[Paper Review] Na-O Anticorrelation and HB. II. The Na-O anticorrelation in the globular cluster NGC 6752

E. Carretta, A. Bragaglia+3 more|ArXiv.org|Jan 8, 2007

Stellar, planetary, and galactic studies37 references73 citations

TL;DR

This study presents high-resolution spectroscopic analysis of ~150 red giant stars in NGC 6752 using FLAMES/VLT, measuring Fe, O, and Na abundances to investigate the Na-O anticorrelation's link to horizontal branch (HB) morphology. It finds a skewed Na-O distribution toward Na-poor stars, resembling M 13 more than NGC 2808, suggesting chemical inhomogeneities may influence HB morphology, though mass loss and helium enrichment remain key uncertainties in explaining the cluster's blue HB tail.

ABSTRACT

We are studying the Na-O anticorrelation in several globular clusters of different Horizontal Branch (HB) morphology in order to derive a possible relation between (primordial) chemical inhomogeneities and morphological parameters of the cluster population. We used the multifiber spectrograph FLAMES on the ESO Very Large Telescope UT2 and derived atmospheric parameters and elemental abundances of Fe, O and Na for about 150 red giant stars in the Galactic globular cluster NGC 6752. The average metallicity we derive is [Fe/H]=-1.56, in agreement with other results from red giants, but lower than obtained for dwarfs or early subgiants. In NGC 6752 there is not much space for an intrinsic spread in metallicity: on average, the rms scatter in [Fe/H] is 0.037+/-0.003 dex, while the scatter expected on the basis of the major error sources is 0.039+/-0.003 dex. The distribution of stars along the Na-O anticorrelation is different to what was found in the first paper of this series for the globular cluster NGC 2808: in NGC 6752 it is skewed toward more Na-poor stars, and it resembles more the one in M 13. Detailed modeling is required to clarify whether this difference may explain the very different distributions of stars along the HB.

Motivation & Objective

• To investigate the relationship between the Na-O anticorrelation and horizontal branch (HB) morphology in globular clusters, particularly NGC 6752.
• To determine whether chemical inhomogeneities—specifically Na-O anticorrelation—can explain the observed HB morphology, including the prominent blue tail in NGC 6752.
• To establish a homogeneous, large-sample dataset of Fe, O, and Na abundances in red giants to test the primordial origin hypothesis of abundance anomalies.
• To compare the distribution of stars along the Na-O anticorrelation in NGC 6752 with other clusters like NGC 2808 and M 13 to assess its role in the second parameter effect.

Proposed method

• Spectroscopic observations were obtained using the FLAMES multifiber spectrograph on the ESO Very Large Telescope (VLT) UT2.
• Atmospheric parameters (Teff, log g, [Fe/H]) were derived from photometry and spectral synthesis.
• Equivalent widths of Na I doublets at 5682–88 Å and 6154–60 Å, and forbidden O I lines at 6300.3 and 6363.8 Å, were measured to determine [O/Fe] and [Na/Fe].
• Abundance analysis was performed using consistent atomic parameters, NLTE corrections, and solar reference abundances across all stars.
• The distribution function of stars along the Na-O anticorrelation was derived and compared across clusters.
• The R parameter (ratio of HB to RGB stars) was evaluated to assess consistency with mass loss and helium enrichment models.

Experimental results

Research questions

• RQ1 Does the distribution of stars along the Na-O anticorrelation in NGC 6752 differ significantly from that in NGC 2808, and if so, what does this imply for HB morphology?
• RQ2 Can the observed Na-O anticorrelation in NGC 6752 be explained by primordial chemical inhomogeneities from a previous generation of stars?
• RQ3 Is the skewness of the Na-O distribution in NGC 6752 toward Na-poor stars related to its extreme blue horizontal branch morphology?
• RQ4 To what extent do mass loss and helium enrichment processes explain the discrepancy between RGB and HB star counts in NGC 6752?
• RQ5 How does the R parameter in NGC 6752 compare to other clusters with similar metallicities, and does it support the existence of RGB-manquè stars?

Key findings

• The average metallicity in NGC 6752 is [Fe/H] = -1.56 ± 0.04 dex, with no intrinsic scatter (rms = 0.037 dex), indicating no significant metallicity spread among red giants.
• The Na-O anticorrelation is present and uniform across the red giant branch, with no evidence of mixing effects, confirming its primordial origin.
• The distribution of stars along the Na-O anticorrelation in NGC 6752 is skewed toward Na-poor, O-rich stars, resembling M 13 more than NGC 2808.
• The R parameter (HB/RGB ratio) in NGC 6752 is 1.56 ± 0.18, which is higher than average and could explain the observed 12% excess of blue HB stars.
• The degree of chemical anomalies in NGC 6752 is less severe than in M 13, suggesting differences in the efficiency or history of pollution processes.
• The observed discrepancy between RGB and HB star counts may be reconciled by the existence of RGB-manquè stars, possibly due to enhanced mass loss in O-rich, Na-poor stars.