Stellar Magnetic Cycles in the Solar-Like Stars KEPLER-17 and KEPLER-63 (original) (raw)
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Using planetary transits to estimate magnetic cycles lengths in Kepler stars
2017
Observations of various solar-type stars along decades revealed that they can have magnetic cycles, just like our Sun. An investigation of the relation between their cycle length and rotation period can shed light on the dynamo mechanisms operating in these stars. Previous works on this relation suggested that the stars could be separated into active and inactive branches, with the Sun falling between them. In this work, we determined short magnetic activity cycles for 6 active solar-type stars observed by the Kepler telescope. The method adopted here estimates the activity from the excess in the residuals of the transitlight curves. This excess is obtained by subtracting a spotless model transit from the light curve, and then integrating over all the residuals during the transit. The presence of long term periodicity is estimated from the analysis of a Lomb-Scargle periodogram of the complete time series. Finally, we investigate the rotation-cycle period relation for the stars anal...
Magnetic activity of six young solar analogues I. Starspot cycles from long-term photometry
Astronomy & Astrophysics, 2002
The present paper is the second of a series dedicated to the study of the magnetic activity in a selected sample of young solar analogues. The sample includes five single G0-G5V stars with ages between 130 Myr and 700 Myr: EK Dra, π 1 UMa, HN Peg, k 1 Cet and BE Cet. In this study we also include the Pleiades-age ( 130 Myr) K0V star DX Leo. Our analysis is based on high precision photometric observations carried out as part of The Sun in Time project, aimed at a multiwavelength study of stars with solar-like global properties, but with different ages and thus at different stages of evolution. In the first papers of this series we presented the photometric observations and determined the existence of starspot cycles and their correlation with the global stellar properties. In the present paper we investigate the surface differential rotation (SDR). The periodogram analysis of the photometric data time series has allowed us to determine the rotational periods and to derive the following results: i) all the selected stars show variations of the rotational period. Such variations are definitely periodic and in phase with the starspot cycle for BE Cet and DX Leo. They are likely periodic and in phase also for π 1 UMa, EK Dra and HN Peg, but still need confirmation. By analogy with the solar butterfly diagram, the rotational period variations are interpretable in terms of surface differential rotation, that is, they are attributable to the existence of active latitude belts migrating during the activity cycle on a differentially rotating star; ii) BE Cet, π 1 UMa and EK Dra show a solar-like pattern of SDR, that is the rotational period steadily decreases along the activity cycle, jumping back to higher values at the beginning of the next cycle; on the contrary, DX Leo, k 1 Cet and HN Peg show an antisolar pattern; iii) the amplitude of the rotational period variations shows a power law dependence on the rotational period similar to that found in previous studies. Contrary to theoretical predictions, the cycle length is not correlated to the Dynamo number, it is indeed positively correlated to the SDR amplitude. More precisely, stars tend to concentrate along three different branches with the cycle length increasing with increasing ∆Ω/Ω. Moreover, we found that the SDR amplitude changes from cycle to cycle, which is reminiscent of a wave of excess rotation propagating in latitude; iiii) the apparently different solar and antisolar behaviours are probably due to different inclinations of the stellar rotation axis under which the star is seen. The long-term photometry of the young single star LQ Hya, although not included in the initial project, is also used in the present analysis to enlarge the investigated sample. We determined for LQ Hya three different starspot cycles and an antisolar pattern of SDR.
Further Analysis of Stellar Magnetic Cycle Periods
2001
We further investigate relationships between activity cycle periods in cool stars and rotation to include new cycle data, and explore different parameterizations of the problem. We find that relations between cycle and rotational frequencies (ω cyc vs. Ω) and between their ratio and the inverse Rossby number (ω cyc /Ω vs. Ro −1 ) show many similarities, including three branches and similar rms scatter. We briefly discuss some implications for dynamo models.
Stellar magnetic activity and the butterfly diagram of Kepler-63
Astronomy and Astrophysics, 2020
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.
Starspots properties and stellar activity from planetary transits
Proceedings of the International Astronomical Union
Magnetic 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.
Time Evolution of the Magnetic Activity Cycle Period. II. Results for an Expanded Stellar Sample
The Astrophysical Journal, 1999
We further explore nondimensional relationships between the magnetic dynamo cycle period the P cyc , rotational period the activity level (as observed in Ca II HK), and other stellar properties by P rot , expanding the stellar sample studied in the Ðrst paper in this series. We do this by adding photometric and other cycles seen in active stars and the secondaries of CV systems and by selectively adding less certain cycles from the Mount Wilson HK survey ; evolved stars, long-term HK trends and secondary are also considered. We conÐrm that most stars with age Gyr occupy two roughly parallel P cyc t Z 0.1 branches, separated by a factor of D6 in with the ratio of cycle and rotational frequencies P cyc , where Ro is the Rossby number. Using the model of the Ðrst paper in this series, this u cyc /) P Ro~0.5, result implies that the a e †ect increases with mean magnetic Ðeld (contrary to the traditional aquenching concept) and that a and decrease with t. Stars are not strictly segregated onto one or the u cyc other branch by activity level, though the branch is primarily composed of inactive stars. high-u cyc /) The expanded data set suggests that for Gyr, stars can have cycles on one or both branches, t Z 1 though among older stars, those with higher (lower) mass tend to have their primary on the lower P cyc (upper) branch. The SunÏs D80 yr Gleissberg cycle agrees with this scenario, suggesting that long-u cyc /) term activity "" trends ÏÏ in many stars may be segments of long yr) cycles not yet resolved (P cyc D 50È100 by the data. Most very active stars days) appear to occupy a new, third branch with (P rot \ 3 u cyc /) P Ro0.4. Many RS CVn variables lie in a transition region between the two most active branches. We compare our results with various models, discuss their implications for dynamo theory and evolution, and use them to predict for three groups : stars with long-term HK trends, stars in young open P cyc clusters, and stars that may be in Maunder-like magnetic minima.
Proceedings of the International Astronomical Union
The solar activity cycle is a manifestation of the hydromagnetic dynamo working inside our star. The detection of activity cycles in solar-like stars and the study of their properties allow us to put the solar dynamo in perspective, investigating how dynamo action depends on stellar parameters and stellar structure. Nevertheless, the lack of spatial resolution and the limited time extension of stellar data pose limitations to our understanding of stellar cycles and the possibility to constrain dynamo models. I briefly review some results obtained from disc-integrated proxies of stellar magnetic fields and discuss the new opportunities opened by space-borne photometry made available by MOST, CoRoT, Kepler, and GAIA, and by new ground-based spectroscopic or spectropolarimetric observations. Stellar cycles have a significant impact on the energetic output and circumstellar magnetic fields of late-type active stars which affects the interaction between stars and their planets. On the ot...
Magnetic activities on active solar-type stars
arXiv: Solar and Stellar Astrophysics, 2019
We present results obtained from the studies of magnetic activities on four solar-type stars (F-type star KIC 6791060, K-type star LO Peg, and two M-type planet-hosting stars K2-33 and EPIC 211901114) by using optical observations from several ground- and space-based telescopes. In this study, we investigate magnetic activities such as spot-topographic evolution and flaring events in these stars. We compare the results obtained from this study with that of the Sun. In the surface temperature maps, one active longitude has been detected in KIC 6791060, whereas in each of the other three cases two active longitudes are seen. The spottedness was found to vary in the range of 0.07--0.44%, 9--26%, 3.6--4.2%, and 4.5--5.3% for KIC 6791060, LO Peg, K2-33, and EPIC 211901114, respectively. Several flaring events have been identified in each star. An increasing trend in flaring frequency per stellar rotation has been found in the stars with a change in spectral type from F to M. These findin...
Research in Astronomy and Astrophysics, 2016
The atmospheric activity of the Sun and Sun-like stars is analyzed involving observations from HK-project at the Mount Wilson Observatory, the California and Carnegie Planet Search Program at the Keck and Lick Observatories and the Magellan Planet Search Program at the Las Campanas Observatory. We show that for stars of F, G and K spectral classes, the cyclic activity, similar to the 11-yr solar cycles, is different: it becomes more prominent in K-stars. Comparative study of Sun-like stars with different levels of the chromospheric and coronal activity confirms that the Sun belongs to stars with the low level of the chromospheric activity and stands apart among these stars by the minimum level of its coronal radiation and the minimum level of its variations of the photospheric flux.