On the reliability of measuring differential rotation of spotted stars

TL;DR

This paper evaluates the reliability of the ACCORD method, which uses averaged cross-correlations of Doppler images, for measuring surface differential rotation on spotted stars, addressing issues caused by spot distribution and configuration changes.

Contribution

The paper introduces and demonstrates the effectiveness of the ACCORD technique for more reliable differential rotation measurements in Doppler imaging.

Findings

ACCORD improves DR measurement reliability over single cross-correlation.

Spot distribution can distort shear measurements if not properly accounted for.

ACCORD is validated on artificial data showing consistent results.

Abstract

Cross-correlation of consecutive Doppler images is one of the most common techniques used to detect surface differential rotation (hereafter DR) on spotted stars. The disadvantage of a single cross-correlation is, however, that the expected DR pattern can be overwhelmed by sudden changes in the apparent spot configuration. Another way to reconstruct the image shear using Doppler imaging is to include a predefined latitude-dependent rotation law in the inversion code (`sheared image method'). However, special but not unusual spot distributions, such like a large polar cap or an equatorial belt (e.g., small random spots evenly distributed along the equator), can distort the rotation profile similarly as the DR does, consequently, yielding incorrect measure of the DR from the sheared image method. To avoid these problems, the technique of measuring DR from averaged cross-correlations using time-series Doppler images (`ACCORD') is introduced and the reliability of this tool is demonstrated on artificial data.