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The Sun is less active than other solar-like stars (2005.01401v1)

Published 4 May 2020 in astro-ph.SR

Abstract: Magnetic activity of the Sun and other stars causes their brightness to vary. We investigate how typical the Sun's variability is compared to other solar-like stars, i.e. those with near-solar effective temperatures and rotation periods. By combining four years of photometric observations from the Kepler space telescope with astrometric data from the Gaia spacecraft, we measure photometric variabilities of 369 solar-like stars. Most of the solar-like stars with well-determined rotation periods show higher variability than the Sun and are therefore considerably more active. These stars appear nearly identical to the Sun, except for their higher variability. Their existence raises the question of whether the Sun can also experience epochs of such high variability.

Citations (65)

Summary

  • The paper compares the Sun's photometric variability using Kepler data to 369 solar-like stars, finding the Sun is significantly less active than the vast majority of the sample.
  • Using the Rvar measure, the study found the solar-like star sample averaged 0.36% variability, compared to the Sun's historical maximum of 0.20% and median of 0.07%, demonstrating lower solar activity.
  • These findings suggest the Sun may be in a current low-activity phase or that the dataset represents a range of activity levels the Sun could exhibit over much longer evolutionary timescales.

Analysis of Solar Activity in Comparison to Solar-Like Stars

In this paper, the authors delve into the variability of solar-like stars, focusing on comparing the Sun’s photometric variability with other stars that share similar fundamental parameters and rotation periods. Utilizing data over a span of four years from the Kepler space telescope, along with astrometric data from the Gaia spacecraft, the paper measures and analyzes the photometric variabilities of a carefully curated sample of 369 solar-like stars. These stars exhibit near-solar effective temperatures and rotation periods, yet display significantly greater variability than the Sun, suggesting they possess higher magnetic activity.

Key aspects of the methodology include selection criteria that confidently classify the examined stars as solar-like or pseudo-solar, depending on the determination of their rotation periods. Of the select samples, 369 stars have verified rotational data aiding their classification as solar-like, while 2529 stars, lacking detectable rotation periods, are classified as pseudo-solar. The variability measure, RvarR_\text{var}, is calculated to assess this photometric variability, derived as the difference between the 95th and 5th percentile of normalized stellar flux values. The data utilizes Kepler's Presearch Data Conditioning (PDC) signals, verified to accurately reflect the intrinsic astrophysical variability.

A detailed comparative analysis indicates that the periodic stars (solar-like) generally show variability distributions significantly exceeding that observed in the Sun. Specifically, the variability for the periodic sample, corrected for dependencies based on fundamental parameters such as effective temperature, rotation period, and metallicity, averages at 0.36%, which is considerably greater than the Sun's median variability of 0.07% and even its maximum in observed history of 0.20%. This observation implies the presence of a factor not encompassed by the fundamental parameters which influences stellar activity.

The broader implications of this research underscore the variability potential within solar-like stars which suggests two interpretations: either the Sun is currently in a less active phase possibly due to evolutionary changes in its internal processes, or alternatively, the dataset may represent the spectrum of possible activity levels the Sun could potentially exhibit over longer epochs, well beyond the constraints of modern observation periods. This spurs ongoing inquiry into the dynamo mechanisms potentially transitioning the Sun through differing magnetic activity epochs.

Furthermore, complementary findings include the indication that the non-periodic stars—which would include the Sun under conditions similar to those of Kepler's observation due to its undetected sidereal rotation parameters—align more closely with lower variability, demonstrating a complex interaction between stellar dynamics not strictly determined by immediate fundamental attributes.

Potential future directions for this area of research include longitudinal studies that integrate longer timescales of observation, potentially leveraging cosmological isotope analysis to extend understanding of the Sun's activity beyond typical observational timeframes. This could provide refined calibration of solar variability models against actual stellar evolution tracks. Additionally, enhancing detection sensitivity to discern rotation periods of more stars could redefine criteria for photometric variability across broader stellar samples leading to enriched comprehension of the underlying astrophysical causes of variability.

In conclusion, this research contributes a detailed empirical dataset and analysis distinguishing solar variability among an equivalent cohort of stellar analogs, promoting a nuanced understanding of stellar magnetic dynamics and solar astronomy. Such findings effectively counsel further evaluation into stochastic variability phenomena among celestial bodies historically analogous to our Sun.

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