Power spectra of solar brightness variations at various inclinations

N.-E Nèmec,N A Krivova,R H Cameron,S Dreizler,R V Tagirov,S K Solanki,A I Shapiro

doi:10.1051/0004-6361/202037588

Abstract

Context. Magnetic features on the surfaces of cool stars lead to variations in their brightness. Such variations on the surface of the Sun have been studied extensively. Recent planet-hunting space telescopes have made it possible to measure brightness variations in hundred thousands of other stars. The new data may undermine the validity of setting the sun as a typical example of a variable star. Putting solar variability into the stellar context suffers, however, from a bias resulting from solar observations being carried out from its near-equatorial plane, whereas stars are generally observed at all possible inclinations. Aims. We model solar brightness variations at timescales from days to years as they would be observed at different inclinations. In particular, we consider the effect of the inclination on the power spectrum of solar brightness variations. The variations are calculated in several passbands that are routinely used for stellar measurements. Methods. We employ the surface flux transport model to simulate the time-dependent spatial distribution of magnetic features on both the near and far sides of the Sun. This distribution is then used to calculate solar brightness variations following the Spectral And Total Irradiance REconstruction approach. Results. We have quantified the effect of the inclination on solar brightness variability at timescales down to a single day. Thus, our results allow for solar brightness records to be made directly comparable to those obtained by planet-hunting space telescopes. Furthermore, we decompose solar brightness variations into components originating from the solar rotation and from the evolution of magnetic features.

Highlights

Recent planet-hunting missions such as CNES’ Convection, Rotation and planetary Transit (CoRoT, Baglin et al 2006; Bordé et al 2003), NASA’s Kepler (Borucki et al 2010), and the Transiting Exoplanet Survey Satellite (TESS, Ricker et al 2014) have opened up new possibilities for studying stellar variability up to timescales of the rotational period and, in some cases, even beyond (Reinhold et al 2017; Montet et al 2017)
We built our method based on the SATIRE model, in which brightness variations on timescales longer than a day are attributed to the emergence and evolution of magnetic field on the surface of the Sun, as well as on solar rotation (Fligge et al 2000; Krivova et al 2003)
We found our best set of parameters by comparing the power spectra obtained with the output of our model to those obtained with the PhysikalischMeterologisches Observatorium Davos (PMOD) composite

Summary

Introduction

Recent planet-hunting missions such as CNES’ Convection, Rotation and planetary Transit (CoRoT, Baglin et al 2006; Bordé et al 2003), NASA’s Kepler (Borucki et al 2010), and the Transiting Exoplanet Survey Satellite (TESS, Ricker et al 2014) have opened up new possibilities for studying stellar variability up to timescales of the rotational period and, in some cases, even beyond (Reinhold et al 2017; Montet et al 2017). One of the possible approaches for such an approach to modelling is to rely on the solar paradigm; that is, to take a model which reproduces the observed variability of solar brightness and extend it to other stars Such an approach has been used by Witzke et al (2018), who extended the Spectral And Total Irradiance REconstruction (SATIRE, Fligge et al 2000; Krivova et al 2003) model of solar brightness variability to calculate brightness variations over the timescale of the activity cycle in stars with different metallicities and effective temperatures. Witzke et al (2020) utilised a similar model to investigate how the amplitude of the rotational stellar brightness variability as well as the detectability of stellar rotation periods depend on the metallicity. We perform one more extension of the SATIRE model to study how the amplitude of solar brightness variability depends on the angle between solar rotation axis and directions to the observer (hereafter, the inclination)

Objectives

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Conclusion