Bisectors of the HARPS Cross-Correlation-Function. The dependence on stellar atmospheric parameters

TL;DR

This study investigates how HARPS CCF bisectors vary with stellar parameters like gravity and temperature, aiming to improve the discrimination of planetary signals from stellar activity in radial velocity measurements.

Contribution

It introduces a new bisector measure, CBS, and analyzes its relation to stellar parameters, enhancing understanding of stellar atmosphere effects on CCF bisectors.

Findings

Bisector measures correlate with log g and T_eff in complex ways.

The new CBS measure provides additional diagnostic power.

Results align with 3D stellar atmosphere models for the Sun.

Abstract

Bisectors of the HARPS cross-correlation function (CCF) can discern between planetary radial-velocity (RV) signals and spurious RV signals from stellar magnetic activity variations. However, little is known about the effects of the stellar atmosphere on CCF bisectors or how these effects vary with spectral type and luminosity class. Here we investigate the variations in the shapes of HARPS CCF bisectors across the HR diagram in order to relate these to the basic stellar parameters, surface gravity and temperature. We use archive spectra of 67 well studied stars observed with HARPS and extract mean CCF bisectors. We derive previously defined bisector measures (BIS, v_bot, c_b) and we define and derive a new measure called the CCF Bisector Span (CBS) from the minimum radius of curvature on direct fits to the CCF bisector. We show that the bisector measures correlate differently, and non-linearly with log g and T_eff. The resulting correlations allow for the estimation of log g and T_eff from the bisector measures. We compare our results with 3D stellar atmosphere models and show that we can reproduce the shape of the CCF bisector for the Sun.