Cool Stars 14 - Submitted Abstract # 128
This version created on 05 October 2006

On the boundry temperatures of K and M giants


George Wallerstein, University of Washington
Tamara Mishenina, University of Odessa, Ukraine
Heinz Edelmann, University of Texas
David Lambert, University of Texas

The temperature minimum is an important quantity in model atmospheres
of cool stars.  It is difficult to establish because the coolest
layers contribute very little to the emergent flux and may be affected
by back-warming from the chromosphere.  At the same time the strength
of lines formed by pure absorption (rather than scattering) depend on
the temperature gradient throughout the entire atmosphere.  In
addition the percentage of an element in the neutral state for
elements that are largely ionized depends sensitively on the local
temperature.  We have investigated the sensitivity of the
singlet-triplet transition of MgI at 4571.10  A to the boundry
temperature in normal giants of type K and early M.  Spectra of
resolving power about 100,000  and S/N > 100  of 27 giants were
obtained with the McDonald 2.7-m telescope and coude spectrograph.
The data were reduced with standard programs at the Odessa Observatory
and compared with line profiles from model atmospheres with the
appropriate Teff and logg values.  The line profiles were generally
significantly less deep than predicted by the models.  This indicates
that the chromosphere is contributing to the flux at the bottom of the
line either by warming the outermost layers of the photosphere or by
emission from the chromosphere itself at the center of the line.

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