Astrometric effects of solar-like magnetic activity in late-type stars and their relevance for the detection of extrasolar planets

TL;DR

This paper models how magnetic activity in sun-like stars causes photocenter shifts, affecting astrometric planet detection, and emphasizes the need for simultaneous flux and activity monitoring to avoid false positives.

Contribution

It introduces a law quantifying maximum photocenter shifts due to active regions and analyzes their impact on astrometric planet detection methods.

Findings

Surface brightness inhomogeneities cause measurable photocenter shifts.

Activity-induced shifts can mimic planetary signals in astrometry.

Monitoring stellar activity is essential to distinguish true planets from false signals.

Abstract

Using a simple model based on the characteristics of sunspots and faculae in solar active regions, the effects of surface brightness inhomogeneities on the position of the photocentre of the disk of a solar-like, magnetically active star, are studied. A general law is introduced, giving the maximum amplitude of the photocentre excursion produced by a distribution of active regions with a given surface filling factor. The consequences for the detection of extrasolar planets by means of the astrometric method are investigated with some applications to forthcoming space missions, such as GAIA and SIM, as well as to ground-based interferometric measurements. Spurious detections of extrasolar planets can indeed be caused by activity-induced photocentre oscillations, requiring a simultaneous monitoring of the optical flux and a determination of the rotation period and of the level of activity of the target stars for an appropriate discrimination.