scholarly journals Stellar magnetic activity and the butterfly diagram of Kepler-63

2020 ◽  
Vol 635 ◽  
pp. A78 ◽  
Author(s):  
Y. Netto ◽  
A. Valio
Keyword(s):  
Light Curve ◽  
Reference Frame ◽  
Differential Rotation ◽  
Total Duration ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Dark Spot ◽  
Latitudinal Distribution ◽  
Latitude Distribution ◽  
Butterfly Diagram

Context. The study of young solar-type stars is fundamental for better understanding the magnetic activity of the Sun. Most commonly, this activity manifests itself in the form of spots and faculae. As a planet in transit crosses in front of its host star, a dark spot on the stellar surface may be occulted, causing a detectable variation in the light curve. Kepler-63 is a young solar-like star with an age of only 210 Myr that exhibits photometric variations compatible with spot signatures. Because the planet that orbits it is in an almost polar orbit, different latitudes of the star can be probed by the method of spot transit mapping. Aims. The goal of this work is to characterise the spots of Kepler-63 and thus decipher the behaviour of the young Sun. Because planetary orbit is highly oblique, the latitudinal distribution and thus the differential rotation of the spots may be determined. Methods. A total of 150 transits of Kepler-63b were observed in the short-cadence light curve, corresponding to a total duration of about four years. Each transit light curve was fit by a model that simulates planetary transits and allows including starspots on the surface of the host star. This enables the physical characterisation of the spot size, intensity, and location. We determined the spot position in a reference frame that rotates with the star, and thus obtained the latitudinal distribution of the spots. Results. We fit a total of 297 spots and determined their sizes, intensities, and positions. The longitude and latitude of the spots were calculated in a reference frame that rotated with the star. The latitude distribution of spots exhibits a bimodality with a lack of spots around 34°. Moreover, the spot sizes tend to be larger close to the equator, but decrease toward the latitude distribution gap, after which they again increase toward the poles. High-latitude spots dominate the magnetic cycle of Kepler-63. For a mean stellar rotation period of 5.400 d, 59 spots were found at approximately the same longitude and latitude on a later transit. Some of these spots were detected eight transits later. This shows that the lifetimes of spots can be at least 75 d. Conclusions. The geometry of the Kepler-63 system, enabled us to build a starspot butterfly diagram, similar to that of sunspots. It was also possible to infer the differential rotation of Kepler-63 from the spots at different latitudes. This star was found to rotate almost rigidly with a period of 5.400 d and a relative shear close to 0.01% for latitudes lower than 34°, whereas the high latitudes do not follow a well-behaved pattern.

scholarly journals Starspots properties and stellar activity from planetary transits

2016 ◽  
Vol 12 (S328) ◽  
pp. 69-76
Author(s):  
Adriana Valio
Keyword(s):  
Magnetic Field ◽  
Time Series ◽  
Light Curve ◽  
Physical Properties ◽  
Differential Rotation ◽  
Magnetic Activity ◽  
Stellar Surface ◽  
Stellar Rotation ◽  
Long Time ◽  
Magnetic Cycles

AbstractMagnetic activity of stars manifests itself in the form of dark spots on the stellar surface. This in turn will cause variations of a few percent in the star light curve as it rotates. When an orbiting planet eclipses its host a star, it may cross in front of one of these spots. In this case, a “bump” will be detected in the transit lightcurve. By fitting these spot signatures with a model, it is possible to determine the spots physical properties such as size, temperature, location, magnetic field, and lifetime. Moreover, the monitoring of the spots longitude provides estimates of the stellar rotation and differential rotation. For long time series of transits during multiple years, magnetic cycles can also be determined. This model has been applied successfully to CoRoT-2, CoRoT-4, CoRot-5, CoRoT-6, CoRoT-8, CoRoT-18, Kepler-17, and Kepler-63.

scholarly journals Decoding the radial velocity variations of HD 41248 with ESPRESSO

2020 ◽  
Vol 635 ◽  
pp. A13 ◽  
Author(s):  
J. P. Faria ◽  
V. Adibekyan ◽  
E. M. Amazo-Gómez ◽  
S. C. C. Barros ◽  
J. D. Camacho ◽  
...  
Keyword(s):  
Light Curve ◽  
Radial Velocity ◽  
Gaussian Process ◽  
Process Model ◽  
Moving Average ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Noise Model ◽  
Stellar Rotation ◽  
Gaussian Process Model

Context. Twenty-four years after the discoveries of the first exoplanets, the radial-velocity (RV) method is still one of the most productive techniques to detect and confirm exoplanets. But stellar magnetic activity can induce RV variations large enough to make it difficult to disentangle planet signals from the stellar noise. In this context, HD 41248 is an interesting planet-host candidate, with RV observations plagued by activity-induced signals. Aims. We report on ESPRESSO observations of HD 41248 and analyse them together with previous observations from HARPS with the goal of evaluating the presence of orbiting planets. Methods. Using different noise models within a general Bayesian framework designed for planet detection in RV data, we test the significance of the various signals present in the HD 41248 dataset. We use Gaussian processes as well as a first-order moving average component to try to correct for activity-induced signals. At the same time, we analyse photometry from the TESS mission, searching for transits and rotational modulation in the light curve. Results. The number of significantly detected Keplerian signals depends on the noise model employed, which can range from 0 with the Gaussian process model to 3 with a white noise model. We find that the Gaussian process alone can explain the RV data while allowing for the stellar rotation period and active region evolution timescale to be constrained. The rotation period estimated from the RVs agrees with the value determined from the TESS light curve. Conclusions. Based on the data that is currently available, we conclude that the RV variations of HD 41248 can be explained by stellar activity (using the Gaussian process model) in line with the evidence from activity indicators and the TESS photometry.

scholarly journals Magnetic field and prominences of the young, solar-like, ultra-rapid rotator V530 Persei

2020 ◽  
Vol 643 ◽  
pp. A39
Author(s):  
T.-Q. Cang ◽  
P. Petit ◽  
J.-F. Donati ◽  
C. P. Folsom ◽  
M. Jardine ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Differential Rotation ◽  
Large Scale ◽  
Rotation Period ◽  
Dark Spot ◽  
Field Energy ◽  
Data Set ◽  
Large Scale Magnetic Field ◽  
Rapid Rotator

Context. Young solar analogs reaching the main sequence experience very strong magnetic activity, generating angular momentum losses through wind and mass ejections. Aims. We investigate signatures of magnetic fields and activity at the surface and in the prominence system of the ultra-rapid rotator V530 Per, a G-type solar-like member of the young open cluster α Persei. This object has a rotation period that is shorter than all stars with available magnetic maps. Methods. With a time-series of spectropolarimetric observations gathered with ESPaDOnS over two nights on the Canada-France-Hawaii Telescope, we reconstructed the surface brightness and large-scale magnetic field of V530 Per using the Zeeman-Doppler imaging method, assuming an oblate stellar surface. We also estimated the short term evolution of the brightness distribution through latitudinal differential rotation. Using the same data set, we finally mapped the spatial distribution of prominences through tomography of the Hα emission. Results. The brightness map is dominated by a large, dark spot near the pole, accompanied by a complex distribution of bright and dark features at lower latitudes. Taking the brightness map into account, the magnetic field map is reconstructed as well. Most of the large-scale magnetic field energy is stored in the toroidal field component. The main radial field structure is a positive region of about 500 G, at the location of the dark polar spot. The brightness map of V530 Per is sheared by solar-like differential rotation, with roughly a solar value for the difference in rotation rate between the pole and equator. It is important to note that Hα is observed in emission and it is mostly modulated by the stellar rotation period over one night. The prominence system is organized in a ring at the approximate location of the corotation radius, and displays significant evolution between the two observing nights. Conclusions. V530 Per is the first example of a solar-type star to have its surface magnetic field and prominences mapped together, which will bring important observational constraints to better understand the role of slingshot prominences in the angular momentum evolution of the most active stars.

scholarly journals Is the primary CoRoT target HD 43587 under a Maunder minimum phase?

2020 ◽  
Vol 640 ◽  
pp. A46 ◽  
Author(s):  
R. R. Ferreira ◽  
R. Barbosa ◽  
M. Castro ◽  
G. Guerrero ◽  
L. de Almeida ◽  
...  
Keyword(s):  
Light Curve ◽  
Rotation Period ◽  
Evolutionary Model ◽  
Magnetic Activity ◽  
Maunder Minimum ◽  
Activity Level ◽  
Cycle Period ◽  
Minimum Phase ◽  
Mhd Simulations ◽  
Activity Measurements

Context. One of the most enigmatic phenomena related to solar activity is the so-called Maunder minimum phase. It consists in the lowest sunspot count ever registered for the Sun and has not been confirmed for other stars to date. Since the spectroscopic observations of stellar activity at the Mount Wilson Observatory, the solar analog HD 43587 has shown a very low and apparently invariant activity level, which makes it a Maunder minimum candidate. Aims. We aim to analyze the chromospheric activity evolution of HD 43587 and its evolutive status, with the intention of unraveling the reasons for this low and flat activity. Methods. We used an activity measurements dataset available in the literature, and computed the S-index from HARPS and NARVAL spectra to infer a cycle period. Additionally, we analyzed the CoRoT light curve of HD 43587, and applied gyrochronology and activity calibrations to determine its rotation period. Finally, based on an evolutionary model and the inferred rotation period, we used the EULAG-MHD code to perform global MHD simulations of HD 43587 to get some insight into its dynamo process. Results. We confirm the almost flat activity profile, with a cycle period Pcyc = 10.44 ± 3.03 yr deduced from the S-index time series, and a long-term trend that might be a period of more than 50 yr. It was impossible to define a rotation period from the light curve, however gyrochronology and activity calibrations allow us to infer an indirect estimate of ¯Prot = 22.6 ± 1.9 d. Furthermore, the MHD simulations confirm an oscillatory dynamo with a cycle period in good agreement with the observations and a low level of surface magnetic activity. Conclusions. We conclude that this object might be experiencing a “natural” decrease in magnetic activity as a consequence of its age. Nevertheless, the possibility that HD 43587 is in a Maunder minimum phase cannot be ruled out.

scholarly journals Rotation Profile of Kepler-63 from Planetary Transits

2015 ◽  
Vol 10 (S314) ◽  
pp. 259-261
Author(s):  
Yuri Netto ◽  
Adriana Valio
Keyword(s):  
Light Curve ◽  
Differential Rotation ◽  
Rotation Period ◽  
Stellar Rotation ◽  
Planetary Transits

AbstractCurrently it is possible to estimate the rotation profile of a star that harbours a planet in an orbit such that it eclipses the star periodically. During one of these transits, the planet may occult a spot on the photosphere of the star, causing small variations in its light curve. By detecting the same spot in a later transit, it is possible to estimate the stellar rotation period. Here we present the results of this model for the case of the star Kepler-63, which has a planet in an orbit with high obliquity. This means that the planetary eclipse occults many latitude bands of the star, from near the equator to the poles. The results show that Kepler-63 has differential rotation of 0.133 rd/d and a relative differential rotation of 11.4%.

scholarly journals Learning about Stellar Dynamos from Long–term Photometry of Starspots

1991 ◽  
Vol 130 ◽  
pp. 353-369 ◽  
Author(s):  
Douglas S. Hall
Keyword(s):  
Differential Rotation ◽  
Rotation Period ◽  
Turnover Time ◽  
Rotating Stars ◽  
Photometric Variability ◽  
Solar Type ◽  
Magnetic Cycles ◽  
The Many ◽  
Rapidly Rotating Stars

AbstractSpottedness, as evidenced by photometric variability in 277 late-type binary and single stars, is found to occur when the Rossby number is less than about 2/3. This holds true when the convective turnover time versus B–V relation of Gilliland is used for dwarfs and also for subgiants and giants if their turnover times are twice and four times longer, respectively, than for dwarfs. Differential rotation is found correlated with rotation period (rapidly rotating stars approaching solid-body rotation) and also with lobe-filling factor (the differential rotation coefficient k is 2.5 times larger for F = 0 than F = 1). Also reviewed are latitude extent of spottedness, latitude drift during a solar-type cycle, sector structure and preferential longitudes, starspot lifetimes, and the many observational manifestations of magnetic cycles.

scholarly journals Comparison of the power-2 limb-darkening law from the STAGGER-grid to Kepler light curves of transiting exoplanets

2018 ◽  
Vol 616 ◽  
pp. A39 ◽  
Author(s):  
P. F. L. Maxted
Keyword(s):  
Light Curve ◽  
Probability Distributions ◽  
Binary Star ◽  
Magnetic Activity ◽  
Light Curves ◽  
Practical Implementation ◽  
Light Curve Analysis ◽  
Stagger Grid ◽  
Least Squares Fit ◽  
Limb Darkening

Context. Inaccurate limb-darkening models can be a significant source of error in the analysis of the light curves for transiting exoplanet and eclipsing binary star systems, particularly for high-precision light curves at optical wavelengths. The power-2 limb-darkening law, Iλ(µ) = 1 − c(1−µα), has recently been proposed as a good compromise between complexity and precision in the treatment of limb-darkening. Aims. My aim is to develop a practical implementation of the power-2 limb-darkening law and to quantify the accuracy of this implementation. Methods. I have used synthetic spectra based on the 3D stellar atmosphere models from the STAGGER-grid to compute the limb-darkening for several passbands (UBVRI, CHEOPS, TESS, Kepler, etc.). The parameters of the power-2 limb-darkening laws are optimized using a least-squares fit to a simulated light curve computed directly from the tabulated Iλ(μ) values. I use the transformed parameters h1 = 1 − c(1 − 2−α) and h2 = c2−α to directly compare these optimized limb-darkening parameters to the limb darkening measured from Kepler light curves of 16 transiting exoplanet systems. Results. The posterior probability distributions (PPDs) of the transformed parameters h1 and h2 resulting from the light curve analysis are found to be much less strongly correlated than the PPDs for c and α. The agreement between the computed and observed values of (h1, h2) is generally very good but there are significant differences between the observed and computed values for Kepler-17, the only star in the sample that shows significant variability between the eclipses due to magnetic activity (star spots). Conclusions. The tabulation of h1 and h2 provided here can be used to accurately model the light curves of transiting exoplanets. I also provide estimates of the priors that should be applied to transformed parameters h1 and h2 based on my analysis of the Kepler light curves of 16 stars with transiting exoplanets.

scholarly journals Surface magnetic activity of the fast-rotating G5 giant IN Comae, central star of the faint planetary nebula LoTr 5

2019 ◽  
Vol 624 ◽  
pp. A83 ◽  
Author(s):  
Zs. Kővári ◽  
K. G. Strassmeier ◽  
K. Oláh ◽  
L. Kriskovics ◽  
K. Vida ◽  
...  
Keyword(s):  
Time Series ◽  
Differential Rotation ◽  
Planetary Nebula ◽  
Spectral Synthesis ◽  
Binary Star ◽  
Rotation Period ◽  
Planetary Nebulae ◽  
Asymptotic Giant Branch ◽  
Stellar Surface ◽  
Average Lifetime

Context. On the asymptotic giant branch, low to intermediate mass stars blow away their outer envelopes, forming planetary nebulae. Dynamic interaction between the planetary nebula and its central progenitor is poorly understood. The interaction is even more complex when the central object is a binary star with a magnetically active component, as is the case for the target in this paper. Aims. We aim to quantify the stellar surface activity of the cool binary component of IN Com and aim to explain its origin. In general, we need a better understanding of how central binary stars in planetary nebulae evolve and how this evolution could develop such magnetically active stars as IN Com. Methods. We present a time series of 13 consecutive Doppler images covering six months in 2017 that we used to measure the surface differential rotation with a cross-correlation method. Hitherto unpublished high-precision photometric data from 1989 to 2017 are presented. We applied Fourier-transformation-based frequency analysis to both photometry and spectra. Very high resolution (R ≈ 200 000) spectra were used to update IN Com’s astrophysical parameters by means of spectral synthesis. Results. Our time-series Doppler images show cool and warm spots coexisting with an average surface temperature contrast of −1000 K and +300 K with respect to the effective temperature. Approximately 8% of the stellar surface is covered with cool spots and ∼3% with warm spots. A consistent cool polar spot is seen in all images. The average lifetime of the cool spots is not much more than a few stellar rotations (one month), while the warm spots appear to live longer (three months) and are mostly confined to high latitudes. We found anti-solar surface differential rotation with a shear coefficient of α = −0.026 ± 0.005 suggesting an equatorial rotation period of 5.973 ± 0.008 d. We reconfirm the 5.9 day rotation period of the cool star from photometry, radial velocities, and Hα line-profile variations. A long-term V-brightness variation with a likely period of 7.2 yr is also found. It appears in phase with the orbital radial velocity of the binary system in the sense that it is brightest at highest velocity and faintest at lowest velocity, that is, at the two phases of quadrature. We redetermine [Ba/Fe], [Y/Fe], and [Sr/Fe] ratios and confirm the overabundance of these s-process elements in the atmosphere of IN Com.

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 Magnetic activity in the HARPS M dwarf sample

2017 ◽  
Vol 600 ◽  
pp. A13 ◽  
Author(s):  
N. Astudillo-Defru ◽  
X. Delfosse ◽  
X. Bonfils ◽  
T. Forveille ◽  
C. Lovis ◽  
...  
Keyword(s):  
Monitoring Program ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Spectral Lines ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
The Galaxy ◽  
Low Mass ◽  
M Dwarf ◽  
The Relationship

Context. Atmospheric magnetic fields in stars with convective envelopes heat stellar chromospheres, and thus increase the observed flux in the Ca ii H and K doublet. Starting with the historical Mount Wilson monitoring program, these two spectral lines have been widely used to trace stellar magnetic activity, and as a proxy for rotation period (Prot) and consequently for stellar age. Monitoring stellar activity has also become essential in filtering out false-positives due to magnetic activity in extra-solar planet surveys. The Ca ii emission is traditionally quantified through the R'HK-index, which compares the chromospheric flux in the doublet to the overall bolometric flux of the star. Much work has been done to characterize this index for FGK-dwarfs, but M dwarfs – the most numerous stars of the Galaxy – were left out of these analyses and no calibration of their Ca ii H and K emission to an R'HK exists to date. Aims. We set out to characterize the magnetic activity of the low- and very-low-mass stars by providing a calibration of the R'HK-index that extends to the realm of M dwarfs, and by evaluating the relationship between R'HK and the rotation period. Methods. We calibrated the bolometric and photospheric factors for M dwarfs to properly transform the S-index (which compares the flux in the Ca ii H and K lines to a close spectral continuum) into the R'HK. We monitored magnetic activity through the Ca ii H and K emission lines in the HARPS M dwarf sample. Results. The R'HK index, like the fractional X-ray luminosity LX/Lbol, shows a saturated correlation with rotation, with saturation setting in around a ten days rotation period. Above that period, slower rotators show weaker Ca ii activity, as expected. Under that period, the R'HK index saturates to approximately 10-4. Stellar mass modulates the Ca ii activity, with R'HK showing a constant basal activity above 0.6 M⊙ and then decreasing with mass between 0.6 M⊙ and the fully-convective limit of 0.35 M⊙. Short-term variability of the activity correlates with its mean level and stars with higher R'HK indexes show larger R'HK variability, as previously observed for earlier spectral types.

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