Chemical evolution of the Milky Way: the origin of phosphorus

Context. Recently, for the first time the abundance of P has been measured in disk stars. This provides the opportunity of comparing the observed abundances with predictions from theoretical models. Aims. We aim at predicting the chemical evolution of P in the Milky Way and compare our results with the observed P abundances in disk stars in order to put constraints on the P nucleosynthesis. Methods. To do that we adopt the two-infall model of galactic chemical evolution, which is a good model for the Milky Way, and compute the evolution of the abundances of P and Fe. We adopt stellar yields for these elements from different sources. The element P should have been formed mainly in Type II supernovae. Finally, Fe is mainly produced by Type Ia supernovae. Results. Our results confirm that to reproduce the observed trend of [P/Fe] vs. [Fe/H] in disk stars, P is formed mainly in massive stars. However, none of the available yields for P can reproduce the solar abundance of this element. In other words, to reproduce the data one should assume that massive stars produce more P than predicted by a factor of ~ 3. Conclusions. We conclude that all the available yields of P from massive stars are largely underestimated and that nucleosynthesis calculations should be revised. We also predict the [P/Fe] expected in halo stars.Comment: Accepted for publication in A&A (minor changes with respect to the submitted version

The Earliest Phases of Galaxy Evolution

In this paper we study the very early phases of the evolution of our Galaxy by means of a chemical evolution model which reproduces most of the observational constraints in the solar vicinity and in the disk. We have restricted our analysis to the solar neighborhood and present the predicted abundances of several elements (C, N, O, Mg, Si, S, Ca, Fe) over an extended range of metallicities $[Fe/H] = -4.0$ to $[Fe/H] = 0.0$ compared to previous models. We adopted the most recent yield calculations for massive stars taken from different authors (Woosley & Weaver 1995 and Thielemann et al. 1996) and compared the results with a very large sample of data, one of the largest ever used to this purpose. These data have been analysed with a new and powerful statistical method which allows us to quantify the observational spread in measured elemental abundances and obtain a more meaningful comparison with the predictions from our chemical evolution model. Our analysis shows that the ``plateau'' observed for the [$\alpha$/Fe] ratios at low metallicities ($-3.0< [Fe/H] <-1.0$) is not perfectly constant but it shows a slope, especially for oxygen. This slope is very well reproduced by our model with both sets of yields. This is not surprising since realistic chemical evolution models, taking into account in detail stellar lifetimes, never predicted a completely flat plateau. This is due either to the fact that massive stars of different mass produce a slightly different O/Fe ratio or to the often forgotten fact that supernovae of type Ia, originating from white dwarfs, start appearing already at a galactic age of 30 million years and reach their maximum at 1 Gyr.Comment: 32 pages, 9 figures, to be published in Ap

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&

Testing the universal stellar IMF on the metallicity distribution in the bulges of the Milky Way and M31

We test whether the universal initial mass function (UIMF) or the integrated galaxial IMF (IGIMF) can be employed to explain the metallicity distribution (MD) of giants in the Galactic bulge. We make use of a single-zone chemical evolution model developed for the Milky Way bulge in the context of an inside-out model for the formation of the Galaxy. We checked whether it is possible to constrain the yields above 80 M_{\sun} by forcing the UIMF and required that the resulting MD matches the observed ones. We also extended the analysis to the bulge of M31 to investigate a possible variation of the IMF among galactic bulges. Several parameters that have an impact on stellar evolution (star-formation efficiency, gas infall timescale) are varied. We show that it is not possible to satisfactorily reproduce the observed metallicity distribution in the two galactic bulges unless assuming a flatter IMF ($x \leq 1.1$) than the universal one. We conlude that it is necessary to assume a variation in the IMF among the various environments.Comment: 9 pages, 4 figures, accepted for publication in A&

IUE observations of oxygen-rich supernova remnants

The IUE observations were used to determine the composition of the ejecta (especially C and Si abundances) and to test models for the ionization and excitation of the ejecta of two oxygen-rich supernova remnants (N132D in the Large Magellanic Cloud and 1E 0102-7219 in the Small Magellanic Cloud). Time-dependent photoionization by the EUV and X-ray radiation from 1E 0102-7219 can qualitatively explain its UV and optical line emission, but the density and ionization structures are complex and prevent a unique model from being specified. Many model parameters are poorly constrained, including the time dependence and shape of the ionizing spectrum. Moreover, the models presented are not self-consistent in that the volumes and densities of the optically emitting gas imply optical depths of order unity in the EUV, but absorption of the ionizing radiation was ignored. It is possible that these shortcomings reflect a more fundamental limitation of the model assumptions. It is assumed that the electron velocity distribution is Maxwellian and that the energy deposited by photoionization heats the electrons directly. The 500 eV electrons produced by the Auger process may excite or ionize other ions before they slow down enough to share their energy with other electrons. Many of the excitations would produce photons that could ionize lower ionization stages

A Chandra archival study of the temperature and metal abundance profiles in hot Galaxy Clusters at 0.1 < z < 0.3

We present the analysis of the temperature and metallicity profiles of 12 galaxy clusters in the redshift range 0.1--0.3 selected from the Chandra archive with at least ~20,000 net ACIS counts and kT>6 keV. We divide the sample between 7 Cooling-Core (CC) and 5 Non-Cooling-Core (NCC) clusters according to their central cooling time. We find that single power-laws can describe properly both the temperature and metallicity profiles at radii larger than 0.1 r_180 in both CC and NCC systems, showing the NCC objects steeper profiles outwards. A significant deviation is only present in the inner 0.1 r_180. We perform a comparison of our sample with the De Grandi & Molendi BeppoSAX sample of local CC and NCC clusters, finding a complete agreement in the CC cluster profile and a marginally higher value (at ~1sigma) in the inner regions of the NCC clusters. The slope of the power-law describing kT(r) within 0.1 r_180 correlates strongly with the ratio between the cooling time and the age of the Universe at the cluster redshift, being the slope >0 and tau_c/tau_age<=0.6 in CC systems.Comment: 12 pages, 6 figures, Accepted for publication by the Astrophysical Journa

Effects of the integrated galactic IMF on the chemical evolution of the solar neighbourhood

The initial mass function determines the fraction of stars of different intial mass born per stellar generation. In this paper, we test the effects of the integrated galactic initial mass function (IGIMF) on the chemical evolution of the solar neighbourhood. The IGIMF (Weidner & Kroupa 2005) is computed from the combination of the stellar intial mass function (IMF), i.e. the mass function of single star clusters, and the embedded cluster mass function, i.e. a power law with index beta. By taking into account also the fact that the maximum achievable stellar mass is a function of the total mass of the cluster, the IGIMF becomes a time-varying IMF which depends on the star formation rate. We applied this formalism to a chemical evolution model for the solar neighbourhood and compared the results obtained by assuming three possible values for beta with the results obtained by means of a standard, well-tested, constant IMF. In general, a lower absolute value of beta implies a flatter IGIMF, hence a larger number of massive stars and larger metal ejection rates. This translates into higher type Ia and II supernova rates, higher mass ejection rates from massive stars and a larger amount of gas available for star formation, coupled with lower present-day stellar mass densities. (abridged) We also discuss the importance of the present day stellar mass function (PDMF) in providing a way to disentangle among various assumptions for beta. Our results indicate that the model adopting the IGIMF computed with beta ~2 should be considered the best since it allows us to reproduce the observed PDMF and to account for most of the chemical evolution constraints considered in this work.Comment: 22 pages, 19 figure

Loss of star forming gas in SDSS galaxies

Using the star formation rates from the SDSS galaxy sample, extracted using the MOPED algorithm, and the empirical Kennicutt law relating star formation rate to gas density, we calculate the time evolution of the gas fraction as a function of the present stellar mass. We show how the gas-to-stars ratio varies with stellar mass, finding good agreement with previous results for smaller samples at the present epoch. For the first time we show clear evidence for progressive gas loss with cosmic epoch, especially in low-mass systems. We find that galaxies with small stellar masses have lost almost all of their cold baryons over time, whereas the most massive galaxies have lost little. Our results also show that the most massive galaxies have evolved faster and turned most of their gas into stars at an early time, thus strongly supporting a downsizing scenario for galaxy evolution.Comment: 29 pages, 9 figures, ApJ, accepte