Studies of the r-process enhanced stars are important for understanding the nature and origin of the r-process better. We present a detailed abundance analysis of a very metal-poor giant star ...discovered in the HERES project, HE 2252-4225, which exhibits overabundances of the r-process elements with r/Fe = +0.80. No firm conclusion can be drawn about the relationship between the first neutron-capture peak elements, Sr to Ru, in HE 2252-4225 and the solar r-process, owing to the uncertainty in the solar r-process. The investigated star has an anomalously high Th/Eu abundance ratio, so that radioactive dating results in a stellar age of tau = 1.5 + or - 1.5 Gyr that is not expected for a very metal-poor halo star.
I describe the method of nucleochronometry for determining individual ages of stars, and report on results of the application of this method to old, metal‐poor stars belonging to the Galactic halo ...population. I discuss uncertainties and caveats of this age determination method.
The advent of high-resolution spectrographs and detailed stellar atmosphere modelling has strengthened the need for accurate molecular data. Carbon-enhanced metal-poor (CEMP) stars spectra are ...interesting objects with which to study transitions from the CH molecule. We combine programs for spectral analysis of molecules and stellar-radiative transfer codes to build an extensive CH linelist, including predissociation broadening as well as newly identified levels. We show examples of strong predissociation CH lines in CEMP stars, and we stress the important role played by the CH features in the Bond-Neff feature depressing the spectra of barium stars by as much as 0.2 mag in the λ = 3000−5500 Å range. Because of the extreme thermodynamic conditions prevailing in stellar atmospheres (compared to the laboratory), molecular transitions with high energy levels can be observed. Stellar spectra can thus be used to constrain and improve molecular data.
We present a homogeneous chemical abundance analysis of 16 elements in 190 metal-poor Galactic halo stars (38 program and 152 literature objects). The sample includes 171 stars with Fe/H < or =, ...slant -2.5, of which 86 are extremely metal poor, Fe/H < or =, slant -3.0. Our program stars include 10 new objects with Fe/H < or =, slant -3.5. We identify a sample of "normal" metal-poor stars and measure the trends between X/Fe and Fe/H, as well as the dispersion about the mean trend for this sample. Using this mean trend, we identify objects that are chemically peculiar relative to "normal" stars at the same metallicity. These chemically unusual stars include CEMP-no objects, one star with high Si/Fe, another with high Ba/Sr, and one with unusually low X/Fe for all elements heavier than Na. The Sr and Ba abundances indicate that there may be two nucleosynthetic processes at lowest metallicity that are distinct from the main r-process. Finally, for many elements, we find a significant trend between X/Fe versus T sub(eff), which likely reflects non-LTE and/or three-dimensional effects. Such trends demonstrate that care must be exercised when using abundance measurements in metal-poor stars to constrain chemical evolution and/or nucleosynthesis predictions.
Context. Carbon-enhanced metal-poor stars (CEMP) form a significant proportion of the metal-poor stars, their origin is not well understood, and this carbon-enhancement appears in stars that exhibit ...different abundance patterns. Aims. Three very metal-poor C-rich turnoff stars were selected from the SDSS survey, observed with the ESO VLT (UVES) to precisely determine the element abundances. In turnoff stars (unlike giants) the carbon abundance has not been affected by mixing with deep layers and is therefore easier to interpret. Methods. The analysis was performed with one dimensional (1D) local thermodynamical equilibrium (LTE) static model atmospheres. When available, non-LTE corrections were applied to the classical LTE abundances. The 3D effects on the CH and CN molecular bands were computed using hydrodynamical simulations of the stellar atmosphere (CO5BOLD) and are found to be very important. Results. To facilitate a comparison with previous results, only 1D abundances are used in the discussion. The abundances (or upper limits) of the elements enable us to place these stars in different CEMP classes. The carbon abundances confirm the existence of a plateau at A(C)= 8.25 for Fe/H ≥ −3.4. The most metal-poor stars (Fe/H < −3.4) have significantly lower carbon abundances, suggesting a lower plateau at A(C) ≈ 6.5. Detailed analyses of a larger sample of very low metallicity carbon-rich stars are required to confirm (or refute) this possible second plateau and specify the behavior of the CEMP stars at very low metallicity.
We examine the metallicity distribution function (MDF) and fraction of carbon-enhanced metal-poor (CEMP) stars in a sample that includes 86 stars with Fe/H < or =, slant -3.0, based on ...high-resolution, high signal-to-noise spectroscopy, of which some 32 objects lie below Fe/H = -3.5. After accounting for the completeness function, the "corrected" MDF does not exhibit the sudden drop at Fe/H = -3.6 that was found in recent samples of dwarfs and giants from the Hamburg/ESO survey. Rather, the MDF decreases smoothly down to Fe/H = -4.1. Similar results are obtained from the "raw" MDF. We find that the fraction of CEMP objects below Fe/H = -3.0 is 23% + or - 6% and 32% + or - 8% when adopting the Beers & Christlieb and Aoki et al. CEMP definitions, respectively. The former value is in fair agreement with some previous measurements, which adopt the Beers & Christlieb criterion.
We present a high-resolution elemental-abundance analysis for a sample of 23 very metal-poor (Fe/H < -2.0) stars, 12 of which are extremely metal-poor (Fe/H < -3.0), and 4 of which are ...ultra-metal-poor (Fe/H < -4.0). These stars were targeted to explore differences in the abundance ratios for elements that constrain the possible astrophysical sites of element production, including Li, C, N, O, the alpha-elements, the iron-peak elements, and a number of neutron-capture elements. This sample substantially increases the number of known carbon-enhanced metal-poor (CEMP) and nitrogen-enhanced metal-poor (NEMP) stars-our program stars include eight that are considered "normal" metal-poor stars, six CEMP-no stars, five CEMP-s stars, two CEMP-r stars, and two CEMP-r/s stars. One of the CEMP-r stars and one of the CEMP-r/s stars are possible NEMP stars. We detect lithium for three of the six CEMP-no stars, all of which are Li depleted with respect to the Spite plateau. The majority of the CEMP stars have C/N > 0. The stars with C/N < 0 suggest a larger degree of mixing; the few CEMP-no stars that exhibit this signature are only found at Fe/H < -3.4, a metallicity below which we also find the CEMP-no stars with large enhancements in Na, Mg, and Al. We confirm the existence of two plateaus in the absolute carbon abundances of CEMP stars, as suggested by Spite et al. We also present evidence for a "floor" in the absolute Ba abundances of CEMP-no stars at A(Ba)~ -2.0.
We report an abundance analysis for the highly r-process-enhanced (r-II) star CS 29497-004, a very metal-poor giant with solar system Teff = 5013 K and Fe/H = −2.85, whose nature was initially ...discovered in the course of the HERES project. Our analysis is based on high signal-to-noise ratio, high-resolution (R ~ 75 000) VLT/UVES spectra and MARCS model atmospheres under the assumption of local thermodynamic equilibrium, and obtains abundance measurements for a total of 46 elements, 31 of which are neutron-capture elements. As is the case for the other ~25 r-II stars currently known, the heavy-element abundance pattern of CS 29497-004 well-matches a scaled solar system second peak r-process-element abundance pattern. We confirm our previous detection of Th, and demonstrate that this star does not exhibit an “actinide boost”. Uranium is also detected (log ϵ(U) = −2.20 ± 0.30), albeit with a large measurement error that hampers its use as a precision cosmo-chronometer. Combining the various elemental chronometer pairs that are available for this star, we derive a mean age of 12.2 ± 3.7 Gyr using the theoretical production ratios from published waiting-point approximation models. We further explore the high-entropy wind model (Farouqi et al. 2010, ApJ, 712, 1359) production ratios arising from different neutron richness of the ejecta (Ye), and derive an age of 13.7 ± 4.4 Gyr for a best-fitting Ye = 0.447. The U/Th nuclei-chronometer is confirmed to be the most resilient to theoretical production ratios and yields an age of 16.5 ± 6.6 Gyr. Lead (Pb) is also tentatively detected in CS 29497-004, at a level compatible with a scaled solar r-process, or with the theoretical expectations for a pure r-process in this star.
Context. The oldest stars born before the onset of the main s-process are expected to have a pure r-process Ba/Eu abundance ratio. Aims. We revised barium and europium abundances of selected very ...metal-poor (VMP) and strongly r-process enhanced (r-II) stars to evaluate an empirical r-process Ba/Eu ratio. Methods. Our calculations were based on non-local thermodynamic equilibrium (NLTE) line formation for Ba II and Eu II in the classical 1D MARCS model atmospheres. Homogeneous stellar abundances were determined from the Ba II subordinate and resonance lines by applying a common Ba isotope mixture. We used high-quality VLT/UVES spectra and observational material from the literature. Results. For most investigated stars, NLTE leads to a lower Ba, but a higher Eu abundance. The resulting elemental ratio of the NLTE abundances amounts to, on average, log(Ba/Eu) = 0.78+ or -0.06. This is a new constraint to pure r-process production of Ba and Eu. The obtained Ba/Eu abundance ratio of the r-II stars supports the corresponding solar system r-process ratio as predicted by recent Galactic chemical evolution calculations of Bisterzo, Travaglio, Gallino, Wiescher, and Kappeler. We present the NLTE abundance corrections for the Ba II and Eu II lines in the grid of VMP model atmospheres.
This work presents a large consistent study of molybdenum (Mo) and ruthenium (Ru) abundances in the Milky Way. These two elements are important nucleosynthetic diagnostics. In our sample of 71 ...Galactic metal-poor field stars, we detect Ru and/or Mo in 52 of these (59 including upper limits). The sample consists of high-resolution, high signal-to-noise spectra covering both dwarfs and giants from Fe/H = −0.63 down to −3.16. Thus we provide information on the behaviour of Mo I and Ru I at higher and lower metallicity than is currently known. In this sample we find a wide spread in the Mo and Ru abundances, which is typical of heavy elements. We confirm earlier findings of Mo enhanced stars around Fe/H = −1.5 and add ~15 stars both dwarfs and giants with normal (<0.3 dex) Mo and Ru abundances, as well as more than 15 Mo and Ru enhanced (>0.5 dex) stars to the currently known stellar sample. This indicates that several formation processes, in addition to high entropy winds, can be responsible for the formation of elements like Mo and Ru. We trace the formation processes by comparing Mo and Ru to elements (Sr, Zr, Pd, Ag, Ba, and Eu) with known formation processes. Based on how tight the two elements correlate with each other, we are able to distinguish if they share a common formation process and how important this contribution is to the element abundance. We find clear indications of contributions from several different formation processes, namely the p-process, and the slow (s-), and rapid (r-) neutron-capture processes. From these correlations we find that Mo is a highly convolved element that receives contributions from both the s-process and the p-process and less from the main and weak r-processes, whereas Ru is mainly formed by the weak r-process as is silver. We also compare our absolute elemental stellar abundances to relative isotopic abundances of presolar grains extracted from meteorites. Their isotopic abundances can be directly linked to the formation process (e.g. r-only isotopes) providing a unique comparison between observationally derived abundances and the nuclear formation process. The comparison to abundances in presolar grains shows that the r-/s-process ratios from the presolar grains match the total elemental chemical composition derived from metal-poor halo stars with Fe/H around −1.5 to −1.1 dex. This indicates that both grains and stars around and above Fe/H = −1.5 are equally (well) mixed and therefore do not support a heterogeneous presolar nebula. An inhomogeneous interstellar medium (ISM) should only be expected at lower metallicities. Our data, combined with the abundance ratios of presolar grains, could indicate that the AGB yields are less efficiently mixed into stars than into presolar grains. Finally, we detect traces of s-process material at Fe/H = −1.5, indicating that this process is at work at this and probably at even lower metallicity.
PDF
