Stellar Surface Magneto-convection as a Source of Astrophysical Noise. II. Center-to-limb Parameterization of Absorption Line Profiles and Comparison to Observations

Abstract

Manifestations of stellar activity (such as star-spots, plage/faculae, and convective flows) are well known to induce spectroscopic signals often referred to as astrophysical noise by exoplanet hunters. For example, setting an ultimate goal of detecting true Earth-analogs demands reaching radial velocity (RV) precisions of ∼9 cm s −1 . While this is becoming technically feasible with the latest generation of highly stabilised spectrographs, it is astrophysical noise that sets the true fundamental barrier on attainable RV precisions. In this paper we parameterise the impact of solar surface magneto-convection on absorption line profiles, and extend the analysis from the solar disc centre (Paper I) to the solar limb. Off disc-centre, the plasma flows orthogonal to the granule tops begin to lie along the lineof-sight and those parallel to the granule tops are no longer completely aligned with the observer. Moreover, the granulation is corrugated and the granules can block other granules, as well as the intergranular lane components. Overall, the visible plasma flows and geometry of the corrugated surface significantly impact the resultant line profiles and induce centre-to-limb variations in shape and net position. We detail these herein, and compare to various solar observations. We find our granulation parameterisation can recreate realistic line profiles and induced radial velocity shifts, across the stellar disc, indicative of both those found in computationally heavy radiative 3D magnetohydrodynamical simulations and empirical solar observations.

Convective blueshifts in the solar atmosphere

“…In Fig. 21, we compare the observed line shifts with theoretical syntheses for Fe i 6301.5 Å (de la Cruz Rodríguez et al 2011) and Fe i 6302.5 Å (Cegla et al 2018). The synthesized line shifts agree with the observations within a margin of around ±100 m s −1 .…”

“…However, it fails to reproduce the slope of the center-to-limb variation. In case of Fe i 6302.5 Å, the synthesis of Cegla et al (2018) manages to largely reproduce the slope of the observed center-to-limb variation. In anticipation of Sect.…”

“…From disk center toward the solar limb, the convective shifts describe a systematic variation of decreasing blueshifts, or increasing redshifts. Model syntheses by de la Cruz Rodríguez et al (2011) and Cegla et al (2018) yield deviations by less than 100 m s −1 , but need refinement to better reproduce the observed slope of the center-to-limb variation. We found the center-tolimb variation to be highly dependent on the line depth and the linked atmospheric height.…”

“…21. By lowering Lower 5%: R=700,000 R=250,000 R=180,000 R=100,000 Synthesis (Cegla et al 2018) Lower 91%: R=700,000 R=250,000 R=180,000 R=100,000 Synthesis (Cegla et al 2018) olution from 700 000 to 100 000, the convective blueshift of the line core increases by more than 100 m s −1 at disk center for both spectral lines. Toward the solar limb, the difference diminishes monotonically to around 50 m s −1 .…”

“…In addition, we included the 5250 Å and 6173 Å for the calibration of Dopplergrams from IMAX (Imaging Magnetograph eXperiment, Martínez Pillet et al 2011), CRISP (CRisp Imaging SpectroPolarimeter, Scharmer 2006) and HMI. Further, our line selection enabled a direct comparison with theoretical syntheses of the solar convective blueshift by de la Cruz Rodríguez et al (2011) and Cegla et al (2018). With a spectral sampling of less than 3 mÅ pixel −1 on a 2048 pixel wide detector, each spectral region covered a range of around 5.6 Å.…”