Kepler allows the measurement of starspot variability in a large sample of field red giants for the first time. With a new method that combines autocorrelation and wavelet decomposition, we measure ...361 rotation periods from the full set of 17 377 oscillating red giants in our sample. This represents 2.08% of the stars, consistent with the fraction of spectroscopically detected rapidly rotating giants in the field. The remaining stars do not show enough variability to allow us to measure a reliable surface rotation period. Because the stars with detected rotation periods have measured oscillations, we can infer their global properties, e.g. mass and radius, and quantitatively evaluate the predictions of standard stellar evolution models as a function of mass. Consistent with results for cluster giants when we consider only the 4881 intermediate-mass stars, M > 2.0 M⊙ from our full red giant sample, we do not find the enhanced rates of rapid rotation expected from angular momentum conservation. We therefore suggest that either enhanced angular momentum loss or radial differential rotation must be occurring in these stars. Finally, when we examine the 575 low-mass (M< 1.1 M⊙) red clump stars in our sample, which were expected to exhibit slow (non-detectable) rotation, 15% of them actually have detectable rotation. This suggests a high rate of interactions and stellar mergers on the red giant branch.
Context. Thanks to recent asteroseismic observations, it has been possible to infer the radial differential rotation profile of subgiants and red giants. Aims. We want to reproduce the observed ...rotation profile of the early red giant KIC 7341231 through modeling and constrain the physical mechanisms responsible for angular momentum transport in stellar interiors. Methods. We computed models of KIC 7341231 including a treatment of shellular rotation, and we compared the rotation profiles obtained with the one derived from observations. We then modify some modeling parameters in order to quantify their effect on the obtained rotation profile. Moreover, we mimicked a powerful angular momentum transport during the main sequence and studied its effect on the evolution of the rotation profile during the subgiant and red giant phases. Results. We show that meridional circulation and shear mixing alone produce a rotation profile for KIC 7341231 that is too steep compared to the observed one. An additional mechanism is then needed to increase the internal transport of angular momentum. We find that this undetermined mechanism has to be efficient not only during the main sequence but also during the much quicker subgiant phase. Moreover, we point out the importance of studying the whole rotational history of a star to explain its rotation profile during the red giant evolution.
The rotation rates in the deep interior and at the surface of 22 main-sequence stars with masses between 1.0 and 1.6 M⊙ are constrained by combining asteroseismological analysis with spectroscopic ...measurements. The asteroseismic data of each star are taken by the Kepler or CoRoT space mission. It is found that the difference between the surface rotation rate and the average rotation rate (excluding the convective core) of most of stars is small enough to suggest that an efficient process of angular momentum transport operates during and/or before the main-sequence stage of stars. If each of the surface convective zone and the underlying radiative zone, for individual stars, is assumed to rotate uniformly, the difference in the rotation rate between the two zones turns out to be no more than a factor of 2 in most of the stars independently of their ages.
We present the results of a blind exercise to test the recoverability of stellar rotation and differential rotation in Kepler light curves. The simulated light curves lasted 1000 d and included ...activity cycles, Sun-like butterfly patterns, differential rotation and spot evolution. The range of rotation periods, activity levels and spot lifetime were chosen to be representative of the Kepler data of solar-like stars. Of the 1000 simulated light curves, 770 were injected into actual quiescent Kepler light curves to simulate Kepler noise. The test also included five 1000-d segments of the Sun's total irradiance variations at different points in the Sun's activity cycle. Five teams took part in the blind exercise, plus two teams who participated after the content of the light curves had been released. The methods used included Lomb–Scargle periodograms and variants thereof, autocorrelation function and wavelet-based analyses, plus spot modelling to search for differential rotation. The results show that the ‘overall’ period is well recovered for stars exhibiting low and moderate activity levels. Most teams reported values within 10 per cent of the true value in 70 per cent of the cases. There was, however, little correlation between the reported and simulated values of the differential rotation shear, suggesting that differential rotation studies based on full-disc light curves alone need to be treated with caution, at least for solar-type stars. The simulated light curves and associated parameters are available online for the community to test their own methods.
The solar analogues 16 Cyg A and B are excellent asteroseismic targets in the Kepler field of view and together with a red dwarf and a Jovian planet form an interesting system. For these more evolved ...Sun-like stars we cannot detect surface rotation with the current Kepler data but instead use the technique of asteroseimology to determine rotational properties of both 16 Cyg A and B. We find the rotation periods to be ... and ..., and the angles of inclination to be ... and ..., for A and B, respectively. Together with these results we use the published mass and age to suggest that, under the assumption of a solar-like rotation profile, 16 Cyg A could be used when calibrating gyrochronology relations. In addition, we discuss the known 16 Cyg B star-planet eccentricity and measured low obliquity which is consistent with Kozai cycling and tidal theory. (ProQuest: ... denotes formulae/symbols omitted.)
Kepler ultra-high precision photometry of long and continuous observations provides a unique dataset in which surface rotation and variability can be studied for thousands of stars. Because many of ...these old field stars also have independently measured asteroseismic ages, measurements of rotation and activity are particularly interesting in the context of age-rotation-activity relations. We study the surface rotation and photometric magnetic activity of a subset of 540 solar-like stars on the main-sequence and the subgiant branch for which stellar pulsations have been measured. The photometric magnetic activity levels of these stars were computed, and for 61.5% of the dwarfs, this level is similar to the range, from minimum to maximum, of the solar magnetic activity. We demonstrate that hot dwarfs, cool dwarfs, and subgiants have very different rotation-age relationships, highlighting the importance of separating out distinct populations when interpreting stellar rotation periods. Our sample of cool dwarf stars with age and metallicity data of the highest quality is consistent with gyrochronology relations reported in the literature.
Context. We still do not understand which physical mechanisms are responsible for the transport of angular momentum inside stars. The recent detection of mixed modes that contain the clear signature ...of rotation in the spectra of Kepler subgiants and red giants gives us the opportunity to make progress on this question. Aims. Our aim is to probe the radial dependence of the rotation profiles for a sample of Kepler targets. For this purpose, subgiants and early red giants are particularly interesting targets because their rotational splittings are more sensitive to the rotation outside the deeper core than is the case for their more evolved counterparts. Methods. We first extracted the rotational splittings and frequencies of the modes for six young Kepler red giants. We then performed a seismic modeling of these stars using the evolutionary codes Cesam2k and astec. By using the observed splittings and the rotational kernels of the optimal models, we inverted the internal rotation profiles of the six stars. Results. We obtain estimates of the core rotation rates for these stars, and upper limits to the rotation in their convective envelope. We show that the rotation contrast between the core and the envelope increases during the subgiant branch. Our results also suggest that the core of subgiants spins up with time, while their envelope spins down. For two of the stars, we show that a discontinuous rotation profile with a deep discontinuity reproduces the observed splittings significantly better than a smooth rotation profile. Interestingly, the depths that are found to be most probable for the discontinuities roughly coincide with the location of the H-burning shell, which separates the layers that contract from those that expand. Conclusions. We characterized the differential rotation pattern of six young giants with a range of metallicities, and with both radiative and convective cores on the main sequence. This will bring observational constraints to the scenarios of angular momentum transport in stars. Moreover, if the existence of sharp gradients in the rotation profiles of young red giants is confirmed, it is expected to help in distinguishing between the physical processes that could transport angular momentum in the subgiant and red giant branches.
Context. The study of stellar activity is important because it can provide new constraints for dynamo models when combined with surface rotation rates and the depth of the convection zone. We know ...that the dynamo mechanism, which is believed to be the main process that rules the magnetic cycle of solar-like stars, results from the interaction between (differential) rotation, convection, and magnetic field. The Kepler mission has already been collecting data for a large number of stars during four years allowing us to investigate magnetic stellar cycles. Aims. We investigated the Kepler light curves to look for magnetic activity or even hints of magnetic activity cycles. Based on the photometric data we also looked for new magnetic indexes to characterise the magnetic activity of the stars. Methods. We selected a sample of 22 solar-like F stars that have a rotation period shorter than 12 days. We performed a time-frequency analysis using the Morlet wavelet yielding a magnetic proxy for our sample of stars. We computed the magnetic index Sph as the standard deviation of the whole time series and the index ⟨ Sph ⟩, which is the mean of standard deviations measured in subseries of length five times the rotation period of the star. We defined new indicators, such as the contrast between high and low activity, to take into account the fact that complete magnetic cycles are not observed for all the stars. We also inferred the Rossby number of the stars and studied their stellar background. Results. This analysis shows different types of behaviour in the 22 F stars. Two stars show behaviour very similar to magnetic activity cycles. Five stars show long-lived spots or active regions suggesting the existence of active longitudes. Two stars in our sample seem to have a decreasing or increasing trend in the temporal variation of the magnetic proxies. Finally, the last group of stars shows magnetic activity (with the presence of spots) but no sign of cycle.
Context. Constraints on the internal rotation of red giants are now available thanks to asteroseismic observations. Preliminary comparisons with rotating stellar models indicate that an undetermined ...additional process for the internal transport of angular momentum is required in addition to purely hydrodynamic processes. Aims. We investigate how asteroseismic measurements of red giants can help us characterize the additional transport mechanism. Methods. We first determine the efficiency of the missing transport mechanism for the low-mass red giant KIC 7341231 by computing rotating models that include an additional viscosity corresponding to this process. We then discuss the change in the efficiency of this transport of angular momentum with the mass, metallicity, and evolutionary stage in the light of the corresponding viscosity determined for the more massive red giant KIC 8366239. Results. In the case of the low-mass red giant KIC 7341231, we find that the viscosity corresponding to the additional mechanism is constrained to the range νadd = 1 × 103–1.3 × 104 cm2 s-1. This constraint on the efficiency of the unknown additional transport mechanism during the post-main sequence is obtained independently of any specific assumption about the modeling of rotational effects during the pre-main sequence and the main sequence (in particular, the braking of the surface by magnetized winds and the efficiency of the internal transport of angular momentum before the post-main-sequence phase). When we assume that the additional transport mechanism is at work during the whole evolution of the star together with a solar-calibrated braking of the surface by magnetized winds, the range of νadd is reduced to 1–4 × 103 cm2 s-1. In addition to being sensitive to the evolutionary stage of the star, the efficiency of the unknown process for internal transport of angular momentum increases with the stellar mass.
Context. The NASA Kepler mission has observed more than 190 000 stars in the constellations of Cygnus and Lyra. Around 4 years of almost continuous ultra high-precision photometry have been obtained ...reaching a duty cycle higher than 90% for many of these stars. However, almost regular gaps due to nominal operations are present in the light curves on different time scales. Aims. In this paper we want to highlight the impact of those regular gaps in asteroseismic analyses, and we try to find a method that minimizes their effect on the frequency domain. Methods. To do so, we isolate the two main time scales of quasi regular gaps in the data. We then interpolate the gaps and compare the power density spectra of four different stars: two red giants at different stages of their evolution, a young F-type star, and a classical pulsator in the instability strip. Results. The spectra obtained after filling the gaps in the selected solar-like stars show a net reduction in the overall background level, as well as a change in the background parameters. The inferred convective properties could change as much as ~200% in the selected example, introducing a bias in the p-mode frequency of maximum power. When asteroseismic scaling relations are used, this bias can lead to a variation in the surface gravity of 0.05 dex. Finally, the oscillation spectrum in the classical pulsator is cleaner than the original one.
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