The Solar Twin Planet Search
Universidade de São Paulo, Departamento de Astronomia do IAG/USP, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP, Brazil
2 University of Chicago, Department of Astronomy and Astrophysics, Chicago, IL, USA
3 Tacoma Community College, 6501 South 19th Street, Tacoma, WA 98466, USA
4 The Australian National University, Research School of Astronomy and Astrophysics, Cotter Road, Weston, ACT 2611, Australia
5 Institut für Astrophysik, Universität Göttingen, Germany
6 Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil
7 Center for Computational Astrophysics, Flatiron Institute, 162 5th Ave., New York, NY 10010, USA
8 Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800, Australia
Accepted: 20 June 2018
Context. It is well known that the magnetic activity of solar-type stars decreases with age, but it is widely debated in the literature whether there is a smooth decline or if there is an early sharp drop until 1–2 Gyr that is followed by a relatively inactive constant phase.
Aims. We revisited the activity-age relation using time-series observations of a large sample of solar twins whose precise isochronal ages and other important physical parameters have been determined.
Methods. We measured the Ca II H and K activity indices using ≈9000 HARPS spectra of 82 solar twins. In addition, the average solar activity was calculated through asteroids and Moon reflection spectra using the same instrumentation. Thus, we transformed our activity indices into the S Mount Wilson scale (SMW), recalibrated the Mount Wilson absolute flux and photospheric correction equations as a function of Teff, and then computed an improved bolometric flux normalized activity index log R′HK (Teff) for the entire sample.
Results. New relations between activity and the age of solar twins were derived by assessing the chromospheric age-dating limits using log R′HK (Teff). We measured an average solar activity of SMW = 0.1712 ± 0.0017 during solar magnetic cycles 23–24 covered by HARPS observations, and we also inferred an average of SMW = 0.1694 ± 0.0025 for cycles 10–24, anchored on a sunspot number correlation of S index versus. We also found a simple relation between the average and the dispersion of the activity levels of solar twins. This enabled us to predict the stellar variability effects on the age-activity diagram, and consequently, to estimate the chromospheric age uncertainties that are due to the same phenomena. The age-activity relation is still statistically significant up to ages around 6–7 Gyr, in agreement with previous works using open clusters and field stars with precise ages.
Conclusions. Our research confirms that Ca II H & K lines remain a useful chromospheric evolution tracer until stars reach ages of at least 6–7 Gyr. We found evidence that for the most homogenous set of old stars, the chromospheric activity indices seem to continue to decrease after the solar age toward the end of the main sequence. Our results indicate that a significant part of the scatter observed in the age-activity relation of solar twins can be attributed to stellar cycle modulations effects. The Sun seems to have a normal activity level and variability for its age.
Key words: stars: solar-type / stars: evolution / stars: fundamental parameters / magnetic fields
Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programs 188.C-0265, 183.D-0729, 292.C-5004, 097.C-0571, 092.C-0721, 093.C-0409, 072.C-0488, 183.C-0972, 091.C-0936, 192.C-0852, 196.C-1006, 076.C-0155, 096.C-0499, 185.D-0056, 192.C-0224, 075.C-0332, 090.C-0421, 091.C-0034, 077.C-0364, 089.C-0415, 60.A-9036, 092.C-0832, 295.C-5035, 295.C-5031, 60.A-9700, 289.D-5015, 096.C-0210, 086.C-0284, 088.C-0323, 0100.D-0444, and 099.C-0491.
Tables 1 and 2 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/619/A73
© ESO 2018