[Paper Review] Na-O Anticorrelation And HB I. The Na-O anticorrelation in NGC 2808

E. Carretta, A. Bragaglia+4 more|ArXiv.org|Nov 30, 2005

Stellar, planetary, and galactic studiesPhysics and Astronomy40 references144 citations

TL;DR

This study presents the Na-O anticorrelation in NGC 2808 using high-resolution FLAMES/GIRAFFE spectroscopy of 120 red giants, revealing three distinct stellar populations: O-normal, O-poor, and super O-poor stars. The key finding is that while NGC 2808 hosts a significant fraction of O-rich stars—unlike the O-poor-dominated M 13—the distribution of [O/Na] along the anticorrelation is remarkably similar, supporting a second-generation star formation scenario via pollution from intermediate-mass AGB stars.

ABSTRACT

We derived atmospheric parameters and elemental abundances of Fe, O and Na for about 120 red giant stars in the Galactic globular cluster NGC 2808. Our results are based on the analysis of medium-high resolution (R=22000-24000) GIRAFFE spectra acquired with the FLAMES spectrograph at VLT-UT2 as a part of a project aimed at studying the Na-O anticorrelation as a function of physical parameters in globular clusters. We present here the anticorrelation of Na and O abundances in NGC 2808, and we discuss the distribution function of stars along this relation. Besides a bulk of O-normal stars, with composition typical of field halo stars, NGC 2808 seems to host two other groups of O-poor and super O-poor stars. In this regard, NGC 2808 is similar to M 13, the template cluster for the Na-O anticorrelation. However, at variance with M 13, most stars in NGC 2808 are O-rich. This might be related to the horizontal branch morphologies which are very different in these two clusters. The average metallicity we found for NGC 2808 is [Fe/H]=-1.10 (rms=0.065 dex, from 123 stars). We also found some evidence of a small intrinsic spread in metallicity, but more definitive conclusions are hampered by the presence of a small differential reddening.

Motivation & Objective

• To investigate the Na-O anticorrelation in NGC 2808 as a signature of second-generation star formation in globular clusters.
• To determine the distribution function of stars along the Na-O anticorrelation and compare it with other clusters like M 13.
• To assess the connection between the Na-O anticorrelation, helium enrichment, and the extended blue horizontal branch (BHB) morphology in NGC 2808.
• To examine the metallicity distribution and intrinsic spread in NGC 2808, accounting for differential reddening effects.
• To test whether the observed abundance patterns are consistent with pollution from intermediate-mass AGB stars via hot bottom burning.

Proposed method

• Acquired medium-high resolution (R ≈ 22,000–24,000) GIRAFFE spectra using the FLAMES spectrograph on ESO VLT-UT2 for 120 red giant stars in NGC 2808.
• Measured Na and O abundances using the [O I] forbidden line at 6300 Å and the Na I doublets at 5682–5688 Å and 6154–6160 Å.
• Determined atmospheric parameters (Teff, log g, [Fe/H]) via spectral synthesis and line-by-line analysis of Fe-peak elements.
• Constructed the [O/Na] distribution function to map the Na-O anticorrelation and identify distinct stellar populations.
• Applied a Kolmogorov-Smirnov test to compare the [O/Na] distribution in NGC 2808 with that of M 13, the canonical template cluster.
• Corrected abundances for non-LTE effects using the same prescriptions as in Gratton et al. (1999), ensuring homogeneity with prior M 13 data.

Experimental results

Research questions

• RQ1 Does NGC 2808 exhibit a Na-O anticorrelation similar to that observed in M 13, the canonical template cluster?
• RQ2 What is the relative abundance of O-normal, O-poor, and super O-poor stars in NGC 2808, and how does this compare to M 13?
• RQ3 Is there a correlation between the distribution of stars along the Na-O anticorrelation and the bimodal horizontal branch morphology in NGC 2808?
• RQ4 What evidence exists for intrinsic metallicity spread or helium enrichment in the different stellar populations of NGC 2808?
• RQ5 Can the observed abundance patterns in NGC 2808 be explained by pollution from intermediate-mass AGB stars undergoing hot bottom burning?

Key findings

• NGC 2808 hosts three distinct groups of stars along the Na-O anticorrelation: O-normal (typical of field halo stars), O-poor, and super O-poor stars.
• Unlike M 13, which is dominated by O-poor stars, most stars in NGC 2808 are O-normal, indicating a different population distribution despite similar anticorrelation shape.
• The [O/Na] distribution function in NGC 2808 is statistically indistinguishable from that of M 13 at the 3% level (Kolmogorov-Smirnov test), suggesting similar nucleosynthetic processes.
• The average metallicity of NGC 2808 is [Fe/H] = −1.10 ± 0.065 dex, based on 123 stars, with tentative evidence for a small intrinsic metallicity spread.
• The observed abundance patterns are consistent with theoretical models of pollution by He-rich, O-depleted material from intermediate-mass AGB stars, supporting a second-generation star formation scenario.
• The presence of O-normal stars in NGC 2808 with no corresponding RHB stars—unlike in M 13—suggests that HB morphology may not be solely determined by initial composition, requiring further modeling.