danielski · Katia Biazzo · Giada Casali · Mathieu Van der Swaelmen · Monica Rainer · Vardan Adibekyan · Elisa Delgado-Mena · Nicoletta Sanna Received: date / Accepted: date Abstract Ariel has been selected as the next ESA M4 three different spectroscopic techniques used to deter- science mission and it is expected to be launched in mine stellar parameters for a selected number of targets 2028. During its 4-year mission, Ariel will observe the belonging to the Ariel reference sample. We aim to con- atmospheres of a large and diversified population of solidate a method that will be used to homogeneously transiting exoplanets. A key factor for the achievement determine the stellar parameters of the complete Ariel of the scientific goal of Ariel is the selection strategy reference sample. Homogeneous, accurate and precise for the definition of the input target list. A meaningful derivation of stellar parameters is crucial for character- choice of the targets requires an accurate knowledge of ising exoplanet-host stars and in turn is a key factor for the planet hosting star properties and this is necessary the accuracy of the planet properties. to be obtained well before the launch. In this work, we Keywords Exoplanet atmospheres · Space missions · present the results of a bench-marking analysis between Optical and IR Spectroscopy A. Brucalassi INAF-Osservatorio Astronomico Arcetri, 1 Introduction Largo Enrico Fermi 5, 50125 Firenze Tel.: +39-055-2752 243 Fax: +39-055-220039 So far, more than 4000 planets (with more than 3000 E-mail: [email protected] transiting their stars) have been detected showing an M. Tsantaki, L. Magrini, G. Casali, M. Van der Swaelmen, incredible diversity in terms of masses, sizes and orbits. M. Rainer, N. Sanna Moreover, thousands of Jupiter-size down to Earth-size INAF-Osservatorio Astronomico di Arcetri, planets are expected to be discovered in the next few Largo Enrico Fermi 5, 50125 Firenze, Italy years by Gaia [51,33], TESS [38], CHEOPS [11] and the upcoming space surveys, such as PLATO [37], along G. Casali Dipartimento di Fisica e Astronomia, Universit`adegli Studi with ground-based surveys, like WASP [35], NGTS [66], di Firenze, TRAPPIST [24], HARPS [29] and ESPRESSO [32], arXiv:2101.02242v1 [astro-ph.EP] 6 Jan 2021 via G. Sansone 1, 50019 Sesto Fiorentino (Firenze), Italy CARMENES[36] and SPIRoU[6]. The recent success of ground-based, space transit and radial velocity searches K. Biazzo has ushered exoplanet research into an era of character- INAF-Osservatorio Astronomico di Roma isation studies with the goal to investigate the nature, Via di Frascati 33, 00044 Monte Porzio Catone, Italy formation, and evolutionary history of the detected ob- jects. S. Sousa, V. Adibekyan, E. Delgado-Mena Instituto de Astrof´ısica e Ciˆencias do Espa¸co, Universidade At first, these studies have been focused on understand- do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal ing the internal structure of exoplanets. From the tran- C. Danielski sit light-curve, the planet radius can be measured and UCL Centre for Space Exochemistry Data from spectroscopic Doppler measurements, the planet Atlas Building, Fermi Avenue, Harwell Campus l Didcot l mass is obtained. From the bulk density we have the OX11 0QR (UK) first hints of the internal structure of the exoplanet 2 Anna Brucalassi et al. and the gas/ice/rock ratios. However, to have a reli- allowing to measure planetary atmospheric signals of able estimate of the density, an accurate knowledge of 10-100 ppm relative to the star. Such small signals re- both radius (with a precision of up to 5%) and plane- quire an exact knowledge of the host star spectrum, tary mass (up to 10%) is necessary [65,8]. Additionally, at least at the same level of the planetary signal, to the absolute value of planetary radius and mass relies map any stellar intrinsic variation (i.e. due to magnetic on the precise determination of the radius and mass activity and convective turbulence) in order to avoid of the exoplanet-host star. The derivation of these last misleading results with planetary features [39]. two values is in turn strongly connected to the effec- Another important point is that information on the tive temperature (Teff ), surface gravity (log g), and the host star composition is critical to separate the sig- metallicity of the star. Thus, the planetary properties natures left on the planet by its formation, evolution are critically dependent on their stellar host properties and migration processes, from those due to the spe- [57,50,2]. cific chemistry of the host star [63]. Indeed, recent stud- Furthermore, transiting planets provide us one of ies suggest that planetary O/H, C/H, C/O ratios and the best ways of characterising their atmospheres. In- metallicity with respect to the stellar values could pro- transit spectroscopy as well as secondary transit stud- vide stronger constraints on the planet formation region ies [12,54,44,20,62,68,34] using space observatories, as and their migration mechanisms (see [27] and references Hubble and Spitzer, and some ground-based observa- therein for a recent review), but similar considerations tories, have yielded the detection of some important apply to other elements (e.g., N, S, Ti, Al, [63]). molecules present in the planetary atmospheres for a Finally, an increasing number of studies have pointed limited number of targets, or have identified the pres- towards the existence of correlations between the prop- ence of clouds, probing the thermal structure and pro- erties of the host stars and the characteristics and fre- viding some constraints on the planet properties. How- quency of their planetary systems. In this respect, the ever, the data available is still too sparse to provide a correlation between the stellar metallicity and the fre- consistent interpretation and the achieved results point quency of giant planets [41,64,49], the connection be- out the main limitations of the existing facilities: very tween radius vs metallicity [13,43], eccentricity vs metal- narrow wavelength coverage, observations usually not licity [3,67], the role of the abundances of other ele- simultaneous for a wider spectral range with the intro- ments [4,16,17] in the host stars are only few examples duction of systematic noise, insufficient time allocated of different results that take a clear shape as the new to exoplanet science, and more in general the lack of planet discoveries increase, shading light on many de- a dedicated space-based exoplanet spectroscopy mis- tails still missing concerning planet formation and evo- sion. Thus, our current knowledge of exoplanetary at- lution. mospheric and thermal characteristics is still very lim- Such works rely upon homogeneously and precisely de- ited. rived stellar parameters. Therefore, homogeneous deriva- Ariel (Atmospheric Remote-sensing Infrared Exo- tion of stellar parameters using high-quality data is planet Large-survey) has been selected as the next ESA- crucial for characterising exoplanet-host stars [42,5,47], M4 science mission [55] and it is expected to be launched and in turn, is fundamental to improve the accuracy of in 2028. During its 4-year mission, Ariel will observe the planet properties. the atmospheres of a statistically representative sam- It is important to underline how a well-defined target ple (∼1000) of transiting gaseous (Jupiters, Saturns, selection strategy and the definition of the input target Neptunes) and rocky (super-Earths and sub-Neptunes) list with a statistically significant dataset in the range of planets using transit spectroscopy in the 1.10-7.8µm relevant stellar/planet parameters have a fundamental spectral range and three narrow-bands photometry in role for maximising the scientific yield and the achieve- the optical. The wavelength range proposed covers all ment of scientific goals of Ariel. A meaningful choice the expected major atmospheric gases from H2O, CO2, of the targets requires an accurate study of the stel- CH4, NH3, HCN, H2S up to the more exotic metallic lar properties that need to be derived in advance and compounds, such as TiO, VO, and condensed species. continuously updated as the mission approaches launch Ariel is designed as a dedicated survey mission for tran- and the target list evolves with the new exoplanet dis- sit, eclipse and phase-curved spectroscopy, providing a coveries. homogeneous dataset, with a consistent pipeline and In this context, we have started a benchmarking anal- an well-defined target selection strategy, maximising ysis between three different spectroscopic techniques the scientific yield. Focusing on transit, eclipse spec- used to determine stellar parameters for selected tar- troscopy, the methods are based on the differential anal- gets belonging to the Ariel Reference Sample [19]. Our ysis of the star and planet spectra in and out of transit, goal is to consolidate a method that will be applied to Title Suppressed Due to Excessive Length 3 homogeneously determine the stellar parameters for the complete Ariel Reference Sample. More generally, we refer also to the works of [26] and [45] where compar- 50 isons between the results from different spectroscopic 40 techniques are discussed. For a global approach to characterize the stars of 30 Ariel Reference Sample see also [15], where an overview on the methods used to determine stellar fundamental 20 Star Sample parameters, elemental abundances, activity indices, and stellar ages for the Ariel Reference Sample is given and 10 in particular, results for the homogeneous estimation 0 of elemental abundances of Al, Mg, Si, C, N, and the 4 6 8 10 12 14 16 activity indices S and log(R’ HK) are presented.