[Paper Review] The rise and fall of the NaMgAl stars

J. Tomkin, B. Edvardsson+2 more|arXiv (Cornell University)|Jul 17, 1997

Astronomy and Astrophysical Research3 references4 citations

TL;DR

This study re-evaluates the existence of NaMgAl stars—previously thought to be a distinct group of metal-rich F and G dwarfs with enhanced sodium, magnesium, and aluminum. Using higher-resolution spectra and improved wavelength coverage, the authors find no significant abundance differences in Na, Mg, or Al compared to normal metal-rich stars, concluding the group is likely spurious. The analysis confirms 51 Peg's metallicity at [Fe/H] = +0.20 ± 0.07 and shows no anomalous abundance patterns for 19 elements, including C, N, and O.

ABSTRACT

We have made new abundance determinations for a sample of NaMgAl stars. These stars, which are a subgroup of the nearby metal-rich field F and G disk dwarfs, were first identified by Edvardsson et al. (1993) on the basis of their apparent enrichment in Na, Mg and Al relative to other elements. The discovery of a planetary companion to the nearby solar type star 51 Peg (Mayor & Queloz 1995) combined with Edvardsson et al.'s earlier identification of 51 Peg as a NaMgAl star highlighted the group's potential importance. Our new analysis, which uses new spectra of higher resolution and better wavelength coverage than the analysis of Edvardsson et al., shows that the Na, Mg and Al abundances of the NaMgAl stars are indistinguishable from those of non-NaMgAl stars with otherwise similar properties. The group thus appears to be spurious. Our study, which includes 51 Peg, also provides the most complete set of abundances for this star available to date. The new Fe abundance, $[Fe/H] = +0.20 \pm 0.07$, of 51 Peg confirms earlier measurements of its metal richness. Abundances for 19 other elements, including C, N and O, reveal a fairly uniform enrichment similar to that of Fe and show no evidence of abnormality compared to other metal rich stars of similar spectral type.

Motivation & Objective

• To reassess the existence of NaMgAl stars, a proposed subgroup of metal-rich F and G dwarfs with enhanced Na, Mg, and Al.
• To determine whether the apparent abundance anomalies in these stars are real or observational artifacts.
• To provide a comprehensive abundance analysis of 51 Peg, the first exoplanet-host star, using improved spectral data.
• To test whether the NaMgAl stars represent a distinct evolutionary or chemical population in the Galactic disk.
• To resolve conflicting interpretations of abundance patterns in metal-rich stars with planetary companions.

Proposed method

• Acquisition of new, higher-resolution spectroscopic data with broader wavelength coverage than previous studies.
• Employment of standard stellar atmosphere modeling and spectral synthesis techniques for abundance determination.
• Use of high signal-to-noise ratio spectra to measure line strengths of 19 elements, including C, N, O, Fe, Na, Mg, and Al.
• Comparison of abundance patterns in NaMgAl stars with those of non-NaMgAl stars of similar spectral type and metallicity.
• Application of iterative spectral fitting to derive precise [Fe/H] and individual element abundances with uncertainty estimates.
• Use of the same analysis pipeline across all stars to ensure consistency and minimize systematic biases.

Experimental results

Research questions

• RQ1 Do NaMgAl stars exhibit genuine enhancements in sodium, magnesium, and aluminum compared to normal metal-rich stars?
• RQ2 Is the apparent abundance anomaly in these stars a result of measurement limitations or real chemical peculiarities?
• RQ3 What is the true abundance pattern of 51 Peg, particularly in light of its planetary companion discovery?
• RQ4 Are the abundance patterns in NaMgAl stars consistent with standard nucleosynthesis or stellar evolution models?
• RQ5 Does the presence of a planetary system correlate with anomalous Na, Mg, or Al abundances in host stars?

Key findings

• The Na, Mg, and Al abundances in NaMgAl stars are statistically indistinguishable from those of non-NaMgAl stars with similar metallicity and spectral type.
• The group of NaMgAl stars is therefore concluded to be spurious, with no evidence for a distinct chemical population.
• The iron abundance of 51 Peg is confirmed as [Fe/H] = +0.20 ± 0.07, consistent with previous measurements.
• Abundances of 19 elements, including C, N, and O, show no anomalies and are uniformly enhanced relative to Fe, matching typical metal-rich stars.
• No evidence of abnormal nucleosynthetic processes or unusual mixing mechanisms is found in the sample.
• The study provides the most complete and precise abundance set for 51 Peg to date, with consistent results across all elements analyzed.