Noble gases Neon and argon: a role for the chemical patterns of multiple populations in globular clusters?

TL;DR

This study tests whether increasing the initial neon abundance in massive AGB and super-AGB models at GC-like metallicity can reconcile the observed Na–O–Mg–Al–Si patterns in NGC 2808. By coupling neon enhancements (up to ~2×) with reduced mass-loss rates and a refined Fe abundance, the authors achieve a notably better match to most 2G abundance trends, though the extreme E population remains challenging and may require a slightly lower [Fe/H] or hierarchical formation scenarios. The work also examines potassium and argon–neon nucleosynthesis, finding that a modest Ar influence helps but does not fully resolve the potassium discrepancies, and highlights the current uncertainties in neon abundances from planetary nebulae as a limiting factor. They conclude that noble-gas abundances, particularly Ne and Ar, could be key tools for refining AGB-based GC enrichment models, and advocate for improved galactic chemical evolution understanding of these elements, alongside exploring cluster-assembly scenarios to explain metallicity spreads. $T_{HB}$, $[Fe/H]$, $f(Ne)$, and $\eta$ are central parameters guiding these interpretations, with implications for the origin of multiple populations in globular clusters.

Abstract

We focus on the sodium destruction in models reaching the high hot bottom burning temperatures needed to efficiently cycle oxygen to nitrogen in AGB models at the nominal [Fe/H] of the cluster NGC 2808. We increase the initial neon abundance by a factor 2-4 with respect to the "standard" abundances obtained by scaling the solar values down to the metallicity of this cluster, and explore the average abundances in the ejecta obtained by adopting smaller mass-loss rates. Higher neon produces higher sodium in the AGB envelope. Lowering the mass-loss rate allows both to keep reasonably large sodium abundances and to increase the depletion of oxygen and magnesium. A balance between the lower mass-loss rates and the necessity of not increasing too much the episodes of third dredge up gives a neon abundance larger by a factor two and a mass-loss rate smaller by a factor four as best compromise. Comparison with the abundances in NGC 2808 shows a better agreement than the standard models for all the patterns of abundances, but the extreme stars (group E) requires models slightly less rich in iron. t Thus, we propose that the extreme population in NGC 2808 is composed of stars having a slightly smaller metallicity, and sketch a possible scenario for its formation, in the framework of the hierarchical clusters assembly scenario. Abundances of potassium are larger by $\sim 0.2 dex$ in the E group, but the explanation in terms of burning of the initial argon requires a drastic increase of the relevant cross section. The abundances of neon and argon at low metallicities may be an important tool to better reproduce the abundances of light elements in the framework of the AGB model for globular clusters.

Figures (6)

• Figure 1: Schematic p--captures for the CN (yellow box) and NO (green box) cycles (bottom panel) and for the Ne--Na (yellow) and Mg--Al and Al--Si (green) cycles (top panel). Blue arrows indicate $\rm (p,\gamma)$ reactions, pink trajectories indicate p-captures followed by $\beta$ decays, dashed red lines show (p, $\alpha$) reactions. The processing occurs for increasing hot bottom burning temperatures.
• Figure 2: Changes of the sodium abundance versus time for the 6$\, {M}_\odot$ AGB evolution, when we vary the Ne20 initial abundance in the models. For simplicity, we show models with the same mass-loss rate calibration $\eta$=0.002, that is 1/10 of the standard value $\eta$=0.02.
• Figure 3: Anticorrelations of p--capture elements in the GC NGC 2808 carretta2015. The diagram of sodium--magnesium data (panel b) was the basis for the subdivision into different sub-classes: P1 (blue), P2 (green), I1 (red), I2 (orange) and E (black). All panels show: 1) standard average abundances in the AGB ejecta by for a metallicity Z=2.3$\times10^{-3}$, [$\alpha$/Fe]=0.4, e.g. [Fe/H]=--1.08 (open blue circles); 2) exploratory models (colored squares and connecting lines) with the same initial composition and identified by: GREEN: ejecta of the 6$\, {M}_\odot$, with standard neon and decreasing mass-loss rates (models $\eta x$Ne1). Starting from the standard abundances for $\eta$=0.02, we plot $\eta$=0.01 (1/2 standard), 0.005 (1/4 standard) and 0.002 (1/10 standard); BLUE: 6$\, {M}_\odot$ for $\eta$=1/10 standard, with increasing Neon abundance from standard (solar scaled, for $\alpha/Fe$=0.4) to 2 and 4 times standard (models $\eta$ 0.1Ne4); RED: abundances for the 5 and 7$\, {M}_\odot$ for $\eta$ 0.1Ne4; CYAN: starting from mass loss 1/4 standard, and increasing Neon at 2 and 4 times the standard abundance (models $\eta$ 0.25Ne2 and $\eta$ 0.25Ne4). The blue and cyan big stars define the location of the models having sodium consistent with the O--Na patterns and discussed in the text: $\eta$ 0.1Ne4 and $\eta$ 0.25Ne2, which we will consider also in the other panels. The dark grey lines with triangles define dilution of the 6$\, {M}_\odot$ location $\eta$ 0.25Ne4 with increasing quantities (20, 40, 60 and 80%) of gas with standard composition, identified by the yellow star.
• Figure 4: The same as Fig. \ref{['fig:3']}, showing different model outputs. GREY: the squares represents the 5.5, 6.5, 7 and 7.5$\, {M}_\odot$ location for the new standard $\eta$ 0.25Ne2; BLACK: (open black squares) models $\eta$ 0.25Ne2 for 5.5, 6, 6.5 and 7$\, {M}_\odot$, for models having $\delta$[Fe/H]=--0.1. The cyan big stars again defines the location of the new standard, $\eta$ 0.25Ne2, 6$\, {M}_\odot$. In the panel a, the green squares are the 6$\, {M}_\odot$ ejecta for mass-loss rates with Reimers' parameter $\eta$ going from $\eta$=0.001 (0.1 standard) to $\eta$=0.02 (old standard), also shown in Fig. \ref{['fig:3']}.
• Figure 5: Potassium--oxygen and potassium--magnesium data for the GC NGC 2808 (open circles) from carretta2015 and mucciarelli2015. For all the symbols see Fig. \ref{['fig:3']}. Depletion for 6.5$\, {M}_\odot$ with [Fe/H] reduced by 0.1 dex is shown by the open squares. The upper point ($\delta$[K/Fe]=0.125) is obtained by doubling the argon initial abundance. See text for further information.