Water in Betelgeuse and Antares

Abstract

Absorption lines of hot water have been identified in the infrared spectra of Betelgeuse (α Orionis) and Antares (α Scorpii) near 12.3 micrometers (811 to 819 wavenumbers). The water lines originate in the atmospheres of the stars, not in their circumstellar material. The spectra are similar in structure to umbral sunspot spectra. Pure rotation water lines of this type will occur throughout the spectra of cool stars at wavelengths greater than 10 micrometers. From the water spectra, the upper limit for the temperature in the line formation region in both stars is 2800 kelvin. The water column density in both stars is (3 ± 2) × 10 18 molecules per square centimeter, yielding an abundance relative to atomic hydrogen of n (H 2 O)/ n (H) ≈ 10 − 7 .