Constraints on Helium Enhancement in the Globular Cluster M4 (Ngc 6121): The Horizontal Branch Test

Valcarce, A. A. R.; Catelan, M.; Alonso-García, J.; Cortés, C.; Medeiros, J. R. De

doi:10.1088/0004-637x/782/2/85

Cited by 13 publications

(14 citation statements)

References 50 publications

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“…As pointed out by our referee, this is however not a matter of concern for M 4. Determinations of ∆Y in the literature range from negligible values (Valcarce et al 2014), up to just ∼0.02 (Villanova et al 2012;Nardiello et al 2015). The fraction of SP stars in this cluster is approximately the same between the central (Milone et al 2016) and external (Marino et al 2008;Nardiello et al 2015) regions analyzed so far, being equal to 30-40 %; this fraction, together with a maximum ∆Y =0.02 would cause systematic effects on the reddening below 0.01 mag.…”

Section: Photometric Datamentioning

confidence: 64%

Multiple populations along the asymptotic giant branch of the globular cluster M4

Lardo

Salaris

Savino

et al. 2016

Mon. Not. R. Astron. Soc.

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Nearly all Galactic globular clusters host stars that display characteristic abundance anti-correlations, like the O-rich/Na-poor pattern typical of field halo stars, together with O-poor/Na-rich additional components. A recent spectroscopic investigation questioned the presence of O-poor/Na-rich stars amongst a sample of asymptotic giant branch stars in the cluster M 4, at variance with the spectroscopic detection of a O-poor/Na-rich component along both the cluster red giant branch and horizontal branch. This is contrary to what is expected from the cluster horizontal branch morphology and horizontal branch stellar evolution models. Here we have investigated this issue by employing the C UBI = (U − B) − (B − I) index, that previous studies have demonstrated to be very effective in separating multiple populations along both the red giant and asymptotic giant branch sequences. We confirm previous results that the RGB is intrinsically broad in the V −C UBI diagram, with the presence of two components which nicely correspond to the two populations identified by high-resolution spectroscopy. We find that AGB stars are distributed over a wide range of C UBI values, in close analogy with what is observed for the RGB, demonstrating that the AGB of M 4 also hosts multiple stellar populations.

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Section: Photometric Datamentioning

confidence: 64%

Multiple populations along the asymptotic giant branch of the globular cluster M4

Lardo

Salaris

Savino

et al. 2016

Mon. Not. R. Astron. Soc.

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“…A higher He enhancement, as suggested by, for example, Villanova et al (2012), would imply a higher mass difference (see e.g. Valcarce et al 2014, for an exhaustive review on recent results). The precision in the average mass determined here is insufficient to detect this difference.…”

Section: Comparison With Independent Estimates Of Mass and Distancementioning

confidence: 98%

Detection of solar-like oscillations in relics of the Milky Way: asteroseismology of K giants in M4 using data from the NASA K2 mission

Miglio

Chaplin

Brogaard

et al. 2016

Mon. Not. R. Astron. Soc.

116

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Asteroseismic constraints on K giants make it possible to infer radii, masses and ages of tens of thousands of field stars. Tests against independent estimates of these properties are however scarce, especially in the metal-poor regime. Here, we report the detection of solar-like oscillations in 8 stars belonging to the red-giant branch and red-horizontal branch of the globular cluster M4. The detections were made in photometric observations from the K2 Mission during its Campaign 2. Making use of independent constraints on the distance, we estimate masses of the 8 stars by utilising different combinations of seismic and non-seismic inputs. When introducing a correction to the ∆ν scaling relation as suggested by stellar models, for RGB stars we find excellent agreement with the expected masses from isochrone fitting, and with a distance modulus derived using independent methods. The offset with respect to independent masses is lower, or comparable with, the uncertainties on the average RGB mass (4 − 10%, depending on the combination of constraints used). Our results lend confidence to asteroseismic masses in the metal poor regime. We note that a larger sample will be needed to allow more stringent tests to be made of systematic uncertainties in all the observables (both seismic and non-seismic), and to explore the properties of RHB stars, and of different populations in the cluster.

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“…For the helium mass-fraction of SP1 stars in M 4 we adopted Y = 0.245 (Valcarce et al 2014), and for SP2 stars we adopted Y = 0.275 (so ∆Y = 0.03, see Table 11). For Figure 14.…”

Section: Expectations From Theoretical Stellar Evolution Modelsmentioning

confidence: 99%

“…It is accepted that some GC stars are significantly enriched in helium (by more than ∆Y = 0.15 in some clusters, for example NGC2808; D' Antona et al 2005). Villanova et al (2012) determined helium abundances for a sample of blue HB stars in M 4 (assumed to represent the most He-rich stars in the cluster), and found ∆Y to be of the order of 0.03-0.04, while Valcarce et al (2014) and Nardiello et al (2015) determined ∆Y values of 0.01 and 0.02, respectively.…”

Section: Helium Enriched Model Testmentioning

confidence: 99%

On the AGB stars of M 4: a robust disagreement between spectroscopic observations and theory

MacLean

Campbell

Amarsi

et al. 2018

Monthly Notices of the Royal Astronomical Society

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Several recent spectroscopic investigations have presented conflicting results on the existence of Na-rich asymptotic giant branch (AGB) stars in the Galactic globular cluster M 4 (NGC 6121). The studies disagree on whether or not Na-rich red giant branch (RGB) stars evolve to the AGB. For a sample of previously published HER-MES/AAT AGB and RGB stellar spectra we present a re-analysis of O, Na, and Fe abundances, and a new analysis of Mg and Al abundances; we also present CN band strengths for this sample, derived from low-resolution AAOmega spectra. Following a detailed literature comparison, we find that the AGB samples of all studies consistently show lower abundances of Na and Al, and are weaker in CN, than RGB stars in the cluster. This is similar to recent observations of AGB stars in NGC 6752 and M 62. In an attempt to explain this result, we present new theoretical stellar evolutionary models for M 4; however, these predict that all stars, including Na-rich RGB stars, evolve onto the AGB. We test the robustness of our abundance results using a variety of atmospheric models and spectroscopic methods; however, we do not find evidence that systematic modelling uncertainties can explain the apparent lack of Narich AGB stars in M 4. We conclude that an unexplained, but robust, discordance between observations and theory remains for the AGB stars in M 4.

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Constraints on Helium Enhancement in the Globular Cluster M4 (Ngc 6121): The Horizontal Branch Test

Cited by 13 publications

References 50 publications

Multiple populations along the asymptotic giant branch of the globular cluster M4

Multiple populations along the asymptotic giant branch of the globular cluster M4

Detection of solar-like oscillations in relics of the Milky Way: asteroseismology of K giants in M4 using data from the NASA K2 mission

On the AGB stars of M 4: a robust disagreement between spectroscopic observations and theory

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