E. Spitoni

The Effect of Different Type Ia Supernova Progenitors on Galactic Chemical Evolution

Aims. Our aim is to show how different hypotheses about type Ia supernova progenitors can affect Galactic chemical evolution. Supernovae Ia are believed to be the main producers of Fe and the timescale with which Fe is restored into the interstellar medium depends on the assumed supernova progenitor model. This is a way of selecting the most appropriate progenitor model for supernovae Ia, a still debated issue. Methods. We include different type Ia SN progenitor models, identified by their distribution of time delays, in a very detailed chemical evolution model for the Milky Way which follows the evolution of several chemical species. We test the single degenerate and the double degenerate models for supernova Ia progenitors, as well as other more empirical models based on differences in the time delay distributions. Results. We find that assuming the single degenerate or the double degenerate scenario produces negligible differences in the predicted [O/Fe] vs. [Fe/H] relation. On the other hand, assuming a percentage of prompt (exploding in the first 100 Myr) type Ia supernovae of 50%, or that the maximum type Ia rate is reached after 3–4 Gyr from the beginning of star formation, as suggested by several authors, produces more noticeable effects on the [O/Fe] trend. However, given the spread still existing in the observational data, no model can be firmly excluded on the basis of only the [O/Fe] ratios. On the other hand, when the predictions of the different models are compared with the G-dwarf metallicity distribution, the scenarios with very few prompt type Ia supernovae can be excluded. Conclusions. Models including the single degenerate or double degenerate scenario with a percentage of 10–13% of prompt type Ia supernovae produce results in very good agreement with the observations. A fraction of prompt type Ia supernovae larger than 30% worsens the agreement with observations and the same occurs if no prompt type Ia supernovae are allowed. In particular, two empirical models for the type Ia SN progenitors can be excluded: the one without prompt type Ia supernovae and the one assuming a delay time distribution that is ∝t −0.5 . We conclude that the typical timescale for the Fe enrichment in the Milky Way is around 1–1.5 Gyr and that type Ia supernovae already should appear during the halo phase.

The Origin of the Mass-Metallicity relation: an analytical approach

Context. The mass-metallicity (MZ) relation in star-forming galaxies at all redshifts has been recently established. It is therefore important to understand the physical mechanisms underlying such a relation. Aims. We aim at studying some possible physical mechanisms contributing to the MZ relation by adopting analytical solutions of chemical evolution models, including infall and outflow. Methods. Analytical models assume the instantaneous recycling approximation, which is still an acceptable assumption for elements produced on short timescales such as oxygen, which is the measured abundance in the MZ relation.We explore the hypotheses of a variable galactic wind rate, infall rate, and yield per stellar generation (i.e. a variation in the IMF), as possible causes for the MZ relation. Results. By means of analytical models, we computed the expected O abundance for galaxies of a given total baryonic mass and gas mass.The stellar mass was derived observationally and the gas mass was derived by inverting the Kennicutt law of star formation, once its rate is known. Then we tested how the parameters describing the outflow, infall, and IMF should vary to reproduce the MZ relation, and we exclude the cases where such a variation leads to unrealistic situations. Conclusions. We find that a galactic wind rate increasing with decreasing galactic mass or a variable IMF are both viable solutions for the MZ relation. A variable infall rate instead is not acceptable. It is difficult to differentiate among the outflow and IMF solutions by only considering the MZ relation, and other observational constraints should be taken into account in selecting a specific solution. For example, a variable efficiency of star formation increasing with galactic mass can also reproduce the MZ relation and explain the downsizing in star formation suggested for ellipticals. The best solution could be a variable efficiency of star formation coupled with galactic winds, which are indeed observed in low-mass galaxies.

Galactic fountains and their connection with high and intermediate velocity clouds

Context. Sequential supernova explosions create supershells which can break out a stratified medium, producing bipolar outflows. The gas of the supershells can fragment into clouds which eventually fall toward the disk producing so-called galactic fountains. Aims. The aim of this paper is to calculate the expansion law and chemical enrichment of a supershell powered by the energetic feedback of a typical Galactic OB association at various galactocentric radii. We study the orbits of the fragments created when the supershell breaks out and we compare their kinetic and chemical properties with the available observations of high – and intermediate – velocity clouds. Methods. We use the Kompaneets (1960, Soviet Phys. Dokl., 5, 46) approximation for the evolution of the superbubble driven by sequential supernova explosions and we compute the abundances of oxygen and iron residing in the thin cold supershell. We assume that supershells are fragmented by Rayleigh-Taylor instabilities and we follow the orbit of the clouds either ballistically or by means of a hybrid model considering viscous interaction between the clouds and the extra-planar gas. Results. Given the self-similarity of the solutions, clouds are always formed ∼448 pc above the plane. If the initial metallicity is solar, the pollution from dying stars of the OB association has a negligible effect on the chemical composition of the clouds. The maximum height reached by the clouds above the plane seldom exceeds 2 kpc and when averaging over different throwing angles, the landing coordinate differs from the throwing coordinate by ∼1 kpc at most. Conclusions. The range of heights and [O/Fe] ratios spanned by our clouds suggest that the high velocity clouds cannot have a Galactic origin, whereas intermediate velocity clouds have kinematic properties similar to our modeled clouds but have overabundances observed for the [O/Fe] ratios that can be reproduced only with initial metallicities that are too low compared to those of the Galaxy disk.

Abundance gradients in spiral discs: is the gradient inversion at high redshift real?

We compute the abundance gradients along the disk of the Milky Way by means of the two-infall model: in particular, the gradients of oxygen and iron and their temporal evolution. First, we explore the effects of several physical processes which influence the formation and evolution of abundance gradients. They are: i) the inside-out formation of the disk, ii) a threshold in the gas density for star formation, iii) a variable star formation efficiency along the disk, iv) radial flows and their speed, and v) different total surface mass density (gas plus stars) distributions for the halo. We are able to reproduce at best the present day gradients of oxygen and iron if we assume an inside-out formation, no threshold gas density, a constant efficiency of star formation along the disk and radial gas flows. It is particularly important the choice of the velocity pattern for radial flows and the combination of this velocity pattern with the surface mass density distribution in the halo. Having selected the best model, we then explore the evolution of abundance gradients in time and find that the gradients in general steepen in time and that at redshift z~3 there is a gradient inversion in the inner regions of the disk, in the sense that at early epochs the oxygen abundance decreases toward the Galactic center. This effect, which has been observed, is naturally produced by our models if an inside-out formation of the disk and and a constant star formation efficiency are assumed. The inversion is due to the fact that in the inside-out formation a strong infall of primordial gas, contrasting chemical enrichment, is present in the innermost disk regions at early times. The gradient inversion remains also in the presence of radial flows, either with constant or variable speed in time, and this is a new result.

The Effect of Stellar Migration on Galactic Chemical Evolution: A Heuristic Approach

In the last years, stellar migration in galactic discs has been the subject of several investigations. However, its impact on the chemical evolution of the Milky Way still needs to be fully quantified. In this paper, we aim at imposing some constraints on the significance of this phenomenon by considering its influence on the chemical evolution of the Milky Way thin disc. We do not investigate the physical mechanisms underlying the migration of stars. Rather, we introduce a simple, heuristic treatment of stellar migration in a detailed chemical evolution model for the thin disc of the Milky Way, which already includes radial gas flows and reproduces several observational constraints for the solar vicinity and the whole Galactic disc. When stellar migration is implemented according to the results of chemo-dynamical simulations by Minchev et. al. (2013) and finite stellar velocities of 1 km s

The galactic habitable zone of the Milky Way and M31 from chemical evolution models with gas radial flows

^{-1}

Galactic and cosmic Type Ia supernova (SNIa) rates: is it possible to impose constraints on SNIa progenitors?

are taken into account, the high-metallicity tail of the metallicity distribution function of long-lived thin-disc stars is well reproduced. By exploring the velocity space, we find that the migrating stars must travel with velocities in the range 0.5 -2 km s

New chemical evolution analytical solutions including environment effects

^{-1}

The AMBRE project: chemical evolution models for the Milky Way thick and thin discs

to properly reproduce the high-metallicity tail of the metallicity distribution. We confirm previous findings by other authors that the observed spread in the age-metallicity relation of solar neighbourhood stars can be explained by the presence of stars which originated at different Galactocentric distances, and we conclude that the chemical properties of stars currently observed in the solar vicinity do suggest that stellar migration is present to some extent.

Are ancient dwarf satellites the building blocks of the Galactic halo

The galactic habitable zone is defined as the region with sufficient abundance of heavy elements to form planetary systems in which Earth-like planets could be born and might be capable of sustaining life, after surviving to close supernova explosion events. Galactic chemical evolution models can be useful for studying the galactic habitable zones in different systems. We apply detailed chemical evolution models including radial gas flows to study the galactic habitable zones in our Galaxy and M31. We compare the results to the relative galactic habitable zones found with “classical” (independent ring) models, where no gas inflows were included. For both the Milky Way and Andromeda, the main effect of the gas radial inflows is to enhance the number of stars hosting a habitable planet with respect to the “classical” model results, in the region of maximum probability for this occurrence, relative to the classical model results. These results are obtained by taking into account the supernova destruction processes. In particular, we find that in the Milky Way the maximum number of stars hosting habitable planets is at 8 kpc from the Galactic center, and the model with radial flows predicts a number which is 38% larger than what predicted by the classical model. For Andromeda we find that the maximum number of stars with habitable planets is at 16 kpc from the center and that in the case of radial flows this number is larger by 10 % relative to the stars predicted by the classical model.