Generation of longitudinal flux tube waves in theoretical main-sequence stars: effects of model parameters

Abstract

Aims. We compute the wave energy fluxes carried by longitudinal tube waves along vertically oriented thin magnetic fluxes tubes embedded in the atmospheres of theoretical main-sequence stars based on stellar parameters deduced by Kurucz and Gray. In addition, we present a fitting formula for the wave energy flux based on the governing stellar and magnetic parameters. Methods. A modified theory of turbulence generation based on the mixing-length concept is combined with the magnetohydrodynamic equations to numerically account for the wave energies generated at the base of magnetic flux tubes. Results. The results indicate a stiff dependence of the generated wave energy on the stellar and magnetic parameters in principal agreement with previous studies. The wave energy flux F LTW decreases by about a factor of 1.7 between G0 V and K0 V stars, but drops by almost two orders of magnitude between K0 V and M0 V stars. In addition, the values for F LTW are significantly higher for lower in-tube magnetic field strengths. Both results are consistent with the findings from previous studies. Conclusions. Our study complements existing descriptions of magnetic energy generation in late-type main-sequence stars. Our results will be helpful for calculating theoretical atmospheric models for stars of different levels of magnetic activity.

Testing a Predictive Theoretical Model for the Mass Loss Rates of Cool Stars

“…These fits are similar in form to those given by Fawzy & Cuntz (2011) for longitudinal MHD waves. Figure 2 shows a comparison between the above fitting formula and the plotted results of Musielak & Ulmschneider (2002a) for log g = 3, 4, and 5.…”

Solar magnetic flux tube simulations with time-dependent ionization

“…According to the updated value of α ML , the initial wave energy flux for the acoustic model is given as F M = 1.09 × 10 8 erg cm −2 s −1 (Ulmschneider et al ), see Table . For the wave energy flux of the longitudinal flux tube wave, we use F M = 2.80 × 10 8 erg cm −2 s −1 (Fawzy & Cuntz ); this value assumes a pressure ratio between outside and inside of the tube of ε = 3 (see equation ). It is based on the assumption of an inside magnetic field strength of 1700 G, noting that in our model the equipartition magnetic field strength is given as G with B / B eq ≃ 0.82.…”

The Secret Lives of Cepheids: δ Cep—The Prototype of a New Class of Pulsating X-Ray Variable Stars*

“…The role of short-scale magnetoacoustic convective modes, entailing the generation of longitudinal tube waves, has also been explored in detail, as pursued in the framework of time-dependent simulations. By considering photospheric-level magnetic parameters (Fawzy & Cuntz 2011), as well as the detailed treatment of shock formation and dissipation (Fawzy et al 2012), it was found that processes associated with those modes are insufficient to explain the X-ray observations. Another possible mechanism for generating magnetic fields and the resulting X-ray emission in δ Cep (and perhaps other Cepheids) are via a convective zone, or through a combination of convective and pulsation-driven motions and turbulence, within the stellar interior (Narain & Ulmschneider 1996, and subsequent work).…”