The Solar Wind in Time II: 3D stellar wind structure and radio emission

Abstract

In this work, we simulate the evolution of the solar wind along its main sequence lifetime and compute its thermal radio emission. To study the evolution of the solar wind, we use a sample of solar mass stars at different ages. All these stars have observationally-reconstructed magnetic maps, which are incorporated in our 3D magnetohydrodynamic simulations of their winds. We show that angular-momentum loss and mass-loss rates decrease steadily on evolutionary timescales, although they can vary in a magnetic cycle timescale. Stellar winds are known to emit radiation in the form of thermal bremsstrahlung in the radio spectrum. To calculate the expected radio fluxes from these winds, we solve the radiative transfer equation numerically from first principles. We compute continuum spectra across the frequency range 100 MHz -100 GHz and find maximum radio flux densities ranging from 0.05 -8.3 µJy. At a frequency of 1 GHz and a normalised distance of d = 10 pc, the radio flux density follows 0.24 (Ω/Ω ) 0.9 (d/[10pc]) 2 µJy, where Ω is the rotation rate. This means that the best candidates for stellar wind observations in the radio regime are faster rotators within distances of 10 pc, such as κ 1 Ceti (2.83 µJy) and χ 1 Ori (8.3 µJy). These flux predictions provide a guide to observing solar-type stars across the frequency range 0.1 -100 GHz in the future using the next generation of radio telescopes, such as ngVLA and SKA.

The evolution of the solar wind

“…In particular, the densest region in a stellar wind (its innermost region) can emit at radio wavelengths, thus providing a way to directly detect the wind in radio (Güdel 2002). For large enough densities, the innermost regions can become optically thick to radio wavelengths, creating a radio photosphere (Ó Fionnagáin et al 2019;Kavanagh et al 2019), that, if detected, can allow us to quantify the mass-loss rate of the wind. In this case, the underlying non-thermal radio emission from the star cannot be seen.…”

“…The X-ray emitting stellar corona, set by energy input, varies with the properties of the star (Jardine 2004;Güdel 2004), as do the stellar wind properties (Ó Fionnagáin et al 2019).…”

Impact of space weather on climate and habitability of terrestrial-type exoplanets

“…(2016) and Ó Fionnagáin et al . (2018). Specifically, the young Sun's wind speed at 1 AU was twice as fast, five times hotter with the mass loss rate of >50 times greater as compared to the current solar-wind properties (see Fig.…”

MOVES – II. Tuning in to the radio environment of HD189733b

“…We calculate the free-free radio emission of the stellar wind of HD189733, implementing the numerical code developed byÓ Fionnagáin et al (2019). In this model, we solve the equations of radiative transfer for our wind models, assuming that it emits as a blackbody.…”

“…However, the detection of planetary radio emission could be inhibited due to dense stellar winds, which can absorb emission at radio wavelengths (Vidotto & Donati 2017). The winds of low-mass stars are known to be sources of radio emission, arising through thermal free-free processes (Panagia & Felli 1975;Wright & Barlow 1975;Güdel 2002;O Fionnagáin et al 2019), which depend on both the density and temperature of the wind. By the same process, the wind can self-absorb the generated free-free radio emission if the optical depth is high enough.…”