Thermonuclear Reaction Rates for the $^2^4$Mg(p,\gamma)$^2^5$Al Reaction

Zaidins, C.S.; Langer, G. E.

doi:10.1086/133881

Cited by 8 publications

(1 citation statement)

References 18 publications

(37 reference statements)

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“…Instead, we explore what changes in the 24 Mg proton-capture rate would be sufficient to match these limited observations. Zaidins & Langer (1997) suggest that theoretically, there is room for a factor of 35 increase in the 24 Mg proton-capture rate based on a poorly known width of the lowest resonance at 2468 keV in 25 Al. In Figure 28 we present the effect of increasing this rate by various amounts on the one model which most closely resembles the brightest M13 giants.…”

Section: Mgal Cycle Reactionsmentioning

confidence: 97%

Proton‐Capture Nucleosynthesis in Globular Cluster Red Giant Stars

Cavallo

Sweigart

Bell

1998

ApJ

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Observational evidence suggests that many of the variations of the surface abundances of light to intermediate mass elements (A < 28) in globular cluster red-giant-branch (RGB) stars can be attributed to non-canonical mixing between the surface and the deep stellar interior during the RGB phase. As a first step to studying this mixing in more detail, we have combined a large nuclear reaction network with four detailed stellar evolutionary sequences of different metallicities in order to follow the production and destruction of the C, N, O, Ne, Na, Mg, and Al isotopes around the hydrogen-burning shell (H shell) of globular cluster RGB stars. The abundance distributions determined by this method allow for the variation in the temperature and density around the H shell as well as for the dependence on both the stellar luminosity and cluster metallicity. Because our nuclear reaction network operates separately from the stellar evolution code, we are able to more readily to explore the effects of the uncertainties in the reaction rates on the calculated abundances.We discuss implications of our results for mixing in the context of the observational data. Our results are qualitatively consistent with the observed C vs. N, O vs. N, Na vs. O, and Al vs. O anticorrelations and their variations -2with both luminosity and metallicity. We see evidence for variations in Na without requiring changes in O, independent of metallicity, as observed by Norris &Da Costa (1995a) andBriley et al. (1997). Also, we derive 12 C/ 13 C ratios near the observed equilibrium value of 4 for all sequences, and predict the temperature-dependent 16 O/ 17 O equilibrium ratio based on new data for the 17 O(p, α) 14 N reaction rate. Additionally, we discuss the Mg isotopic abundances in light of the recent observations of M13 (Shetrone 1996b) and NGC 6752 (Shetrone 1997).

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Section: Mgal Cycle Reactionsmentioning

confidence: 97%

Proton‐Capture Nucleosynthesis in Globular Cluster Red Giant Stars

Cavallo

Sweigart

Bell

1998

ApJ

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Reaction rate of 24Mg(p,γ)25Al

et al. 1999

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Abundance Ratios in Metal-Poor Globular Clusters: Deep Mixing and its Effect on Stellar Populations of the Galactic Halo

Kraft

1998

Highlights astron.

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Only a bit more than 25 years ago, it seemed possible to assume that all Galactic globular clusters were chemically homogeneous. There were indications that star-to-star Fe abundance variations existed in ω Cen, but this massive cluster appeared to be unique. Following Osborn’s (1971) initial discovery, Zinn’s (1973) observation that M92 asymptotic giant branch (AGB) stars had weaker G-bands than subgiants with equivalent temperatures provided the first extensive evidence that there might be variations in the abundances of the light elements in an otherwise “normal” cluster. Since then star-to-star variations in the abundances of C, N, O, Na, Mg and Al have been observed in all cases in which sample sizes have exceeded 5-10 stars, e.g., in clusters such as M92, M15, M13, M3, ω Cen, MIO and M5. Among giants in these clusters one finds large surface O abundance differences, and these are intimately related to differences of other light element abundances, not only of C and N, but also of Na, Mg and Al (cf. reviews by Suntzeff 1993, Briley et al 1994, and Kraft 1994). The abundances of Na and O, as well as Al and Mg, are anticorrelated. Prime examples are found among giants in M15 (Sneden et al 1997), M13 (Pilachowski et al 1996; Shetrone 1996a,b; and Kraft et al 1997) and ω Cen (Norris & Da Costa 1995a,b). These observed anticorrelations almost certainly result from proton- capture chains that convert C to N, 0 to N, Ne to Na and Mg to Al in or near the hydrogen fusion layers of evolved cluster stars. But which stars? An appealing idea is that during the giant branch lifetimes of the low-mass stars that we now observe, substantial portions of the stellar envelopes have been cycled through regions near the H-burning shell where proton-capture nucleosynthesis can occur. This so-called “evolutionary” scenario involving deep envelope mixing in first ascent red giant branch (RGB) stars has been studied by Denissenkov & Denissenkova (1990), Langer & Hoffman (1995), Cavallo et al (1996, 1997) and Langer et al (1997). The mixing mechanism that brings proton-capture products to the surface is poorly understood (Denissenkov & Weiss 1996, Denissenkov et al 1997, Langer et al 1997), but deep mixing driven by angular momentum has been suggested (Sweigart & Mengel 1979, Kraft 1994, Langer & Hoffman 1995, Sweigart 1997).

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Thermonuclear Reaction Rates for the $^2^4$Mg(p,\gamma)$^2^5$Al Reaction

Cited by 8 publications

References 18 publications

Proton‐Capture Nucleosynthesis in Globular Cluster Red Giant Stars

Proton‐Capture Nucleosynthesis in Globular Cluster Red Giant Stars

Reaction rate of 24Mg(p,γ)25Al

Abundance Ratios in Metal-Poor Globular Clusters: Deep Mixing and its Effect on Stellar Populations of the Galactic Halo

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