9 research outputs found
New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
(abridged) The heating mechanism at high densities during M dwarf flares is
poorly understood. Spectra of M dwarf flares in the optical and
near-ultraviolet wavelength regimes have revealed three continuum components
during the impulsive phase: 1) an energetically dominant blackbody component
with a color temperature of T 10,000 K in the blue-optical, 2) a smaller
amount of Balmer continuum emission in the near-ultraviolet at lambda 3646
Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer
lines. These properties are not reproduced by models that employ a typical
"solar-type" flare heating level in nonthermal electrons, and therefore our
understanding of these spectra is limited to a phenomenological interpretation.
We present a new 1D radiative-hydrodynamic model of an M dwarf flare from
precipitating nonthermal electrons with a large energy flux of erg
cm s. The simulation produces bright continuum emission from a
dense, hot chromospheric condensation. For the first time, the observed color
temperature and Balmer jump ratio are produced self-consistently in a
radiative-hydrodynamic flare model. We find that a T 10,000 K
blackbody-like continuum component and a small Balmer jump ratio result from
optically thick Balmer and Paschen recombination radiation, and thus the
properties of the flux spectrum are caused by blue light escaping over a larger
physical depth range compared to red and near-ultraviolet light. To model the
near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer
lines, we include the extra Balmer continuum opacity from Landau-Zener
transitions that result from merged, high order energy levels of hydrogen in a
dense, partially ionized atmosphere. This reveals a new diagnostic of ambient
charge density in the densest regions of the atmosphere that are heated during
dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar
Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015):
updated to include comments by Guest Editor. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-
Superflares on Giant Stars
The Kepler mission identified huge flares on various stars including some of
solar type. These events are substantially more energetic than solar flares,
and so they are referred to as superflares. Even a small probability of such a
superflare on the Sun would be a menace to modern society. A flare comparable
in energy with that of superflares was observed on 24th and 25th September on
the binary HK Lac. Unlike the Kepler stars, there are observations of
differential rotation for HK Lac. This differential rotation appears to be
anti-solar. For anti-solar differential rotation, dynamo models can give
magnetic activity waves of dipole symmetry as well as quasi-stationary magnetic
configurations with quadrupole symmetry. The magnetic energy of such stationary
configurations is usually about two orders of magnitude higher than that
associated with activity waves. We believe that this mechanism could provide
sufficient energy to produce superflares on late type stars, and present some
simple models in support of this idea.Comment: 7 pages, 3 figures, 1 table. Accepted to Astronomy Reports, 2018,
Vol.62, No.
Two Strong White-Light Solar Flares in AR NOAA 12673 as Potential Clues for Stellar Superflares
Observing Dynamos in Cool Stars
The main aim of this paper is to introduce the most important observables
that help us to investigate stellar dynamos and compare those to the modeling
results. We give an overview of the available observational methods and data
processing techniques that are suitable for such purposes, with touching upon
examples of inadequate interpretations as well. Stellar observations are
compared to the solar data in such a way, which ensures that the measurements
are comparable in dimension, wavelength, and timescale. A brief outlook is
given to the future plans and possibilities. A thorough review of this topic
was published nearly a decade ago (Berdyugina 2005), now we focus on the
experience that have been gathered since that time.Comment: 47 pages, accepted for publication in Space Science Review