Stellar surface information from the Ca II H&K lines -- II. Defining better activity proxies

TL;DR

This paper develops improved stellar activity proxies from Ca II H&K lines using principal component analysis, enhancing radial velocity correction methods across various spectral types.

Contribution

It introduces a novel application of PCA to Ca II H&K spectra, providing more effective activity proxies than classical indicators for stellar activity analysis.

Findings

PCA disentangles different sources in Ca II H&K spectra.

Second PCA component effectively corrects RVs for stellar activity.

First principal component resembles the classical S-index.

Abstract

In our former paper I, we showed on the Sun that different active regions possess unique intensity profiles on the Ca II H & K lines. We now extend the analysis by showing how those properties can be used on real stellar observations, delivering more powerful activity proxies for radial velocity correction. More information can be extracted on rotational timescale from the Ca II H & K lines than the classical indicators: S-index and log(R'HK). For high-resolution HARPS observations of alpha Cen B, we apply a principal and independent component analysis on the Ca II H & K spectra time-series to disentangle the different sources that contribute to the disk-integrated line profiles. While the first component can be understood as a denoised version of the Mount-Wilson S-index, the second component appears as powerful activity proxies to correct the RVs induced by the inhibition of the convective blueshift in stellar active regions. However, we failed to interpret the extracted component into a physical framework. We conclude that a more complex kernel or bandpass than the classical triangular of the Mount Wilson convention should be used to extract activity proxies. To this regard, we provide the first principal component activity profile obtained across the spectral type sequence between M1V to F9V type stars.