Context. Characterising the time evolution of magnetic activity on Sun-like stars is important not only for stellar physics but also for determining the environment in which planets evolve. Aims. In ...recent decades, many surveys of open clusters have produced extensive rotation periods measurements on Sun-like stars of different ages. The present study uses this information with the aim to improve the description of their magnetic activity evolution. Methods. I present a method that infers the long-term evolution of Ca II chromospheric (R′HK) and X-ray coronal (LX) emission on solar mass stars by combining a best fit parametric model of their rotation evolution with empirical rotation-activity relationships. Results. The inferred scenario reproduces the high chromospheric and coronal emission levels around R′HK ≈ 10−4 and LX ≈ 1030 erg s−1 that are observed on pre-main sequence (PMS) stars. At the end of the PMS contraction phase around the age of ~30 Myr, the slowest rotating stars experience a rapid transition of their magnetic activity to more moderate levels around R′HK ≈ 4 × 10−5 and L5 ≈ 1029 erg s−1. This transition occurs later on more rapidly rotating stars, up to an age of ~600 Myr for the fastest rotators. After this brief episode of large magnetic activity decay, the average chromospheric and coronal emission levels of solar-mass stars decrease steadily converging towards similar values (R′HK ≈ 10−5 and LX ≈ 1027 erg s−1) by the age of the Sun. Conclusion. The study suggests that solar mass stars in open clusters with ages between ~30 and ~600 Myr exhibit bimodal distributions of their R′HK chromospheric activity indices and coronal X-ray to bolometric luminosity ratios that can be traced back to their rotation evolution. This conclusion is consistent with available measurements of activity indices from Sun-like stars in nearby open clusters.
Context. Observations of young open clusters show a bimodal distribution of stellar rotation. In those clusters, Sun-like stars group into two main populations of fast and slow rotators. Beyond an ...age of approximately 600 Myr, the two populations converge towards a single sequence of slow rotators. Aims. The present study addresses the origin of this bimodal distribution and the cause of its observed evolution. Methods. New prescriptions of mass-loss rate and Alfven radius dependences on Rossby number suggested by observations are implemented in a phenomenological model of angular-momentum loss and redistribution. The obtained model is used to calculate the time evolution of a rotation-period distribution of solar-mass stars similar to that observed in the 5 Myr-old NGC2362 open cluster. The simulated distributions at subsequent ages are compared with those of h Per, the Pleiades, M50, M35, and M37. Results. The model is able to reproduce the appearance and disappearance of a bimodal rotation-period distribution in open clusters providing that a brief episode of large-angular-momentum loss is included in the early evolution of Sun-like stars. Conclusions. I argue that a transitory episode of large-angular-momentum loss occurs on Sun-like stars with Rossby numbers between 0.13 and 0.3. This phenomenon of enhanced magnetic braking by stellar wind would be mainly driven by a rapid increase of mass loss at a critical rotation rate. This scenario accounts for the bimodal distribution of stellar rotation in open clusters with ages between 20-30 Myr and approximately 600 Myr. The mass-loss rate increase could account for a significant fraction of the X-ray luminosity decay of Sun-like stars in the 0.13-0.3 Rossby number range where a transition from the saturated to the non-saturated regime of X-ray emission is observed. Observed correlations between Li abundance and rotation sequences in the Pleiades and M34 clusters support this scenario.
Context. K-type stars of similar effective temperatures in clusters younger than about 250 Myr are characterised by a wide dispersion in lithium abundance whose origin is not understood. Aims. The ...present study addresses this question in light of rotation period measurements recently conducted in the Pleiades and M 34 open clusters. Methods. The study investigates the dependence of lithium abundance with effective temperature distinguishing stars that belong to different rotation sequences in diagrams where measured rotation periods are plotted against B − V colour indices. Results. At any given effective temperature below 5500 K, stars on the fast rotator sequence in the Pleiades and M 34 have significantly higher lithium abundances that stars with same masses and ages located on the slow rotator sequence. Conclusions. This correlation suggests that shear-induced turbulence at the base of the convection zone accelerates the depletion of lithium in rapidly rotating late G- and K-type stars during a brief episode of strong rotational braking by stellar wind in their early evolution on the main sequence. The epoch at which this strong rotational braking occurs does not depend on stellar mass only but also on the rotation history of each individual star. I argue that the large dispersion in surface lithium abundance of K stars in the Pleiades and M 34 could be a consequence of this phenomenon.
The X-ray emission from late-type stars in open clusters exhibits two kinds of dependences on stellar rotation. While fast rotators have a relatively constant X-ray emission level, slower rotators ...show a decline of their X-ray emission with decreasing rotation rate. The purpose of the present study is to look for the X-ray signatures of such possible modifications, to search for correlations with rotation and stellar parameters, and to propose a parameterization of the X-ray activity evolution on the main sequence. The aim is to provide observational constraints on the time evolution of dynamo processes in the interiors of late-type stars. The author argues that the drop of LX/Lbol by one order of magnitude observed in M34 around a Rossby number of 0.3 is indicative of a change in dynamo efficiency. He concludes that the transition from the saturated to the non-saturated regime of X-ray emission among main-sequence stars is the result of a dynamo regime transition, possibly between a turbulent dynamo and an interface-type dynamo.
Aims.The time variability of the broadband solar irradiance depends not only on the intrinsic evolution and visibility modulation of sunspots but also on that of faculae that become brighter near the ...limb during the solar rotation. Sun-like faculae around spots could also play a significant role in modulating the broadband visible flux of active dwarfs. It is the aim of the present study to test this hypothesis. Methods.I analyzed high accuracy light-curves of two active dwarfs obtained with the MOST satellite during several stellar rotation periods. The observed time series were fitted using a model that takes into account not only starspot contributions but also the areas of faculae in active regions and their bolometric contrast. Results.A low value of the mean ratio of faculae to cool spot areas in active regions provides the best description of ϵ Eri and κ Ceti light curves. Conclusions.Although not conclusive, this result suggests that the ratio of faculae to cool spot areas decreases in stars somewhat more active than the Sun.
Contact.
Investigating how the Milky Way formed and has evolved is an important topic in astrophysics that requires the determination of precise ages for large samples of stars over long periods.
...Aims.
The present study addresses the formation history of nearby Sun-like stars using the emission reversal in the cores of their Ca
II
H&K Fraunhofer lines as an age indicator.
Methods.
I used an empirical age–activity relationship derived from stellar rotation period measurements in intermediate-age open clusters to infer the age distribution of a representative sample of nearby 0.85−1.0
M
⊙
stars with −0.2 ≤ Fe/H ≤ +0.2. The evolution of the dispersion of their velocity components and of the mean iron abundance as a function of age is estimated.
Results.
The inferred age distribution shows a steep rise in star formation in the solar neighbourhood between 7 and 6 Gyr ago, with a maximum formation rate ∼5 Gyr ago. This rate then decays until ∼2 Gyr and rises again in the recent past. The dispersion of the radial and vertical velocity components of the sample stars is the largest at the time of maximum star formation. Their mean iron abundance first decays from a super-solar value (Fe/H ∼ +0.05) ∼ 6 Gyr ago to a sub-solar value (Fe/H ≤ −0.05) ∼ 4 Gyr ago and rises again in the recent past.
Conclusions.
This timeline is consistent with a scenario where the steep rise in the age distribution of nearby Sun-like stars around 7−6 Gyr is related to an external perturbation induced by a first close pericentric passage of the Sgr galaxy ∼6.5 Gyr ago. The Sgr galaxy would have been significantly stripped from its gas in this first encounter, thus explaining the weaker star formation during a more recent encounter ∼2 Gyr ago. The gas infall from the satellite galaxy onto the MilkyWay disc would have diluted its metallicity over an extended period of time after the first encounter. The turbulence induced in this initial encounter may be partly responsible for the increased dispersion of velocity components of the stars born around the age of maximum star formation. A continuous metal enrichment of the disc would have progressively compensated the decaying infall of low-metallicity gas leading to an increase in the mean stellar metallicity in the last 4 Gyr.
Context.
The chromospheric emission in the cores of the Ca
II
H & K lines of late-type dwarfs is a well known indicator of magnetic activity that decreases with increasing stellar age.
Aims.
I use ...this indicator to investigate the formation history of nearby G- and early K-type stars with origins at galactocentric distances similar to that of the region where the Sun was born.
Methods.
A parent sample of single main-sequence stars with near-solar metallicity and known magnetic activity levels is built from catalogues of stellar atmospheric parameters and chromospheric activity indices. A kinematical approach uses
Gaia
astrometric data to differentiate thin disc stars from thick disc stars. Measured distributions of
R
′
HK
chromospheric activity indices are compared with Monte Carlo simulations based on an empirical model of chromospheric activity evolution.
Results.
The thin disc includes a significant fraction of Sun-like stars with intermediate activity levels (2 × 10
−5
≤
R
′
HK
≤ 6 × 10
−5
), while most early K- and G-type stars from the thick disc are inactive (
R
′
HK
< 2 × 10
−5
). The chromospheric activity distribution among nearby Sun-like dwarfs from the thin disc can be explained by a combination of an old (>6–7 Gyr) star formation event (or events) and a more recent (<3 Gyr) burst of star formation. Such an event is not required to account for the
R
′
HK
index distributions of nearby thick disc stars.
Conclusions.
The distribution of magnetic activity among local G- and early K-type stars with a near-solar metallicity bears the imprint of an important star formation event that occurred ~1.9–2.6 Gyr ago in the thin disc of the Milky Way.
Aims.The present study aims (i) to test the existence of a correlation between magnetic activity and rotation among G and K giants in binary systems and (ii) to test whether parameters other than ...rotation play a role in determining the X-ray emission level of intermediate-mass giants. Methods.The method consists in testing the existence of correlations between measured stellar parameters including the X-ray surface flux, rotation period, Rossby number and surface gravity of a sample of G and K giants with masses included between 1.5 $M_\odot$ and 3.8 $M_\odot$. Results.I found evidence that the X-ray surface flux FX of intermediate-mass G and K giants is correlated with their rotation period P as previously observed on single G giants. Confidence in the degree of correlation is not higher when the Rossby number is used in place of the rotation period, but it significantly improves when stellar gravity g is taken into account. The empirical relation given by $\log\,(F_{\rm X}) = -0.73$ $\times$ $\log\, (P) + 0.64$ $\times$ $\log\, (g/g_{\odot})+ 7.9$ differs from the power-law dependence with an index of about -2 between X-ray to bolometric luminosity ratio and the rotation period that is observed for main-sequence stars. The X-ray surface flux of single G giants and of intermediate-mass G and K giants in close binary systems, such as RS CVn systems, also depends on the stellar gravity. This dependence could result from the effect of gravity on the electron density and emission measure of the X-ray emitting plasmas, as well as on the characteristic sizes of coronal magnetic loops. The measured X-ray surface-flux dependence on gravity is, however, not as steep as the one predicted by simple models of hydrostatic loops that assume a fixed ratio between the coronal energy losses by thermal conduction and by radiation. Conclusions.I conclude that (i) a relation exists between the rotation and X-ray activity level in giants, (ii) that this relation is not directly dependent on the presence of a companion and applies to all intermediate-mass giants with either G or K spectral type, and (iii) that gravity is an important stellar parameter in determining the X-ray surface flux of intermediate-mass giants.
I study the relation between X-ray activity and rotation among intermediate-mass single G giants. The results show evidence that the quiescent coronal activity of these stars, as measured by their ...X-ray surface flux, increases linearly with the angular rotation velocity and with the inverse of the Rossby number. Even the most rapidly rotating G giants do not reach the canonical log($L_{\rm X}/L_{\rm bol}$) ≈ –3 saturation level. The effect of rapid rotation on these stars could result mainly in an increased coverage of their surface with magnetic close loop structures. The empirical activity-rotation relationship accounts for the occurrence of a maximum of magnetic activity in the atmosphere of intermediate-mass stars as they evolve off the main-sequence near the bottom of the red giant branch. Remarkably, the relation between X-ray to bolometric luminosity ratio and the Rossby number or rotation period for G giants differs from the power law dependence with an index of about –2 that is observed for main-sequence stars. Possible implications for the dynamo generation of magnetic fields on giants are discussed.
Context. The chromospheric emission in the cores of the Ca II H & K lines of late-type dwarfs is a well known indicator of magnetic activity that decreases with increasing stellar age. Aims. I use ...this indicator to investigate the formation history of nearby G- and early K-type stars with origins at galactocentric distances similar to that of the region where the Sun was born. Methods. A parent sample of single main-sequence stars with near-solar metallicity and known magnetic activity levels is built from catalogues of stellar atmospheric parameters and chromospheric activity indices. A kinematical approach uses Gaia astrometric data to differentiate thin disc stars from thick disc stars. Measured distributions of R′HK chromospheric activity indices are compared with Monte Carlo simulations based on an empirical model of chromospheric activity evolution. Results. The thin disc includes a significant fraction of Sun-like stars with intermediate activity levels (2 × 10−5 ≤ R′HK ≤ 6 × 10−5), while most early K- and G-type stars from the thick disc are inactive (R′HK < 2 × 10−5). The chromospheric activity distribution among nearby Sun-like dwarfs from the thin disc can be explained by a combination of an old (>6–7 Gyr) star formation event (or events) and a more recent (<3 Gyr) burst of star formation. Such an event is not required to account for the R′HK index distributions of nearby thick disc stars. Conclusions. The distribution of magnetic activity among local G- and early K-type stars with a near-solar metallicity bears the imprint of an important star formation event that occurred ~1.9–2.6 Gyr ago in the thin disc of the Milky Way.
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