scholarly journals An application of a solar-type dynamo model for ε Eridani

2020 ◽  
Vol 497 (3) ◽  
pp. 3968-3975
Author(s):  
A P Buccino ◽  
L Sraibman ◽  
P M Olivar ◽  
F O Minotti
Keyword(s):  
Activity Cycle ◽  
Maunder Minimum ◽  
Dynamo Model ◽  
Dynamo Theory ◽  
Great Opportunity ◽  
Non Linear ◽  
Solar Type ◽  
Short And Long Term ◽  
Activity Cycles

ABSTRACT During the last decade, the relation between activity cycle periods and stellar parameters has received special attention. The construction of reliable registries of activity reveals that solar-type stars exhibit activity cycles with periods from few years to decades and, in some cases, long and short activity cycles coexist suggesting that two dynamos could operate in these stars. In particular, ε Eridani is an active young K2V star (0.8 Gyr), which exhibits short and long-term chromospheric cycles of ∼3 and ∼13-yr periods. Additionally, between 1985 and 1992, the star went through a broad activity minimum, similar to the solar Maunder Minimum state. Motivated by these results, we found in ε Eridani a great opportunity to test the dynamo theory. Based on the model developed in Sraibman & Minotti, in this work we built a non-linear axisymmetric dynamo for ε Eridani. The time series of the simulated magnetic field components near the surface integrated in all the stellar disc exhibits both the long and short activity cycles with periods similar to the ones detected from observations and also time intervals of low activity that could be associated with the broad Minimum. The short activity cycle associated with the magnetic reversal could be explained by the differential rotation, while the long cycle is associated with the meridional mass flows induced by the Lorentz force. In this way, we show that a single non-linear dynamo model derived from first principles with accurate stellar parameters could reproduce coexisting activity cycles.

scholarly journals Activity–rotation in the dM4 star Gl 729

2020 ◽  
Vol 644 ◽  
pp. A2
Author(s):  
R. V. Ibañez Bustos ◽  
A. P. Buccino ◽  
S. Messina ◽  
A. F. Lanza ◽  
P. J. D. Mauas
Keyword(s):  
Differential Rotation ◽  
Activity Cycle ◽  
Magnetic Activity ◽  
Power Law Distribution ◽  
Dynamo Theory ◽  
Strong Shear ◽  
Solar Type ◽  
Activity Cycles ◽  
The One

Aims. Recently, new debates about the role of layers of strong shear have emerged in stellar dynamo theory. Further information on the long-term magnetic activity of fully convective stars could help determine whether their underlying dynamo could sustain activity cycles similar to the solar one. Methods. We performed a thorough study of the short- and long-term magnetic activity of the young active dM4 star Gl 729. First, we analyzed long-cadence K2 photometry to characterize its transient events (e.g., flares) and global and surface differential rotation. Then, from the Mount Wilson S-indexes derived from CASLEO spectra and other public observations, we analyzed its long-term activity between 1998 and 2020 with four different time-domain techniques to detect cyclic patterns. Finally, we explored the chromospheric activity at different heights with simultaneous measurements of the Hα and the Na I D indexes, and we analyzed their relations with the S-Index. Results. We found that the cumulative flare frequency follows a power-law distribution with slope ~−0.73 for the range 1032–1034 erg. We obtained Prot = (2.848 ± 0.001) days, and we found no evidence of differential rotation. We also found that this young active star presents a long-term activity cycle with a length of about 4 yr; there is less significant evidence of a shorter cycle of 0.8 yr. The star also shows a broad activity minimum between 1998 and 2004. We found a correlation between the S index, on the one hand, and the Hα the Na I D indexes, on the other hand, although the saturation level of these last two indexes is not observed in the Ca lines. Conclusions. Because the maximum-entropy spot model does not reflect migration between active longitudes, this activity cycle cannot be explained by a solar-type dynamo. It is probably caused by an α2-dynamo.

scholarly journals HD 38858: a solar-type star with an activity cycle of ∼10.8 yr

2018 ◽  
Vol 620 ◽  
pp. A34 ◽  
Author(s):  
M. Flores ◽  
J. F. González ◽  
M. Jaque Arancibia ◽  
C. Saffe ◽  
A. Buccino ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Activity Cycle ◽  
Earth Planet ◽  
Stellar Activity ◽  
Balmer Lines ◽  
Solar Type ◽  
Activity Cycles ◽  
Novel Strategy ◽  
Solar Type Star

Context. The detection of chromospheric activity cycles in solar-analogue and twin stars can be used to place the solar cycle in a wider context. However, relatively few of these stars with activity cycles have been detected. It is well known that the cores of the Ca II H&K lines are modulated by stellar activity. The behaviour of the Balmer and other optical lines with stellar activity is not yet completely understood. Aims. We search for variations in the Ca II H&K, Balmer, and Fe II lines modulated by stellar activity. In particular, we apply a novel strategy to detect possible shape variations in the Hα line. Methods. We analysed activity signatures in HD 38858 using HARPS and CASLEO spectra obtained between 2003 and 2017. We calculated the Mount Wilson index (SMW), log(R′HK), and the statistical moments of the Ca II H&K, Balmer, and other optical lines. We searched for periodicities using the generalized Lomb-Scargle periodogram. Results. We detect a long-term activity cycle of 10.8 yr in Ca II H&K and Hα in the solar-analogue star HD 38858. In contrast, this cycle is marginally detected in the Fe II lines. We also detect a noticeable variation in radial velocity that seems to be produced by stellar activity. Conclusions. HD 38858 is the second solar-analogue star where we find a clear activity cycle that is replicated in the Balmer lines. Spectral indexes based on the shape of Hα line seem to be more reliable than the fluxes in the same line for detecting activity variations. The cyclic modulation we detected gives place to a variation in radial velocity that previously has been associated with a super-Earth planet. Finally, due to the similarity of HD 38858 with the Sun, we recommend to continue monitoring this star.

scholarly journals Helioseismic inferences

2009 ◽  
Vol 5 (S264) ◽  
pp. 33-38
Author(s):  
Hiromoto Shibahashi
Keyword(s):  
Solar Activity ◽  
Linear Approximation ◽  
Activity Cycle ◽  
Solar Activity Cycle ◽  
Convective Zone ◽  
Dynamo Model ◽  
Kinematic Dynamo ◽  
Wide Range ◽  
Dynamo Mechanism ◽  
Solar Type

AbstractThe brilliant outcome of some 30 years of helioseismology spreads over a wide range of topics. Some highlights relevant to the cause of the solar activity cycle are listed up. The rotation profile in the solar convective zone is discussed as an important source of the dynamo mechanism. The kinematic dynamo model is described in the linear approximation, and the condition for the solar type dynamo is derived. It is shown that comparison of this condition with the rotation profile determined from helioseismology is useful to identify the possible seats of the dynamo.

scholarly journals Discovery of short-term activity cycles in F-type stars

2019 ◽  
Vol 621 ◽  
pp. A136 ◽  
Author(s):  
M. Mittag ◽  
J. H. M. M. Schmitt ◽  
A. Hempelmann ◽  
K.-P. Schröder
Keyword(s):  
Activity Cycle ◽  
Total Activity ◽  
Short Term ◽  
Cyclic Activity ◽  
Solar Type ◽  
Cyclic Variations ◽  
Activity Cycles ◽  
F Stars ◽  
One Year ◽  
Index Time Series

Previous studies have revealed a 120 day activity cycle in the F-type star τ Boo, which represents the shortest activity cycle discovered until now. The question arises as to whether or not short-term activity cycles are a common phenomenon in F-type stars. To address this question, we analyse S-index time series of F-type stars taken with the TIGRE telescope to search for periodic variations with a maximal length of 2 years using the generalised Lomb-Scargle periodogram method. In our sample, we find four F-type stars showing periodic variations shorter than one year. However, the amplitude of these variations in our sample of F-star type stars appears to be smaller than that of solar-type stars with well-developed cyclic activity, and apparently represents only a part of the total activity. We conclude that among F-stars, the time-behaviour of activity differs from that of the Sun and cooler main sequence stars, as short-term cyclic variations with shallow amplitude of the cycle seem to prevail, rather than cycles with 10+ years periods and a larger cycle amplitude.

scholarly journals Global evolution of solar magnetic fields and prediction of activity cycles

2019 ◽  
Vol 15 (S354) ◽  
pp. 147-156
Author(s):  
Irina N. Kitiashvili
Keyword(s):  
Solar Activity ◽  
Magnetic Fields ◽  
Activity Cycle ◽  
Dynamo Model ◽  
Solar Magnetic Fields ◽  
Solar Cycles ◽  
Multiscale Dynamics ◽  
Activity Cycles ◽  
Solar Dynamics ◽  
Solar Activity Cycles

AbstractPrediction of solar activity cycles is challenging because physical processes inside the Sun involve a broad range of multiscale dynamics that no model can reproduce and because the available observations are highly limited and cover mostly surface layers. Helioseismology makes it possible to probe solar dynamics in the convective zone, but variations in differential rotation and meridional circulation are currently available for only two solar activity cycles. It has been demonstrated that sunspot observations, which cover over 400 years, can be used to calibrate the Parker-Kleeorin-Ruzmaikin dynamo model, and that the Ensemble Kalman Filter (EnKF) method can be used to link the modeled magnetic fields to sunspot observations and make reliable predictions of a following activity cycle. However, for more accurate predictions, it is necessary to use actual observations of the solar magnetic fields, which are available only for the last four solar cycles. In this paper I briefly discuss the influence of the limited number of available observations on the accuracy of EnKF estimates of solar cycle parameters, the criteria to evaluate the predictions, and application of synoptic magnetograms to the prediction of solar activity.

scholarly journals Sunspot cycles and Grand Minima

2009 ◽  
Vol 5 (S264) ◽  
pp. 111-119 ◽  
Author(s):  
Dmitry Sokoloff ◽  
Rainer Arlt ◽  
David Moss ◽  
Steven H. Saar ◽  
Ilya Usoskin
Keyword(s):  
Magnetic Activity ◽  
Dynamo Model ◽  
Isotopic Data ◽  
Dynamo Theory ◽  
History Of ◽  
Solar Magnetic Activity ◽  
Basic Features ◽  
Random Fluctuations ◽  
Grand Minima

AbstractObservational data concerning the long-term history of cyclic solar activity as recorded in sunspot and isotopic data are discussed in the context of solar dynamo theory. In particular, a simple dynamo model based on differential rotation and the mirror asymmetry of convection with random fluctuations of dynamo governing parameters is shown to reproduce some basic features of the solar magnetic activity evolution.

scholarly journals Short- and long-term variations in non-linear dynamics of heart rate variability

1996 ◽  
Vol 31 (3) ◽  
pp. 400-409 ◽  
Author(s):  
J. K. Kanters ◽  
M. V. Hojgaard ◽  
E. Agner ◽  
N.-H. Holstein-Rathlou
Keyword(s):  
Heart Rate ◽  
Heart Rate Variability ◽  
Linear Dynamics ◽  
Non Linear ◽  
Short And Long Term ◽  
Long Term Variations

Short and long term non-linear analysis of RR variability series

2002 ◽  
Vol 24 (1) ◽  
pp. 21-32 ◽  
Author(s):  
G. Baselli ◽  
S. Cerutti ◽  
A. Porta ◽  
M.G. Signorini
Keyword(s):  
Linear Analysis ◽  
Non Linear ◽  
Short And Long Term ◽  
Rr Variability

scholarly journals Ross 128 – GL 447

2019 ◽  
Vol 628 ◽  
pp. L1 ◽  
Author(s):  
R. V. Ibañez Bustos ◽  
A. P. Buccino ◽  
M. Flores ◽  
P. J. D. Mauas
Keyword(s):  
Time Series ◽  
Activity Cycle ◽  
Magnetic Activity ◽  
Extrasolar Planet ◽  
Low Mass Stars ◽  
Chromospheric Activity ◽  
Low Mass ◽  
Activity Cycles

Context. Long-term chromospheric activity in slow-rotating fully convective stars has scarcely been explored. Ross 128 (Gl 447) is a slow-rotator and inactive dM4 star that has been extensively observed. It hosts the fourth closest extrasolar planet. Aims. Ross 128 is an ideal target to test dynamo theories in slow-rotating low-mass stars. Methods. To characterize the magnetic activity of Ross 128, we studied the SK-indexes derived from CASLEO, HARPS, FEROS, UVES, and X-shooter spectra. Using the generalized Lomb-Scargle and CLEAN periodograms, we analyzed the whole SK time-series obtained between 2004 and 2018. We performed a similar analysis for the Na I-index, and we analyzed its relation with the SK-index. Results. From both indexes, we obtain a possible activity cycle with a period of about five years, which is one of a small handful of activity cycles that have been reported for a slow-rotating fully convective star.

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