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. ----------------------------------