New photometric data for the old galactic cluster NGC 188 : the presence of a gap chemical composition and distance modulus.
Abstract
New photoelectric UB V photometry has been combined with rereduced previous photometry for the old galactic cluster NGC 188. A gap on the rising branch of the evolving main sequence has been found in a place similar, relative to the luminosity at the turnoff from the main sequence, to the gap previously found in M67. The gap in NGC 188 occurs at 4.33 »= Mv »= 4.53, which is fainter than gaps found in other clusters of old to intermediate age and which cannot be explained by some of the available theoreti~ cal evolutionary tracks. The C-M diagram extends from V = 9.8 (M~° = -1.3) to fainter than V = 19 (M~° = 7.9). The main sequence termination point occurs near M~ +3.8, (B - V)00 0.62. These values are 0.8 mag fainter and 0 08 mag redder than the termination point in M67. The two-color diagram shows the presence of a small ultraviolet excess relative to the Hyades for stars near the main sequence. Separation of the effects of reddening and blanketing gives E(B - V) = 0.09 mag and L~(B - V)bl = 0.03 (in the sense of line weakening for main-sequence stars). A photo- metric modulus of (m - = 10.85 ± 0.15 is obtained by fitting the C-M diagram to the zero-age main sequence by the use of a new curve of evolutionary deviation. Comparison of NGC 188 and M67 with old subgiants of known chemical composition reaffirms pre- vious conclusions that old disk stars whose ages are between S X 1O~ and 1010 years have solar abundances ~n the mean, but with variations of Fe/H between 0.5 and 2 times the solar value. About 85 percent of all old disk stars in the solar neighborhood lie within this range. The data emphasize again that the metal content of the galactic disk has been nearly fixed since the time of its formation, with no systematic trend of Fe/H with age. Photometry of three of the four known contact binaries in NGC 188 is discussed in the Appendix
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1969
- DOI:
- 10.1086/150228
- Bibcode:
- 1969ApJ...158..669E