Rotation, convection, and magnetic activity in lower main-sequence stars (original) (raw)
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On the Cycle Periods of Stellar Dynamos
Astronomy and Astrophysics, 1997
We show that the cycle periods (P cyc ) of slowly rotating lower main-sequence stars with well-defined periodic chromospheric activity can be parametrized by the rotation period P rot and the convective turnover time τ c through a relation of the form P cyc ∝ P b rot τ c c , with b = 2.0 ± 0.3 and c = −2.0 ± 0.3. This suggests a common dynamo mechanism for slowly rotating stars lower main-sequence stars. Using a simple linear meanfield dynamo model, we are able to reproduce the observed relation if ∆Ω, the total difference in angular velocity along the radial direction, scales as ∆Ω ∝ P p rot with p = 1.1 ± 0.2, and if the α-coefficient scales as α ∝ Ro q with q = −5.1 ± 0.6. This would suggest that, with increasing rotation rate, differential rotation decreases while |α| rapidly increases.
A Dynamo Model of Magnetic Activity in Solar-Like Stars with Different Rotational Velocities
The Astrophysical Journal, 2014
We attempt to provide a quantitative theoretical explanation for the observations that Ca II H/K emission and X-ray emission from solar-like stars increase with decreasing Rossby number (i.e., with faster rotation). Assuming that these emissions are caused by magnetic cycles similar to the sunspot cycle, we construct flux transport dynamo models of 1M ⊙ stars rotating with different rotation periods. We first compute the differential rotation and the meridional circulation inside these stars from a mean-field hydrodynamics model. Then these are substituted in our dynamo code to produce periodic solutions. We find that the dimensionless amplitude f m of the toroidal flux through the star increases with decreasing rotation period. The observational data can be matched if we assume the emissions to go as the power 3-4 of f m. Assuming that the Babcock-Leighton mechanism saturates with increasing rotation, we can provide an explanation for the observed saturation of emission at low Rossby numbers. The main failure of our model is that it predicts an increase of magnetic cycle period with increasing rotation rate, which is the opposite of what is found observationally. Much of our calculations are based on the assumption that the magnetic buoyancy makes the magnetic flux tubes to rise radially from the bottom of the convection zone. On taking account of the fact that the Coriolis force diverts the magnetic flux tubes to rise parallel to the rotation axis in rapidly rotating stars, the results do not change qualitatively.
The Astrophysical Journal
We study the rotation-activity correlations (RACs) in a sample of stars from spectral type dK4 to dM4. We study RACs using chromospheric data and coronal data. We study the Ca II line surface fluxes-P i sin RACs. We fit the RACs with linear homoscedastic and heteroscedastic regression models. We find that these RACs differ substantially from one spectral sub-type to another. For dM3 and dM4 stars, we find that the RACs cannot be described by a simple model, but instead that there may exist two distinct RAC behaviors for the low-activity and the high-activity stellar sub-samples, respectively. Although these results are preliminary and will need confirmation, the data suggest that these distinct RACs may be associated with different dynamo regimes. We also study ¢ R HK as a function of the Rossby number R 0. We find (i) for dK4 stars, ¢ R HK as a function of R 0 agrees well with previous results for F-G-K stars and (ii) in dK6, dM2, dM3, and dM4 stars, at a given R 0 , the values of ¢ R HK lie at a factor of 3, 10, 20, and 90, respectively, below the F-G-K RAC. Our results suggest a significant decrease in the efficiency of the dynamo mechanism(s) as regards chromospheric heating before and at dM3, i.e., before and at the transition to complete convection. We also show that the ratio of coronal heating to chromospheric heating L X / L HK increases by a factor of 100 between dK4 and dM4 stars.
Asymptotic dynamos in late-type stars
Monthly Notices of the Royal Astronomical Society, 2002
The behaviour of a simple thin-shell αdynamo model is considered in the asymptotic regime, characterized by dynamo numbers much larger than the critical ones, in order to derive scaling relationships connecting the properties of dynamo waves with global stellar parameters. The proposed approach is applied to stellar models of subgiant and giant stars from K0IV to K1III spectral types in the Hertzsprung-Russell diagram, to predict some characteristics of activity cycles in very active stars. We found that the strength of the dynamo action in such stars is higher than in the Sun. Therefore, larger magnetic field energy and larger spot filling factors are expected, in agreement with observations. The periods of stellar cycles are also estimated and compared with observations. The characteristic times of migration of the star-spot belts relative to the cycle period, namely the Hale number, together with the ratio of toroidal to poloidal dynamo magnetic fields are estimated. From our simplified analysis we can only derive general trends, but cannot perform a direct comparison with the observed properties of particular active stars. These general trends indicate that the cycle periods have a large spread for stars with low rotation rates (∼1-5 times the solar one), while they tend to be saturated for stars with high rotation rates (∼5-15 times solar), for which the periods range from 10 to 20 yr. For such stars, Hale numbers range from 1.5 to approximately 4 (the Hale number for the Sun is approximately 1.1), denoting the possible existence of cycles with different periodicities present simultaneously, the ratio of toroidal to poloidal dynamo fields tends to become smaller for increasing rotation rates, indicating a transition from the αto the α 2type of dynamo. Moreover, the magnetic field filling factors tend to become larger for faster rotation rates, though the effect of the convection zone depth should not be neglected. Our results show a reasonable agreement with available observations of a sample of active stars we have considered.
Theoretical estimates of the convective turnover time for low-mass, rotating pre-main sequence stars
Proceedings of the International Astronomical Union, 2006
The Rossby number Ro is an important quantity related to the well-known magnetic activity-rotation correlation for main sequence, solar-type stars. For such stars, Ro can be obtained by a semi-empirical relationship between the convective turnover time τc and the B-V colour index, but an equivalent activity-rotation correlation seems not to exist for pre-main sequence stars. In this work we report theoretical estimates of τc for low-mass, rotating pre-main sequence stars under either the Full Spectrum of Turbulence (FST) or the classical Mixing Length Theory (MLT) convection models. The results for the MLT models show that the lower the convection efficiency the higher τc, while the FST models give τc lower than those for the MLT. The presence of a parametric magnetic field lowers the convection efficiency, resulting in smaller τc values.
Magnetic fields on young, moderately rotating Sun-like stars-I: HD~ 35296 and HD~ 29615
Observations of the magnetic fields of young solar-type stars provide a way to investigate the signatures of their magnetic activity and dynamos. Spectropolarimetry enables the study of these stellar magnetic fields and was thus employed at the Télescope Bernard Lyot and the Anglo-Australian Telescope to investigate two moderately rotating young Sun-like stars, namely HD 35296 (V119 Tau, HIP 25278) and HD 29615 (HIP 21632). The results indicate that both stars display rotational variation in chromospheric indices consistent with their spot activity, with variations indicating a probable long-term cyclic period for HD 35296. Additionally, both stars have complex, and evolving, large-scale surface magnetic fields with a significant toroidal component. High levels of surface differential rotation were measured for both stars. For the F8V star HD 35296 a rotational shear of ∆Ω = 0.22 +0.04 −0.02 rad d −1 was derived from the observed magnetic profiles. For the G3V star HD 29615 the magnetic features indicate a rotational shear of ∆Ω = 0.48 +0.11 −0.12 rad d −1 , while the spot features, with a distinctive polar spot, provide a much lower value of ∆Ω of 0.07 +0.10 −0.03 rad d −1 . Such a significant discrepancy in shear values between spot and magnetic features for HD 29615 is an extreme example of the variation observed for other lower-mass stars. From the extensive and persistent azimuthal field observed for both targets it is concluded that a distributed dynamo operates in these moderately rotating Sun-like stars, in marked contrast to the Sun's interface-layer dynamo.
Dynamo models and differential rotation in late-type rapidly rotating stars
Astronomy and Astrophysics, 2005
Increasing evidence is becoming available about not only the surface differential rotation of rapidly rotating cool stars but, in a small number of cases, also about temporal variations, which possibly are analogous to the solar torsional oscillations. Given the present difficulties in resolving the precise nature of such variations, due to both the short length and poor resolution of the available data, theoretical input is vital to help assess the modes of behaviour that might be expected, and will facilitate interpretation of the observations. Here we take a first step in this direction by studying the variations in the convection zones of such stars, using a two dimensional axisymmetric mean field dynamo model operating in a spherical shell in which the only nonlinearity is the action of the azimuthal component of the Lorentz force of the dynamo generated magnetic field on the stellar angular velocity. We consider three families of models with different depths of dynamo-active regions. For moderately supercritical dynamo numbers we find torsional oscillations that penetrate all the way down to the bottom of the convection zones, similar to the case of the Sun. For larger dynamo numbers we find fragmentation in some cases and sometimes there are other dynamical modes of behaviour, including quasi-periodicity and chaos. We find that the largest deviations in the angular velocity distribution caused by the Lorentz force are of the order of few percent, implying that the original assumed 'background' rotation field is not strongly distorted.
In a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained highresolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this second paper, we present projected rotation velocities, average magnetic field strengths, and chromospheric emission from the Hα line. We confirm earlier results that the mean level of normalized Hα luminosity decreases with lower temperature, and we find that the scatter among Hα luminosities is larger at lower temperature. We measure average magnetic fields between 0 and 4 kG with no indication for a dependence on temperature between M7 and M9.5. For a given temperature, Hα luminosity is related to magnetic field strength, consistent with results in earlier stars. A few very slowly rotating stars show very weak magnetic fields and Hα emission, all stars rotating faster than our detection limit show magnetic fields of at least a few hundred Gauss. In contrast to earlier-type stars, we observe magnetic fields weaker than 1 kG in stars rotating faster than ∼ 3 km s −1 , but we find no correlation between rotation and magnetic flux generation among them. We interpret this as a fundamental change in the dynamo mechanism; in ultracool dwarfs, magnetic field generation is predominantly achieved by a turbulent dynamo, while other mechanisms can operate more efficiently at earlier spectral type.
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%.