Stellar surface information from the Ca ii H&K lines – I. Intensity profiles of the solar activity components

Abstract

The detection of Earth-like planets with the radial-velocity (RV) method is currently limited by the presence of stellar activity signatures. On rotational time-scales, spots and plages (or faculae) are known to introduce different RV signals, but their corrections require better activity proxies. The best-known chromospheric activity proxies in the visible are the Ca ii H&K lines, but the physical quantities measured by their profiles need to be clarified. We first investigate resolved images of the Sun in order to better understand the spectrum of plages, spots, and the network using the Meudon spectroheliogram. We show that distinct line profiles are produced by plages, spots, and by the network component and we also derived the centre-to-limb variations of the three profiles. Some care is required to disentangle their contributions due to their similarities. By combining disc-integrated spectra from the ISS high-resolution spectrograph with SDO direct images of the Sun, we managed to extract a high-resolution emission spectrum of the different components, which tend to confirm the spectra extracted from the Meudon spectroheliogram datacubes. Similar results were obtained with the HARPS-N Sun-as-a-star spectra. We concluded using a three-component model that the temporal variation of the popular Sindex contains, on average for the 24th solar cycle: $70\pm 12{{\ \rm per\ cent}}$ of plage, $26\pm 12{{\ \rm per\ cent}}$ of network, and $4\pm 4 {{\ \rm per\ cent}}$ of spots. This preliminary investigation suggests that a detailed study of the Ca ii H&K profiles may provide rich information about the filling factor and distribution of different types of active regions.

Correlations between Ca ii H and K Emission and the Gaia M Dwarf Gap

“…Further, various authors have shown that the strength of calcium II H and K emission may evolve over month to year timescales (e.g., Rauscher & Marcy 2006;Perdelwitz et al 2021;Cretignier et al 2024). Targets from Boudreaux et al (2022) were observed an average of only four times and over yearlong timescales.…”

GRASS. II. Simulations of Potential Granulation Noise Mitigation Methods

“…That different lines exhibit different degrees of variability is unsurprising; past works have shown that measured RVs change with the sets of lines used to measure velocities (e.g., Meunier et al 2017;Al Moulla et al 2022. Lines that trace global activity have been the focus of many studies, and good progress has been made toward understanding the physical mechanisms underpinning their ability to trace this activity (e.g., Vitas et al 2009;Wise et al 2022;Cretignier et al 2024).…”