CCD Photometry of the RR Lyrae Variable in the LMC Cluster NGC 2257 and the Adjacent Field
Abstract
Extensive B and V band CCD photometry for 33 cluster and nine field RR Lyrae variables near the old LMC cluster NGC 2257 provide accurate mean magnitudes for the stars, and hence distances for the cluster and field populations. The mean magnitudes for the two groups differ by 0.17 mag. This is interpreted as a distance effect rather than a difference in metal abundance or evolutionary state. The field stars thus average 3.7 kpc more distant from us than the cluster stars, and 4.1 kpc more distant than the plane of the LMC as defined by the de Vaucouleurs and Freeman incline plane model. In addition, the field stars occupy less than 9 kpc in depth; however, the data are not inconsistent with the LMC field RR Lyraes, taken as a whole, forming a spherical rather than a planar system. From the period-amplitude diagram, both the cluster and field variables have metal abundance of [Fe/H] = -1.8 +/- 0.1. There is little evidence for any gross spread in metal abundance among the field variables, although periods for two short-period and, presumably, metal-rich RRab stars are confirmed. The new light curves extend previous work on period changes for the cluster variables and have allowed revised periods and rates of period change to be calculated. Two possible double-mode (RRd) field variables were found to have incorrect periods and are actually RRab stars. Photometry for 462 stars, reaching to V~21 and sampling the cluster to its very center, have been used to prepare a color-magnitude diagram of the evolved stars in NGC 2257. This shows a richly populated blue horizontal branch, the width of which is consistent with measurements of the dispersion of absolute magnitudes of RR Lyraes in galactic globular clusters by Sandage. The asymptotic giant branch is clearly separated from the red giant branch allowing accurate numbers of stars in the various stages of evolution to be counted. These counts are compared with those for galactic globular clusters, with the result that NGC 2257 appears to be indistinguishable from its local counterparts such as M3.
- Publication:
-
The Astronomical Journal
- Pub Date:
- December 1989
- DOI:
- 10.1086/115282
- Bibcode:
- 1989AJ.....98.2086W
- Keywords:
-
- Astronomical Photometry;
- Blue Stars;
- Charge Coupled Devices;
- Giant Stars;
- Globular Clusters;
- Magellanic Clouds;
- Color-Magnitude Diagram;
- Interstellar Extinction;
- Light Curve;
- Metallicity;
- Stellar Evolution;
- Stellar Magnitude;
- Astronomy;
- GALAXIES: MAGELLANIC CLOUDS;
- STARS: RR LYRAE