A Re-Evaluation of the Evolved Stars in the Globular Cluster M13

Abstract

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

Differences in the rotational properties of multiple stellar populations in M13: a faster rotation for the ‘extreme’ chemical subpopulation

“…VandenBerg et al 2013). In any case, M13 is a clear outlier that does not fit this picture with a large A rot /σ 0 despite its bimodal and extremely blue horizontal branch with B−R B+R+V = 0.976 (Sandquist et al 2010). Milone 2015) and also found to play a role in bi-variate correlations involving parameters related to the anticorrelation phenomenon (Carretta et al 2009), all these possible relations hint at a complex interplay between cluster mass, rotation, and the chemical properties of multiple populations.…”

Agb Sodium Abundances in the Globular Cluster 47 Tucanae (Ngc 104)

“…Despite having significantly different metallicities and horizontal branch morphologies, the AGB/RGB composition difference in 47 Tuc may be more analagous to the case of M13 where Johnson & Pilachowski (2012) found that only the most Na-rich and O-poor stars (presumably also the most He-rich stars) failed to reach the AGB. 18 We note that the paucity of O-poor stars on M13ʼs AGB, albeit from a sample of only ∼10 AGB stars, is observed despite the failure (so far) of investigators to find a strongly He-enhanced population (Sandquist et al 2010;Smith et al Figure 6. Sample Na I spectrum synthesis for a typical 47 Tuc AGB star.…”

New atmospheric parameters and spectral interpolator for the MILES cool stars

“…MILES 934: Sandquist et al (2010) Bessell (1983) as a probable interloper on the basis of its velocity. From temperature-colour relations, the author reports T eff = 4260 K and from a spectroscopic analysis The second one matches the previous measurements for NGC 6838 1077 and we believe that the two targets were swapped.…”