Solar photospheric spectrum microvariability

Abstract

Context. Extreme precision radial-velocity spectrometers enable extreme precision in stellar spectroscopy. Searches for low-mass exoplanets around solar-type stars are limited by various types of physical variability in stellar spectra, such as the short-term jittering of apparent radial velocities on levels of ∼2 m s−1. Aims. To understand the physical origins of radial-velocity jittering, the solar spectrum is assembled, as far as possible, from basic principles. Solar surface convection is modeled with time-dependent 3D hydrodynamics, followed by the computation of high-resolution spectra during numerous instances of the simulation sequence. The behavior of different classes of photospheric spectral lines is monitored throughout the simulations to identify commonalities or differences between different classes of lines: weak or strong, neutral or ionized, high or low excitation, atomic or molecular. Methods. Synthetic spectra were examined. With a wavelength sampling λ/Δλ ∼ 1 000 000, the changing shapes and wavelength shifts of unblended and representative Fe I and Fe II lines were followed during the simulation sequences. The radial-velocity jittering over the small simulation area typically amounts to ±150 m s−1, scaling to ∼2 m s−1 for the full solar disk. Flickering within the G-band region and in photometric indices of the Strömgren uvby system were also measured, and synthetic G-band spectra from magnetic regions are discussed. Results. Most photospheric lines vary in phase, but with different amplitudes among different classes of lines. Amplitudes of radial-velocity excursions are greater for stronger and for ionized lines, decreasing at longer wavelengths. Matching precisely measured radial velocities to such characteristic patterns should enable us to remove a significant component of the stellar noise originating in granulation. Conclusions. The granulation-induced amplitudes in full-disk sunlight amount to ∼2 m s−1; the differences between various line groups are an order of magnitude less. To mitigate this jittering, a matched filter must recognize dissimilar lineshifts among classes of diverse spectral lines with a precision of ∼10 cm s−1 for each line group. To verify the modeling toward the filter, predictions of center-to-limb dependences of jittering amplitudes for different classes of lines are presented, testable with spatially resolving solar telescopes connected to existing radial-velocity instruments.

GRASS. II. Simulations of Potential Granulation Noise Mitigation Methods

“…Of the existing suite of high-resolution RV spectrographs, it is most plausible that ESPRESSO (in UHR mode) or PEPSI could resolve the submeters per second stellar line-shape variations that characterize granulation-driven jitter, assuming an adequate number of similarly varying lines can be identified and binned. As discussed in Section 4.3.2, a recent work based on HD simulations (Dravins & Ludwig 2023) demonstrated that subsets of lines do indeed vary in phase, motivating our claim that variability in granulation signals would need to be coherently binned across lines. Line lists derived from or informed by such HD simulations could provide a valuable starting point for observational studies of convective variability in other stars.…”

“…Some initial work (Dravins et al 2017a(Dravins et al , 2017b has already successfully shown that "similar" Fe lines can be binned in order to enable retrievals of time-averaged spatially resolved stellar line profiles during planet transits. Additionally, HD simulations performed by Dravins & Ludwig (2023) show that different absorption lines fluctuate in phase (with some dependence on line strength and spectral region), suggesting that it is indeed plausible that convective variability can be coherently binned across lines.…”

“…In comparison to studies of global activity, the magnetoconvective variability of individual lines has been somewhat understudied. Studies utilizing HD and MHD codes (e.g., Dravins et al 2017a;Cegla et al 2019;Dravins & Ludwig 2023) have made important contributions to our understanding of this "microvariability" (borrowing the language of Dravins & Ludwig 2023), but are limited by computational costs and our current lack of disk-resolved line profiles for other stars to use as a basis for validation. A growing number of works have cataloged and investigated the absolute convective blueshift of lines both in the Sun and in other stars.…”