Limits of ultra-high-precision optical astrometry: Stellar surface structures

TL;DR

This paper analyzes how stellar surface structures limit ultra-high-precision optical astrometry, especially affecting exoplanet detection, by quantifying the expected astrometric jitter across different stellar types and activity levels.

Contribution

It provides a theoretical and simulation-based assessment of stellar surface effects on astrometric measurements, highlighting their impact on exoplanet detection capabilities.

Findings

Astrometric jitter due to surface structures is typically around 10 micro-AU.

Jitter exceeds the signal from Earth-like planets in most stars, complicating their detection.

Stars with very low activity levels may allow detection of Earth-sized planets via astrometry.

Abstract

We investigate the astrometric effects of stellar surface structures as a practical limitation to ultra-high-precision astrometry, e.g. in the context of exoplanet searches, and to quantify the expected effects in different regions of the HR-diagram. Stellar surface structures are likely to produce fluctuations in the integrated flux and radial velocity of the star, as well as a variation of the observed photocentre, i.e. astrometric jitter, and closure phase. We use theoretical considerations supported by Monte Carlo simulations to derive statistical relations between the corresponding astrometric, photometric, and radial-velocity effects. For most stellar types the astrometric jitter due to stellar surface structures is expected to be of order 10 micro-AU or greater. This is more than the astrometric displacement typically caused by an Earth-size exoplanet in the habitable zone, which is about 1-4 micro-AU for long-lived main-sequence stars. Only for stars with extremely low photometric variability (<0.5 mmag) and low magnetic activity, comparable to that of the Sun, will the astrometric jitter be of order 1 micro-AU, suffcient to allow the astrometric detection of an Earth-sized planet in the habitable zone. While stellar surface structure may thus seriously impair the astrometric detection of small exoplanets, it has in general negligible impact on the detection of large (Jupiter-size) planets and on the determination of stellar parallax and proper motion. From the starspot model we also conclude that the commonly used spot filling factor is not the most relevant parameter for quantifying the spottiness in terms of the resulting astrometric, photometric and radial-velocity variations.