Cool Stars 14 - Submitted Abstract # 318
This version created on 05 October 2006

Spectral Diagnostics of Hot Plasma from Cool Stars


Nancy Brickhouse, Harvard-Smithsonian Center for Astrophysics

EUV and X-ray spectra tell us about the physical conditions in the hot
coronae, in particular temperature, density, elemental abundances,
opacity, and velocity. The values derived from spectroscopy inform our
models of magnetic structure, accretion shocks, and hot winds. But two
issues must always be kept in mind: (1) the methods used are often
indirect and inferential, and make a lot of assumptions; and (2) the
fundamental atomic data have flaws which are often not taken into
account. The proposed splinter session will address both of these
issues for topics of recent interest. This session also emphasizes the
solar/stellar connection. A summary of this session will be provided
to the CS14 Proceedings by N. Brickhouse.

The session will address three critical problems. Each problem will be
discussed by a panel of 3 scientists. The panelists will briefly
describe how spectroscopic measurements are used to determine a
physical property or to infer some physical process. Particular
emphasis will be given to the underlying systematic
uncertainties. Following their presentations, the audience will be
asked for comments.

I. Coronal structure (temperature, density, opacity)

The recent literature has pointed out problems with the ``classic''
He-like G-ratio, differences among densities derived using different
diagnostics (are stellar coronae really as dense as solar flares?),
and temperature discrepancies among Fe lines from the same ionization
state. A concerted effort in laboratory astrophysics and atomic theory
to address problems in fundamental X-ray spectroscopy was highlighted
at NASA's Laboratory Astrophysics Workshop in Las Vegas this
spring. Atomic theory is converging on experiment for some important
diagnostic line ratios. What is the status of these calculations? What
is the impact on coronal research?  What does the sun tell us about
individual loop structures?  Can these be extrapolated to stars?  For
the case where we think the atomic data are ok, how well do our
methods work?

II. Elemental abundances in solar and stellar coronae

For reasons still not well understood, coronal abundances do not in
general reflect the underlying photospheric abundances. How reliable
are the measurements of abundances? How much do absolute or relative
abundances depend on the emission measure distribution modeling?

III. Velocity measurements for high temperature plasma

Chandra HETG can measure Doppler shifts to an accuracy of <25 km/s.
The SOHO spectrometers routinely measure velocities of a few to 10's
of km/s.  Radial velocity studies are useful to constrain the
location[s] of the emitting regions in stars. In conjunction with
eclipse mapping or flare occultation, even more information can be
obtained. Doppler shift measurements indicate the detailed dynamics of
solar structures.  Line widths are also potentially useful diagnostics
for non-thermal broadening. What can we learn about coronae from
velocity studies?

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