Radiation transfer of gyrosynchrotron emission in stellar environments

Abstract

Non-thermal radio emission has been detected from several kinds of active stars. Polarization and intensity measurements of the quiescent (i.e. non-flaring) emission indicate gyrosynchmtron crnission. A three-dimensional magnetic lield model for the stellar field is presented anti the gyrosynchrolron intensity and polarization emerging from such a model is calculalcd and compared with obserwations. Model spectra agree well with observations. Model polarization restdts indicate that the emission region is morc complex than the simple model and may indicate the prcscncc ~yf multipolc components in the field, small loops of llux near the stellar surface and significanl energy loss in the radiating electron distribution over time.

Nonthermal Radio Emission from the Precataclysmic Binary System V471 Tauri

“…A model for the emission region of RS CVn binary systems incorporating a dipole magnetosphere and a radial magnetized stellar wind outside the dipole Ðeld lines produces spectra in good agreement with observations of RS CVn binary systems (Storey 1996).…”

“…This is consistent with magnetic Ðeld values derived for RS CVn binary systems and other similar active stars (Morris et al 1990 ;Kuijpers & van der Hulst 1985 ;Mutel et al 1985 ;Lestrade et al 1988 ;PCS). We use the fact that in our RS CVn modeling we have found that the model circular polarization is lower than that observed (Storey 1996) and the fact that circular polarization of gyrosynchrotron emission increases with magnetic Ðeld strength (Dulk 1985) to restrict still further the range of magnetic Ðelds investigated to greater than 10~3 T (Storey 1996). Since we are assuming the active star in V471 Tau is similar to the active stars in RS CVn binary systems, we assume a magnetic Ðeld value of 3.5 ] 10~3 T, a value at the high end of the range of validity of the analytic expressions used.…”

“…In this section we extend the model, used previously to describe the radio emission from RS CVn binary systems (Storey 1996), to develop a model for the nonthermal radio emission from V471 Tau. We assume that the K dwarf (the secondary) is surrounded by a symmetric magnetic dipole Ðeld, and that beyond the closed Ðeld lines is a magnetized stellar wind.…”

“…However, as mentioned above, the magnetic Ðelds of localized active regions would decrease with height above the surface faster than the dipole component, resulting in the dipole Ðeld being most likely to dominate at a signiÐcant distance from the surface. Such regions may make a contribution to the emission close to the surface, but results from RS CVn binary systems indicate that small active regions do not dominate the quiescent emission (Storey 1996 ;Morris et al 1990). Moreover, models of the system incorporating a loop whose plane is oriented perpendicular to the magnetic dipole axis are not able to reproduce the observations, nor are models incorporating a Ñux tube of small cross-sectional area and limited azimuthal extent (see below).…”

“…Using the method of Storey (1996 ;outlined in°3) and the basic system conÐguration illustrated in Figure 1, we have calculated the gyrosynchrotron emission from our binary system model as a function of rotation phase for several values of system parameters and for several models of the mildly relativistic electron distribution in the enhanced region.…”