The magnetic future of the Sun

TL;DR

This study compares UV and X-ray data of old solar analogs and the Sun, revealing convergence in magnetic activity for stars older than 2-4 Gyr and highlighting gaps in solar data for better stellar comparison.

Contribution

It provides new insights into the magnetic activity convergence of G2 stars over billions of years and emphasizes the need for high-resolution solar UV spectra for accurate stellar comparisons.

Findings

UV spectra of old G2 stars are similar, indicating magnetic activity convergence.

The X-ray flux of 16 Cyg B is significantly below known minimum levels.

Solar UV spectra at similar resolution are lacking but crucial for stellar studies.

Abstract

We analyze space- and ground-based data for the old ($7.0\pm0.3$~Gyr) solar analogs 16 Cyg A and B. The stars were observed with the Cosmic Origins UV Spectrographs on the Hubble Space Telescope (HST) on 23 October 2015 and 3 February 2016 respectively, and with the Chandra X-ray Observatory on 7 February 2016. Time-series data in \ion{Ca}{2} data are used to place the UV data in context. The UV spectra of 18 Sco (3.7$\pm0.5$ Gyr), the Sun (4.6$\pm0.04$ Gyr) and $\alpha$ Cen A ($5.4_{-0.2}^{+1.2}$ Gyr), appear remarkably similar, pointing to a convergence of magnetic heating rates for G2 main-sequence stars older than $\approx 2-4$ Gyr. But the B component's X-ray (0.3-2.5 keV) flux lies 20$\times$ below a well-known minimum level reported by Schmitt. As reported for $\alpha$~Cen~A, the coronal temperature probably lies below that detectable in soft X-rays. No solar UV flux spectra of comparable resolution to stellar data exist, but they are badly needed for comparison with stellar data. Center-to-limb (C-L) variations are re-evaluated for lines such as \ion{Ca}{2} through to X-rays, with important consequences for observing activity cycles in such features. We also call into question work that has mixed solar intensity-intensity statistics with flux-flux relations of stars.