Infrared photometry, bolometric magnitudes, and effective temperatures for giants in M3, M13, M92, and M67
Broad-band infrared J-, H-, and K-magnitudes and narrow-band CO and H_2O indices are presented for a selection of giants reaching 3 mag below the red-giant tip in the globular clusters M3, M13, and M92, and in the old open cluster M67. Comparison of these data with a calculated grid of model atmospheres gives the following results: (1) the models satisfactorily predict the broad-band colors; (2) V — K measurements accurate to ±0.1 mag can be used to give effective temperatures to ± 100 K independent of surface gravity or metal abundance for metal-poor stars; (3) there is disagreement with a previous (T_(eff), spectral type)-calibration based on DDO photometry. Empirical bolometric magnitudes are derived by integrating the observed energy distributions out to 2.2 /µm. The derived luminosities and effective temperatures are plotted in a H-R diagram and are compared with a set of evolutionary tracks for metal-poor stars due to Rood. The agreement is good. The CO index, which is sensitive primarily to luminosity and effective temperature for Population I giants, becomes sensitive to metal abundance for very metal-poor stars. The relative metal abundance of M3 and M13 derived from CO is reversed from that derived by some other methods. Some possible explanations are considered. Because of the importance of Rayleigh scattering in these cluster stars, B — V depends on surface gravity. Gravities determined from B — V colors are used to derive crude masses for the stars. These masses (~0.6 M_⊙) are roughly consistent with estimates of the turnoff mass. The CNO abundances are suggested as the second parameter affecting the color-magnitude diagrams of globular clusters
Additional Information© American Astronomical Society • Provided by the NASA Astrophysics Data System. Received 1977 September 16; accepted 1977 November 9. We are grateful to Dr. R. L. Kurucz for permission to use his ATLAS6 code and to Dr. Ruth Peterson for her help in getting the code running on the KPNO computer. We appreciate the criticisms and suggestions of several of our colleagues at CTIO. We thank S. Beckwith for help in making some of the observations. This work was supported in part by NSF grant AST 74-18555A and NASA grant NGL 05-002-207. Operated by AURA, Inc., under NSF contract AST 74-04129.
Published - 1978ApJ___222__165C.pdf