Mid-IR Spectroscopic Observations of Water in Betelgeuse and Antares

We have performed high-resolution (lambda /Delta lambda ~ 10,000) spectroscopic observations of Betelgeuse (alpha Orionis) and Antares (alpha Scorpii) near 12.3 mu m on March 21--25 and May 21--23, 1997. We used CELESTE, a Goddard-developed cryogenic echelle spectrometer, in conjunction with the McMath-Pierce Solar Telescope (NSO/NOAO) at Kitt Peak National Observatory. Absorption lines of hot water have been identified in the infrared spectra of both stars. These spectra compare closely in structure with umbral sunspot spectra. The transitions were identified thanks to new laboratory studies of water spectra taken at high temperatures. These water lines originate in the lower chromosphere of each star, near the temperature minimum. We find the temperature minima in both stars to be 2800 K. The water column density in both stars is 3(+/-2)x10(18) cm(-2) , yielding an abundance relative to atomic hydrogen of n(H2O)/n(H) ~ 10(-7) . This relative abundance agrees well with the expected value for 2700 K atmospheres in oxygen-rich supergiants. Chromospheric hydroxyl (OH) was also observed in both stars. These stars undergo irregular variability over periods of months and years which are expected to change the spectral structure and intensities of the observed molecular lines. We compare the 812 cm(-1) region of the spectrum in alpha Orionis with previous observations made with CELESTE and the 3.5-m telescope at Apache Point Observatory on March 10--11, 1995.