Toward ground-based detection of exoEarths. Spectral microvariability of exoplanet host stars

The radial-velocity method is a plausible tool toward ground-based detection of Earth-like planets around solar-type stars. Instrumental precisions are approaching the required level but limitations remain in understanding the complexities of stellar atmospheric dynamics that modulate spectral-line displacements. Solar-type stars are covered by a million granules, each with velocities on order 2 km/s, producing full-disk jittering of the apparent radial velocity of perhaps 2 m/s. From 3D hydrodynamic modeling, time-variable stellar spectra can now be computed, showing correlated variability in different spectral-line parameters (Dravins et al., 2021ab). Blueshifted line components from hot granules in rising motion carry greater photon fluxes while steeper atmospheric gradients enhance local line-strengths. Dependences differ between high-excitation lines from ionized species and those from easily dissociated molecules. Observational calibration is enabled by the recent release of HARPS-N spectra of the Sun seen-as-a-star, where lines with specific astrophysical parameters can be examined for how their fluctuations correlate with excursions in radial velocity. Their calibration might allow to adjust low-noise observations of bright stars to attain their very accurate velocities. Dravins, Ludwig & Freytag: A&A 649, A16 (2021a); A&A 649, A17 (2021b