arXiv subjects

S. W. Campbell

Publications and source records attributed to S. W. Campbell.

23 records · Page 2Linked to original sources

arXiv2008

Fluorine in carbon-enhanced metal-poor stars: a binary scenario

A super-solar fluorine abundance was observed in the carbon-enhanced metal-poor (CEMP) star HE 1305+0132 ([F/Fe] = +2.90, [Fe/H] = -2.5). We propose that this observation can be explained using a binary model that involve mass transfer from an asymptotic giant branch (AGB) star companion and, based on this model, we predict F abundances in CEMP stars in general. We discuss wether F can be used to discriminate between the formation histories of most CEMP stars: via binary mass transfer or from the ejecta of fast-rotating massive stars. We compute AGB yields using different stellar evolution and nucleosynthesis codes to evaluate stellar model uncertainties. We use a simple dilution model to determine the factor by which the AGB yields should be diluted to match the abundances observed in HE 1305+0132. We further employ a binary population synthesis tool to estimate the probability of F-rich CEMP stars. The abundances observed in HE 1305+0132 can be explained if this star accreted 3-11% of the mass lost by its former AGB companion. The primary AGB star should have dredged-up at least 0.2 Msun of material from its He-rich region into the convective envelope via third dredge-up, which corresponds to AGB models of Z ~ 0.0001 and mass ~ 2 Msun. Many AGB model uncertainties, such as the treatment of convective borders and mass loss, require further investigation. We find that in the binary scenario most CEMP stars should also be FEMP stars, that is, have [F/Fe] > +1, while fast-rotating massive stars do not appear to produce fluorine. We conclude that fluorine is a signature of low-mass AGB pollution in CEMP stars, together with elements associated with the slow neutron-capture process.

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arXiv2007

Origin of Abundance Inhomogeneity in Globular Clusters

We numerically investigate abundance properties of the Galactic globular clusters (GCs) by adopting a new ``external pollution'' scenario. In this framework, GCs are assumed to originate in forming low-mass dwarfs embedded in dark matter subhalos at very high redshifts (z) and thus be chemically influenced by field AGB stars of the dwarfs during early GC formation processes. In this external pollution scenario, the ratio of the total mass of infalling gas to that of AGB ejecta during GC formation in a dwarf (s) and the time scale of gas infall (sigma_I) are the most important key parameters that can determine abundance properties of GCs. We mainly investigate the abundance inhomogeneity among light elements (e.g., C, N, O, Na, and Al) of stars in GCs by using the latest stellar yield models of metal-poor AGB stars with and without third dredge-up. Our principal results for the models with no third dredge-up, which are more consistent with observations, are as follows. Both [N/Fe] and [C/Fe] can be diverse among stars within a GC owing to chemical pollution from field AGB stars. [N/Fe] distributions in some GCs can clearly show bimodality whereas [C/Fe] is monomodal in most models. [N/Fe] distributions depend on s such that models with smaller $s$ (i.e., larger mass fraction of AGB ejecta used for GC formation) show the [N/Fe] bimodality more clearly. N-rich, C-poor stars in GCs also have higher He abundances owing to pollution from massive AGB stars with He-rich ejecta. The number fraction of He-rich stars (Y >0.30) is higher for the models with smaller s and shorter sigma_I for 3 < s <24 and 10^5 < sigma_I < 10^7 yr.Our models are in strong disagreement with the observed O-Na anticorrelation.

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arXiv2006

Are There Radical Cyanogen Abundance Differences Between Galactic Globular Cluster RGB and AGB Stars?

[Abridged] Norris et al. (1981) found that there was a distinct lack of cyanogen-strong (CN-strong) stars in their sample of AGB stars, as compared to their sample of RGB stars (which had roughly equal numbers of CN-normal and CNstrong stars). Similar features have been discovered in the AGB populations of other clusters. Unfortunately all of these studies suffer from low AGB star counts so the conclusions are not necessarily robust -- larger, statistically signifcant, sample sizes are needed. We outline the results of a literature search for relevant CN observations and describe our observing proposal to test the suggestion that there are substantial abundance differences between the AGB and RGB in galactic globular clusters. The literature search revealed that the AGB star counts for all studies (which are not, in general, studies about AGB stars in particular) are low, usually being < or = 10. The search also revealed that the picture may not be consistent between clusters. Although most clusters appear to have CN-weak AGBs, at least two seem to have CN-strong AGBs (M5 & 47 Tuc). To further complicate the picture, clusters often appear to have a combination of both CN-strong and CN-weak stars on their AGBs -- although one population tends to dominate. Again, all these assertions are however based on small sample sizes.We aim to increase the sample sizes by an order of magnitude using existing high quality photometry in which the AGB and RGB can be reliably separated. For the observations we will use a wide-field, low- to mid-resolution multi-object spectroscope. With the new information we hope to ascertain whether significant abundance differences really exist.

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arXiv2005

Abundance Anomalies in NGC6752 - Do AGB Stars Have a Role?

We are in the process of testing a popular theory that the observed abundance anomalies in the Globular Cluster NGC 6752 are due to `internal pollution' from intermediate mass asymptotic giant branch stars. To this end we are using a chemical evolution model incorporating custom-made stellar evolution yields calculated using a detailed stellar evolution code. By tracing the chemical evolution of the intracluster gas, which is polluted by two generations of stars, we are able to test the internal pollution scenario in which the Na- and Al-enhanced ejecta from intermediate mass stars is either accreted onto the surfaces of other stars, or goes toward forming new stars. In this paper we focus mainly on the nucleosynthetic yields of the AGB stars and discuss whether these stars are the source of the observed Na-O anticorrelation. Comparing our preliminary results with observational data suggests that the qualitative theory is not supported by this quantitative study.

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arXiv2003

Nucleosynthesis in Early Stars

We present preliminary results of stellar structure and nucleosynthesis calculations for some early stars. The study (still in progress) seeks to explore the expected chemical signatures of second generation low- and intermediate-mass stars that may have formed out of a combination of Big Bang and Pop III (Z=0) supernovae material. Although the study is incomplete at this stage, we find some important features in our models. The initial chemical composition of these early stars is found to be significantly different to that given by just scaling the solar composition. The most notable difference is the lack of nitrogen. This should not affect the structural evolution significantly as nitrogen will be quickly produced through the CNO cycle due to the presence of carbon (and oxygen). It should however effect the nucleosynthetic yields. We also find that our very low metallicity five solar-mass model, with [Fe/H]=-4.01, does not reach the RGB - it goes directly to the helium burning phase. It does not experience the first dredge-up either. This is not a new finding but it will have an effect on the surface chemical evolution of the stars and should alter the nucleosynthetic yields that we are currently calculating. Our higher metallicity stars, with a globular cluster composition at [Fe/H]= -1.40, do experience all the standard phases of evolution but also have significantly higher surface temperatures and luminosities compared to solar metallicity stars. Their internal temperatures are also higher which should again effect the final nucleosynthetic yields.

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