scholarly journals Magnetic activity of the solar-like star HD 140538

2019 ◽  
Vol 628 ◽  
pp. A107 ◽  
Author(s):  
M. Mittag ◽  
J. H. M. M. Schmitt ◽  
T. S. Metcalfe ◽  
A. Hempelmann ◽  
K.-P. Schröder
Keyword(s):  
Rotation Period ◽  
Activity Cycle ◽  
Magnetic Activity ◽  
Main Sequence Stars ◽  
Age Relation ◽  
Activity Cycles ◽  
Index Time Series ◽  
Evolution With Time ◽  
High Cadence

The periods of rotation and activity cycles are among the most important properties of the magnetic dynamo thought to be operating in late-type, main-sequence stars. In this paper, we present a SMWO-index time series composed from different data sources for the solar-like star HD 140538 and derive a period of 3.88 ± 0.02 yr for its activity cycle. Furthermore, we analyse the high-cadence, seasonal SMWO data taken with the TIGRE telescope and find a rotational period of 20.71 ± 0.32 days. In addition, we estimate the stellar age of HD 140538 as 3.7 Gyrs via a matching evolutionary track. This is slightly older than the ages obtained from gyrochronology based on the above rotation period, as well as the activity-age relation. These results, together with its stellar parameters that are very similar to a younger Sun, make HD 140538 a relevant case study for our understanding of solar activity and its evolution with time.

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 Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars

2018 ◽  
Vol 618 ◽  
pp. A48 ◽  
Author(s):  
M. Mittag ◽  
J. H. M. M. Schmitt ◽  
K.-P. Schröder
Keyword(s):  
Main Sequence ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Turnover Time ◽  
Rossby Number ◽  
Stellar Rotation ◽  
Main Sequence Stars ◽  
Rotation Periods ◽  
Period Distribution ◽  
Cool Stars

The connection between stellar rotation, stellar activity, and convective turnover time is revisited with a focus on the sole contribution of magnetic activity to the Ca II H&K emission, the so-called excess flux, and its dimensionless indicator R+HK in relation to other stellar parameters and activity indicators. Our study is based on a sample of 169 main-sequence stars with directly measured Mount Wilson S-indices and rotation periods. The R+HK values are derived from the respective S-indices and related to the rotation periods in various B–V-colour intervals. First, we show that stars with vanishing magnetic activity, i.e. stars whose excess flux index R+HK approaches zero, have a well-defined, colour-dependent rotation period distribution; we also show that this rotation period distribution applies to large samples of cool stars for which rotation periods have recently become available. Second, we use empirical arguments to equate this rotation period distribution with the global convective turnover time, which is an approach that allows us to obtain clear relations between the magnetic activity related excess flux index R+HK, rotation periods, and Rossby numbers. Third, we show that the activity versus Rossby number relations are very similar in the different activity indicators. As a consequence of our study, we emphasize that our Rossby number based on the global convective turnover time approaches but does not exceed unity even for entirely inactive stars. Furthermore, the rotation-activity relations might be universal for different activity indicators once the proper scalings are used.

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 Multi-wavelength variability of the young solar analog ι Horologii

2019 ◽  
Vol 631 ◽  
pp. A45 ◽  
Author(s):  
J. Sanz-Forcada ◽  
B. Stelzer ◽  
M. Coffaro ◽  
S. Raetz ◽  
J. D. Alvarado-Gómez
Keyword(s):  
Light Curve ◽  
Thermal Structure ◽  
Rotation Period ◽  
Activity Cycle ◽  
X Rays ◽  
X Ray ◽  
Multi Wavelength ◽  
Activity Cycles ◽  
Irregular Behavior

Context. Chromospheric activity cycles are common in late-type stars; however, only a handful of coronal activity cycles have been discovered. ι Hor is the most active and youngest star with known coronal cycles. It is also a young solar analog, and we are likely facing the earliest cycles in the evolution of solar-like stars, at an age (~600 Myr) when life appeared on Earth. Aims. Our aim is to confirm the ~1.6 yr coronal cycle and characterize its stability over time. We use X-ray observations of ι Hor to study the corona of a star representing the solar past through variability, thermal structure, and coronal abundances. Methods. We analyzed multi-wavelength observations of ι Hor using XMM-Newton, TESS, and HST data. We monitored ι Hor throughout almost seven years in X-rays and in two UV bands. The summed RGS and STIS spectra were used for a detailed thermal structure model, and the determination of coronal abundances. We studied rotation and flares in the TESS light curve. Results. We find a stable coronal cycle along four complete periods, more than covered in the Sun. There is no evidence for a second longer X-ray cycle. Coronal abundances are consistent with photospheric values, discarding any effects related to the first ionization potential. From the TESS light curve we derived the first photometric measurement of the rotation period (8.2 d). No flares were detected in the TESS light curve of ι Hor. We estimate the probability of having detected zero flares with TESS to be ~2%. Conclusions. We corroborate the presence of an activity cycle of ~1.6 yr in ι Hor in X-rays, more regular than its Ca II H&K counterpart. A decoupling of the activity between the northern and southern hemispheres of the star might explain the disagreement. The inclination of the system would result in an irregular behavior in the chromospheric indicators. The more extended coronal material would be less sensitive to this effect.

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 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.

scholarly journals Determination of the star-spot covering fraction as a function of stellar age from observational data

2019 ◽  
Vol 491 (2) ◽  
pp. 2706-2714
Author(s):  
Fiona Nichols-Fleming ◽  
Eric G Blackman
Keyword(s):  
Power Law ◽  
Main Sequence ◽  
Full Range ◽  
Rotation Period ◽  
Main Sequence Stars ◽  
Rotation Periods ◽  
Fraction Versus ◽  
Activity Cycles ◽  
Rotation Age

ABSTRACT The association of star-spots with magnetic fields leads to an expectation that quantities which correlate with magnetic field strength may also correlate with star-spot coverage. Since younger stars spin faster and are more magnetically active, assessing whether star-spot coverage correlates with shorter rotation periods and stellar youth tests these principles. Here, we analyse the star-spot covering fraction versus stellar age for M-, G-, K-, and F-type stars based on previously determined variability and rotation periods of over 30 000 Kepler main-sequence stars. We determine the correlation between age and variability using single and dual power-law best fits. We find that star-spot coverage does indeed decrease with age. Only when the data are binned in an effort to remove the effects of activity cycles of individual stars, do statistically significant power-law fits emerge for each stellar type. Using bin averages, we then find that the star-spot covering fraction scales with the X-ray to bolometric ratio to the power λ with 0.22 ± 0.03 < λ < 0.32 ± 0.09 for G-type stars of rotation period below 15 d and for the full range of F- and M-type stars. For K-type stars, we find two branches of λ separated by variability bins, with the lower branch showing nearly constant star-spot coverage and the upper branch λ ∼ 0.35 ± 0.04. G-type stars with periods longer than 15 d exhibit a transition to steeper power law of λ ∼ 2.4 ± 1.0. The potential connection to previous rotation-age measurements suggesting a magnetic breaking transition at the solar age, corresponding to period of 24.5 is also of interest.

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 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.

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