Donatella Romano

Abundance Gradients and the Formation of the Milky Way

In this paper we adopt a chemical evolution model, which is an improved version of the Chiappini, Matteucci, & Gratton model, assuming two main accretion episodes for the formation of the Galaxy, the first forming the halo and bulge in a short timescale and the second one forming the thin disk, with a timescale that is an increasing function of the Galactocentric distance (being of the order of 7 Gyrs at the solar neighborhood). The present model takes into account in more detail than previously the halo density distribution and explores the effects of a threshold density in the star formation process during both the halo and disk phases. The model also includes the most recent nucleosynthesis prescriptions concerning supernovae of all types, novae, and single stars dying as white dwarfs. In the comparison between model predictions and available data, we have focused our attention on abundance gradients as well as gas, stellar, and star formation rate distributions along the disk, since this kind of model has already proven to be quite successful in reproducing the solar neighborhood characteristics. We suggest that the mechanism for the formation of the halo leaves detectable imprints on the chemical properties of the outer regions of the disk, whereas the evolution of the halo and the inner disk are almost completely disentangled. This is due to the fact that the halo and disk densities are comparable at large Galactocentric distances and therefore the gas lost from the halo can substantially contribute to building up the outer disk. We also show that the existence of a threshold density for the star formation rate, both in the halo and disk phase, is necessary to reproduce the majority of observational data in the solar vicinity and in the whole disk. In particular, a threshold in the star formation implies the occurrence of a gap in the star formation at the halo-disk transition phase, in agreement with recent data. We conclude that a relatively short halo formation timescale (0.8 Gyr), in agreement with recent estimates for the age differences among Galactic globular clusters, coupled with an inside-out formation of the Galactic disk, where the innermost regions are assumed to have formed much faster than the outermost ones, represents, at the moment, the most likely explanation for the formation of the Milky Way. This scenario allows us to predict abundance gradients and other radial properties of the Galactic disk in very good agreement with observations. Moreover, as a consequence of the adopted inside-out scenario for the disk, we predict that the abundance gradients along the Galactic disk must have increased with time and that the average ratios in stars (halo plus disk) slightly decrease going from 4 to 10 kpcs from the Galactic center. We also show that the same ratios increase substantially toward the outermost disk regions and the expected scatter in the stellar ages decreases, because the outermost regions are dominated by halo stars. More observations at large Galactocentric distances are needed to test these predictions.

Oxygen, carbon and nitrogen evolution in galaxies

We discuss the evolution of oxygen, carbon and nitrogen in galaxies of different morphological type by adopting detailed chemical evolution models with different star formation histories (continuous star formation or starbursts). In all the models detailed nucleosynthesis prescriptions from supernovae of all types and low- and intermediate-mass stars are taken into account. We start by computing chemical evolution models for the Milky Way with different stellar nucleosynthesis prescriptions. Then, a comparison between model results and ‘key’ observational constraints allows us to choose the best set of stellar yields. Once the best set of yields is identified for the Milky Way, we apply the same nucleosynthesis prescriptions to other spirals (in particular M101) and dwarf irregular galaxies. We compare our model predictions with the [C,N,O/Fe] versus [Fe/H], log(C/O) versus 12 + log(O/H), log(N/O) versus 12+ log(O/H) and [C/O] versus [Fe/H] relations observed in the solar vicinity and draw the following conclusions. (i) There is no need to invoke strong stellar winds in massive stars in order to explain the evolution of the C/O ratio, as often claimed in the literature. (ii) The predicted [O/Fe] ratio as a function of metallicity is in very good agreement with the most recent data available for the solar vicinity, especially for halo stars. This fact again suggests that the oxygen stellar yields in massive stars computed by either Woosley & Weaver or Thielemann, Nomoto & Hashimoto without taking into account mass loss, reproduce the observations well. (iii) We predict that the gap observed in the [Fe/O] versus [O/H] at [O/H] ∼− 0.3 dex should be observed also in C/O versus O/H. The existence of such a gap is predicted by our model for the Milky Way and is caused by a halt in the star formation between the end of the thick disc and the beginning of the thin disc phase. Such a halt is produced by the adopted threshold gas density for the star formation rate. (iv) This threshold is also responsible for the prediction of a very slow chemical enrichment between the time of formation of the solar system (4.5 Gyr ago) and the present time, in agreement with new abundance measurements. (v) The chemical evolution models for dwarf irregulars and spirals, adopting the same nucleosynthesis prescriptions of the best model for the solar neighbourhood, well reproduce the available constraints for these objects. (vi) By taking into account the results obtained for all the studied galaxies (Milky Way, M101, dwarf galaxies and DLAs) we conclude that there is no need for claiming a strong primary component of N produced in massive stars (M > 10 M� ). (vii) Moreover, there is a strong indication that C and N are mainly produced in low- and intermediate-mass stars, at variance with recent suggestions that most of the C should come from massive stars. In particular, intermediate-mass stars with masses between 4 and 8 M� contribute mostly to N (both primary and secondary) whereas those with masses between 1 and

Chemical evolution in a model for the joint formation of quasars and spheroids

Direct and indirect pieces of observational evidence point to a strong connection between high-redshift quasars and their host galaxies. In the framework of a model where the shining of the quasar is the episode that stops the formation of the galactic spheroid inside a virialized halo, it has been proven possible to explain the submillimetre source counts together with their related statistics and the local luminosity function of spheroidal galaxies. The time delay between the virialization and the quasar manifestation required to fit the counts is short and incresing with decresing the host galaxy mass. In this paper we compute the detailed chemical evolution of gas and stars inside virialized haloes in the framework of the same model, taking into account the combined effects of cooling and stellar feedback. Under the assumption of negligible angular momentum, we are able to reproduce the main observed chemical properties of local ellipticals. In particular, by using the same duration of the bursts which are required in order to fit the submillimetre source counts, we recover the observed increase of the Mg/Fe ratio with galactic mass. Since for the most massive objects the assumed duration of the burst is Tburst < 0.6 Gyr, we end up with a picture for elliptical galaxy formation in which massive spheroids complete their assembly at early times, thus resembling a monolithic collapse, whereas smaller galaxies are allowed for a more prolonged star formation, thus allowing for a more complicated evolutionary history. In the framework of the adopted scenario, only quasar activity can provide energies large enough to stop the star formation very soon after virialization in the most massive galactic haloes. The chemical abundance of the gas that we estimate at the end of the burst matches well the metallicity inferred from the quasar spectra. Therefore, the assumption that quasar activity interrupts the main episode of star formation in elliptical galaxies turns out to be quite reasonable. In this scenario, we also point out that non-dusty extremely red objects are the best targets for searching for high-redshift Type Ia supernovae.

Nova nucleosynthesis and Galactic evolution of the CNO isotopes

In this paper we study the role played both by novae and single stars in enriching the interstellar medium of the Galaxy with CNO group nuclei, in the framework of a detailed successful model for the chemical evolution of both the Galactic halo and disc. First, we consider only the nucleosynthesis from single low-mass, intermediate-mass and massive stars. In particular, the nucleosynthesis prescriptions in the framework of the adopted model are such that (i) lowand intermediate-mass stars are responsible for the production of most of the Galactic 12 C and 14 N; (ii) massive stars produce the bulk of the Galactic 16 O; (iii) 13 C and 17 O originate mostly in intermediate-mass stars, with only a minor contribution from low-mass and massive stars. In this context, we show that the behaviour of the 12 C/ 13 C, 14 N/ 15 N and 16 O/ 17 O isotopic ratios, as inferred from observations, can be explained only allowing for a substantial revision of the available stellar yields. On the other hand, the introduction of nova nucleosynthesis allows us to better explain the temporal evolution of the CNO isotopic ratios in the solar neighbourhood as well as their trends across the Galactic disc. Once all the nucleosynthesis sources of CNO elements are taken into account, we conclude that 13 C, 15 N and 17 O are likely to have both a primary and a secondary origin, in contrast to previous beliefs. We show that, when adopting the most recent 17 O yields from intermediate-mass stars published in the literature so far, we still get a too large solar abundance for this element, a problem already encountered in the past by other authors using different yield sets. Therefore, we conclude that in computing the 17 O yields from intermediate-mass stars some considerable sink of 17 O is probably neglected. The situation for 15 N is less clear than that for 13 C and 17 O, mainly due to contradictory observational findings. However, a stellar factory restoring 15 N on quite long time-scales seems to be needed in order to reproduce the observed positive gradient of 14 N/ 15 N across the disc, and novae are, at present, the best candidates for this factory. Given the uncertainties still present in the computation of theoretical stellar yields, our results can be used to put constraints on stellar evolution and nucleosynthesis models.

The Mass Surface Density in the Local Disk and the Chemical Evolution of the Galaxy

We have studied the effect of adopting different values of the total baryonic mass surface density in the local disk at the present time, Σ(R☉, tGal), on the model of the chemical evolution of the Galaxy. We have compared our model results with the G dwarf metallicity distribution, the amounts of gas, stars, and stellar remnants, the infall rate, and the supernova rate in the solar vicinity, and with the radial abundance gradients and gas distribution in the disk. This comparison strongly suggests that the value of Σ(R☉, tGal) that best fits the observational properties should lie in the range 50-75 M☉ pc-2 and that values of the total disk mass surface density outside this range should be ruled out.

The stellar origin of

We adopt up-to-date 7Li yields from asymptotic giant branch stars in order to study the temporal evolution of 7Li in the solar neighbourhood in the context of a revised version of the two-infall model for the chemical evolution of our galaxy. We consider several lithium stellar sources besides the asymptotic giant branch stars such as Type II supernovae, novae, low-mass giants as well as Galactic cosmic rays and low-mass X-ray binaries. We conclude that asymptotic giant branch stars cannot be considered as important 7Li producers as believed in so far and that the contribution of low-mass giants and novae is necessary to reproduce the steep rise of the 7Li abundance in disk stars as well as the meteoritic 7Li abundance. Lithium production in low-mass X-ray binaries hardly affects the temporal evolution of 7Li in the solar neighbourhood.As programmers we have worked with many Application Development Interface API development kits. They are well suited for interaction with a particular system. A vast source of information can be made accessible by using the http protocol through the web as an API. This setup has many advantages including the vast knowledge available on setting web servers and services. Also, these tools are available on most hardware and operating system combinations. In this paper I will cover the various types of systems that can be developed this way, their advantages and some drawbacks of this approach. Index Terms--Application Programmer Interface, Distributed applications, Hyper Text Transfer Protocol, Web.

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We adopt up-to-date yields of7Li,13C,15N from classical novae and use a well tested model for the chemical evolution of the Milky Way in order to predict the temporal evolution of these elemental species in the solar neighborhood. In spite of major uncertainties due to our lack of knowledge of metallicity effects on the final products of explosive nucleosynthesis in nova outbursts, we find a satisfactory agreement between theoretical predictions and observations for7Li and13C. On the contrary,15N turns out to be overproduced by about an order of magnitude

Li - Do AGB stars contribute a substantial fraction of the local Galactic lithium abundance?

We discuss the role of novae as producers of 7Li and CNO isotopes in the Milky Way. A detailed model for the chemical evolution of the Milky Way including novae, Type Ia supernovae, Type II supernovae as well as single low‐ and intermediate‐mass stars is adopted and the results are compared with the available observational constraints. It is shown that novae are among the most promising candidates in order to explain the steep rise off the Li plateau in the log e(7Li)‐[Fe/H] diagram observed for stars in the solar vicinity. We also find that novae are likely to be the main producers of 15N, whereas they should only partly contribute to 13C and 17O.

The Influence of Nova Nucleosynthesis on the Chemical Evolution of the Galaxy

In this paper we review the chemical evolution models for the Galactic bulge: in particular, we discuss the predictions of models as compared with the available abundance data and infer the mechanism as well as the time scale for the formation of the Galactic bulge. We show that good chemical evolution models reproducing the observed metallicity distribution of stars in the bulge predict that the [α/Fe] > 0 over most of the metallicity range. This is a very important constraint indicating that the bulge of our Galaxy formed at the same time and even faster than the inner Galactic halo. We also discuss predictions for the evolution of light elements such as D and 7Li and conclude that the D astration should be maximum due to the high star formation rate required for the bulge whereas the evolution of the abundance of Li should be similar to that observed in the solar neighbourhood, but with an higher Li abundance in the interstellar medium at the present time.