Determination of stellar parameters for Ariel targets: a comparison analysis between different spectroscopic methods

Experimental Astronomy https://doi.org/10.1007/s10686-020-09695-4 ORIGINAL ARTICLE Determination of stellar parameters for Ariel targets: a comparison analysis between different spectroscopic methods Anna Brucalassi1 · Maria Tsantaki1 · Laura Magrini1 · Sergio Sousa2 · Camilla Danielski3 · Katia Biazzo4 · Giada Casali1,5 · Mathieu Van der Swaelmen1 · Monica Rainer1 · Vardan Adibekyan2 · Elisa Delgado-Mena2 · Nicoletta Sanna1 Received: 30 June 2020 / Accepted: 15 December 2020 / © The Author(s) 2021 Abstract Ariel has been selected as the next ESA M4 science mission and it is expected to be launched in 2028. During its 4-year mission, Ariel will observe the atmospheres of a large and diversified population of transiting exoplanets. A key factor for the achievement of the scientific goal of Ariel is the selection strategy for the definition of the input target list. A meaningful choice of the targets requires an accurate knowledge of the planet hosting star properties and this is necessary to be obtained well before the launch. In this work, we present the results of a bench-marking analysis between three different spectroscopic techniques used to determine stellar parameters for a selected number of targets belonging to the Ariel reference sample. We aim to consolidate a method that will be used to homogeneously determine the stellar parameters of the complete Ariel reference sample. Homogeneous, accurate and precise derivation of stellar parameters is crucial for characterising exoplanet-host stars and in turn is a key factor for the accuracy of the planet properties. Keywords Exoplanet atmospheres · Space missions · Optical and IR spectroscopy 1 Introduction So far, more than 4000 planets (with more than 3000 transiting their stars) have been detected showing an incredible diversity in terms of masses, sizes and orbits. Moreover, thousands of Jupiter-size down to Earth-size planets are expected to be  Anna Brucalassi Extended author information available on the last page of the article. Experimental Astronomy discovered in the next few years by Gaia [33, 51], TESS [38], CHEOPS [11] and the upcoming space surveys, such as PLATO [37], along with ground-based surveys, like WASP [35], NGTS [66], TRAPPIST [24], HARPS [29] and ESPRESSO [32], CARMENES [36] and SPIRoU [6]. The recent success of ground-based, space transit and radial velocity searches has ushered exoplanet research into an era of characterisation studies with the goal to investigate the nature, formation, and evolutionary history of the detected objects. At first, these studies have been focused on understanding the internal structure of exoplanets. From the transit light-curve, the planet radius can be measured and from spectroscopic Doppler measurements, the planet mass is obtained. From the bulk density we have the first hints of the internal structure of the exoplanet and the gas/ice/rock ratios. However, to have a reliable estimate of the density, an accurate knowledge of both radius (with a precision of up to 5%) and planetary mass (up to 10%) is necessary [8, 65]. Additionally, the absolute value of planetary radius and mass relies on the precise determination of the radius and mass of the exoplanethost star. The derivation of these last two values is in turn strongly connected to the effective temperature (Teff ), surface gravity (log g), and the metallicity of the star. Thus, the planetary properties are critically dependent on their stellar host properties [2, 50, 57]. Furthermore, transiting planets provide us one of the best ways of characterising their atmospheres. In-transit spectroscopy as well as secondary transit studies [12, 20, 34, 44, 54, 62, 68] using space observatories, as Hubble and Spitzer, and some ground-based observatories, have yielded the detection of some important molecules present in the planetary atmospheres for a limited number of targets, or have identified the presence of clouds, probing the thermal structure and providing some constraints on the planet properties. However, the data available is still too sparse to provide a consistent interpretation and the achieved results point out the main limitations of the existing facilities: very narrow wavelength coverage, observations usually not simultaneous for a wider spectral range with the introduction of systematic noise, insufficient time allocated to exoplanet science, and more in general the lack of a dedicated space-based exoplanet spectroscopy mission. Thus, our current knowledge of exoplanetary atmospheric and thermal characteristics is still very limited. Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) has been selected as the next ESA-M4 science mission [55] and it is expected to be launched in 2028. During its 4-year mission, Ariel will observe the atmospheres of a statistically representative sample (∼1000) of transiting gaseous (Jupiters, Saturns, Neptunes) and rocky (super-Earths and sub-Neptunes) planets using transit spectroscopy in the 1.10-7.8μm spectral range and three narrow-bands photometry in the optical. The wavelength range proposed covers all the expected major atmospheric gases from H2 O, CO2 , CH4 , NH3 , HCN, H2 S up to the more exotic metallic compounds, such as TiO, VO, and condensed species. Ariel is designed as a dedicated survey mission for transit, eclipse and phase-curved spectroscopy, providing a homogeneous dataset, with a consistent pipeline and an well-defined target selection strategy, maximising the scientific yield. Focusing on transit, eclipse spectroscopy, the methods are based on the differential analysis of the star and planet spectra in and out of transit, allowing Experimental Astronomy to measure planetary atmospheric signals of 10-100 ppm relative to the star. Such small signals require an exact knowledge of the host star spectrum, at least at the same level of the planetary signal, to map any stellar intrinsic variation (i.e. due to magnetic activity and convective turbulence) in order to avoid misleading results with planetary features [39]. Another important point is that information on the host star composition is critical to separate the signatures left on the planet by its formation, evolution and migration processes, from those due to the specific chemistry of the host star [63]. Indeed, recent studies suggest that planetary O/H, C/H, C/O ratios and metallicity with respect to the stellar values could provide stronger constraints on the planet formation region and their migration mechanisms (see [27] and references therein for a recent review), but similar considerations apply to other elements (e.g., N, S, Ti, Al, [63]). Finally, an increasing number of studies have pointed towards the existence of correlations between the properties of the host stars and the characteristics and frequency of their planetary systems. In this respect, the correlation between the stellar metallicity and the frequency of giant planets [41, 49, 64], the conne (...truncated)