Stellar Surface Magnetoconvection as a Source of Astrophysical Noise. III. Sun-as-a-Star Simulations and Optimal Noise Diagnostics

Abstract

Stellar surface magnetoconvection (granulation) creates asymmetries in the observed stellar absorption lines that can subsequently manifest themselves as spurious radial velocities shifts. In turn, this can then mask the Doppler-reflex motion induced by orbiting planets on their host stars, and represents a particular challenge for determining the masses of low-mass, long-period planets. Herein, we study this impact by creating Sun-as-a-star observations that encapsulate the granulation variability expected from 3D magnetohydrodynamic simulations. These Sun-as-a-star model observations are in good agreement with empirical observations of the Sun, but may underestimate the total variability relative to the quiet Sun due to the increased magnetic field strength in our models. We find numerous line profile characteristics linearly correlate with the disc-integrated convection-induced velocities. Removing the various correlations with the line bisector, equivalent width, and the V asy indicator may reduce ∼50-60% of the granulation noise in the measured velocities. We also find that simultaneous photometry may be a key diagnostic, as our proxy for photometric brightness also allowed us to remove ∼50% of the granulation-induced radial velocity noise. These correlations and granulation-noise mitigations breakdown in the presence of low instrumental resolution and/or increased stellar rotation, as both act to smooth the observed line profile asymmetries.

The Impact of Stellar Surface Magnetoconvection and Oscillations on the Detection of Temperate, Earth-Mass Planets Around Sun-Like Stars

“…In contrast to [82], the series by [35,[88][89][90] focuses on the impact of magnetoconvection alone; their hypothesis is that the convection-induced line profile asymmetries responsible for this noise source can also be used to diagnose and disentangle it from planetary signals. State-of-the-art three-dimensional magnetohydrodynamic (MHD) simulations form the backbone of the model Sun-as-a-star observations in [90]; similar to [66], the simulation box size is too small to permit supergranulation, as such the authors focus on the small-scale granulation. These 3D MHD simulations are coupled with 1D radiative transport to synthesis an absorption line profile for each granulation snapshot.…”

“…All profiles of a given category are binned together to create four time-average components for each limb angle; note, it is creating these time-averages that kills the oscillation signature. Then the probability distributions of the component filling factors can be used in conjunction with the four time-average component profiles to generate new line profiles, with the same fundamental convection characteristics as the computationally intensive 3D radiative MHD simulation [89,90].…”

“…Using this parameterization, Ref. [90] can create a stellar grid with each tile having an independent realization of granulation and integrate over the disc to mimic stellar observations. As such, Ref.…”

“…As such, Ref. [90] created 1000 Sun-as-a-star model observations, and searched for correlations between the line profile shape and the net RV. Each model observation was assumed to have been separated by at least one granulation turnover, such that each instance is independent.…”

“…For these reasons, Ref. [90] quote the fractional reduction in the RV rms as this should give an idea of how much telescope time could be saved by not needing to average as heavily and should scale with the true RV rms if the fundamental physics is correct. For example, a Figure 10.…”