The place of the Sun among the Sun-like stars

Abstract

Context. Monitoring of the photometric and chromospheric HK emission data series of stars similar to the Sun in age and average activity level showed that there is an empirical correlation between the average stellar chromospheric activity level and the photometric variability. In general, more active stars show larger photometric variability. Interestingly, the measurements and reconstructions of the solar irradiance show that the Sun is significantly less variable than indicated by the empirical relationship. Aims. We aim to identify possible reasons for the Sun to be currently outside of this relationship. Methods. We employed different scenarios of solar HK emission and irradiance variability and compared them with available time series of Sun-like stars. Results. We show that the position of the Sun on the diagram of photometric variability versus chromospheric activity changes with time. The present solar position is different from its temporal mean position as the satellite era of continuous solar irradiance measurements has accidentally coincided with a period of unusually high and stable solar activity. Our analysis suggests that although present solar variability is significantly smaller than indicated by the stellar data, the temporal mean solar variability might be in agreement with the stellar data. We propose that the continuation of the photometric program and its expansion to a larger stellar sample will ultimately allow us to constrain the historical solar variability.

Variability of Sun-like stars: reproducing observed photometric trends

“…10). Models and measurements indicate that the solar irradiance variability on the 11 year cycle timescale is smaller than given by the Lockwood et al (2007) empirical regression of the stellar variability vs. activity (see, e.g., Knaack et al 2001;Judge et al 2012;Shapiro et al 2013a, and references therein). Because our model is based on SATIRE, which accurately reproduces the measured TSI variability, it is no surprise that it also leads to the same deviation.…”

“…This approach is starting to attract attention and has already been employed in the literature to constrain solar variability (Preminger et al 2011;Judge et al 2012;Shapiro et al 2013a). The model presented below provides a quantitative basis for this comparison.…”

“…Most of the stars in the program were deemed variable on both the stellar rotation and the cycle timescales (Lockwood et al 1997(Lockwood et al , 2007Radick et al 1998). While the solar 11 year variability in the Strömgren b and y filters is estimated to be around 0.0002 mag (Shapiro et al 2013a), the variability of several stars was found to exceed 0.01 mag. Throughout this paper we calculate the variability in terms of root mean squares (rms), which is more appropriate for the analysis of the very broad variety of stellar light curves than the amplitude (Radick et al 1998).…”

“…The Sun is located so close to the threshold between direct and inverse correlations of visible irradiance (e.g., as measured in the Strömgren b and y filters) and activity, that there is evidence for both the direct (e.g., Lean et al 2005;Krivova & Solanki 2008;Krivova et al 2010;Ball et al 2011) and inverse (Harder et al 2009;Preminger et al 2011) correlations. Lockwood et al (2007) showed that while the solar 11 year cycle in chromospheric activity is more vigorous than the solar analogs, its photometric variability is significantly weaker than indicated by the stellar data (see also Shapiro et al 2013a, and references therein). At the same time, Hall et al (2007aHall et al ( , 2009 showed that both chromospheric and photometric variabilities of 18 Scorpii, which is believed to be the closest bright solar analog (Soubiran & Triaud 2004;Petit et al 2008), are very similar to the solar values.…”

Magnetic cycles of Sun-like stars with different levels of coronal and chromospheric activity — comparison with the Sun

“…It was found that on a long time scale the position of the Sun on the diagram of photometric variability versus CA changers is not constant in time. So Shapiro et al (2013) suggested that the temporal mean solar variability might be in agreement with stellar data.…”