J/A+A/622/A85 Starspot rotation rates vs. activity cycle phase (Nielsen+, 2019)
Starspot rotation rates vs. activity cycle phase:
Butterfly diagrams of Kepler stars are unlike that of the Sun.
Nielsen M.B., Gizon L., Cameron R.H., Miesch M.
<Astron. Astrophys. 622, A85 (2019)>
=2019A&A...622A..85N 2019A&A...622A..85N (SIMBAD/NED BibCode)
ADC_Keywords: Space observations ; Stellar distribution ; Magnetic fields
Keywords: methods: data analysis - techniques: photometric - stars: activity -
stars: rotation - starspots
Abstract:
During the solar magnetic activity cycle the emergence latitudes of
sunspots change, leading to the well-known butterfly diagram. This
phenomenon is poorly understood for other stars since starspot
latitudes are generally unknown. The related changes in starspot
rotation rates caused by latitudinal differential rotation can however
be measured.
Using the set of 3093 Kepler stars with activity cycles identified by
Reinhold et al. (2017A&A...603A..52R 2017A&A...603A..52R, Cat. J/A+A/603/A52), we aim to
study the temporal change in starspot rotation rates over magnetic
activity cycles, and how this relates to the activity level, the mean
rotation rate of the star, and its effective temperature.
We measured the photometric variability as a proxy for the magnetic
activity and the spot rotation rate in each quarter over the duration
of the Kepler mission. We phase-fold these measurements with the cycle
period. To reduce random errors we perform averages over stars with
comparable mean rotation rates and effective temperature at fixed
activity-cycle phases.
We detect a clear correlation between the variation of activity level
and the variation of the starspot rotation rate. The sign and
amplitude of this correlation depends on the mean stellar rotation and
-- to a lesser extent -- on the effective temperature. For slowly
rotating stars (rotation periods between 15-28 days) the starspot
rotation rates are clearly anti-correlated with the level of activity
during the activity cycles. A transition is observed around rotation
periods of 10-15 days, where stars with effective temperature above
4200K instead show positive correlation.
Our measurements can be interpreted in terms of a stellar "butterfly
diagram", but these appear different from the Sun's since the starspot
rotation rates are either in phase or anti-phase with the activity
level. Alternatively, the activity cycle periods observed by Kepler
are short (around 2.5 years) and may therefore be secondary cycles,
perhaps analogous to the solar quasi-biennial oscillations.
Description:
Activity cycle parameters for 3093 stars observed by Kepler, with
measured cycle periods from Reinhold et al. (2017A&A...603A..52R 2017A&A...603A..52R, Cat.
J/A+A/603/A52). The integral, A, of the power density spectrum around
the mean rotation rate (nurot, from McQuillan et al.
(2014ApJS..211...24M 2014ApJS..211...24M, Cat. J/ApJS/211/24)) is used as proxy for
magnetic activity. This and the rotation rate, nu, are traced from
quarters Q1 to Q17 of Kepler observations.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
cyclepar.dat 313 3093 Activity cycle parameters for 3093 stars
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See also:
V/133 : Kepler Input Catalog (Kepler Mission Team, 2009)
J/ApJS/211/2 : Q1-16 Kepler targets revised stellar properties (Huber+, 2014)
J/ApJS/211/24 : Rotation periods of Kepler MS stars (McQuillan+, 2014)
J/A+A/603/A52 : Activity cycles in 3203 Kepler stars (Reinhold+, 2017)
Byte-by-byte Description of file: cyclepar.dat
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Bytes Format Units Label Explanations
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1- 8 I8 --- KIC Kepler Input Catalog number
10- 13 I4 K Teff Effective surface temperature from Huber et al.
(2014ApJS..211....2H 2014ApJS..211....2H, Cat. J/ApJS/211/2),
Table 4
15- 19 F5.3 [cm/s2] logg Surface gravity from Huber et al.
(2014ApJS..211....2H 2014ApJS..211....2H, Cat. J/ApJS/211/2),
Table 4
21- 27 F7.2 d Pcyc Activity cycle period from Reinhold et al.
(2017A&A...603A..52R 2017A&A...603A..52R, Cat. J/A+A/603/A52)
29- 35 F7.2 d phi0 Time of zero phase from Reinhold et al.
(2017A&A...603A..52R 2017A&A...603A..52R, Cat. J/A+A/603/A52)
37- 41 F5.3 uHz nurot Mean rotation rate from McQuillan et al.
(2014ApJS..211...24M 2014ApJS..211...24M, Cat. J/ApJS/211/24) (1)
43- 47 F5.3 uHz nu1 ? Rotation rate in Q1
49- 53 F5.3 uHz nu2 ? Rotation rate in Q2
55- 59 F5.3 uHz nu3 ? Rotation rate in Q3
61- 65 F5.3 uHz nu4 ? Rotation rate in Q4
67- 71 F5.3 uHz nu5 ? Rotation rate in Q5
73- 77 F5.3 uHz nu6 ? Rotation rate in Q6
79- 83 F5.3 uHz nu7 ? Rotation rate in Q7
85- 89 F5.3 uHz nu8 ? Rotation rate in Q8
91- 95 F5.3 uHz nu9 ? Rotation rate in Q9
97-101 F5.3 uHz nu10 ? Rotation rate in Q10
103-107 F5.3 uHz nu11 ? Rotation rate in Q11
109-113 F5.3 uHz nu12 ? Rotation rate in Q12
115-119 F5.3 uHz nu13 ? Rotation rate in Q13
121-125 F5.3 uHz nu14 ? Rotation rate in Q14
127-131 F5.3 uHz nu15 ? Rotation rate in Q15
133-137 F5.3 uHz nu16 ? Rotation rate in Q16
139-143 F5.3 uHz nu17 ? Rotation rate in Q17
145-153 F9.3 ppm+2 A1 ? Integral of power spectral density in Q1
155-163 F9.3 ppm+2 A2 ? Integral of power spectral density in Q2
165-173 F9.3 ppm+2 A3 ? Integral of power spectral density in Q3
175-183 F9.3 ppm+2 A4 ? Integral of power spectral density in Q4
185-193 F9.3 ppm+2 A5 ? Integral of power spectral density in Q5
195-203 F9.3 ppm+2 A6 ? Integral of power spectral density in Q6
205-213 F9.3 ppm+2 A7 ? Integral of power spectral density in Q7
215-223 F9.3 ppm+2 A8 ? Integral of power spectral density in Q8
225-233 F9.3 ppm+2 A9 ? Integral of power spectral density in Q9
235-243 F9.3 ppm+2 A10 ? Integral of power spectral density in Q10
245-253 F9.3 ppm+2 A11 ? Integral of power spectral density in Q11
255-263 F9.3 ppm+2 A12 ? Integral of power spectral density in Q12
265-273 F9.3 ppm+2 A13 ? Integral of power spectral density in Q13
275-283 F9.3 ppm+2 A14 ? Integral of power spectral density in Q14
285-293 F9.3 ppm+2 A15 ? Integral of power spectral density in Q15
295-303 F9.3 ppm+2 A16 ? Integral of power spectral density in Q16
305-313 F9.3 ppm+2 A17 ? Integral of power spectral density in Q17
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Note (1): See Fig. 2 of the paper.
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Acknowledgements:
Martin Bo Nielsen, mbn4(at)nyu.edu
(End) Patricia Vannier [CDS] 17-Dec-2018