Key science questions

The following have been identified as key science questions for the Targets and their Environments theme:

1. What are the physical properties (including mass and age) of the target stars? This requires the determination of the fundamental stellar properties such as the mass, radius, chemical composition, and age. The possible techniques to obtain this information are the use of accurate calibrations of mass and radius, stellar evolution models and asteroseismology.
2. What are the radiative properties (light and particles, T_eff, L_bol of the target stars? The overall radiative properties are the effective temperature and the luminosity, although a detailed spectral energy distribution covering from high (X-rays) to low energies (IR, radio) is essential. Spectroscopy is required to characterize the the radiation field. Particle emissions in the form of stellar wind are also a requirement to fully describe the stellar radiation. Related to this, it is important to characterize the magnetic properties of the star (including flares and mass ejections) since they are basic to understand star-planet interactions.
3. What is the time-variation of such emissions ? The stellar emissions need to be characterized also in their time variation. This is especially relevant to the high-energy and particle components. The characterization must include all timescales, including those of minutes and hours (microvariability, flares), days (spot modulations), years (spot cycles), centuries (Maunder-like minima) and Gyr (rotational spin down). The targets themselves can be used to investigate the short timescales, but the long-term changes can only be determined using stellar ensembles or stellar proxies to reconstruct the overall history.
4. What are the characteristics of the stellar immediate surroundings (i.e., zodiacal dust, companion stars, brown dwarfs or giant planets)? The properties of the stellar surroundings include the zodiacal dust (parameterized as the surface brightness vs. wavelength) and the multiplicity. The latter comprises stellar and brown dwarf companions, but also the presence of Jupiter-size planets, which may influence the presence of telluric planets.
5. What are the stellar properties (mass, chemical composition) influencing the existence of telluric planets? This question boils down to determining eta_Earth(M,Z). That is, the fraction of stars with telluric planets as a function of stellar mass and chemical composition. This question is essential to decide what stellar types are the optimum targets for detailed searches. The correlation with parameters such as multiplicity and age must also be investigated. Additionally, putting our own Sun in context is highly relevant. Questions such as the comparison between the Sun and other solar-like stars or the existence of chemical abundance patterns may provide important clues on the process of planet formation. In the latter case, it is important to compare the abundances of biogenic elements (C,N,O,P,S) in the Sun to those measured in other solar like stars.
6. What is the census of telluric planets in the solar neighborhood? A survey of the solar neighborhood should be conducted to uncover the presence of telluric planets and especially those in the habitable zones of their parent stars.
7. What are the properties of stars and their circumstellar environments (disks, outflows) in which planets are actually forming? What are the earliest stages when this happens, and what environments are conducive to planet formation? A survey of disk-surrounded classical T Tauri stars and disk-less weak-lined T Tauri stars in the nearest star-forming regions should be conducted. The presence of planets may be evidenced by disk properties (gaps, etc), but radial velocity measurements may be more reliable.