The Magnetic Future of the Sun

Abstract

We analyze space-and ground-based data for the old (7.0 ± 0.3 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 Ca II data are used to place the UV data in context. The UV spectra of 18 Sco (3.7±0.5 Gyr), the Sun (4.6±0.04 Gyr) and α Cen A (5.4 +1.2 −0.2 Gyr), appear remarkably similar, pointing to a convergence of magnetic heating rates for G2 main-sequence stars older than ≈ 2 − 4 Gyr. But the B component's X-ray (0.3-2.5 keV) flux lies 20× below a well-known minimum level reported by Schmitt. As reported for α 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 Ca II 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.

A Chandra/LETGS Survey of Main-sequence Stars

“…For an energy range of 0.3 − 2.5 keV we find log L X = 25.37 and log F X = 2.59 for the QS distribution, a full order of magnitude lower than the 0.1 − 2.4 keV values quoted above. This 0.3 − 2.5 keV QS luminosity is not inconsistent with the very low 16 Cyg B limit of Judge et al (2017). The luminosities inferred from the Schonfeld et al (2017) distributions decrease to log L X = 26.15 − 26.96 in the 0.3 − 2.5 keV range, more consistent with the α Cen AB luminosities quoted by Ayres (2014), which are for 0.2 − 2 keV.…”

“…However, the models of Suzuki (2018) suggest that such stars may end up brighter in X-rays due to much higher coronal densities. Judge et al (2017) recently found a very low upper limit for the X-ray luminosity of 16 Cyg B (G3 V), log L X < 25.5, which is much lower than the minimum QS level of log L X = 26.4 just quoted in the previous paragraph. However, the 16 Cyg observations are Chandra ACIS-I data, which covers an energy range of 0.3 − 2.5 keV, compared to the more traditional 0.1 − 2.4 keV ROSAT/PSPC range that we are using.…”

Sounding stellar cycles with Kepler – III. Comparative analysis of chromospheric, photometric, and asteroseismic variability

“…Though this result has received some criticism, mainly focusing on selection bias and underestimation of the age uncertainties (Barnes et al 2016), the result has later been confirmed by another independent study (dos Santos et al 2016). Lately, a number of theoretical explanations for the small angular momentum loss of old solar-type stars have also emerged (Metcalfe et al 2016;Brandenburg et al 2017;Metcalfe & van Saders 2017;Judge et al 2017).…”

Stellar Cycles in Fully Convective Stars and a New Interpretation of Dynamo Evolution

“…In the end, however, after further evolution along the I branch, below a critical Ro −1 , the dynamo may begin to sputter (van Saders et al 2016), the star begins to experience magnetic grand minima (Metcalfe et al 2016), differential rotation weakens (Metcalfe et al 2022) or may even reverse sign (Gastine et al 2014;Karak et al 2020), and the dynamo dies, with P cyc going to infinity (see, e.g., the lengthening P cyc in the evolutionary sequence of G2 stars 18 Sco, the Sun, and α Cen A; Judge et al 2017). With the cycling dynamo shut down, large-scale fields driving spindown vanish, stopping further rotational evolution, and the star evolves vertically off the diagram at constant Ro (e.g., Metcalfe & van Saders 2017, shown by a vertical gold arrow in Figure 18).…”