Lambda And: A post-main sequence wind from a solar-mass star

TL;DR

This study models the stellar wind of the evolved solar-mass star lambda And using spectropolarimetric data and MHD simulations, constraining its mass-loss rate through radio observations and wind scenario comparisons.

Contribution

It presents the first detailed magnetic field reconstruction and wind simulation for lambda And, linking magnetic topology to wind properties and radio emission.

Findings

Magnetic field of lambda And is stronger than the Sun's, dominated by poloidal component.

Estimated mass-loss rate of lambda And is approximately 3×10⁻⁹ solar masses per year.

Radio observations suggest possible wind-related emission, constraining the star's mass-loss rate.

Abstract

We investigate the wind of lambda And, a solar-mass star that has evolved off the main sequence becoming a sub-giant. We present spectropolarimetric observations and use them to reconstruct the surface magnetic field of lambda And. Although much older than our Sun, this star exhibits a stronger (reaching up to 83 G) large-scale magnetic field, which is dominated by the poloidal component. To investigate the wind of lambda And, we use the derived magnetic map to simulate two stellar wind scenarios, namely a polytropic wind (thermally-driven) and an Alfven-wave driven wind with turbulent dissipation. From our 3D magnetohydrodynamics simulations, we calculate the wind thermal emission and compare it to previously published radio observations and more recent VLA observations, which we present here. These observations show a basal sub-mJy quiescent flux level at ~5 GHz and, at epochs, a much larger flux density (>37 mJy), likely due to radio flares. By comparing our model results with the radio observations of lambda And, we can constrain its mass-loss rate Mdot. There are two possible conclusions. 1) Assuming the quiescent radio emission originates from the stellar wind, we conclude that lambda And has Mdot ~ 3e-9 Msun/yr, which agrees with the evolving mass-loss rate trend for evolved solar-mass stars. 2) Alternatively, if the quiescent emission does not originate from the wind, our models can only place an upper limit on mass-loss rates, indicating that Mdot <~ 3e-9 Msun/yr.