scholarly journals Starspot rotation rates versus activity cycle phase: Butterfly diagrams of Kepler stars are unlike that of the Sun

2019 ◽  
Vol 622 ◽  
pp. A85 ◽  
Author(s):  
M. B. Nielsen ◽  
L. Gizon ◽  
R. H. Cameron ◽  
M. Miesch
Keyword(s):  
Effective Temperature ◽  
Rotation Rate ◽  
Activity Cycle ◽  
Magnetic Activity ◽  
Activity Level ◽  
The Sun ◽  
Rotation Rates ◽  
Rotation Periods ◽  
Activity Cycles ◽  
The Mean

Context. 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. Aims. Using the set of 3093 Kepler stars with measured activity cycles, 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. Methods. 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-folded these measurements with the cycle period. To reduce random errors, we performed averages over stars with comparable mean rotation rates and effective temperature at fixed activity-cycle phases. Results. 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 an effective temperature above 4200 K instead show positive correlation. Conclusions. Our measurements can be interpreted in terms of a stellar “butterfly diagram”, but these appear different from that of the Sun 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.

scholarly journals Transition from spot to faculae domination

2018 ◽  
Vol 621 ◽  
pp. A21 ◽  
Author(s):  
Timo Reinhold ◽  
Keaton J. Bell ◽  
James Kuszlewicz ◽  
Saskia Hekker ◽  
Alexander I. Shapiro
Keyword(s):  
Time Series ◽  
Phase Difference ◽  
Process Model ◽  
Activity Cycle ◽  
Magnetic Activity ◽  
Cycle Period ◽  
Stellar Activity ◽  
Rotation Periods ◽  
Active Stars ◽  
Activity Cycles

Context. The study of stellar activity cycles is crucial to understand the underlying dynamo and how it causes magnetic activity signatures such as dark spots and bright faculae. Having knowledge about the dominant source of surface activity might allow us to draw conclusions about the stellar age and magnetic field topology, and to put the solar cycle in context. Aims. We investigate the underlying process that causes magnetic activity by studying the appearance of activity signatures in contemporaneous photometric and chromospheric time series. Methods. Lomb-Scargle periodograms are used to search for cycle periods present in the photometric and chromospheric time series. To emphasize the signature of the activity cycle we account for rotation-induced scatter in both data sets by fitting a quasi-periodic Gaussian process model to each observing season. After subtracting the rotational variability, cycle amplitudes and the phase difference between the two time series are obtained by fitting both time series simultaneously using the same cycle period. Results. We find cycle periods in 27 of the 30 stars in our sample. The phase difference between the two time series reveals that the variability in fast-rotating active stars is usually in anti-phase, while the variability of slowly rotating inactive stars is in phase. The photometric cycle amplitudes are on average six times larger for the active stars. The phase and amplitude information demonstrates that active stars are dominated by dark spots, whereas less-active stars are dominated by bright faculae. We find the transition from spot to faculae domination to be at the Vaughan–Preston gap, and around a Rossby number equal to one. Conclusions. We conclude that faculae are the dominant ingredient of stellar activity cycles at ages ≳2.55 Gyr. The data further suggest that the Vaughan–Preston gap cannot explain the previously detected dearth of Kepler rotation periods between 15 and 25 days. Nevertheless, our results led us to propose an explanation for the lack of rotation periods to be due to the non-detection of periodicity caused by the cancelation of dark spots and bright faculae at ∼800 Myr.

scholarly journals C I 5380 Å: A SPECTRAL LINE WITH LOW INFLUENCE FROM STARSPOTS DYNAMICS AND MAGNETIC CYCLES

2012 ◽  
Vol 18 ◽  
pp. 178-181
Author(s):  
D. SOUTO ◽  
J. D. DO NASCIMENTO
Keyword(s):  
Radial Velocity ◽  
Activity Cycle ◽  
Magnetic Activity ◽  
Spectral Lines ◽  
The Sun ◽  
Total Irradiance ◽  
Modulation Signal ◽  
Solar Magnetic Activity ◽  
Activity Cycles ◽  
Magnetic Cycles

In the Sun-as-a-star Project, the sun was observed spectroscopically and photometrically for more than 25 years in order to determine variability and luminosity changes. This project detected systematic longterm decrease in the total irradiance as a consequence of the solar magnetic activity cycle (scale of years) and variability on solar activity from a time scale of days-months. The solar magnetic activity cycles could mimic the radial velocity modulation signal of a long-period companion in several spectral lines. This effect is an important limitation for the exoplanet searches programs using the radial velocity technique. The Lomb-Scargle periodogram analysis of the Sun-as-a-star spectroscopic data shows that the photospheric line C I 5380 Å and other 11 lines seems to not show significant influence from the rotational or cromospheric magnetic activity modulation. Thus, our analysis suggest that C I 5380 Å line could be used in programs that require extremely line stability.

scholarly journals The effects of rotation on the lithium depletion of G- and K-dwarfs in Messier 35

2020 ◽  
Vol 500 (1) ◽  
pp. 1158-1177
Author(s):  
R D Jeffries ◽  
R J Jackson ◽  
Qinghui Sun ◽  
Constantine P Deliyannis
Keyword(s):  
Rotation Rate ◽  
Binary Systems ◽  
Main Sequence ◽  
Strong Relationship ◽  
Magnetic Activity ◽  
Rich Cluster ◽  
Stellar Radius ◽  
Rotation Periods ◽  
Probability Zero ◽  
Lithium Depletion

ABSTRACT New fibre spectroscopy and radial velocities from the WIYN telescope are used to measure photospheric lithium in 242 high-probability, zero-age main-sequence F- to K-type members of the rich cluster M35. Combining these with published rotation periods, the connection between lithium depletion and rotation is studied in unprecedented detail. At Teff < 5500 K there is a strong relationship between faster rotation and less Li depletion, although with a dispersion larger than measurement uncertainties. Components of photometrically identified binary systems follow the same relationship. A correlation is also established between faster rotation rate (or smaller Rossby number), decreased Li depletion and larger stellar radius at a given Teff. These results support models where star-spots and interior magnetic fields lead to inflated radii and reduced Li depletion during the pre-main-sequence (PMS) phase for the fastest rotators. However, the data are also consistent with the idea that all stars suffered lower levels of Li depletion than predicted by standard PMS models, perhaps because of deficiencies in those models or because saturated levels of magnetic activity suppress Li depletion equally in PMS stars of similar Teff regardless of rotation rate, and that slower rotators subsequently experience more mixing and post-PMS Li depletion.

scholarly journals Stellar activity and rotation of the planet host Kepler-17 from long-term space-borne photometry

2019 ◽  
Vol 626 ◽  
pp. A38 ◽  
Author(s):  
A. F. Lanza ◽  
Y. Netto ◽  
A. S. Bonomo ◽  
H. Parviainen ◽  
A. Valio ◽  
...  
Keyword(s):  
Maximum Entropy ◽  
Differential Rotation ◽  
Activity Cycle ◽  
Magnetic Activity ◽  
Model Parameters ◽  
The Sun ◽  
Level Of Activity ◽  
Spot Model ◽  
Rotational Evolution ◽  
Solar Type

Context. The study of young Sun-like stars is fundamental to understanding the magnetic activity and rotational evolution of the Sun. Space-borne photometry by the Kepler telescope provides unprecedented datasets to investigate these phenomena in Sun-like stars. Aims. We present a new analysis of the entire Kepler photometric time series of the moderately young Sun-like star Kepler-17 accompanied by a transiting hot Jupiter. Methods. We applied a maximum-entropy spot model to the long-cadence out-of-transit photometry of the target to derive maps of the starspot filling factor versus the longitude and the time. These maps are compared to the spots occulted during transits to validate our reconstruction and derive information on the latitudes of the starspots. Results. We find two main active longitudes on the photosphere of Kepler-17, one of which has a lifetime of at least ∼1400 days although with a varying level of activity. The latitudinal differential rotation is of solar type, that is, with the equator rotating faster than the poles. We estimate a minimum relative amplitude ΔΩ/Ω between ∼0.08 ± 0.05 and 0.14 ± 0.05, our determination being affected by the finite lifetime of individual starspots and depending on the adopted spot model parameters. We find marginal evidence of a short-term intermittent activity cycle of ∼48 days and an indication of a longer cycle of 400−600 days characterized by an equatorward migration of the mean latitude of the spots as in the Sun. The rotation of Kepler-17 is likely to be significantly affected by the tides raised by its massive close-by planet. Conclusion. We confirm the reliability of maximum-entropy spot models to map starspots in young active stars and characterize the activity and differential rotation of this young Sun-like planetary host.

scholarly journals Seasonal variations in the incidence of auroral radio absorption events at very high latitude, and the influence of the magnetotail

2007 ◽  
Vol 25 (3) ◽  
pp. 711-720 ◽  
Author(s):  
J. K. Hargreaves
Keyword(s):  
Statistical Analysis ◽  
Seasonal Variations ◽  
Polar Region ◽  
Winter Season ◽  
Magnetic Activity ◽  
Time Of Day ◽  
Activity Level ◽  
The Sun ◽  
South Pole ◽  
Very High

Abstract. A statistical analysis has been made of the incidence of auroral radio absorption events at South Pole, and of its dependence on basic geophysical parameters such as season, time of day, and magnetic activity level. It is found that at low and moderate levels of activity the incidence of events in the winter season is at least twice that in the summer. However, at high activity no events at all occurred during the local summer night, which appears to be explicable as the effect of the magnetotail and the consequent distortion of the magnetosphere when the southern polar region is tilted strongly towards the Sun. Previous results from even higher latitudes show the effect in an even more exaggerated form, in that both the day and night periods of absorption activity exhibit strong seasonal variations.

scholarly journals Dynamo Action in Evolved Stars

1991 ◽  
Vol 130 ◽  
pp. 336-341
Author(s):  
David F. Gray
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Rotation Rate ◽  
Magnetic Activity ◽  
Dynamo Action ◽  
Rotation Rates ◽  
Evolved Stars ◽  
Solar Type ◽  
Limiting Value

AbstractEvolved stars tell us a great deal about dynamos. The granulation boundary shows us where solar-type convection begins. Since activity indicators also start at this boundary, it is a good bet that solar-type convection is an integral part of dynamo activity for all stars. The rotation boundary tells us where the magnetic fields of dynamos become effective in dissipating angular momentum, and rotation beyond the boundary tells us the limiting value needed for a dynamo to function. The observed uniqueness of rotation rates after the rotation boundary is crossed can be understood through the rotostat hypothesis. Quite apart from the reason for the unique rotation rate, its existence can be used to show that magnetic activity of giants is concentrated to the equatorial latitudes, as it is in the solar case. The coronal boundary in the H-R diagram is probably nothing more than a map of where rotation becomes too low to sustain dynamo activity.

scholarly journals The effect of stellar activity on the spectroscopic stellar parameters of the young solar twin HIP 36515

2019 ◽  
Vol 490 (1) ◽  
pp. L86-L90 ◽  
Author(s):  
Jhon Yana Galarza ◽  
Jorge Meléndez ◽  
Diego Lorenzo-Oliveira ◽  
Adriana Valio ◽  
Henrique Reggiani ◽  
...  
Keyword(s):  
Effective Temperature ◽  
Signal To Noise Ratio ◽  
Activity Cycle ◽  
Magnetic Activity ◽  
Resolving Power ◽  
Strongly Correlated ◽  
Chemical Abundances ◽  
Stellar Activity ◽  
Time Variations ◽  
First Time

ABSTRACT Spectroscopic equilibrium allows us to obtain precise stellar parameters in Sun-like stars. It relies on the assumption of the iron excitation and ionization equilibrium. However, several works suggest that magnetic activity may affect chemical abundances of young active stars, calling into question the validity of this widely used method. We have tested, for the first time, variations in stellar parameters and chemical abundances for the young solar twin HIP 36515 (∼0.4 Gyr), along its activity cycle. This star has stellar parameters very well established in the literature and we estimated its activity cycle in ∼6 yr. Using HARPS spectra with high resolving power (115 000) and signal-to-noise ratio (∼270), the stellar parameters of six different epochs in the cycle were estimated. We found that the stellar activity is strongly correlated with the effective temperature, metallicity, and microturbulence velocity. The possibility of changes in the Li i 6707.8 Å line due to flares and star-spots was also investigated. Although the core of the line profile shows some variations with the stellar cycle, it is compensated by changes in the effective temperature, resulting in a non-variation of the Li abundance.

scholarly journals Applegate mechanism in post-common-envelope binaries: Investigating the role of rotation

2018 ◽  
Vol 615 ◽  
pp. A81 ◽  
Author(s):  
F. H. Navarrete ◽  
D. R. G. Schleicher ◽  
J. Zamponi Fuentealba ◽  
M. Völschow
Keyword(s):  
Orbital Period ◽  
Rotation Rate ◽  
Main Sequence ◽  
Magnetic Activity ◽  
Main Sequence Star ◽  
Common Envelope ◽  
Rotation Rates ◽  
Critical Rotation ◽  
Period Variations ◽  
Time Variations

Context. Eclipsing time variations are observed in many close binary systems. In particular, for several post-common-envelope binaries (PCEBs) that consist of a white dwarf and a main sequence star, the observed-minus-calculated (O–C) diagram suggests that real or apparent orbital period variations are driven by Jupiter-mass planets or as a result of magnetic activity, the so-called Applegate mechanism. The latter explains orbital period variations as a result of changes in the stellar quadrupole moment due to magnetic activity. Aims. In this work we explore the feasibility of driving eclipsing time variations via the Applegate mechanism for a sample of PCEB systems, including a range of different rotation rates. Methods. We used the MESA code to evolve 12 stars with different masses and rotation rates. We applied simple dynamo models to their radial profiles to investigate the scale at which the predicted activity cycle matches the observed modulation period, and quantifiy the uncertainty. We further calculated the required energies to drive the Applegate mechanism. Results. We show that the Applegate mechanism is energetically feasible in 5 PCEB systems. In RX J2130.6+4710, it may be feasible as well considering the uncertainties. We note that these are the systems with the highest rotation rate compared to the critical rotation rate of the main-sequence star. Conclusions. The results suggest that the ratio of physical to critical rotation rate in the main sequence star is an important indicator for the feasibility of Applegate’s mechanism, but exploring larger samples will be necessary to probe this hypothesis.

scholarly journals Semi-empirical modelling of stellar magnetic activity

2011 ◽  
Vol 7 (S286) ◽  
pp. 307-316
Author(s):  
Adriana Valio
Keyword(s):  
Activity Cycle ◽  
Solar Activity Cycle ◽  
Magnetic Activity ◽  
Maunder Minimum ◽  
The Sun ◽  
Cyclic Activity ◽  
Empirical Modelling ◽  
Level Of Activity ◽  
Total Luminosity ◽  
Semi Empirical

AbstractSince Galileo, for four hundred years, dark spots have been observed systematically on the surface of the Sun. The monitoring of the sunspot number has shown that their number varies periodically every 11 years. This is the well-known solar activity cycle that is caused by the periodic changes of the magnetic field of the Sun. Not only do spots vary in number on a timescale of a decade, but the total luminosity and other signatures of activity such as flares and coronal mass ejections also increase and decrease with the 11-year cycle. Still unexplained to the present date are periods of decades with almost an absence of activity, where the best known example is the Maunder Minimum. Other stars also exhibit signs of cyclic activity, however the level of activity is usually thousand times higher than the solar one. Obviously, this is due to the difficulty of observing activity at the solar level on most stars. Presently, a method has been developed to detect and study individual solar like spots on the surface of planet-harbouring stars. As the planet eclipses dark patches on the surface of the star, a detectable signature can be observed in the light curve of the star during the transit. The study of a different variety of stars allows for a better understanding of magnetic cycles and the evolution of stars.

Stabilization and destabilization of turbulent shear flow in a rotating fluid

1992 ◽  
Vol 241 ◽  
pp. 503-523 ◽  
Author(s):  
D. J. Tritton
Keyword(s):  
Experimental Data ◽  
Shear Flow ◽  
Reynolds Stress ◽  
Rotation Rate ◽  
Turbulent Shear ◽  
Rotating Fluid ◽  
Mean Flow ◽  
Rotation Rates ◽  
Principal Axes ◽  
The Mean

We consider turbulent shear flows in a rotating fluid, with the rotation axis parallel or antiparallel to the mean flow vorticity. It is already known that rotation such that the shear becomes cyclonic is stabilizing (with reference to the non-rotating case), whereas the opposite rotation is destabilizing for low rotation rates and restabilizing for higher. The arguments leading to and quantifying these statement are heuristic. Their status and limitations require clarification. Also, it is useful to formulate them in ways that permit direct comparison of the underlying concepts with experimental data.An extension of a displaced particle analysis, given by Tritton & Davies (1981) indicates changes with the rotation rate of the orientation of the motion directly generated by the shear/Coriolis instability occurring in the destabilized range.The ‘simplified Reynolds stress equations scheme’, proposed by Johnston, Halleen & Lezius (1972), has been reformulated in terms of two angles, representing the orientation of the principal axes of the Reynolds stress tensor (αa) and the orientation of the Reynolds stress generating processes (αb), that are approximately equal according to the scheme. The scheme necessarily fails at large rotation rates because of internal inconsistency, additional to the fact that it is inapplicable to two-dimensional turbulence. However, it has a wide range of potential applicability, which may be tested with experimental data. αa and αb have been evaluated from numerical data for homogeneous shear flow (Bertoglio 1982) and laboratory data for a wake (Witt & Joubert 1985) and a free shear layer (Bidokhti & Tritton 1992). The trends with varying rotation rate are notably similar for the three cases. There is a significant range of near equality of αa and αb. An extension of the scheme, allowing for evolution of the flow, relates to the observation of energy transfer from the turbulence to the mean flow.

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