Abundance anomalies in hot horizontal-branch stars of the globular cluster NGC 6752

Glaspey, J. W.; Michaud, G.; Moffat, A.F.J.; Demers, S.

doi:10.1086/167346

Cited by 61 publications

(59 citation statements)

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“…21), with hints of a slight rise, no more than 0.5 dex on average, among the hottest stars of M13 and M15. This pattern of overabundant iron and normal magnesium that we observe in our stars matches the prior results of Glaspey et al (1989) and the recent work by Moehler et al (1999) on the southern metal-poor globular NGC 6752. Moehler et al observe 42 BHB stars at medium spectral resolution and derive [Fe/H] and [Mg/H] from the strongest lines of each species.…”

Section: à0561supporting

confidence: 92%

“…The first hint that photospheric chemical abundances might also change significantly as a function of position on the HB came from Glaspey et al (1989), who measured iron and helium line strengths for two stars in NGC 6752. The cooler one, at T eff ' 10;000 K, showed [Fe/H ¼ À1:5, as expected for this cluster, but the hotter star, at 16,000 K, was found to be strongly enhanced in iron and depleted in helium.…”

Section: Introductionmentioning

confidence: 99%

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Chemical Abundances and Rotation Velocities of Blue Horizontal‐Branch Stars in Six Globular Clusters

Behr

2003

ASTROPHYS J SUPPL S

144

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High-resolution spectroscopic measurements of blue horizontal-branch stars in six metal-poor globular clusters-M3, M13, M15, M68, M92, and NGC 288-reveal remarkable variations in photospheric composition and rotation velocity as a function of a star's position along the horizontal branch. For the cooler stars (T eff < 11;200 K), the derived abundances are in good agreement with the canonical cluster metallicities, and we find a wide range of v sin i rotation velocities, some as high as 40 km s À1 . In the hotter stars, however, most metal species are strongly enhanced, by as much as 3 dex, relative to the expected cluster metallicity, while helium is depleted by 2 dex or more. In addition, the hot stars all rotate slowly, with v sin i < 8 km s À1 . The anomalous abundances appear to be due to atomic diffusion mechanisms-gravitational settling of helium, and radiative levitation of metals-in the nonconvective atmospheres of these hot stars. We discuss the influence of these photospheric metal enhancements on the stars' photometric properties and explore possible explanations for the observed distribution of rotation velocities.

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Section: à0561supporting

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Chemical Abundances and Rotation Velocities of Blue Horizontal‐Branch Stars in Six Globular Clusters

Behr

2003

ASTROPHYS J SUPPL S

144

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“…The empty circles, on the contrary, show the result of substituting each data point with the average of the ten adjacent points, in order of temperature. As known for many decades (Baschek 1975;Heber 1987;Glaspey et al 1989), the atmosphere of HB stars hotter than 11 500 K is depleted in helium because it settles toward deeper layers as an effect of diffusion (Greenstein 1967). The observed change is, however, not abrupt, because the surface He abundance smoothly decreases with temperature up to T eff ≈ 15 000 K, where it reaches a minimum.…”

Section: Reddeningmentioning

confidence: 94%

Spectroscopy of horizontal branch stars inω Centauri

Bidin

Villanova

Piotto

et al. 2012

A&A

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Aims. We analyze the reddening, surface helium abundance and spectroscopic mass of 115 blue horizontal branch (HB) and blue hook (BH) stars in ω Centauri, spanning the cluster HB from the blue edge of the instability strip (T eff = 8000 K) to BH objects with T eff ≈ 50 000 K. Methods. The temperatures, gravities, and surface helium abundances were measured on low-resolution spectra fitting the Balmer and helium lines with a grid of synthetic spectra. From these parameters, the mass and reddening were estimated. Results. The mean cluster reddening is E(B − V) = 0.115 ± 0.004, in good agreement with previous estimates, but we evidence a pattern of differential reddening in the cluster area. The stars in the western half are more reddened than in the southwest quadrant by 0.03−0.04 mag. We find that the helium abundances measured on low-resolution spectra are systematically higher by 0.20−0.25 dex than the measurements based on higher resolution. No difference in surface helium abundance is detected between HB stars in ω Centauri and in three comparison clusters, and the stars in the range 11 500−20 000 K follow a trend with temperature, which probably reflects a variable efficiency of the diffusion processes. There is mild evidence that two families of extreme HB (EHB) cluster stars (T eff ≥ 20 000 K) could exist, as observed in the field, with ∼15% of the objects being helium depleted by a factor of ten with respect to the main population. The distribution of helium abundance above 30 000 K is bimodal, but we detect a fraction of He-poor objects lower than previous investigations. The observations are consistent with these being stars evolving off the HB. Their spatial distribution is not uniform across the cluster, but this asymmetric distribution is only marginally significative. We also find that EHB stars with anomalously high spectroscopic mass could be present in ω Centauri, as previously found in other clusters. The derived temperature-color relation reveals that the HB stars hotter than ∼11 000 K are fainter than the expectations of the canonical models in the U band, while no anomaly is detected in B and V. This behavior, not observed in NGC 6752, is a new peculiarity of ω Centauri HB stars. More investigation is needed to reach a full comprehension of this complex observational picture.

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“…In GCs all stars belong to the same population and should therefore have similar primordial chemical compositions. Glaspey et al (1989) were the first to discover a significant change of chemical abundances as function of the position on the HB. While a BHB star with T eff 10 000 K in NGC 6752 showed helium and iron abundances similar to the cluster composition (which is usually derived from abundance studies of red giants), a hotter one (T eff 16 000 K) turned out to show depletion of helium and strong enrichment of iron.…”

Section: Introductionmentioning

confidence: 99%

Hot subdwarf stars in close-up view

Geier

2013

A&A

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Context. Hot subdwarf B stars (sdBs) are considered to be core helium-burning stars with very thin hydrogen envelopes situated on or near the extreme horizontal branch. The formation of sdBs is still unclear as well as the chemical composition of their atmospheres. The observed helium depletion is attributed to atmospheric diffusion. Metal abundances have been determined for about a dozen sdBs only resulting in puzzling patterns with enrichment of heavy metals and depletion of lighter ones. Aims. We present a detailed metal abundance analysis of 106 sdBs. Methods. From high resolution spectra we measured elemental abundances of up to 24 different ions per star. A semi-automatic analysis pipeline was developed to calculate and fit LTE models to a standard set of spectral lines. Results. A general trend of enrichment was found with increasing temperature for most of the heavier elements. The lighter elements like carbon, oxygen, and nitrogen are depleted and less affected by temperature. Although there is considerable scatter from star to star, the general abundance patterns in most sdBs are similar. State-of-the-art diffusion models predict such patterns and are in qualitative agreement with our results. However, the highest enrichments measured cannot be explained with these models. Peculiar line shapes of the strongest metal lines in some stars indicate vertical stratification to be present in the atmospheres. Such effects are not accounted for in current diffusion models and may be responsible for some of the yet unexplained abundance anomalies.

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Abundance anomalies in hot horizontal-branch stars of the globular cluster NGC 6752

Cited by 61 publications

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Chemical Abundances and Rotation Velocities of Blue Horizontal‐Branch Stars in Six Globular Clusters

Chemical Abundances and Rotation Velocities of Blue Horizontal‐Branch Stars in Six Globular Clusters

Spectroscopy of horizontal branch stars inω Centauri

Hot subdwarf stars in close-up view

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