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

We investigate the wind of lambda And, a solar-mass star that has evolved off the main sequence becoming a subgiant. 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 Very Large Array observations, which we present here. These observations show a basal sub-mJy quiescent flux level at similar to 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 (M)over dot. There are two possible conclusions. (1) Assuming the quiescent radio emission originates from the stellar wind, we conclude that lambda And has (M)over dot similar or equal to 3 x 10(-9) M-circle dot yr(-1), 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 (M)over dot less than or similar to 3 x 10(-9) M-circle dot yr(-1).

Automated summary

Researchers reconstructed the surface magnetic field of lambda And using spectropolarimetric observations, and simulated two stellar wind scenarios. By comparing model results with previous radio observations, conclusions were drawn regarding the mass-loss rate of lambda And.