A New Light on the Relation Between Rotation Periods and Cycle Lengths of Stellar Activity (original) (raw)
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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.
2016
A new sample of solar analogs and twin candidates have been constructed and studied, with particular attention to their light curves from NASA's Kepler mission. This letter aims to assess the evolutionary status, derive their rotation and ages and identify those solar analogs or solar twin candidates. We separate out the subgiants that compose a large fraction of the asteroseismic sample, and which show an increase in the average rotation period as the stars ascend the subgiant branch. The rotation periods of the dwarfs, ranging from 6 to 30 days, and averaged 19d, allow us to assess their individual evolutionary states on the main sequence, and to derive their ages using gyrochronology. These ages are found to be in agreement with a correlation coefficient of r = 0.79 with the independent asteroseismic ages, where available. As a result of this investigation, we are able to identify 34 stars as solar analogs and 22 of them as solar twin candidates.
The age-activity-rotation relationship in solar-type stars
Astronomy & Astrophysics, 2004
We present Ca II K line chromospheric fluxes measured from high-resolution spectra in 35 G dwarf stars of 5 open clusters to determine the age-activity-rotation relationship from the young Hyades and Praesepe (0.6 Gyr) to the old M 67 (∼4.5 Gyr) through the two intermediate age clusters IC 4651 and NGC 3680 (∼1.7 Gyr). The full amplitude of the activity index within a cluster is slightly above 60 % for all clusters but one, NGC 3680, in which only two stars were observed. As a comparison, the same Solar Ca II index varies by ∼40% during a solar cycle. Four of our clusters (Hyades and Praesepe, IC 4651 and NGC 3680) are pairs of twins as far as age is concerned; the Hyades have the same chromospheric-activity level as Praesepe, at odds with early claims based on X-ray observations. Both stars in NGC 3680 are indistinguishable, as far as chromospheric activity is concerned, from those in the coeval IC 4651. This is a validation of the existence of an age-activity relationship. On the other hand, the two intermediate age clusters have the same activity level as the much older M 67 and the Sun. Our data therefore shows that a dramatic decrease in chromospheric activity takes place in solar stars between the Hyades and the IC 4651 age, of about 1 Gyr. Afterwards, activity remains virtually constant for more than 3 Gyr. We have also measured v sin i for all of our stars and the average rotational velocity shows the same trend as the chromospheric-activity index. We briefly investigate the impact of this result on the age determinations of field G dwarfs in the solar neighborhood; the two main conclusions are that a consistent group of "young" stars (i.e. as active as Hyades stars) is present, and that it is virtually impossible to give accurate chromospheric ages for stars older than ∼2 Gyr. The observed abrupt decline in activity explains very well the Vaughan-Preston gap.
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.
Stellar cycles: general properties and future directions
Proceedings of the International Astronomical Union, 2011
We discuss the general properties of stellar cycles with emphasis on their amplitudes as a function of stellar parameters, particularly those stellar characteristics relevant to dynamo-driven magnetic activity. We deduce an empirical scaling relation between cycle frequency and differential rotation based on previously established empirical relations. We also compare the recent Cycle 23 to cycles in solar-type stars. We find that the extended minimum of Cycle 23 resembled in its Ca II H…
Stellar Magnetic Cycles in the Solar-Like Stars KEPLER-17 and KEPLER-63
The Astrophysical Journal, 2016
The stellar magnetic field plays a crucial role in the star internal mechanisms, as in the interactions with its environment. The study of starspots provides information about the stellar magnetic field, and can characterise the cycle. Moreover, the analysis of solar-type stars is also useful to shed light onto the origin of the solar magnetic field. The objective of this work is to characterise the magnetic activity of stars. Here, we studied two solar-type stars Kepler-17 and Kepler-63 using two methods to estimate the magnetic cycle length. The first one characterises the spots (radius, intensity, and location) by fitting the small variations in the light curve of a star caused by the occultation of a spot during a planetary transit. This approach yields the number of spots present in the stellar surface and the flux deficit subtracted from the star by their presence during each transit. The second method estimates the activity from the excess in the residuals of the transit lightcurves. This excess is obtained by subtracting a spotless model transit from the lightcurve, and then integrating all the residuals during the transit. The presence of long term periodicity is estimated in both time series. With the first method, we obtained P cycle = 1.12 ± 0.16 yr (Kepler-17) and P cycle = 1.27 ± 0.16 yr (Kepler-63), and for the second approach the values are 1.35 ± 0.27 yr and 1.27 ± 0.12 yr, respectively. The results of both methods agree with each other and confirm their robustness.
Rotation and Activity of Pre–Main‐Sequence Stars
The Astrophysical Journal, 2007
Rotation and activity are key parameters in stellar evolution and can be used to probe basic stellar physics. Here we present a study of rotation (measured as projected rotational velocity v sin i) and chromospheric activity (measured as Hα equivalent width) based on an extensive set of high-resolution optical spectra obtained with the MIKE instrument on the 6.5 m Magellan Clay telescope. Our targets are 74 F-M dwarfs in the young stellar associations η Chamaeleontis, TW Hydrae, β Pictoris, and Tucana-Horologium, spanning ages from 6 to 30 Myr. While the Hα equivalent widths for most F and G stars are consistent with pure photospheric absorption, most K and M stars show measurable chromospheric emission. By comparing Hα equivalent width in our sample to results in the literature, we see a clear evolutionary sequence: Chromospheric activity declines steadily from the T Tauri phase to the main sequence. Using activity as an age indicator, we find a plausible age range for the Tuc-Hor association of 10-40 Myr. Between 5 and 30 Myr, we do not see evidence for rotational braking in the total sample, thus angular momentum is conserved, in contrast to younger stars. This difference indicates a change in the rotational regulation at ∼5-10 Myr, possibly because disk braking cannot operate longer than typical disk lifetimes, allowing -2the objects to spin up. On timescales of ∼ 100 Myr there is some evidence for weak rotational braking, possibly due to stellar winds. The rotation-activity relation is flat in our sample; in contrast to main-sequence stars, there is no linear correlation for slow rotators. We argue that this is because young stars generate their magnetic fields in a fundamentally different way from main-sequence stars, and not just the result of a saturated solar-type dynamo. By comparing our rotational velocities with published rotation periods for a subset of stars, we determine ages of 13 +7 −6 Myr and 9 +8 −2 Myr for the η Cha and TWA associations, respectively, consistent with previous estimates. Thus we conclude that stellar radii from evolutionary models by are in agreement with the observed radii within ±15%.
Multiple and changing cycles of active stars II. Results
2009
We study the time variations of the cycles of 20 active stars based on decades-long photometric or spectroscopic observations. A method of time-frequency analysis, as discussed in a companion paper, is applied to the data. Fifteen stars definitely show multiple cycles; the records of the rest are too short to verify a timescale for a second cycle. The cycles typically show systematic changes. For three stars, we found two cycles in each of them that are not harmonics, and which vary in parallel, indicating that a common physical mechanism arising from a dynamo construct. The positive relation between the rotational and cycle periods is confirmed for the inhomogeneous set of active stars. Stellar activity cycles are generally multiple and variable.
Rotation Periods and Ages of Solar Analogs and Solar Twins Revealed by the Kepler Mission
The Astrophysical Journal, 2014
A new sample of solar analogs and twin candidates have been constructed and studied, with particular attention to their light curves from NASA's Kepler mission. This letter aims to assess the evolutionary status, derive their rotation and ages and identify those solar analogs or solar twin candidates. We separate out the subgiants that compose a large fraction of the asteroseismic sample, and which show an increase in the average rotation period as the stars ascend the subgiant branch. The rotation periods of the dwarfs, ranging from 6 to 30 days, and averaged 19d, allow us to assess their individual evolutionary states on the main sequence, and to derive their ages using gyrochronology. These ages are found to be in agreement with a correlation coefficient of r = 0.79 with the independent asteroseismic ages, where available. As a result of this investigation, we are able to identify 34 stars as solar analogs and 22 of them as solar twin candidates.