Mixing, enhanced helium and blue tails in globular clusters

Caloi, V.

doi:10.1051/0004-6361:20000210

“…Both scenarios predict lower gravities (and larger luminosities) for stars near 15 000 K. For stars cooler than 12 000 K, we find the observed gravities in agreement with canonical models. This result is consistent with the work of Caloi (2001), who compared the HB luminosities of M 3 and M 13 to conclude that there was no evidence for a substantial surface helium abundance increase for HB stars near the temperature of the RR Lyrae stars.…”

Section: Discussionsupporting

confidence: 91%

Hot HB stars in globular clusters – Physical parameters and consequences for theory

Moehler

¹

,

Landsman²,

Sweigart³

et al. 2003

A&A

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Abstract. We present the results of spectroscopic analyses of hot horizontal branch (HB) stars in M 13 and M 3, which form a famous "second parameter" pair. From the spectra and Strömgren photometry we derived -for the first time in M 13 -atmospheric parameters (effective temperature and surface gravity). For stars with Strömgren temperatures between 10 000 and 12 000 K we found excellent agreement between the atmospheric parameters derived from Strömgren photometry and those derived from Balmer line profile fits. However, for cooler stars there is a disagreement in the parameters derived by the two methods, for which we have no satisfactory explanation. Stars hotter than 12 000 K show evidence for helium depletion and iron enrichment, both in M 3 and M 13. Accounting for the iron enrichment substantially improves the agreement with canonical evolutionary models, although the derived gravities and masses are still somewhat too low. This remaining discrepancy may be an indication that scaled-solar metal-rich model atmospheres do not adequately represent the highly non-solar abundance ratios found in blue HB stars affected by diffusion. We discuss the effects of an enhancement in the envelope helium abundance on the atmospheric parameters of the blue HB stars, as might be caused by deep mixing on the red giant branch or primordial pollution from an earlier generation of intermediate mass asymptotic giant branch stars.

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“…They can produce a bluer HB morphology, making it possible to explain the hot HB population found in M13 as well as the difference in HB morphology between M3 and M13. Although the subsequent work of Caloi (2001) and the spectroscopic study on hot stars in M3 and M13 by Moehler et al (2003) could not confirm that the "helium mixing" scenario of Sweigart (1997) is a definite explanation for the origin of the blue HB tail in M13, it seems worth reexamining helium mixing as a partial cause for the morphological difference in the normal HB between M3 and M13.…”

Section: Conclusion and Summarymentioning

confidence: 92%

Different Characteristics of the Bright Branches of the Globular Clusters M3 and M13

Cho¹,

Lee²,

Jeon³

et al. 2005

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We carried out wide-field BV I CCD photometric observations of the globular clusters M3 (NGC 5272) and M13 (NGC 6205) using the Bohyun Optical Astronomy Observatory 1.8-m telescope equipped with a SITe 2K CCD. We present color-magnitude diagrams (V vs. B−V , V vs. V −I, and V vs. B−I) of M3 and M13. We have found asymptotic giant branch (AGB) bumps at V bump AGB = 14.85 ± 0.05 mag for M3 at V bump AGB = 14.25 ± 0.05 mag for M13. It is found that AGB stars in M3 are more concentrated near the bump, while those in M13 are scattered along the AGB sequence. We identified the red giant branch (RGB) bump of M3 at V bump RGB = 15.50 ± 0.05 mag and that of M13 at V bump RGB = 14.80 ± 0.05 mag through luminosity functions and slope changes of the integrated luminosity functions of M3 and M13. We have estimated the ratios R and R 2 for M3 and M13 and found that the value of R for M3 is larger than that for M13 while values of R 2 for M3 and M13 are similar and compatible with the value expected from evolutionary theory when only normal horizontal branch (HB) stars are used for estimation of R and R 2 for M13. However, we found that values of R for M3 and M13 are similar while the value of R 2 for M3 is larger than that for M13 when all the HB stars are included for estimation of R and R 2 for M13. We have compared the observed RGB luminosity functions of M3 and M13 with the theoretical RGB luminosity function of Bergbusch & VandenBerg at the same radial distances from the cluster centers as used in the estimation of the ratios R and R 2 for M3 and M13. We found "extra stars" belonging to M13 in the comparison of the observed RGB luminosity function of M13 and the theoretical RGB luminosity function of Bergbusch & VandenBerg and even in the comparison of the observed RGB luminosity functions of M3 and M13. In the original definition of the ratio R of Buzzoni et al., N HB corresponds to the lifetime of HB stars in the RR Lyrae instability strip at log T eff = 3.85. So, the smaller R value resulting for M13 compared with that for M3 in the case where only normal HB stars are included in the estimation of R and R 2 for M13 may be partially caused by "extra stars", and the similar R values for M3 and M13 in the case where the all HB stars are included in the estimation of R and R 2 for M13 may be caused by "extra stars" in the upper RGB of M13. If "extra stars" in the upper RGB of M13 are caused by an effective "deep mixing" these facts support the contention that an effective "deep mixing" could lead to different HB morphologies between M3 and M13 and subsequent sequences.

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“…However Moehler et al (2003) could not confirm that the "helium mixing" scenario is a definite explanation for the origin of the blue HB tail in M13. Moreover Caloi (2001) argued that there is no evidence for a substantial increase in the surface helium content of HB stars of M3 and M13, which contradicts the deep mixing and chemical inhomogeneities of the red giant branch (RGB) stars of M13, and argued that the most peculiar giants may belong to the asymptotic giant branch (AGB) stars. However, we should note that we cannot directly measure helium abundances of late-type stars (RGB and AGB stars, main sequence stars, and normal HB stars) of GCs and that hot BHB stars in GCs show deficient helium abundances (Heber et al 1986 andMoehler, Heber, &Rupprecht 1997 for NGC 6752;Behr et al 1999 for M13;Behr, Cohen, & McCarthy 2000 for M15;Behr 2003 for M3, M13, M15, M68, M92, and NGC 288) due to helium diffusion (Greenstein, Truran, & Cameron 1967;Michaud, Vauclair, & Vauclair 1983).…”

Section: Introductionmentioning

confidence: 99%

Different Characteristics of the Bright Branches of the Globular Clusters M15 and M92

Cho

¹

,

Lee

²

2007

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We carried out wide-field BV I CCD photometric observations of the globular clusters M3 (NGC 5272) and M13 (NGC 6205) using the Bohyun Optical Astronomy Observatory 1.8-m telescope equipped with a SITe 2K CCD. We present color-magnitude diagrams (V vs. B−V , V vs. V −I, and V vs. B−I) of M3 and M13. We have found asymptotic giant branch (AGB) bumps at V bump AGB = 14.85 ± 0.05 mag for M3 at V bump AGB = 14.25 ± 0.05 mag for M13. It is found that AGB stars in M3 are more concentrated near the bump, while those in M13 are scattered along the AGB sequence. We identified the red giant branch (RGB) bump of M3 at V bump RGB = 15.50 ± 0.05 mag and that of M13 at V bump RGB = 14.80 ± 0.05 mag through luminosity functions and slope changes of the integrated luminosity functions of M3 and M13. We have estimated the ratios R and R 2 for M3 and M13 and found that the value of R for M3 is larger than that for M13 while values of R 2 for M3 and M13 are similar and compatible with the value expected from evolutionary theory when only normal horizontal branch (HB) stars are used for estimation of R and R 2 for M13. However, we found that values of R for M3 and M13 are similar while the value of R 2 for M3 is larger than that for M13 when all the HB stars are included for estimation of R and R 2 for M13. We have compared the observed RGB luminosity functions of M3 and M13 with the theoretical RGB luminosity function of Bergbusch & VandenBerg at the same radial distances from the cluster centers as used in the estimation of the ratios R and R 2 for M3 and M13. We found "extra stars" belonging to M13 in the comparison of the observed RGB luminosity function of M13 and the theoretical RGB luminosity function of Bergbusch & VandenBerg and even in the comparison of the observed RGB luminosity functions of M3 and M13. In the original definition of the ratio R of Buzzoni et al., N HB corresponds to the lifetime of HB stars in the RR Lyrae instability strip at log T eff = 3.85. So, the smaller R value resulting for M13 compared with that for M3 in the case where only normal HB stars are included in the estimation of R and R 2 for M13 may be partially caused by "extra stars", and the similar R values for M3 and M13 in the case where the all HB stars are included in the estimation of R and R 2 for M13 may be caused by "extra stars" in the upper RGB of M13. If "extra stars" in the upper RGB of M13 are caused by an effective "deep mixing" these facts support the contention that an effective "deep mixing" could lead to different HB morphologies between M3 and M13 and subsequent sequences.

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Mixing, enhanced helium and blue tails in globular clusters

Cited by 11 publications

References 52 publications

Hot HB stars in globular clusters – Physical parameters and consequences for theory

Hot HB stars in globular clusters – Physical parameters and consequences for theory

Different Characteristics of the Bright Branches of the Globular Clusters M3 and M13

Different Characteristics of the Bright Branches of the Globular Clusters M15 and M92

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