Magnetic Activity In Late-type Stars Research Articles

A Knee Point in the Rotation–Activity Scaling of Late-type Stars with a Connection to Dynamo Transitions

Abstract The magnetic activity of late-type stars is correlated with their rotation rates. Up to a certain limit, stars with smaller Rossby numbers, defined as the rotation period divided by the convective turnover time, have higher activity. A more detailed look at this rotation–activity relation reveals that, rather than being a simple power-law relation, the activity scaling has a shallower slope for the low-Rossby stars than for the high-Rossby ones. We find that, for the chromospheric Ca II H&K activity, this scaling relation is well modeled by a broken two-piece power law. Furthermore, the knee point of the relation coincides with the axisymmetry to nonaxisymmetry transition seen in both the spot activity and surface magnetic field configuration of active stars. We interpret this knee point as a dynamo transition between dominating axi- and nonaxisymmetric dynamo regimes with a different dependence on rotation and discuss this hypothesis in the light of current numerical dynamo models.

Imaging the cool stars in the interacting binaries AE Aqr, BV Cen and V426 Oph

AbstractIt is well known that magnetic activity in late‐type stars increases with increasing rotation rate. Using inversion techniques akin to medical imaging, the rotationally broadened profiles from such stars can be used to reconstruct ‘Doppler images’ of the distribution of cool, dark starspots on their stellar surfaces. Interacting binaries, however, contain some of the most rapidly rotating late‐type stars known and thus provide important tests of stellar dynamo models. Furthermore, magnetic activity is thought to play a key role in their evolution, behaviour and accretion dynamics. Despite this, we know comparatively little about the magnetic activity and its influence on such binaries. In this review we summarise the concepts behind indirect imaging of these systems, and present movies of the starspot distributions on the cool stars in some interacting binaries. We conclude with a look at the future opportunities that such studies may provide. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Astrometric effects of solar-like magnetic activity in late-type stars and their relevance for the detection of extrasolar planets

Using a simple model based on the characteristics of sunspots and faculae in solar active regions, the effects of surface brightness inhomogeneities on the position of the photocentre of the disk of a solar-like, magnetically active star, are studied. A general law is introduced, giving the maximum amplitude of the photocentre excursion produced by a distribution of active regions with a given surface filling factor. The consequences for the detection of extrasolar planets by means of the astrometric method are investigated with some applications to forthcoming space missions, such as GAIA and SIM, as well as to ground-based interferometric measurements. Spurious detections of extrasolar planets can indeed be caused by activity-induced photocentre oscillations, requiring a simultaneous monitoring of the optical flux and a determination of the rotation period and of the level of activity of the target stars for an appropriate discrimination.

The Evolution of X‐Ray Emission in Young Stars

The evolution of magnetic activity in late-type stars is part of the intertwined rotation-age-activity relation which provides an empirical foundation to the theory of magnetic dynamos. We study the age-activity relation in the pre-main sequence (PMS) regime, for the first time using mass-stratified subsamples. The effort is based on the Chandra Orion Ultradeep Project (COUP) which provides very sensitive and homogenous X-ray data on a uniquely large sample of 481 optically well-characterized low-extinction low-mass members of the Orion Nebula Cluster, for which individual stellar masses and ages could be determined. More than 98 percent of the stars in this sample are detected as X-ray sources. Within the PMS phase for stellar ages in the range ∼ 0.1 − 10 Myr, we

XMM- Newton spectroscopy of stars in open clusters and star forming regions

We discuss observations of open clusters and star forming regions obtained with the RGS and EPIC instruments on board XMM- Newton. These observations provide a powerful tool to investigate the temperature structure, emission measure distribution and elemental abundances of stars from the pre-main sequence phase to post-main sequence evolution. We report in particular on EPIC and RGS spectroscopy of hot and cool stars in the very young cluster around σ Orionis, of two stars in the Hyades (the main-sequence star VB 50 and the clump giant VB 71) and of two pre-main sequence stars in Taurus–Auriga (the classical T Tauri star SU Aur and the weak-lined T Tauri star HD 283572). The implications of these observations for models of magnetic activity in late-type stars, of wind emission in early-type stars, and of X-ray production in the early stages of stellar evolution are briefly discussed.

The Chandra Deep Field–North Survey. XVII. Evolution of Magnetic Activity in Old Late‐Type Stars

The extremely sensitive Chandra Deep Field–North (CDF-N) pencil-beam X-ray survey is used to identify and characterize the X-ray emission from old high-latitude main-sequence Galactic stars. Our principal goal is to investigate the expected long-term decay of magnetic activity of late-type stars due to the gradual spin-down of stellar rotation from a magnetized stellar wind. Thirteen X-ray sources are associated with late-type stars; 11 of these constitute a well-defined sample for statistical analysis. This sample consists of two G, two K0–K4, and seven M2–M5 stars with median V-band magnitude around 19 and median distance around 300 pc. X-ray luminosities are typically logLX ’ 27 ergs s � 1 but are substantially higher in two cases. The combination of large-amplitude variations on timescales of hours and plasma temperatures around 5–30 MK indicates that the observed X-ray emission is dominated by magnetic reconnection flares rather than quiescent coronal emission. These X-ray properties are quantitatively similar to those seen in the active contemporary Sun. The CDF-N stellar sample is compared to simulations based on convolution of X-ray luminosity functions (XLFs) with the known spatial distribution of old-disk stars. The model indicates that the CDF-N stars are the most magnetically active old-disk stars. A substantial decline in X-ray luminosities over the 1 Gyr < t < 11 Gyr age interval is required: 39 rather than 11 stars should have been detected if the XLF does not evolve over this time interval. This is a clear demonstration that the coronal and flaring components of stellar magnetic activity—and presumably the interior magnetic dynamos responsible for the reconnecting fields at the stellar surface—exhibit long-term decay over the age of the Galactic disk. The model that best fits the magnitudes, spectral types, and X-ray luminosities of the sample has LX / t � 2 ergs s � 1 , which is faster than the t � 1 decay rate predicted from widely accepted rotational spin-down rates and X-ray–activity relations. Subject headingg stars: activity — stars: coronae — stars: late-type — stars: magnetic fields — X-rays: stars

Stellar dynamos - perspectives and challenges

In this paper the present status of theoretical understanding - mostly relying on mean-field dynamo models - of global magnetic activity of late-type stars with convective envelopes is reviewed. The principles of the mean-field dynamo theory are introduced, and a discussion on some of the present-day problems and challenges is presented. The remaining sections are devoted to discuss the current status of solar and stellar dynamo models.

Automatic spectral classification of low-mass late-type stars: LI Jin-zeng & HU Jing-yao Beijing Astronomical Observatory, CAS, Beijing 100080

We discuss observations of open clusters and star forming regions obtained with the RGS and EPIC instruments on board XMM-Newton. These observations provide a powerful tool to investigate the temperature structure, emission measure distribution and elemental abundances of stars from the pre-main sequence phase to post-main sequence evolution. We report in particular on EPIC and RGS spectroscopy of hot and cool stars in the very young cluster around σ Orionis, of two stars in the Hyades (the main-sequence star VB 50 and the clump giant VB 71) and of two pre-main sequence stars in Taurus–Auriga (the classical T Tauri star SU Aur and the weak-lined T Tauri star HD 283572). The implications of these observations for models of magnetic activity in late-type stars, of wind emission in early-type stars, and of X-ray production in the early stages of stellar evolution are briefly discussed.

What is a stellar dynamo?

Numerical simulations of turbulent stellar dynamos are now feasible. The characteristic time-scale for kinematic behaviour is related to the turnover time of the turbulent eddies. Results from idealized two-dimensional models show that the Lorentz force alters the velocity field, allowing transient magnetic activity to persist for intervals much longer than the expected turbulent decay time. For three-dimensional flowss, a characteristic time T e is defined for turbulent diffusion to act, based on the rate at which magnetic energy is dissipated, and assert that there is a dynamo only if the field survives for times much longer than T e . This criterion is then applied to cyclic magnetic activity in late-type stars

Magnetic activity in cool stars

Aspects of magnetic activity of late-type stars are reviewed: first, results obtained through the comparison of emissions from various temperature regimes in the outer atmospheres of cool stars, then results from stellar soft X-ray spectroscopy. Spectra obtained with different instruments (Imaging Proportional Counter and Solid State Spectrometer onboard the EINSTEIN Satellite, and the Objective Grating Instruments onboard EINSTEIN and EXOSAT) are interpreted through fits with model spectra consisting of one or two temperature components. Particular attention is given to the analysis of EXOSAT spectra of Procyon, Capella, and σ2 CrB. Finally various relations between rotation and different measures of activity are compared. The effective X-ray temperature, derived from single-temperature fits to soft X-ray IPC spectra, is shown to play a part in the relation between activity and rotation.

Magnetic fields in late-type stars

Observations show that magnetic activity in late-type stars is correlated with rotation rates and that there is a discontinuous change in behavior at a critical rotation period. This can be explained as a consequence of a transition from convection in rolls parallel to the rotation axis to normal convection cells as the angular velocity is decreased.