222 research outputs found
The chemical evolution of Barium and Europium in the Milky Way
We compute the evolution of the abundances of barium and europium in the
Milky Way and we compare our results with the observed abundances from the
recent UVES Large Program "First Stars". We use a chemical evolution model
which already reproduces the majority of observational constraints. We confirm
that barium is a neutron capture element mainly produced in the low mass AGB
stars during the thermal-pulsing phase by the 13C neutron source, in a slow
neutron capture process. However, in order to reproduce the [Ba/Fe] vs. [Fe/H]
as well as the Ba solar abundance, we suggest that Ba should be also produced
as an r-process element by massive stars in the range 10-30 solar masses. On
the other hand, europium should be only an r-process element produced in the
same range of masses (10-30 solar masses), at variance with previous
suggestions indicating a smaller mass range for the Eu producers. As it is well
known, there is a large spread in the [Ba/Fe] and [Eu/Fe] ratios at low
metallicities, although smaller in the newest data. With our model we estimate
for both elements (Ba and Eu) the ranges for the r-process yields from massive
stars which better reproduce the trend of the data. We find that with the same
yields which are able to explain the observed trends, the large spread in the
[Ba/Fe] and [Eu/Fe] ratios cannot be explained even in the context of an
inhomogeneous models for the chemical evolution of our Galaxy. We therefore
derive the amount by which the yields should be modified to fully account for
the observed spread. We then discuss several possibilities to explain the size
of the spread. We finally suggest that the production ratio of [Ba/Eu] could be
almost constant in the massive stars.Comment: 14 pages, 17 figures, accepted for pubblication in A&
Abundance Uncertainties Obtained With the PizBuin Framework For Monte Carlo Reaction Rate Variations
Uncertainties in nucleosynthesis models originating from uncertainties in
astrophysical reaction rates were estimated in a Monte Carlo variation
procedure. Thousands of rates were simultaneously varied within individual,
temperature-dependent errors to calculate their combined effect on final
abundances. After a presentation of the method, results from application to
three different nucleosynthesis processes are shown: the -process and
the s-process in massive stars, and the main s-process in AGB stars
(preliminary results). Thermal excitation of nuclei in the stellar plasma and
the combined action of several reactions increase the final uncertainties above
the level of the experimental errors. The total uncertainty, on the other hand,
remains within a factor of two even in processes involving a large number of
unmeasured rates, with some notable exceptions for nuclides whose production is
spread over several stellar layers and for s-process branchings.Comment: 8 pages, 4 figures; Proceedings of OMEG 2017, Daejeon, Korea, June
27-30, 2017; to appear in AIP Conf. Pro
The Chemical Evolution of Phosphorus
Phosphorus is one of the few remaining light elements for which little is
known about its nucleosynthetic origin and chemical evolution, given the lack
of optical absorption lines in the spectra of long-lived FGK-type stars. We
have identified a P I doublet in the near-ultraviolet (2135/2136 A) that is
measurable in stars of low metallicity. Using archival Hubble Space
Telescope-STIS spectra, we have measured P abundances in 13 stars spanning -3.3
<= [Fe/H] <= -0.2, and obtained an upper limit for a star with [Fe/H] ~ -3.8.
Combined with the only other sample of P abundances in solar-type stars in the
literature, which spans a range of -1 <= [Fe/H] <= +0.2, we compare the stellar
data to chemical evolution models. Our results support previous indications
that massive-star P yields may need to be increased by a factor of a few to
match stellar data at all metallicities. Our results also show that hypernovae
were important contributors to the P production in the early universe. As P is
one of the key building blocks of life, we also discuss the chemical evolution
of the important elements to life, C-N-O-P-S, together.Comment: Accepted for publication in Astrophysical Journal Letters. 6 pages, 4
figures; reference added to earlier versio
The s-process nucleosynthesis : Impact of the uncertainties in the nuclear physics determined by monte carlo variations
We investigated the impact of uncertainties in neutron-capture and weak reactions (on heavy elements) on the s-process nucleosynthesis in low-mass stars and massive stars using a Monte-Carlo based approach. We performed extensive nuclear reaction network calculations that include newly evaluated temperature-dependent upper and lower limits for the individual reaction rates. We found β-decay rate uncertainties affect only a few nuclei near s-process branchings, whereas most of the uncertainty in the final abundances is caused by uncertainties in the neutron capture rates. We suggest a list of uncertain rates as candidates for improved measurement by future experiments.Peer reviewe
Abundances of carbon-enhanced metal-poor stars as constraints on their formation
Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.Context. An increasing fraction of carbon-enhanced metal-poor (CEMP) stars is found as their iron abundance, [Fe/H], decreases below [Fe/H] =−2.0. The CEMP-s stars have the highest absolute carbon abundances, [C/H], and are thought to owe their enrichment in carbon and the slow neutron-capture (s-process) elements to mass transfer from a former asymptotic giant branch (AGB) binary companion. The most Fe-poor CEMP stars are normally single, exhibit somewhat lower [C/H] than CEMP-s stars, but show no s-process element enhancement (CEMP-no stars). Abundance determinations of CNO offer clues to their formation sites. Aims. Our aim is to use the medium-resolution spectrograph X-Shooter/VLT to determine stellar parameters and abundances for C, N, Sr, and Ba in several classes of CEMP stars in order to further classify and constrain the astrophysical formation sites of these stars. Methods. Atmospheric parameters for our programme stars were estimated from a combination of V−K photometry, model isochrone fits, and estimates from a modified version of the SDSS/SEGUE spectroscopic pipeline. We then used X-Shooter spectra in conjunction with the 1D local thermodynamic equilibrium spectrum synthesis code MOOG, 1D ATLAS9 atmosphere models to derive stellar abundances, and, where possible, isotopic 12C/13C ratios. Results. Abundances (or limits) of C, N, Sr, and Ba are derived for a sample of 27 faint metal-poor stars for which the X-Shooter spectra have sufficient signal-to-noise ratios (S/N). These moderate resolution, low S/N (~10−40) spectra prove sufficient to perform limited chemical tagging and enable assignment of these stars into the CEMP subclasses (CEMP-s and CEMP-no). According to the derived abundances, 17 of our sample stars are CEMP-s and 3 are CEMP-no, while the remaining 7 are carbon-normal. For four CEMP stars, the subclassification remains uncertain, and two of them may be pulsating AGB stars. Conclusions. The derived stellar abundances trace the formation processes and sites of our sample stars. The [C/N] abundance ratio is useful for identifying stars with chemical compositions unaffected by internal mixing, and the [Sr/Ba] abundance ratio allows us to distinguish between CEMP-s stars with AGB progenitors and the CEMP-no stars. Suggested formation sites for the latter include faint supernovae with mixing and fallback and/or primordial, rapidly-rotating, massive stars (spinstars). X-Shooter spectra have thus proved to be valuable tools in the continued search for their origin.Peer reviewe
Solar abundance of manganese: a case for near Chandrasekhar-mass Type la supernova progenitors
Manganese is predominantly synthesised in Type Ia supernova (SN Ia) explosions. Owing to the entropy dependence of the Mn yield in explosive thermonuclear burning, SNe Ia involving near Chandrasekhar-mass (MCh) white dwarfs (WDs) are predicted to produce Mn-to-Fe ratios that significantly exceed those of SN Ia explosions involving sub-Chandrasekhar mass primary WDs. Of all current supernova explosion models, only SN Ia models involving near-MCh WDs produce [Mn/Fe] ≳ 0.0
Production Uncertainties of p-Nuclei in the γ-Process in Massive Stars Using a Monte Carlo Approach
T. Rauscher, N. Nishimura, R. Hirschi, G. Cescutti, A. St J. Murphy and A. Heger, 'Production Uncertainties of p-Nuclei in the γ-Process in Massive Stars Using a Monte Carlo Approach', in Proceedings of the 14th International Symposium on Nuclei in the Cosmos (NIC2016). Niigata, Japan, June 19-25, 2016. ISBN: 978-4-89027-118-4. DOI: http://dx.doi.org/10.7566/JPSCP.14.010509. © 2017 The Physical Society of Japan.Proton-rich nuclei, the so-called p-nuclei, are made in photodisintegration processes in outer shells of massive stars in the course of the final supernova explosion. Nuclear uncertainties in the production of these nuclei have been quantified in a Monte Carlo procedure. Bespoke temperature-dependent uncertainties were assigned to different types of reactions involving nuclei from Fe to Bi and all rates were varied randomly within the uncertainties. The resulting total production uncertainties of the p-nuclei are below a factor of two, with few exceptions. Key reactions dominating the final uncertainties have been identified in an automated procedure using correlations between rate and abundance uncertainties. Our results are compared to those of a previous study manually varying reaction rates
Uncertainties in νp-process nucleosynthesis from Monte Carlo variation of reaction rates
It has been suggested that a νp-process can occur when hot, dense, and proton-rich matter is expanding within a strong flux of antineutrinos. In such an environment, proton-rich nuclides can be produced in sequences of proton captures and (n, p) reactions, where the free neutrons are created in situ by νe + p → n + e+ reactions. The detailed hydrodynamic evolution determines where the nucleosynthesis path turns off from N = Z line and how far up the nuclear chart it runs. In this work, the uncertainties on the final isotopic abundances stemming from uncertainties in the nuclear reaction rates were investigated in a large-scale Monte Carlo approach, simultaneously varying more than 10 000 reactions. A large range of model conditions was investigated because a definitive astrophysical site for the νp-process has not yet been identified. The present parameter study provides, for each model, identification of the key nuclear reactions dominating the uncertainty for a given nuclide abundance. As all rates appearing in the νp-process involve unstable nuclei, and thus only theoretical rates are available, the final abundance uncertainties are larger than those for nucleosynthesis processes closer to stability. Nevertheless, most uncertainties remain below a factor of 3 in trajectories with robust nucleosynthesis. More extreme conditions allow production of heavier nuclides but show larger uncertainties because of the accumulation of the uncertainties in many rates and because the termination of nucleosynthesis is not at equilibrium conditions. It is also found that the solar ratio of the abundances of 92Mo and 94Mo could be reproduced within uncertainties
High-resolution abundance analysis of red giants in the globular cluster NGC 6522
The [Sr/Ba] and [Y/Ba] scatter observed in some galactic halo stars that are
very metal-poor stars and in a few individual stars of the oldest known Milky
Way globular cluster NGC 6522,have been interpreted as evidence of early
enrichment by massive fast-rotating stars (spinstars). Because NGC 6522 is a
bulge globular cluster, the suggestion was that not only the very-metal poor
halo stars, but also bulge stars at [Fe/H]~-1 could be used as probes of the
stellar nucleosynthesis signatures from the earlier generations of massive
stars, but at much higher metallicity. For the bulge the suggestions were based
on early spectra available for stars in NGC 6522, with a medium resolution of
R~22,000 and a moderate signal-to-noise ratio. The main purpose of this study
is to re-analyse the NGC 6522 stars previously reported using new
high-resolution (R~45,000) and high signal-to-noise spectra (S/N>100). We aim
at re-deriving their stellar parameters and elemental ratios, in particular the
abundances of the neutron-capture s-process-dominated elements such as Sr, Y,
Zr, La, and Ba, and of the r-element Eu. High-resolution spectra of four giants
belonging to the bulge globular cluster NGC 6522 were obtained at the 8m VLT
UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVESconfiguration.
The spectroscopic parameters were derived based on the excitation and
ionization equilibrium of \ion{Fe}{I} and \ion{Fe}{II}. Our analysis confirms a
metallicity [Fe/H] = -0.95+-0.15 for NGC 6522, and the overabundance of the
studied stars in Eu (with +~0.2 < [Eu/Fe] < +~0.4) and alpha-elements O and Mg.
The neutron-capture s-element-dominated Sr, Y, Zr, Ba, La now show less
pronounced variations from star to star. Enhancements are in the range 0.0 <
[Sr/Fe] < +0.4, +0.23 < [Y/Fe] < +0.43, 0.0 < [Zr/Fe] < +0.4, 0.0 < [La/Fe] <
+0.35,and 0.05 < [Ba/Fe] < +0.55.Comment: date of acceptation: 31/07/2014, in press, 24 pages, 19
figures,Astronomy & Astrophysics, 201
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