Properties of stars hosting exoplanets

(A. Brucalassi, G. Casali, L. Magrini, M. Rainer, G. Sacco, N. Sanna, M. Tsantaki)

The recent success of ground-based and space transit searches (more than 4000 planets have been detected showing an incredible diversity- http://exoplanet.eu/ ) has ushered exoplanet research into an era of characterization studies with the goal to investigate the nature, formation, and evolutionary history of the detected objects. Transiting planets provide us one of the best ways for characterising their atmospheres. Our research group in Arcetri is part of the Ariel project.

Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, has been selected as the fourth medium-class mission in the ESA Cosmic Vision programme. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets. Key factor for the achievement of the scientific goal of Ariel is the selection strategy for the definition of the input target list, that is based on the accurate knowledge of the stellar properties.

Another important point is that information on the host star composition could provide stronger constraints on the planet formation region and their migration mechanisms (Madhusudhan et al. 2016, Turrini et al. 2018).

Finally, an increasing number of studies have pointed towards the existence of correlations between the properties and frequency of the newfound planets and those of their host stars.

In this context, the correlation between the stellar metallicity and the frequency of giant planets (Santos et al, 2004; Valenti & Fischer 2005), the connection between radius vs. metallicity (Buchhave et al., 2014; Schlaufman 2015), eccentricity vs. metallicity (Adibekyan et al, 2013), the role of the abundances of other elements in the host stars are only few examples of different results that are taking a clear shape as the new planet discoveries increase, shading light on many details still missing concerning planet formation and evolution. Such works rely upon homogeneously and precisely derived stellar parameters.

Our research group is focused on the derivation of stellar parameters for the Ariel Reference Target Sample. In addition, it is involved with other working groups in the calculation of stellar abundances of the same sample. So far, we have conducted a benchmarking analysis between three different spectroscopic techniques (SweetCat (Santos et al. 2013) - FAMA (Magrini et al. 2016) - FASMA (Tsantaki et al. 2018)) used to determine stellar parameters for a number of selected targets belonging to the Ariel reference sample. Moreover, external comparison to evaluate the results of the different methods used (Brucalassi et al. 2020, in prep.) have been produced (see Figure 1).

A well-defined target selection strategy and the definition of the input target list have a fundamental role for maximising the scientific yield and the achievement of scientific goals of Ariel. This requires an accurate study of the stellar properties that need to be derived in advance and continuously updated as the mission approaches the launch and the target list evolves with the new exoplanet discoveries.

Figure 1: (From Brucalassi et al. 2020): Comparison of stellar parameters derived by different methods: Sweet-Cat vs FAMA (blue) and FASMA (red) for effective temperature (Teff), surface gravity (log g), metallicity [Fe/H]). The horizontal line is the zero point level. In the parameter space close to Solar values the agreement among the three methods is good. However, at low and high temperatures some methods tend to under/overestimate the surface gravity log g. Right bottom panel: Kiel diagram (log g vs Teff) using values listed in Sweet-Cat, obtained by FAMA, FASMA and derived by Gaia. Over-plotted PARSEC isochrones. The outermost tracks correspond to log(age/yr )= 8.95 (bottom track) and 10.15 (upper track) with [M/H]= 0.058, Z= 0.0198, Y=0.273.

References:

Adibekyan, V.Z., Figueira, P., Santos, N.C. et al. A&A, 560, A51 (2013).

Brucalassi, A., Tsantaki, M., Magrini, L. et al. Experimental Astronomy (2020), in prep.

Magrini, L., Randich, S., Friel, E., Spina, L. et al. A&A, 558, A38 (2013).

Madhusudhan, N., Agundez, M., Moses, J.I., Hu, Y. SSR 205 (1-4), 285–348 (2016).

Santos, N.C., Sousa, S.G., Mortier, A. et al. A&A, 556, A150 (2013).

Schlaufman, K.C. ApJ 799 (2), L26 (2015).

Tinetti, G., Drossart, P., Eccleston, P. et al. Experimental Astronomy 46 (1), 135–209 (2018).

Tsantaki, M., Andreasen, D.T., Teixeira, G.D.C. et al. MNRAS 473(4), 5066–5097 (2018).

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