Temporal variability of the wind from the star {\tau} Bo\"otis

TL;DR

This study models the stellar wind variability of { au} Bo"otis over its magnetic cycle, revealing that magnetic flux changes influence wind properties and planetary magnetosphere stability, with implications for star-planet interactions.

Contribution

First comprehensive analysis linking magnetic cycle variations to stellar wind changes and planetary magnetosphere stability for { au} Bo"otis.

Findings

Mass loss rate varies by ~4% over cycles.

Angular momentum loss rate varies up to ~140%.

Planetary magnetosphere remains stable despite stellar wind changes.

Abstract

We present new wind models for {\tau} Bo\"otis ({\tau} Boo), a hot-Jupiter-host-star whose observable magnetic cycles makes it a uniquely useful target for our goal of monitoring the temporal variability of stellar winds and their exoplanetary impacts. Using spectropolarimetric observations from May 2009 to January 2015, the most extensive information of this type yet available, to reconstruct the stellar magnetic field, we produce multiple 3D magnetohydrodynamic stellar wind models. Our results show that characteristic changes in the large-scale magnetic field as the star undergoes magnetic cycles produce changes in the wind properties, both globally and locally at the position of the orbiting planet. Whilst the mass loss rate of the star varies by only a minimal amount ($\sim$ 4 percent), the rates of angular momentum loss and associated spin-down timescales are seen to vary widely (up to $\sim$ 140 percent), findings consistent with and extending previous research. In addition, we find that temporal variation in the global wind is governed mainly by changes in total magnetic flux rather than changes in wind plasma properties. The magnetic pressure varies with time and location and dominates the stellar wind pressure at the planetary orbit. By assuming a Jovian planetary magnetic field for {\tau} Boo b, we nevertheless conclude that the planetary magnetosphere can remain stable in size for all observed stellar cycle epochs, despite significant changes in the stellar field and the resulting local space weather environment.