J/A+A/711/A261 Solar-type stars MW and Tigre indices (Schmitt+, 2026)
Six decades of TIGRE and Mount Wilson chromospheric monitoring in the H&K lines:
The quest for an understanding of solar-type activity.
Schmitt J.H.M.M., Schroeder K.-P., Mittag M., Hempelmann A.,
Gonzalez-Perez J.N., Jack D.
<Astron. Astrophys. 711, A261 (2026)>
=2026A&A...711A.261S 2026A&A...711A.261S (SIMBAD/NED BibCode)
ADC_Keywords: Stars, G-type ; Spectroscopy ; Optical
Keywords: stars: activity - stars: chromospheres - stars: coronae -
stars: general - stars: late-type - stars: solar-type
Abstract:
The combined time series of the consistently calibrated chromospheric
activity indicator, the S-index - derived from Mount Wilson and
TIGRE data - now spans more than five decades of monitoring
observations of the Sun and of more than one hundred solar-like stars.
For the first time, these data allow us to probe the long-term
stability of solar-type magnetic cycles as well as their possible
transition into other behavioral forms of stellar activity in the time
domain. Such variability studies are an important input for
solarstellar connection research and for our empirical understanding
of the long-term perspective of solar activity as well as of a wider
empirical picture of the coevolution of magnetic activity with stellar
structure. We here present and discuss a representative selection of
cool main-sequence stars, for which such data are available. The
excellent quality of our TIGRE data now more than doubles the number
of known cyclic stars. Grading them (i.e., excellent, good, fair, and
poor) and putting them into an evolutionary perspective, we find that
stars more active than the Sun are also more likely to show cyclic
variability than stars below solar activity. Furthermore, more active
stars tend to have more regular cycles, while the solar mean S-value
fits the lower end of the S-index range found for cyclic stars. This
is consistent with the occurrence of solar Maunder minima episodes,
hinting at a sometimes already unstable solar cycle. We find eighteen
stars with even lower activity and lower variability than the Sun,
which provide a preview into the latter,s distant future. For half of
these stars, we can use sensitive X-ray observations to complement our
chromospheric data and to explore the lower end of stellar activity
also from a coronal perspective. These so-called "Maunder minimum
candidate stars" exhibit what we refer to as "remnant activity", which
expresses itself in a minimal coronal flux and a minimal S-value,
above which only rather small variations occur. Using accurate Gaia
DR3 distances allows us to place our sample stars into an
Hertzsprung-Russell diagram (HRD), which demonstrates that active
cyclic stars and inactive noncyclic stars populate rather different
regions in the HRD.
Description:
In this paper, we reconsider the "historical" Mount Wilson data (taken
between 1966 and 1997) as well as new TIGRE data taken between 2013
and 2024. For our study we specifically focused on those 111 stars
including the Sun presented by Baliunas et al. (1995ApJ...438..269B 1995ApJ...438..269B),
since for the large majority of this stellar sample HK observations
going back to 1966 are available (see Fig. 1 in Baliunas et al.,
1995ApJ...438..269B 1995ApJ...438..269B). By combining the Mount Wilson and TIGRE data,
one then obtains a sample covering a period of almost 60 years in
time.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
stars.dat 33 57 List of studied stars
table.dat 89 124316 Index measurements, phased and unphased
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Byte-by-byte Description of file: stars.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 A9 --- Name HD name
11- 12 I2 h RAh Simbad right ascension (J2000)
14- 15 I2 min RAm Simbad right ascension (J2000)
17- 21 F5.2 s RAs Simbad right ascension (J2000)
23 A1 --- DE- Simbad declination sign (J2000)
24- 25 I2 deg DEd Simbad declination (J2000)
27- 28 I2 arcmin DEm Simbad declination (J2000)
30- 33 F4.1 arcsec DEs Simbad declination (J2000)
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Byte-by-byte Description of file: table.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Name HD Name
11- 15 F5.2 d Per Period
17 A1 --- Cycle Cycle type
19- 36 F18.13 yr Date Date
38- 64 E27.25 --- Phase Phase
66- 85 F20.17 --- S-index S-index
87- 89 A3 --- Inst Instrument (MW for Mount Wilson, TIG for TIGRE)
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Acknowledgements:
Juergen Schmitt, juergen.schmitt(at)uni-hamburg.de
License: CC-BY-4.0
(End) Patricia Vannier [CDS] 15-Jun-2026