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Register a new userThe Chemical Evolution of Magnesium Isotopic Abundances in the Solar Neighbourhood
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Authors
Y. Fenner
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The abundance of the neutron-rich magnesium isotopes observed in metal-poor stars is explained quantitatively with a chemical evolution model of the local Galaxy that considers - for the first time - the metallicity-dependent contribution from intermediate mass stars. Previous models that simulate the variation of Mg isotopic ratios with metallicity in the solar neighbourhood have attributed the production of Mg25 and Mg26 exclusively to hydrostatic burning in massive stars. These models match the data well for [Fe/H]>-1.0 but severely underestimate Mg25/Mg24 and Mg26/Mg24 at lower metallicities. Earlier studies have noted that this discrepancy may indicate a significant role played by intermediate-mass stars. Only recently have detailed calculations of intermediate-mass stellar yields of Mg25 and Mg26 become available with which to test this hypothesis. In an extension of previous work, we present a model that successfully matches the Mg isotopic abundances in nearby Galactic disk stars through the incorporation of nucleosynthesis predictions of Mg isotopic production in asymptotic giant branch stars.
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In spite of many observational efforts aiming to characterize the chemical evolution of our Galaxy, not much is known about the origin of fluorine (F). Models suggest that the F found in the Galaxy might have been produced mainly in three different ways, namely, Type II supernovae, asymptotic giant branch nucleosynthesis, or in the core of Wolf-Rayet stars. Only a few observational measurements of F abundances are available in the literature and mostly for objects whose characteristics might hamper an accurate determination of fluorine abundance (e.g.,complex mixing and nucleosynthesis processes, external/internal contamination). We derive the F abundances for a set of nine cool main-sequence dwarfs in the solar neighbourhood, based on an unblended line of the HF molecule at 2.3 microns. In addition, we study the s-process elements of five of these stars. We acquire data using the high-resolution IR-spectrograph CRIRES and gather FEROS data from the European Southern Observatory archive. Several of the analysed stars seem to be slightly fluorine enhanced with respect to the Sun, although no correlation is found between the F abundance and the iron content. In addition, the most fluorine enriched stars are also yttrium and zirconium enriched, which suggests that AGB fluorine nucleosynthesis is the dominant source of fluorine production for the observed stars. Nevertheless, the correlation between [F/Fe] and the s-elements is rather weak and possibly masked by the uncertainties in the F abundance measurements. Finally, we compare our derived F abundances to previous measurements of alpha-element and iron-peak element abundances. Type II core collapse Supernovae do not appear to be the main site of F production for our targets, as no correlation seems to exist between the [F/Fe] and the [alpha/Fe] ratios.
Atmospheric parameters and chemical compositions for ten stars with metallicities in the region of -2.2< [Fe/H] <-0.6 were precisely determined using high resolution, high signal to noise, spectra. For each star the abundances, for 14 to 27 elements, were derived using both LTE and NLTE approaches. In particular, differences by assuming LTE or NLTE are about 0.10 dex; depending on [Fe/H], Teff, gravity and element lines used in the analysis. We find that the O abundance has the largest error, ranging from 0.10 and 0.2 dex. The best measured elements are Cr, Fe, and Mn; with errors etween 0.03 and 0.11 dex. The stars in our sample were included in previous different observational work. We provide a consistent data analysis. The data dispersion introduced in the literature by different techniques and assumptions used by the different authors is within the observational errors, excepting for HD103095. We compare these results with stellar observations from different data sets and a number of theoretical galactic chemical evolution (GCE) simulations. We find a large scatter in the GCE results, used to study the origin of the elements. Within this scatter as found in previous GCE simulations, we cannot reproduce the evolution of the elemental ratios [Sc/Fe], [Ti/Fe], and [V/Fe] at different metallicities. The stellar yields from core collapse supernovae (CCSN) are likely primarily responsible for this discrepancy. Possible solutions and open problems are discussed.
Helioseismology has shown that the chemical composition of the Sun has changed over its lifetime. The surface abundance of helium and heavy elements is believed to have decreased by up to 10% relative to their initial values. However, this reduction is too small to be tested by direct observations of the photospheric chemical composition. Here, we compare the predicted variations in the solar photospheric composition with precise measurements of abundances in meteorites and the solar wind composition. Although elemental composition ratios can vary by roughly a percent (e. g. for Ca/Mg and Ca/Fe) over the Suns lifetime, their measurements are rife with uncertainties related to uncertainties in the interpretation of meteoritic measurements, photospheric determinations, and the complex fractionation processes occurring between the upper photosphere and lower chromosphere and the corona. On the other hand, isotopic ratios can be measured much more accurately and are not expected to be affected as much by extrasolar processes, although more work is required to quantify their effect. As the isotopic ratios evolve in the Sun proportionally to the mass ratios of the isotopes, light elements yield the highest variations in isotopic ratios. They are predicted to reach as high as 0.6% for $^{18}$O/$^{16}$O and are only slightly lower in the cases of $^{26}$Mg/$^{24}$Mg and $^{30}$Si/$^{28}$Si. Such a value should be well within the sensitivity of new missions such as Genesis.
We present a comprehensive study of the abundance evolution of the elements from H to U in the Milky Way halo and local disk. We use a consistent chemical evolution model, metallicity dependent isotopic yields from low and intermediate mass stars and yields from massive stars which include, for the first time, the combined effect of metallicity, mass loss and rotation for a large grid of stellar masses and for all stages of stellar evolution. The yields of massive stars are weighted by a metallicity dependent function of the rotational velocities, constrained by observations as to obtain a primary-like $^{14}$N behavior at low metallicity and to avoid overproduction of s-elements at intermediate metallicities. We show that the solar system isotopic composition can be reproduced to better than a factor of two for isotopes up to the Fe-peak, and at the 10% level for most pure s-isotopes, both light ones (resulting from the weak s-process in rotating massive stars) and the heavy ones (resulting from the main s-process in low and intermediate mass stars). We conclude that the light element primary process (LEPP), invoked to explain the apparent abundance deficiency of the s-elements with A< 100, is not necessary. We also reproduce the evolution of the heavy to light s-elements abundance ratio ([hs/ls]) - recently observed in unevolved thin disk stars - as a result of the contribution of rotating massive stars at sub-solar metallicities. We find that those stars produce primary F and dominate its solar abundance and we confirm their role in the observed primary behavior of N. In contrast, we show that their action is insufficient to explain the small observed values of C12/C13 in halo red giants, which is rather due to internal processes in those stars.
Period-luminosity sequences have been shown to exist among the Semi-Regular Variables (SRVs) of the Magellanic Clouds (Wood et al, 1999), the Bulge of the Milky Way galaxy (Glass & Schultheis, 2003) and elsewhere. It would clearly be useful to have absolute (trigonometric) calibrations of these relations. This paper investigates whether the sequences can be seen among the M-type giant SRVs of the solar neighbourhood. Mass loss phenomena among these stars and their dependence on period and spectral sub-type are also discussed.
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