Inhomogeneous Galactic halo: a possible explanation for the spread observed in s- and r- process elements

The considerable scatter of the s- and r-process elements observed in low-metallicity stars, compared to the small star to star scatter observed for the alpha elements, is an open question for the chemical evolution studies. We have developed a stochastic chemical evolution model, in which the main assumption is a random formation of new stars, subject to the condition that the cumulative mass distribution follows a given initial mass function. With our model we are able to reproduce the different features of alpha-elements and s-and r-process elements. The reason for this resides in the random birth of stellar masses coupled with the different stellar mass ranges from where alpha-elements and s-and r-process elements originate. In particular, the sites of production of the alpha elements are the whole range of the massive stars, whereas the mass range of production for the s- and r-process elements has an upper limit of 30 solar masses.Comment: 2 pages, 2 figures, proceedings of "From stars to galaxies", Venice October 2006, to be published in the Astronomical Society of the Pacific Conference Serie

Manganese spread in Ursa Minor as a proof of sub-classes of type Ia supernovae

Context. Recently, new sub-classes of Type Ia supernovae (SNe Ia) were discovered, including SNe Iax. The suggested progenitors of SNe Iax are relatively massive, possibly hybrid C+O+Ne white dwarfs, which can cause white dwarf winds at low metallicities. There is another class that can potentially occur at low or zero metallicities; sub-Chandrasekhar mass explosions in single and/or double degenerate systems of standard C+O white dwarfs. These explosions have different nucleosynthesis yields compared to the normal, Chandrasekhar mass explosions. Aims. We test these SN Ia channels using their characteristic chemical signatures. Methods. The two sub-classes of SNe Ia are expected to be rarer than normal SNe Ia and do not affect the chemical evolution in the solar neighbourhood; however, because of the shorter delay time and/or weaker metallicity dependence, they could influence the evolution of metalpoor systems. Therefore, we have included both in our stochastic chemical evolution model for the dwarf spheroidal galaxy Ursa Minor. Results. The model predicts a butterfly-shape spread in [Mn/Fe] in the interstellar medium at low metallicity and - at the same time - a decrease of [alpha/Fe] ratios at lower [Fe/H] than in the solar neighbourhood, both of which are consistent with the observed abundances in stars of Ursa Minor. Conclusions. The surprising agreement between our models and available observations provides a strong indication of the origins of these new sub-classes of SNe Ia. This outcome requires confirmation by future abundance measurements of manganese in stars of other satellite galaxies of ourMilkyWay. It will be vital for this project to measure not the most extreme metal-poor tail, as more commonly happens, but the opposite; the metal-rich end of dwarf spheroidals.Comment: 8 pages, 6 figures, accepted for publication in A&

Explaining the Ba, Y, Sr, and Eu abundance scatter in metal-poor halo stars: constraints to the r-process

Context. Thanks to the heroic observational campaigns carried out in recent years we now have large samples of metal-poor stars for which measurements of detailed abundances exist. [...] These data hold important clues on the nature of the contribution of the first stellar generations to the enrichment of our Galaxy. Aims. We aim to explain the scatter in Sr, Ba, Y, and Eu abundance ratio diagrams unveiled by the metal-poor halo stars. Methods. We computed inhomogeneous chemical evolution models for the Galactic halo assuming different scenarios for the r-process site: the electron-capture supernovae (EC) and the magnetorotationally driven (MRD) supernovae scenario. We also considered models with and without the contribution of fast-rotating massive stars (spinstars) to an early enrichment by the s-process. A detailed comparison with the now large sample of stars with measured abundances of Sr, Ba, Y, Eu, and Fe is provided (both in terms of scatter plots and number distributions for several abundance ratios). Results. The scatter observed in these abundance ratios of the very metal-poor stars (with [Fe/H] < -2.5) can be explained by combining the s-process production in spinstars, and the r-process contribution coming from massive stars. For the r-process we have developed models for both the EC and the MRD scenario that match the observations. Conclusions. With the present observational and theoretical constraints we cannot distinguish between the EC and the MRD scenario in the Galactic halo. Independently of the r-process scenarios adopted, the production of elements by an s-process in spinstars is needed to reproduce the spread in abundances of the light neutron capture elements (Sr and Y) over heavy neutron capture elements (Ba and Eu). We provide a way to test our suggestions by means of the distribution of the Ba isotopic ratios in a [Ba/Fe] or [Sr/Ba] vs. [Fe/H] diagram.Comment: 14 pages, 7 figures, accepted for publication in Astronomy and Astrophysic

Chemical evolution of the Galactic bulge: different stellar populations and possible gradients

We compute the chemical evolution of the Galactic bulge to explain the existence of two main stellar populations recently observed. After comparing model results and observational data we suggest that the old more metal poor stellar population formed very fast (on a timescale of 0.1-0.3 Gyr) by means of an intense burst of star formation and an initial mass function flatter than in the solar vicinity whereas the metal rich population formed on a longer timescale (3 Gyr). We predict differences in the mean abundances of the two populations (-0.52 dex for ) which can be interpreted as a metallicity gradients. We also predict possible gradients for Fe, O, Mg, Si, S and Ba between sub-populations inside the metal poor population itself (e.g. -0.145 dex for ). Finally, by means of a chemo-dynamical model following a dissipational collapse, we predict a gradient inside 500 pc from the Galactic center of -0.26 dex kpc^{-1} in Fe.Comment: 9 pages, 9 figures, accepted for publication in Section 5. of Astronomy and Astrophysic

Phosphorus Abundances in FGK Stars

We measured phosphorus abundances in 22 FGK dwarfs and giants that span --0.55 $<$ [Fe/H] $<$ 0.2 using spectra obtained with the Phoenix high resolution infrared spectrometer on the Kitt Peak National Observatory Mayall 4m telescope, the Gemini South Telescope, and the Arcturus spectral atlas. We fit synthetic spectra to the P I feature at 10581 $\AA$ to determine abundances for our sample. Our results are consistent with previously measured phosphorus abundances; the average [P/Fe] ratio measured in [Fe/H] bins of 0.2 dex for our stars are within $\sim$ 1 $\sigma$ compared to averages from other IR phosphorus studies. Our study provides more evidence that models of chemical evolution using the results of theoretical yields are under producing phosphorus compared to the observed abundances. Our data better fit a chemical evolution model with phosphorus yields increased by a factor of 2.75 compared to models with unadjusted yields. We also found average [P/Si] = 0.02 $\pm$ 0.07 and [P/S] = 0.15 $\pm$ 0.15 for our sample, showing no significant deviations from the solar ratios for [P/Si] and [P/S] ratios.Comment: 11 pages, 5 figures, Accepted to Ap

Chemical evolution of neutron capture elements in our Galaxy and in the dwarf spheroidal galaxies of the Local Group

By adopting a chemical evolution model for the Milky Way already reproducing the evolution of several chemical elements, we compare our theoretical results with accurate and new stellar data of neutron capture elements and we are able to impose strong constraints on the nucleosynthesis of the studied elements. We can suggest the stellar sites of production for each element. In particular, the r-process component of each element (if any) is produced in the mass range from 10 to 30 Msun, whereas the s-process component arises from stars in the range from 1 to 3 Msun. Using the same chemical evolution model, extended to different galactocentric distances, we obtain results on the radial gradients of the Milky Way. We compare the results of the model not only for the neutron capture elements but also for alpha-elements and iron peak elements with new data of Cepheids stars. We give a possible explanation to the considerable scatter of neutron capture elements observed in low metallicity stars in the solar vicinity, compared to the small star to star scatter observed for the alpha-elements. In fact, we have developed a stochastic chemical evolution model, in which the main assumption is a random formation of new stars, subject to the condition that the cumulative mass distribution follows a given initial mass function. With our model we are able to reproduce the different features of neutron capture elements and alpha-elements. Finally, we test the prescriptions for neutron capture elements also for the dwarf spheroidal galaxies of the Local Group. We predict that the chemical evolution of these elements in dwarf spheroidal galaxies is different from the evolution in the solar vicinity and indicates that dwarf spheroidal galaxies (we see nowadays) cannot be the building blocks of our Galaxy.Comment: 182 pages, 74 figures, PhD Thesis. Supervisor: Francesca Matteucci. High quality figures upon reques