Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........27c&link_type=abstract
Thesis (PH.D.)--INDIANA UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 57-01, Section: B, page: 0389.
Other
Color Gradients
Scientific paper
Using data obtained at the Canada-France-Hawaii Telescope (CFHT) in subarcsecond seeing (0^ {''}.5-0^ {''}.6 FWHM), I construct color maps in B-V and Hα-R of the central regions of the globular cluster M15. Comparison in B-V of the region within 6^{' '}.6 (0.31 pc) of the cluster center with a coadded sample taken from the surrounding region demonstrates a color gradient, in the sense that the central region is bluer than the surrounding region at a high level of statistical significance. This is in agreement with the previous finding of a U-B color gradient in M15 by Bailyn et al. [AJ, 98, 882 (1989)] based on a color map with lower angular resolution. Analysis of the H alpha-R color map indicates that the central region is "Hα deficient", i.e. has a stronger Hα absorption line on average than the coadded region. This result argues against the possibility that the excess blue light from the central region is due to cataclysmic variables. Instead, two possible causes of the color gradient are: (1) it may be due to a centrally concentrated population of faint blue stars, possibly resulting from binary-single star interactions, and (2) it may result from a central deficit of low mass main sequence stars. To further investigate the cause of the observed color gradients, I present color-magnitude diagrams of the central region of the central-cusp cluster M15. These diagrams were constructed using DAOPHOT on UBVR and H alpha CCD images taken at the Canada-France -Hawaii Telescope in 0.4-0.8^{' '} seeing. I also perform artificial star experiments, which have been used by others to estimate incompleteness corrections in such crowded fields. Using a non-binning linear least squares method of fitting the artificial star data, I estimate those corrections and also estimate the errors in the measured stellar magnitudes. Several populations of stars show central concentrations; I discuss the relation of these concentrations with the color gradients and existing theories about the dynamics of star clusters.
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