Patterns of Variation for the Sun and Sun-like Stars

Abstract

We compare patterns of variation for the Sun and 72 Sun-like stars by combining total and spectral solar irradiance measurements between 2003 and 2017 from the SORCE satellite, Strömgren b, y stellar photometry between 1993 and 2017 from Fairborn Observatory, and solar and stellar chromospheric Ca ii H+K emission observations between 1992 and 2016 from Lowell Observatory. The new data and their analysis strengthen the relationships found previously between chromospheric and brightness variability on the decadal timescale of the solar activity cycle. Both chromospheric H+K and photometric b, y variability among Sun-like stars are related to average chromospheric activity by power laws on this timescale. Young active stars become fainter as their H+K emission increases, and older, less active, more Sun-age stars tend to show a pattern of direct correlation between photometric and chromospheric emission variations. The directly correlated pattern between total solar irradiance and chromospheric Ca ii emission variations shown by the Sun appears to extend also to variations in the Strömgren b, y portion of the solar spectrum. Although the Sun does not differ strongly from its stellar age and spectral class mates in the activity and variability characteristics that we have now studied for over three decades, it may be somewhat unusual in two respects: (1) its comparatively smooth, regular activity cycle, and (2) its rather low photometric brightness variation relative to its chromospheric activity level and variation, perhaps indicating that facular emission and sunspot darkening are especially well-balanced on the Sun.

Transition from spot to faculae domination

“…For details, we refer the reader to the above publications. Lockwood et al (2007) presented up to 20 years of differential Strömgren b and y photometry of 32 stars with contemporaneous observations of the Mount Wilson S-index (see also Lockwood et al 1997;Radick et al 1998Radick et al , 2018 with the purpose of studying long-term photometric and chromospheric variability. We received data for 26 out of the 32 stars in their sample.…”

Activity time series of old stars from late F to early K

“…However, on average, the difference between the expected value and the final value is much smaller when this inclination dependence is removed (below 0.01 typically). The differences are smaller than the typical uncertainties on Log R HK values as estimated by Radick et al (2018), of about 0.06 dex, and our Log R HK should not be considered to be more precise than this in absolute value (although for a given simulation, the relative variability will be much more precise, of course).…”

Rotation of solar‐like stars in the immediate solar neighborhood

“…Radick et al (2018) provide a concise summary of 35 years of photometry of solar‐like stars and succinctly point out the limitations of ground‐based photometry: Achieving and maintaining the precision needed for stellar observations to make a meaningful comparison to the Sun is next to impossible. The typical precision achievable from traditional photoelectric photometry is about 0.1% for individual observations, and more modern charge coupled device observations reach similar limits for various instrumental (e.g., flat‐fielding) and physical (e.g., scintillation) reasons, thus staying about two orders of magnitude worse than state‐of‐the‐art solar observations.…”

“…The typical precision achievable from traditional photoelectric photometry is about 0.1% for individual observations, and more modern charge coupled device observations reach similar limits for various instrumental (e.g., flat‐fielding) and physical (e.g., scintillation) reasons, thus staying about two orders of magnitude worse than state‐of‐the‐art solar observations. Furthermore, achieving stellar observations on any given star with the same cadence as solar observations is again next to impossible in practical terms, and thus, Radick et al (2018) conclude that “there will probably never be stellar time series with the duration, dense coverage, and the astounding photometric precision of the solar observations (…) for many, or perhaps even any, Sun‐like stars, absent a dedicated, coordinated, long‐term ground‐ and space‐mission to obtain the needed spectroscopic and photometric data.” While space‐based photometry (cf., Section 4) increases the achievable accuracy by at least one order of magnitude, the limitations are cost and duration, and the vast majority of space missions have lifetimes shorter, and often considerably shorter, than typical solar or stellar cycles.…”