The horizontal-branch stars in globular clusters. 2: The second parameter phenomenon

Lee, Young-Wook; Demarque, P.; Zinn, R.

doi:10.1086/173803

Cited by 414 publications

(566 citation statements)

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“…The overall metallicity is generally recognized as the most important parameter, but age, He abundance, stellar rotation, and any parameter affecting the mass loss along the red giant branch (RGB), may have a substantial role (Fusi Pecci et al 1993;Catelan 2009). The realization that metallicity alone was not sufficient to account for the complex behavior of HB morphologies in Galactic GCs led to the so called second parameter problem, that dominated the scientific debate in the field of Galactic astronomy for a couple of decades (see, e.g., Sandage & Wildey 1967;Zinn 1980;Preston et al 1991;Fusi Pecci et al 1993;Lee et al 1994;Catelan & de Freitas Pacheco 1994Fusi Pecci & Bellazzini 1997;Recío-Blanco et al 2006;Dotter et al 2010;Gratton et al 2010, and references therein). The most recent and thorough analyses concluded that no less than three parameters are needed to account for the HB morphology of Galactic GCs (an idea already proposed in the past, see Fusi Pecci et al 1993;Recío-Blanco et al 2006), namely metallicity, age, and He abundance .…”

Section: Introductionmentioning

confidence: 99%

The horizontal branch morphology of M 31 globular clusters

Bellazzini

Buzzoni

Cacciari

et al. 2012

A&A

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We use deep, high quality color magnitude diagrams obtained with the Hubble Space Telescope to compute a simplified version of the Mironov index (SMI; B B+R ) to parametrize the horizontal branch (HB) morphology for 23 globular clusters in the M 31 galaxy (Sample A), all located in the outer halo at projected distances between 10 kpc and 100 kpc. This allows us to compare them with their Galactic counterparts, for which we estimated the SMI exactly in the same way, in the SMI vs. [Fe/H] plane. We find that the majority of the considered M 31 clusters lie in a significantly different locus, in this plane, with respect to Galactic clusters lying at any distance from the center of the Milky Way. In particular they have redder HB morphologies at a given metallicity, or, in other words, clusters with the same SMI value are ≈0.4 dex more metal rich in the Milky Way than in M 31. We discuss the possible origin of this difference and we conclude that the most likely explanation is that many globular clusters in the outer halo of M 31 formed ≈1-2 Gyr later than their counterparts in the outer halo of the Milky Way, while differences in the cluster-to-cluster distribution of He abundance of individual stars may also play a role. The analysis of another sample of 25 bright M 31 clusters (eighteen of them with M V ≤ −9.0, Sample B), whose SMI estimates are much more uncertain as they are computed on shallow color magnitude diagrams, suggests that extended blue HB tails can be relatively frequent among the most massive M 31 globular clusters, possibly hinting at the presence of multiple populations.

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Section: Introductionmentioning

confidence: 99%

The horizontal branch morphology of M 31 globular clusters

Bellazzini

Buzzoni

Cacciari

et al. 2012

A&A

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“…Recent contributions to our knowledge of the Galactic halo suggest that the origins of the halo and the disk are more complex than the elegant picture envisioned in Eggen et al (1962) of formation in the wake of a rapid (%10 8 yr) gravitational collapse. Some examples of these findings include (1) an excess of stars with extreme retrograde orbits (Sandage & Fouts 1987;Norris & Ryan 1989;Carney et al 1994, hereafter C94;Carney et al 1996); (2) the existence of metal-poor, intermediate-age stars that could not have formed in the rarefied gas of the halo (Preston, Beers, & Schectman 1994); and (3) a large dispersion in the ages of halo globular clusters of $10 9 yr (Searle & Zinn 1978;Zinn 1993;Lee, Demarque, & Zinn 1994) and in the ages of halo field stars (Marquez & Schuster 1994).…”

Section: Introductionmentioning

confidence: 99%

Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars

Stephens

Boesgaard

2002

The Astronomical Journal

145

176

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There are stars in the halo of the Galaxy whose orbital properties are distinctly different from the majority of the halo population. One suggestion to account for these kinematically peculiar stars is that they have been gravitationally subsumed by the Milky Way through the breakup of nearby dwarf galaxies. One test of this hypothesis lies in determining the chemical composition of the deviant halo stars relative to that of normal ('' native '') halo field stars. The possibly accreted stars are predicted to have different chemical enrichments due to a slower rate of star formation in nearby low-mass spheroidal or irregular galaxies. We have obtained echelle spectra of metal-poor halo dwarfs with unusual orbital properties: from the outer halo, from the '' high '' halo, and on retrograde orbits. Our spectra are at high spectral resolution (35,000-48,000) and high signal-to-noise ratios (median value 140), primarily from the Keck I 10 m telescope with HIRES (53 stars), but also from the echelle spectrometer of the KPNO Mayall 4 m Telescope (three stars). The spectra cover a large spectral range: $4000-6800 Å . The strengths of approximately 9000 lines have been measured to determine the stellar parameters (T eff , log g, [Fe/H], and ) and the abundances of 10 elements, including Na, -fusion products (Mg, Si, Ca, Ti), iron peak elements (Cr, Fe, Ni), and neutron capture elements (Y, Ba). A model atmosphere was computed for each star, which was used to determine the composition under the assumption of LTE. The comparison of the abundances with the kinematic properties revealed no special trends, except that the ratio of the mean of the -fusion elements to Fe is weakly correlated with stellar apogalactica in the sense that the outermost stars have lower ratios of [ /Fe] than those with smaller apogalactica. The hallmark of an accreted halo star is presumed to be a deficiency (compared with normal stars) of the -elements (e.g., O, Mg, Si, Ca, Ti) with respect to Fe, a consequence of sporadic bursts of star formation within the dwarf galaxies. However [Fe/H]. The kinematically peculiar stars in our sample must have had their origins in localized star-forming regions far removed from the Galactic center. They do not carry the chemical signature of an accreted population. It seems clear that the chemical enrichment in our sample comes primarily from Type II supernovae with only a small component from Type Ia supernovae. Our stars would have formed at most $1 Gyr after the beginning of a star formation episode in the remote halo. The general conclusion extracted from these data is that the formation of the nascent Milky Way was not dominated by the late accretion of dwarf galaxies like the ones that currently orbit the Galaxy. However, the assimilation of fragments early in the evolution of the Galaxy is a natural by-product of hierarchical models of structure formation and can explain many properties of the halo population.

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“…One of the reasons that complicates the identification of the mechanisms -other than metallicity -at work in shaping the observed luminosity and effective temperature distribution of stars along the HB is that there are many possible culprits (mass-loss, age, helium abundance ...; see Rood 1973 for example) and some of them are not well constrained from theory. Age has been identified as the natural global second parameter by many authors in the past years (Lee et al 1988(Lee et al , 1990Lee, Demarque & Zinn 1994;Sarajedini & King 1989;Dotter et al 2010;Gratton et al 2010). According to this interpretation older clusters tend to have bluer HBs, while younger ones should have on average redder HB morphologies.…”

Section: Introductionmentioning

confidence: 99%

Old stellar systems in UV: resolved and integrated properties

Dalessandro¹

2014

Astrophys Space Sci

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The UV properties of old stellar populations have been subject of intense scrutiny from the late sixties, when the UV-upturn in early type galaxies was first discovered. Because of their proximity and relative simplicity, Galactic globular clusters (GGCs) are ideal local templates to understand how the integrated UV light is driven by hot stellar populations, primarily horizontal branch stars and their progeny. Our understanding of such stars is still plagued by theoretical uncertainties, which are partly due to the absence of an accurate, comprehensive, statistically representative homogeneous data-set. To move a step forward on this subject, we have combined the HST and GALEX capabilities and collected the largest data-base ever obtained for GGCs in UV. This data-base is best suited to provide insights on the HB second parameter problem and on the first stages of GCs formation and chemical evolution and to understand how they are linked to the observed properties of extragalactic systems.

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The horizontal-branch stars in globular clusters. 2: The second parameter phenomenon

Cited by 414 publications

References 0 publications

The horizontal branch morphology of M 31 globular clusters

The horizontal branch morphology of M 31 globular clusters

Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars

Old stellar systems in UV: resolved and integrated properties

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