Other
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........26s&link_type=abstract
PhD Dissertation, Michigan State Univ. East Lansing, MI United States
Other
1
Lyra Constellation, Globular Clusters, Cepheid Variables, Stellar Evolution, Stellar Spectrophotometry, Light Curve, Color, Stellar Magnitude, Charge Coupled Devices
Scientific paper
The globular cluster M15 is a canonical Oosterhoff type 2 globular cluster. The RR Lyrae variable stars within M15 have previously been used to determine the age of the cluster as well as the absolute magnitude of the variables. These previous studies have relied on photographic photometry. With the recent advances in electronic cameras (CCD's), it is now possible to obtain higher quality photometry in a much shorter time. These new cameras challenge observers to make new analyses of RR Lyrae stars in globular clusters. To that end, new CCD photometry is presented of RR Lyrae variable stars in the globular cluster M15. This photometry, mainly in V and R, with some additional B and I, is used to construct lightcurves of 44 RR Lyraes and 1 Cepheid (v1) in M15. One new RR Lyrae star was discovered and is tentatively named v113. Using the new (V-R) colors and Kurucz's stellar models, the blue and red edges of the instability strip are found to be at log Teff = 3.875 and log Teff approximately equal to 3.81, respectively. The transition line between the fundamental and first overtone mode pulsators is found to be at log Teff = 3.839. There is no overlap in (V-R) between the fundamental and first overtone mode variables. From pulsation theory and these observations, the absolute magnitude of the RR Lyrae stars in M15 is MV = +0.33 +/- 0.12, for an RR Lyrae mass of 0.75 M(solar mass). With MV = +0.33, the age of M15 is 12.6 x 109 years. However, due to the still present uncertainties in RR Lyrae mass and the Teff scale calibration, the absolute magnitude of the M15 RR Lyrae stars still remains significantly uncertain. We have also investigated the period changes of 49 RR Lyrae variable stars in the globular cluster M15 using observation spanning almost a century. Some of the variables have been too sparsely observed over that time interval for reliable period changes to be determined. Others, which have been well observed, show a wide range of period change behavior. We find a significant excess of increasing over decreasing periods. The median rate of period change, +0.03 days/million years, agrees with the theoretical prediction of Lee. This implies that most M15 RR Lyrae stars are evolving from blue to red through the instability strip. However, it is not certain whether there are sufficient blue horizontal branch stars in M15 to sustain the M15 RR Lyrae population at the observed rate of period change. The dispersion in the rate of period change for RR Lyraes of Bailey type ab is greater than for those of type cd. The origin of that difference in period change behavior is without adequate explanation.
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