[Paper Review] Na-O Anticorrelation and Horizontal Branches. V. The Na-O anticorrelation in NGC 6441 from Giraffe spectra

R. Gratton, S. Lucatello+17 more|HAL (Le Centre pour la Communication Scientifique Directe)|Jan 8, 2007

Stellar, planetary, and galactic studiesPhysics and Astronomy52 references58 citations

TL;DR

This study analyzes FLAMES-Giraffe spectra of 25 red giant stars in the metal-rich globular cluster NGC 6441 to investigate the Na-O anticorrelation. It confirms a well-defined Na-O anticorrelation with ~25% of stars being Na-rich and O-poor, and finds a strong correlation between the [O/Na] distribution on the red giant branch and the color distribution of horizontal branch stars, supporting the link between helium-enhanced populations and blue horizontal branch morphology.

ABSTRACT

We present an analysis of FLAMES-Giraffe spectra for several bright giants in NGC 6441, to investigate the presence and extent of the Na-O anticorrelation in this anomalous globular cluster. The field of NGC 6441 is very crowded, with severe contamination by foreground (mainly bulge) field stars. Appropriate membership criteria were devised to identify a group of 25 likely cluster members among the about 130 stars observed. Combined with the UVES data obtained with the same observations, high dispersion abundance analyses are now available for a total of 30 stars in NGC 6441, 29 of them having data for both O and Na. The spectra were analyzed by a standard line analysis procedure; care was taken to minimize the impact of the differential interstellar reddening throughout the cluster, and to extract reliable information from crowded, and moderately high S/N (30-70), moderately high resolution (R ~ 23,000) spectra. NGC 6441 is very metal-rich ([Fe/H]=$-0.34\pm 0.02\pm0.04$ dex). There is no clear sign of star-to-star scatter in the Fe-peak elements. The alpha-elements Mg, Si, Ca, and Ti are overabundant by rather large factors, suggesting that the cluster formed from material enriched by massive core collapse SNe. The O-Na anticorrelation is well defined, with about 1/4 of the stars being Na-rich and O-poor. One of the stars is a Ba-rich and moderately C-rich star. The distribution of [Na/O] ratios among RGB stars in NGC 6441 appears similar to the distribution of colors of stars along the horizontal branch. The fraction of Na-poor, O-rich stars found in NGC 6441 agrees well with that of stars on the red horizontal branch of this cluster (in both cases about 80%), with a sloping distribution toward lower values of [O/Na] (among RGB stars and bluer colors (among HB stars).

Motivation & Objective

• To investigate the presence and extent of the Na-O anticorrelation in NGC 6441, a metal-rich globular cluster with anomalous horizontal branch morphology.
• To determine whether the observed horizontal branch morphology correlates with the distribution of Na and O abundances in red giant branch stars.
• To identify and characterize rare stars, such as Ba-rich or C-rich stars, in the cluster's stellar population.
• To assess the impact of differential reddening and contamination from field stars on abundance measurements in a crowded field.
• To combine high-resolution UVES and moderate-resolution Giraffe data to achieve robust abundance determinations for 30 stars, including 29 with both O and Na measurements.

Proposed method

• Used FLAMES-Giraffe spectroscopy at the VLT-UT2 to obtain moderate-resolution (R ~ 23,000) spectra for 130 stars in the crowded field of NGC 6441.
• Applied radial velocity and photometric criteria (using 2MASS K magnitudes and (V-K) colors) to identify 25 likely cluster members, minimizing contamination from bulge field stars.
• Performed standard line-by-line abundance analysis on the Giraffe spectra, focusing on [O I] 6300.3, 6363.8 Å and Na I 6154-6160 Å lines to derive O and Na abundances.
• Combined Giraffe data with pre-existing UVES high-resolution data to obtain consistent, high-precision abundance measurements for 30 stars, with 29 having both O and Na abundances.
• Corrected for differential interstellar reddening using the K-band magnitude and (V-K) color relation to ensure accurate abundance determinations.
• Used atmospheric parameters derived from photometry to minimize systematic errors in abundance analysis, particularly in the presence of moderate signal-to-noise (S/N ~ 30–70).

Experimental results

Research questions

• RQ1 Does the Na-O anticorrelation exist in NGC 6441, and what is its extent in terms of the fraction of Na-rich, O-poor stars?
• RQ2 Is there a correlation between the distribution of [O/Na] ratios among red giant branch stars and the color distribution of horizontal branch stars in NGC 6441?
• RQ3 What is the metallicity of NGC 6441, and is there any star-to-star scatter in Fe-peak or α-elements?
• RQ4 Are there any rare stars—such as Ba-rich or C-rich stars—present in NGC 6441, and what do their abundance patterns suggest about their origin?
• RQ5 Can the observed abundance patterns in NGC 6441 be explained by enrichment from massive core-collapse supernovae?

Key findings

• NGC 6441 has a high metallicity of [Fe/H] = -0.34 ± 0.02 ± 0.04 dex, making it one of the most metal-rich globular clusters known.
• No significant star-to-star scatter is observed in Fe-peak elements, indicating chemical homogeneity in these elements.
• The α-elements (Mg, Si, Ca, Ti) are overabundant by factors consistent with enrichment by massive core-collapse supernovae.
• The Na-O anticorrelation is clearly defined, with approximately 25% of the 29 stars showing Na-rich and O-poor characteristics.
• The distribution of [O/Na] ratios among red giant branch stars shows a dominant population of O-rich, Na-poor stars (~70%), with a tail toward lower O and higher Na abundances.
• One star (6007741) is identified as a Ba-rich and moderately C-rich star, a rare class in globular clusters, with abundance patterns consistent with mass transfer from a low-mass AGB star.