Helium enhanced stars and multiple populations along the horizontal branch of NGC 2808: direct spectroscopic measurements★

Marino, A. F.; Milone, A. P.; Przybilla, N.; Bergemann, M.; Lind, Karin; Asplund, Martin; Cassisi, S.; Catelan, M.; Casagrande, Luca; Valcarce, A. A. R.; Bedin, L. R.; Cortés, C.; D’Antona, F.; Jerjen, Helmut; Piotto, G.; Schlesinger, Katharine J.; Zoccali, M.; Angeloni, R.

doi:10.1093/mnras/stt1993

Cited by 153 publications

(170 citation statements)

References 105 publications

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“…More-massive stars in the first generation, having experienced the advanced phases of stellar nucleosynthesis, burn hydrogen via the CNO cycles. The degree of mixing between first-generation ejecta and pristine cluster gas determines whether the second generations are 'extreme' population stars (sometimes formed from pure ejecta) or belong to the 'intermediate' population that give rise to the observed anticorrelations (mixed ejecta and cluster gas, see Carretta et al 2009 Marino et al 2014 and is consistent with observed split main sequences (Piotto et al 2007;Milone et al 2012). …”

Section: Mass and 4 He-enhancementsupporting

confidence: 57%

Diagnostics of stellar modelling from spectroscopy and photometry of globular clusters

Angelou

D’Orazi

Constantino

et al. 2015

Monthly Notices of the Royal Astronomical Society

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We conduct a series of comparisons between spectroscopic and photometric observations of globular clusters and stellar models to examine their predictive power. Data from medium-to-high resolution spectroscopic surveys of lithium allow us to investigate first dredge-up and extra mixing in two clusters well separated in metallicity. Abundances at first dredge-up are satisfactorily reproduced but there is preliminary evidence to suggest that the models overestimate the luminosity at which the surface composition first changes in the lowest-metallicity system. Our models also begin extra mixing at luminosities that are too high, demonstrating a significant discrepancy with observations at low metallicity. We model the abundance changes during extra mixing as a thermohaline process and determine that the usual diffusive form of this mechanism cannot simultaneously reproduce both the carbon and lithium observations. Hubble Space Telescope photometry provides turnoff and bump magnitudes in a large number of globular clusters and offers the opportunity to better test stellar modelling as function of metallicity. We directly compare the predicted main-sequence turn-off and bump magnitudes as well as the distance-independent parameter ∆M V MSTO bump . We require 15 Gyr isochrones to match the main-sequence turn-off magnitude in some clusters and cannot match the bump in low-metallicity systems. Changes to the distance modulus, metallicity scale and bolometric corrections may impact on the direct comparisons but ∆M V MSTO bump , which is also underestimated from the models, can only be improved through changes to the input physics. Overshooting at the base of the convective envelope with an efficiency that is metallicity dependent is required to reproduce the empirically determined value of ∆M V MSTO bump .

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Section: Mass and 4 He-enhancementsupporting

confidence: 57%

Diagnostics of stellar modelling from spectroscopy and photometry of globular clusters

Angelou

D’Orazi

Constantino

et al. 2015

Monthly Notices of the Royal Astronomical Society

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“…Blue HB stars cooler than the Grundahl-jump are (moderately) O-poor and Na-rich. These results have been confirmed by Marino et al (2014) using higher signal-to-noise ratio (S/N) spectra, from which they also derived He abundances and finding quite a high value of Y ∼ 0.34 ± 0.01 ± 0.05 for BHB stars. Even He-richer stars should be present, but they should be hotter than the Grundahl-jump (D'Antona et al 2005), so this cannot be verified directly.…”

Section: Introductionsupporting

confidence: 54%

“…We derived He abundances for M 22 stars using these relations (Col. 2 of Table 6, with errors on the next column), and found an average He abundances by mass of Y = 0.338±0.014± 0.05, where the first error bar is derived from star-to-star scatter, and the second one referring to systematics is simply the one adopted by Marino et al (2014) ΔY ∼ 0.015 : Sweigart 1987). This indicates that the BHB stars of M 22 hotter than 9000 K are moderately He-rich.…”

Section: Helium Abundancesmentioning

confidence: 85%

“…We could correct the helium LTE abundances for NGC6752 (Villanova et al 2009), M4 (Villanova et al 2012) using this formula. When added to the results by Marino et al (2014) for NGC 2808, we thus have a consistent set of He non-LTE abundances for stars over the whole range of temperatures considered. The He abundances for BHB stars obtained in this way are mainly a function of EW and temperature, with a small correction for surface gravity and an even smaller one for metal abundance.…”

Section: Helium Abundancesmentioning

confidence: 99%

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The Na-O anticorrelation in horizontal branch stars

Gratton

Lucatello

Sollima

et al. 2014

A&A

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We obtained high-resolution spectra for 94 candidate stars belonging to the HB of M 22 with FLAMES. Previous works have indicated that this cluster has split subgiant (SGB) and red giant branches (RGB) and hosts two different stellar populations, differing in overall metal abundance and both exhibiting a Na-O anti-correlation. The HB stars we observed span a restricted temperature range (7800 < T eff < 11 000 K), where about 60% of the HB stars of M 22 are. Within our sample, we can distinguish three groups of stars segregated (though contiguous) in colours: Group 1 (49 stars) is metal-poor, N-normal, Na-poor, and O-rich: our abundances for this (cooler) group match those determined for the primordial group of RGB stars (a third of the total) from previous studies very well. Group 2 (23 stars) is still metal-poor, but it is N-and Na-rich, though only very mildly depleted in O. We can identify this intermediate group as the progeny of the metal-poor RGB stars that occupy an intermediate location along the Na-O anti-correlation and include about 10% of the RGB stars. The third group (20 stars) is metal-rich, Na-rich, and O-rich. This hotter group most likely corresponds to the most O-rich component of the previously found metal-rich RGB population (a quarter of the total). We did not observe any severely O-depleted stars and we think that the progeny of these stars falls on the hotter part of the HB. Furthermore, we found that the metal-rich population is also over-abundant in Sr, in agreement with results for corresponding RGB and SGB stars. However, we do not find any significant variation in the ratio between the sum of N and O abundances to Fe. We do not have C abundances for our stars. There is some evidence of an enhancement of He content for Groups 2 and 3 stars (Y = 0.338 ± 0.014 ± 0.05); the error bar due to systematics is large, but a consistent analysis of data for several GCs confirms that stars in these groups within M 22 are probably overabundant in He. We conclude that on the whole, our results agree with the proposition that chemical composition drives the location of stars along the HB of a GC. Furthermore, we found a number of fast rotators. They are concentrated in a restricted temperature range along the HB of M 22. Fast rotating stars might be slightly less massive and bluer than slowly rotating ones, but other interpretations are possible.

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“…Dependence of the initial helium spread on the polluter scenario Despite the many differences between the scenarios that have been developed to explain the abundance properties of GCs, it is widely accepted that the Na-enriched (2P) GC stars have started their life with a higher helium content than their 1P counterparts (this is actually obvious as helium is the main product of hydrogen burning). This has been confirmed by direct spectroscopic measurements of non-local thermodynamic equilibrium He abundances for a subset of blue horizontal branch stars in one GC (NGC 2808;Marino et al 2014). However the initial helium distribution among 2P stars is not well known yet, as predictions for the extent of helium enrichment along the sodium distribution in 2P stars strongly depend on the nature of the invoked 1P polluters.…”

Section: The Importance Of Quantifying Helium Variationsmentioning

confidence: 73%

Evolution of long-lived globular cluster stars

Chantereau

Charbonnel

Decressin

2015

A&A

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Context. Our understanding of the formation and early evolution of globular clusters (GCs) has been totally overthrown with the discovery of the peculiar chemical properties of their long-lived host stars. Aims. As a consequence, the interpretation of the observed color-magnitude diagrams and of the properties of the GC stellar populations requires the use of stellar models computed with relevant chemical compositions.Methods. We present a grid of 224 stellar evolution models for low-mass stars with initial masses between 0.3 and 1.0 M and initial helium mass fraction between 0.248 and 0.8 computed for [Fe/H] = -1.75 with the stellar evolution code STAREVOL. This grid is made available to the community. Results. We explore the implications of the assumed initial chemical distribution for the main properties of the stellar models: evolution paths in the Hertzsprung-Russel diagram (HRD), duration and characteristics of the main evolutionary phases, and the chemical nature of the white dwarf remnants. We also provide the ranges in initial stellar mass and helium content of the stars that populate the different regions of the HRD at the ages of 10 and 13.4 Gyr, which are typical for Galactic GCs.

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Helium enhanced stars and multiple populations along the horizontal branch of NGC 2808: direct spectroscopic measurements★

Cited by 153 publications

References 105 publications

Diagnostics of stellar modelling from spectroscopy and photometry of globular clusters

Diagnostics of stellar modelling from spectroscopy and photometry of globular clusters

The Na-O anticorrelation in horizontal branch stars

Evolution of long-lived globular cluster stars

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