Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........45g&link_type=abstract
PhD Dissertation, Wyoming Univ. Laramie, WY United States
Astronomy and Astrophysics
Astronomy
1
Main Sequence Stars, Open Clusters, Red Giant Stars, Subgiant Stars, Blue Stars, Globular Clusters, Stellar Evolution, Astronomical Photometry, Charge Coupled Devices, Color, Horizontal Branch Stars, Spatial Distribution, Stellar Mass, Stellar Spectra
Scientific paper
New CCD photometry of the evolved stars in the old open cluster NGC 6819 is presented. From isochrone fits, the age of that cluster is found to be 3.16 GYr and its (Fe/H) is -1.0. The reddening and distance modulus of the cluster are found to be Om.24 and 11.5 respectively. The spatial distributions of clump giant, red giant, and subgiant stars in NGC 6819 are shown to be almost identical. It is argued that this similarity indicates that all the evolved stars in NGC 6819 possess approximately the same dynamical mass. The stellar evolutionary masses of the giants and subgiants in this cluster are shown to be the same. For comparison purposes, the spatial distributions and relative masses of the evolved stars in the diffuse globular cluster NGC 288 are examined. The spatial distributions of blue horizontal branch stars (BHB's) are found to be the same as the giants, subgiants, and turn-off stars in this cluster. Isochrone fits to the CMD of NGC 288 including the blue horizontal branch are presented. These indicate that while the giants, subgiants, and turn-off stars have almost identical masses, the BHB's in NGC 288 are approximately 0.2 M(solar mass) less massive than the red giants. A comparison of the half-mass relaxation time with the helium burning main-sequence lifetime in NGC 6819 suggests that the spatial distribution of the clump giants is relaxed. This indicates that no mass has been lost between the red giant and clump giant stages of stellar evolution in this cluster. A similar comparison in NGC 288 indicates that the half-mass relaxation time in NGC 288 is approximately 10 times that of the helium main-sequence lifetime. This shows that the BHB's could not have relaxed after mass loss during the red giant phase. The author suggests that BHB's in NGC 288 display the relaxed distribution of their progenitors. These progenitors were probably red giants that were slightly more massive than those currently near the red giant tip.
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