Infrared photometry of red giants in the globular cluster 47 Tucanae

Abstract

Infrared broad-band JHK and intermediate-band CO and R2O photometry is presented for 64 stars in the globular cluster 47 Tucanae including the long period variables (LPV) V1-4. These data are combined with optical photometric data and compared with evolutionary tracks for giant branch (GB) stars, with models for asymptotic giant branch (AGB) stars, and with determinations of CN band strengths. The main results of this paper are as follows: 1. At a fixed luminosity, the 47 Tuc giants show negligible scatter in V-K or, equivalently, effective temperature. Except for the upper half-magnitude, the slope of the GB is similar to theoretical giant branch slopes of the appropriate metallicity. These two results set an upper limit of 0.2 M_⊙ to the mass lost by stars as they evolve from the level of the horizontal branch to within 0.5 mag of the GB tip. 2. Comparison of theoretical GB tracks with the observed giant branches of M92, Ml3, 47 Tuc, M7 1, and M67 show that shifts in T_(eff) are required to bring theory and observation into agreement. These shifts seem to be a function of [Fe/H] and are in the sense that the observed tracks lie progressively cooler than the theoretical tracks for higher metal abundance. This suggests that the ratio of the mixing length to the pressure scale height in the convective envelope may be a function of metallicity and/or stellar mass. 3. While an independent estimate of the metallicity of 47 Tuc cannot be made from the IR data alone, the close similarity in all observed parameters of the 47 Tuc stars and the M7 1 giants studied previously implies that the two clusters must have essentially the same metallicity. 4. At a given effective temperature, the CO absorption strengths of the 47 Tuc giants have a significant scatter. This scatter can be accounted for by an anticorrelation between the CO strengths and the CN strengths measured by Norris and Freeman. The origin of this anticorrelation is likely to be the effect of CN-blanketing in one of the filter band passes used to measure the CO strengths. 5. At mean light the four LPVs lie sufficiently above the tip of the giant branch that they must be AGB stars. Their luminosities, temperatures, and periods are in qualitative agreement with model predictions for such stars. The periodic behavior of these variables is similar to that of Galactic LPVs. 6. Stellar H_2O absorption in the band passes of the H and K filters is a strong influence on the J-H and H-K colors of some of the stars studied. Furthermore, the H_2O strengths observed in the four LPVs are greater than those in the non-LPV variables which, in turn, are greater than those in the nonvariables. Again, this is similar to the situation for cool Galactic variable and nonvariable giants.

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