Z. E. Musielak

On Stellar Activity Enhancement Due to Interactions with Extrasolar Giant Planets.

We present a first attempt to identify and quantify possible interactions between recently discovered extrasolar giant planets (and brown dwarfs) and their host stars, resulting in activity enhancement in the stellar outer atmospheres. Many extrasolar planets have masses comparable to or larger than Jupiter and are within a distance of 0.5 AU, suggesting the possibility of their significant influence on stellar winds, coronae, and even chromospheres. Beyond the well-known rotational synchronization, the interactions include tidal effects (in which enhanced flows and turbulence in the tidal bulge lead to increased magnetoacoustic heating and dynamo action) and direct magnetic interaction between the stellar and planetary magnetic fields. We discuss relevant parameters for selected systems and give preliminary estimates of the relative interaction strengths.

On sound generation by turbulent convection: A new look at old results

We have revisited the problem of generation of acoustic waves by turbulent convection in an isothermal atmosphere. The theory of sound generated aerodynamically has been originally developed by Lighthill and later modified by Stein to include the effects of stratification. Lighthill and Steins results have been extensively used to estimate the amount of acoustic wave energy generated in the solar and stellar convective zones. We have recently recognized that Steins treatment of turbulence requires some corrections and that these corrections lead to significant changes in the obtained results. In this paper, we present the correct status of computing the acoustic wave energy fluxes by incorporating a physically meaningful description of the spatial and temporal spectrum of the turbulent convection. We show the dependence of the obtained wave fluxes on the nature of the turbulence and discuss the efficiency of acoustic wave generation in the solar convection zone.

Standard and non-standard Lagrangians for dissipative dynamical systems with variable coefficients

Dynamical systems described by equations of motion with the first-order time derivative (dissipative) terms of even and odd powers, and coefficients varying either in time or in space, are considered. Methods to obtain standard and non-standard Lagrangians are presented and used to identify classes of equations of motion that admit a Lagrangian description. It is shown that there are two general classes of equations that have standard Lagrangians and one special class of equations that can only be derived from non-standard Lagrangians. In addition, each general class has a subset of equations with non-standard Lagrangians. Conditions required for the existence of standard and non-standard Lagrangians are derived and a relationship between these two types of Lagrangians is introduced. By obtaining Lagrangians for several dynamical systems and some basic equations of mathematical physics, it is demonstrated that the presented methods can be applied to a broad range of physical problems.

Alfvén wave trapping, network microflaring, and heating in solar coronal holes

Fresh evidence that much of the heating in coronal holes is provided by Alfven waves is presented. This evidence comes from examining the reflection of Alfven waves in an isothermal hydrostatic model coronal hole with an open magnetic field. Reflection occurs if the wavelength is as long as the order of the scale height of the Alfven velocity. For Alfven waves with periods of about 5 min, and for realistic density, magnetic field strength, and magnetic field spreading in the model, the waves are reflected back down within the model hole if the coronal temperature is only slightly less than 1.0 x 10 to the 6th K, but are not reflected and escape out the top of the model if the coronal temperature is only slightly greater than 1.0 x 10 to the 6th K. Because the spectrum of Alfven waves in real coronal holes is expected to peak around 5 min and the temperature is observed to be close to 1.0 x 10 to the 6th K, the sensitive temperature dependence of the trapping suggests that the temperature in coronal holes is regulated by heating by the trapped Alfven waves.

Two-Component Theoretical Chromosphere Models for K Dwarfs of Different Magnetic Activity: Exploring the Ca II Emission-Stellar Rotation Relationship

We compute two-component theoretical chromosphere models for K2 V stars with diUerent levels of magnetic activity. The two components are a nonmagnetic component heated by acoustic waves and a magnetic component heated by longitudinal tube waves. The —lling factor for the magnetic component is determined from an observational relationship between the measured magnetic area coverage and the stellar rotation period. We consider stellar rotation periods between 10 and 40 days. We investigate two diUerent geometrical distributions of magnetic —ux tubes: uniformly distributed tubes, and tubes arranged as a chromospheric network embedded in the nonmagnetic region. The chromosphere models are constructed by performing state-of-the-art calculations for the generation of acoustic and magnetic energy in stellar convection zones, the propagation and dissipation of this energy at the diUerent atmo- spheric heights, and the formation of speci—c chromospheric emission lines that are then compared to the observational data. In all these steps, the two-component structure of stellar photospheres and chromospheres is fully taken into account. We —nd that heating and chromospheric emission is signi—- cantly increased in the magnetic component and is strongest in —ux tubes that spread the least with height, expected to occur on rapidly rotating stars with high magnetic —lling factors. For stars with very slow rotation, we are able to reproduce the basal —ux limit of chromospheric emission previously identi- —ed with nonmagnetic regions. Most importantly, however, we —nd that the relationship between the Ca II H)K emission and the stellar rotation rate deduced from our models is consistent with the relationship given by observations. Subject headings: line: formationMHDstars: activitystars: chromospheres ¨ stars: late-typestars: rotation

Orbital Stability of Terrestrial Planets inside the Habitable Zones of Extrasolar Planetary Systems

We investigate orbital stability of terrestrial planets inside the habitable zones of three stellar systems, i.e., 51 Peg, 47 UMa, and HD 210277, with recently discovered giant planets. These systems have similar habitable zones; however, their giant planets have different masses and significantly different orbital parameters. It is shown that stable orbits of terrestrial planets exist in the entire habitable zone of 51 Peg as well as in the inner part of the habitable zone of 47 UMa, but no stable orbits are found in the habitable zone of HD 210277. The obtained results allow us to draw general conclusions on the existence of stable orbits in the habitable zones of newly found extra-solar planetary systems.

Reflection and trapping of Alfven waves in a spherically symmetric stellar atmosphere

Alfven wave propagation in a spherically symmetric isothermal and stratified stellar atmosphere are analzyed using a time-dependent MHD numerical model. Particular consideration is given to wave reflection and the resultant trapping of the wave due to a peak in the Alfven speed in the atmosphere. Resonance frequencies in the trapping region and the effect of trapping on Alfven wave pressure force and propagation are examined. The data reveal that Alfven wave trapping has a potentially important role in accelerating winds from cool stars.

On the generation of flux tube waves in stellar convection zones. I - Longitudinal tube waves driven by external turbulence

The source functions and the energy fluxes for wave generation in magnetic flux tubes embedded in an otherwise magnetic field-free, turbulent, and compressible fluid are derived. The calculations presented here assume that the tube interior is not itself turbulent, e.g., that motions within the flux tube are due simply to external excitation. Specific results for the generation of longitudinal tube waves are presented. 39 references.

Reflection and trapping of transient Alfven waves propagating in an isothermal atmosphere with constant gravity and uniform magnetic field

A time-dependent linear magnetohydrodynamic numerical model was used to investigate the propagation of Alfven waves in an isothermal and stratified atmosphere with constant gravity and uniform vertical magnetic field. Results show that the Alfven wave transit time for the wave source to infinity is finite and that the wave exhibits continuous partial reflection which becomes total reflection as the front approaches infinity. The total reflection causes the waves to be trapped in the cavity that extends from the wave source to infinity and in which the wave energy is stored. The results suggest that the reflection of Alfven waves (of sufficiently long period) from the outer corona is an intrinsic phenomenon for any stellar atmosphere stratified by gravity and an open magnetic field, and that, therefore, such waves may be trapped in the stellar atmosphere. 15 refs.

On the origin of 'dividing lines' for late-type giants and supergiants

We show how a change in the nature of the stellar dyanmo can lead to a transition in the topological character of stellar magnetic fields of evolved stars, from being mainly closed on the blueward side of the giant tracks in the Hertzsprung-Russell (H-R) diagram to being mainly open on their redward side. If such a topological transition occurs, then these stars naturally segregate into two classes: those having hot coronae on the blueward side, and those having massive cool winds on the redward side, thus leading naturally to the so-called dividing lines.