Solar Photospheric Spectrum Microvariability I. Theoretical searches for proxies of radial-velocity jittering

TL;DR

This study uses 3D hydrodynamic models of the solar spectrum to analyze the origins of radial-velocity jittering caused by stellar surface convection, aiming to improve exoplanet detection precision.

Contribution

It provides a detailed theoretical analysis of spectral line variability and proposes methods to identify and mitigate stellar noise in radial velocity measurements.

Findings

Radial-velocity jittering reaches ±150 m/s on the Sun's surface, scaling to about 2 m/s for the full disk.

Different classes of spectral lines exhibit varying jitter amplitudes and phases.

Matching jittering patterns across line groups can help remove stellar noise from radial velocity data.

Abstract

Extreme precision radial-velocity spectrometers enable extreme precision stellar spectroscopy. Searches for low-mass exoplanets around solar-type stars are limited by the physical variability in stellar spectra, such as the short-term jittering of apparent radial velocities. To understand the physical origins of such jittering, the solar spectrum is assembled, as far as possible, from basic principles. Surface convection is modeled with time-dependent 3D hydrodynamics, followed by the computation of hyper-high resolution spectra during numerous instances of the simulation sequences. The behavior of different classes of photospheric absorption lines is monitored to identify commonalities or differences between different classes of lines: weak or strong, neutral or ionized, high- or low-excitation, atomic or molecular. For Fe I and Fe II lines, the radial-velocity jittering over the small simulation area typically amounts to +-150 m/s, scaling to about 2 m/s for the full solar disk. Most photospheric lines vary in phase but with different amplitudes among different classes of lines. Radial-velocity excursions are greater for stronger and for ionized lines, decreasing at longer wavelengths. The differences between various line-groups are about one order of magnitude less than the full jittering amplitudes. By matching very precisely measured radial velocities to the characteristic jittering patterns between different line-groups should enable to identify and to remove a significant component of the stellar noise originating in granulation. To verify the modeling toward such a filter, predictions of solar center-to-limb dependences of jittering amplitudes are presented for different classes of lines, testable with spatially resolving solar telescopes connected to existing radial-velocity instruments.