67 research outputs found
Winds from Luminous Late-Type Stars: II. Broadband Frequency Distribution of Alfv\'en Waves
We present the numerical simulations of winds from evolved giant stars using
a fully non-linear, time dependent 2.5-dimensional magnetohydrodynamic (MHD)
code. This study extends our previous fully non-linear MHD wind simulations to
include a broadband frequency spectrum of Alfv\'en waves that drive winds from
red giant stars. We calculated four Alfv\'en wind models that cover the whole
range of Alfv\'en wave frequency spectrum to characterize the role of freely
propagated and reflected Alfv\'en waves in the gravitationally stratified
atmosphere of a late-type giant star. Our simulations demonstrate that, unlike
linear Alfv\'en wave-driven wind models, a stellar wind model based on plasma
acceleration due to broadband non-linear Alfv\'en waves, can consistently
reproduce the wide range of observed radial velocity profiles of the winds,
their terminal velocities and the observed mass loss rates. Comparison of the
calculated mass loss rates with the empirically determined mass loss rate for
alpha Tau suggests an anisotropic and time-dependent nature of stellar winds
from evolved giants.Comment: accepted by Ap
Wild Ideas: Sessions in Teaching Astronomy Courses
A new technique of teaching astronomy courses for science and non-science majors, so called personalized wild ideas sessions, is proposed. The brainstorming approach was developed and applied for teaching astronomy courses at ERAU, Eastern Region over the last 3 years. This technique represents an efficient and fun way to learn basic astronomical concepts and methods through a student\u27s involvement in generating \u27wild ideas about a given astronomical fact or set of facts, their analysis and verification. Basic elements of the technique and one sample session are described
The Atmospheric Dynamics of alpha Tau (K5 III) - Clues to Understanding the Magnetic Dynamo in Late-Type Giant Stars
Using HST/GHRS, HST/STIS and FUSE archival data for alpha Tau and the CHIANTI spectroscopic code, we have derived line shifts, volumetric emission measures, and plasma density estimates, and calculated filling factors for a number of UV lines forming between 10,000 K and 300,000 K in the outer atmosphere of this red giant star. The data suggest the presence of low-temperature extended regions and high-temperature compact regions, associated with magnetically open and closed structures in the stellar atmosphere, respectively. The signatures of UV lines from alpha Tau can be consistently understood via a model of upward-traveling Alfven waves in a gravitationally stratified atmosphere. These waves cause non-thermal broadening in UV lines due to unresolved wave motions and downward plasma motions in compact magnetic loops heated by resonant Alfven wave heating
Understanding the Duration of Solar and Stellar Flares at Various Wavelengths
Recent irradiance measurements from numerous heliophysics and astrophysics
missions including SDO, GOES, Kepler, TESS, Chandra, XMM-Newton, and NICER have
provided critical input in understanding the physics of the most powerful
transient events on the Sun and magnetically active stars, solar and stellar
flares. The light curves of flare events from the Sun and stars show remarkably
similar shapes, typically with a sharp rise and protracted decay phase. The
duration of solar and stellar flares has been found to be correlated with the
intensity of the event in some wavelengths, such as white light, but not in
other wavelengths, such as soft X-rays, but it is not evident why this is the
case. In this study, we use a radiative hydrodynamics code to examine factors
affecting the duration of flare emission at various wavelengths. The duration
of a light curve depends on the temperature of the plasma, the height in the
atmosphere at which the emission forms, and the relative importance of cooling
due to radiation, thermal conduction, and enthalpy flux. We find that there is
a clear distinction between emission that forms low in the atmosphere and
responds directly to heating, and emission that forms in the corona, indirectly
responding to heating-induced chromospheric evaporation, a facet of the Neupert
effect. We discuss the implications of our results to a wide range of flare
energies.Comment: Submitted to ApJ. Comments and criticisms are welcomed
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