We have analyzed photometry from space-and ground-based cameras to identify all bright red giant branch (RGB), horizontal branch (HB), and asymptotic giant branch (AGB) stars within 10 ′ of the center of the globular cluster M13.We identify a modest (7%) population of HB stars redder than the primary peak (including RR Lyrae variables at the blue end of the instability strip) that is somewhat more concentrated to the cluster core than the rest of the evolved stars. We find support for the idea that they are noticeably evolved and in the late stages of depleting helium in their cores. This resolves a disagreement between distance moduli derived from the tip of the red giant branch and from stars in or near the RR Lyrae instability strip. We identified disagreements between HB model sets on whether stars with T eff 10000 K (near the "knee" of the horizontal branch in optical CMDs) should evolve redward or blueward, and the differences may depend on the inclusion of diffusion in the stellar interior. The sharp cut at the red end of M13's HB provides strong evidence that stars from the dominant HB group must still be undergoing blue loops, which implies that diffusion is being inhibited. We argue that M13's HB is a somewhat pathological casethe dominant HB population occurs very near the "knee" in optical CMDs, and evolved stars exclusively appear redward of that peak, leading to the incorrect appearance of a continuation of the unevolved HB.We identify two stars as "blue hook" star candidates -the faintest stars in optical bands that remain significantly subluminous in the shortest ultraviolet wavelength photometry available. M13 also has a distinct group of stars previously identified with the "second U jump". Based on far UV photometry, we find that these stars have genuinely high temperatures (probably 26000 K T eff 31000 K), and are not produced by a jump in brightness at lower temperature (T eff ≈ 22000 K) as previously suggested. These stars are brighter Subject headings: stars: AGB and post-AGB -stars: evolution -Hertzsprung-Russell and C-M diagrams -stars: horizontal branch -globular clusters: individual (M13) -ultraviolet: stars
Ornstein and Levine provide documentation regarding the relationship between the socioeconomics of students' lives and their subsequent capacity to perform effectively in school contexts.Research aimed at understanding and prescribing methods of overcoming the problems of urban low achieving students is discussed in the form of classroom and extra-classroom realities.The authors then suggest selected reform measures for ameliorating those realities that plague urban schools.
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