Measuring precise radial velocities on individual spectral lines. III. Dependence of stellar activity signal on line formation temperature

TL;DR

This study explores how stellar activity signals in radial velocity measurements vary with the formation temperature of spectral lines, revealing that convective suppression impacts RV signals differently across temperature ranges, aiding in precise exoplanet detection.

Contribution

It introduces a method to analyze RV signals based on spectral line formation temperatures, enhancing understanding of stellar activity effects on RV measurements.

Findings

Stellar activity signals vary with spectral line formation temperature.

Convective suppression is the main contributor to activity-induced RV variations.

Different temperature ranges show opposite RV variations and correlations.

Abstract

Context. To enable radial velocity (RV) precision on the order of ~0.1 m/s required for the detection of Earth-like exoplanets orbiting solar-type stars, the main obstacle lies in mitigating the impact of stellar activity. Aims. This study investigates the dependence of derived RVs with respect to the formation temperature of spectral line segments. Methods. Using spectral synthesis, we compute for each observed wavelength point of unblended spectral lines the stellar temperature below which 50% of the emergent flux originates. We can then construct RV time series for different temperature ranges, using template matching. Results. With HARPS-N solar data and HARPS $\alpha$ Cen B measurements, we demonstrate on time intervals of prominent stellar activity that the activity-induced RV signal has different amplitude and periodicity depending on the temperature range considered. We compare the solar measurements with simulated contributions from active surface regions seen in simultaneous images, and find that the suppression of convective motion is the dominant effect. Conclusions. From a carefully selected set of spectral lines, we are able to measure the RV impact of stellar activity at various stellar temperatures ranges. We are able to strongly correlate the effect of convective suppression with spectral line segments formed in hotter temperature ranges. At cooler temperatures, the derived RVs exhibit oppositely directed variations compared to the average RV time series and stronger anti-correlations with chromospheric emission.