III. THE STELLAR DATA

A grid of 17 standard stars was set up in a manner similar to that described in Paper I. The H₂O indices of these stars are referred to that of Lyrae, which is defined equal to 0.00, and are given in the first part of Table 1. The internal accuracy of the grid is judged to be close to ± 0.01 mag. The second part of Table 1 lists values of the H₂O index for high luminosity stars, and can facilitate transfer between ours and other photometric systems. Similar observations of dwarf stars are presented by Persson, Aaronson and Frogel (1977). In the last part of Table 1, CO and H₂O indices of a few Mira and carbon stars are listed. Although high resolution scans (Johnson and Mendez 1970; Frogel and Hyland 1972) show that carbon stars have very strong CO and no H₂O absorption, the extremely red continua of these stars causes our measured indices to indicate the opposite.

Table 2 gives the adopted mean relationships between the H₂O index and color and spectral type for giant and dwarf stars. They are based on the data of Table 1 and on unpublished observations of more than 50 other stars. Values of V - K are from Paper I. It is apparent from Table 2 that for K and M dwarfs and giants, the H₂O index depends strongly on effective temperature. Furthermore, for stars with spectral types between MO and M6, it depends strongly on luminosity, but in the opposite sense to that of the CO index; viz., the H₂O index is stronger in dwarfs than in giants.