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

Chemical evolution of the Galactic Center

In recent years, the Galactic Center (GC) region (200 pc in radius) has been studied in detail with spectroscopic stellar data as well as an estimate of the ongoing star formation rate. The aims of this paper are to study the chemical evolution of the GC region by means of a detailed chemical evolution model and to compare the results with high resolution spectroscopic data in order to impose constraints on the GC formation history.The chemical evolution model assumes that the GC region formed by fast infall of gas and then follows the evolution of alpha-elements and Fe. We test different initial mass functions (IMFs), efficiencies of star formation and gas infall timescales. To reproduce the currently observed star formation rate, we assume a late episode of star formation triggered by gas infall/accretion. We find that, in order to reproduce the [alpha/Fe] ratios as well as the metallicity distribution function observed in GC stars, the GC region should have experienced a main early strong burst of star formation, with a star formation efficiency as high as 25 Gyr^{-1}, occurring on a timescale in the range 0.1-0.7 Gyr, in agreement with previous models of the entire bulge. Although the small amount of data prevents us from drawing firm conclusions, we suggest that the best IMF should contain more massive stars than expected in the solar vicinity, and the last episode of star formation, which lasted several hundred million years, should have been triggered by a modest episode of gas infall/accretion, with a star formation efficiency similar to that of the previous main star formation episode. This last episode of star formation produces negligible effects on the abundance patterns and can be due to accretion of gas induced by the bar. Our results exclude an important infall event as a trigger for the last starburst.Comment: 10 pages, 8 figures, accepted for publication in MNRA

Galactic and Cosmic Type Ia SN rates: is it possible to impose constraints on SNIa progenitors?

We compute the Type Ia supernova rates in typical elliptical galaxies by varying the progenitor models for Type Ia supernovae. To do that a formalism which takes into account the delay distribution function (DTD) of the explosion times and a given star formation history is adopted. Then the chemical evolution for ellipticals with baryonic initial masses $10^{10}$, $10^{11}$ and $10^{12} M_{\odot}$ is computed, and the mass of Fe produced by each galaxy is precisely estimated. We also compute the expected Fe mass ejected by ellipticals in typical galaxy clusters (e.g. Coma and Virgo), under different assumptions about Type Ia SN progenitors. As a last step, we compute the cosmic Type Ia SN rate in an unitary volume of the Universe by adopting several cosmic star formation rates and compare it with the available and recent observational data. Unfortunately, no firm conclusions can be derived only from the cosmic SNIa rate, neither on SNIa progenitors nor on the cosmic star formation rate. Finally, by analysing all our results together, and by taking into account previous chemical evolution results, we try to constrain the best Type Ia progenitor model. We conclude that the best progenitor models for Type Ia SNe are still the single degenerate model, the double degenerate wide model, and the empirical bimodal model. All these models require the existence of prompt Type Ia supernovae, exploding in the first 100 Myr since the beginning of star formation, although their fraction should not exceed 15-20% in order to fit chemical abundances in galaxies.Comment: 17 pages, 11 figures, Submitted to MNRA

Chemical evolution of the bulge of M31: predictions about abundance ratios

We aim at reproducing the chemical evolution of the bulge of M31 by means of a detailed chemical evolution model, including radial gas flows coming from the disk. We study the impact of the initial mass function, the star formation rate and the time scale for bulge formation on the metallicity distribution function of stars. We compute several models of chemical evolution using the metallicity distribution of dwarf stars as an observational constraint for the bulge of M31. Then, by means of the model which best reproduces the metallicity distribution function, we predict the [X/Fe] vs. [Fe/H] relations for several chemical elements (O, Mg, Si, Ca, C, N). Our best model for the bulge of M31 is obtained by means of a robust statistical method and assumes a Salpeter initial mass function, a Schmidt-Kennicutt law for star formation with an exponent k=1.5, an efficiency of star formation of $\sim 15\pm 0.27\, Gyr^{-1}$, and an infall timescale of $\sim 0.10\pm 0.03$Gyr. Our results suggest that the bulge of M31 formed very quickly by means of an intense star formation rate and an initial mass function flatter than in the solar vicinity but similar to that inferred for the Milky Way bulge. The [$\alpha$/Fe] ratios in the stars of the bulge of M31 should be high for most of the [Fe/H] range, as is observed in the Milky Way bulge. These predictions await future data to be proven.Comment: Accepted for publication by MNRA

Compartmentalized control of Cdk1 drives mitotic spindle assembly

During cell division, dramatic microtubular rearrangements driven by cyclin B-cdk1 (Cdk1) kinase activity mark the onset of mitosis leading to dismantling of the interphase microtubular cytoskeleton and assembly of the mitotic spindle. During interphase, Cdk1 accumulates in an inactive state, phosphorylated at inhibitory sites by Wee1/Myt1 kinases. At mitosis onset, Cdc25 phosphatase dephosphorylates and activates Cdk1. Once activated, Cdk1 clears cytoplasmic microtubules by inhibiting microtubule-stabilizing and growth-promoting microtubule-associated proteins (MAPs). Nevertheless, some of these MAPs are required for spindle microtubule growth and spindle assembly, creating quite a conundrum. We show here that a Cdk1 fraction bound to spindle structures escapes Cdc25 action and remains inhibited by phosphorylation (i-Cdk1) in mitotic human cells. Loss or restoration of i-Cdk1 inhibits or promotes spindle assembly, respectively. Furthermore, polymerizing spindle microtubules foster i-Cdk1 aggregating with Wee1 and excluding Cdc25. Our data reveal that spindle assembly relies on compartmentalized control of Cdk1 activity

Foraging and its consequences in the breeding season of the Blue Tit (Parus caeruleus)

This dissertation deals with the direct and indirect effects of food availability in different phases of breeding in a small insectivorous bird, the Blue Tit (Parus caeruleus). Previous studies have emphasised the dual nature of food influencing reproductive decisions in birds. On the one hand, food constitutes energy and nutritional resource for the individual. This thesis has focused on the effects of food as a resource in two highly demanding phases: (1) the period of egg laying and (2) the period of brood rearing. On the other hand, food in the laying phase could also function as a cue predicting the best time for rearing the brood. This hypothesis was tested by means of a series of additional feeding experiments in which extra food was offered to the parents throughout the nestling period. Female Blue Tits experiencing additional food during the nestling period laid relatively later the next year than unfed females, controlled for between-year changes in the environment. As a result, those females mis-timed reproduction and raised the brood far from the caterpillar peak the next year. This suggests that food levels experienced during breeding are involved in fine-tuning the timing of breeding the next year. The additional feeding experiments provided the opportunity to investigate the provisioning rules of parents that experience different degrees of food availability. The parents that had access to extra food delivered similar amount of food as control, unfed parents, but with a different combination of feeding frequency and size of prey. They fed the chicks less frequently, but with larger prey items. This suggests that the change in the state of the parents (which spent less time self-feeding because of the food addition) and of the nestlings (because the parents delivered some extra food to them) produced a significant change in the parents' provisioning rules. The availability of more time caused the food-supplemented parents to get access to larger prey, presumably through an increase in selectivity. This is because larger prey could be obtained only by making longer foraging excursions

Metallicity effects on cosmic Type Ib/c supernovae and gamma-ray burst rates

Type Ib/c supernovae (SNe Ib/c) are likely to be associated with long gamma-ray bursts (GRBs), and therefore it is important to compare the SN rate in galaxies with the GRB rate. To do this we computed SN Ib/c rates in galaxies of different morphological types (ellipticals, spirals and irregulars) by assuming different histories of star formation and different SN Ib/c progenitors. We included some recent suggestions about the dependence of the minimum mass of single Wolf-Rayet (WR) stars on the stellar metallicity and therefore on galactic chemical evolution. We adopted several cosmic star formation rates (i.e. relative to a comoving unitary volume of the Universe) as a function of cosmic time, either observationally or theoretically derived, including the one computed with our galaxy models. We then computed the cosmic SN Ib/c rates. Our results show that the predicted SN Ib/c rates in spirals and irregulars can reproduce well the present observed rates if both single WR stars and massive binary systems are taken into account as SN Ib/c progenitors. The metallicity effects on the minimum mass for single WR stars are evident mainly in the early phases of galaxy evolution and do not influence substantially the predicted local Type Ib/c rates. We reached the following conclusions. (i) The ratio cosmic GRB rate /cosmic Type Ib/c rate varies in the range 10−2-10−4 in the whole redshift range, thus suggesting that only a small fraction of all SNe Ib/c gives rise to GRBs. (ii) The metallicity dependence of SN Ib/c progenitors produces lower cosmic SN Ib/c rates at early times, for any chosen cosmic star formation rate. (iii) Different theoretical cosmic star formation rates, computed under different scenarios of galaxy formation, produce SN Ib/c cosmic rates that differ mainly at very high redshifts. However, it is difficult to draw firm conclusions on the high-redshift trend because of the large uncertainties in the data. (iv) GRBs can be important tracers of star formation at high redshifts if their luminosity function does not vary with redshift, and they can help in discriminating among galaxy formation model

Does degradation from selective logging and illegal activities differently impact forest resources? A case study in Ghana

Degradation, a reduction of the ecosystem’s capacity to supply goods and services, is widespread in tropical forests and mainly caused by human disturbance. To maintain the full range of forest ecosystem services and support the development of effective conservation policies, we must understand the overall impact of degradation on different forest resources. This research investigates the response to disturbance of forest structure using several indicators: soil carbon content, arboreal richness and biodiversity, functional composition (guild and wood density), and productivity. We drew upon large field and remote sensing datasets from different forest types in Ghana, characterized by varied protection status, to investigate impacts of selective logging, and of illegal land use and resources extraction, which are the main disturbance causes in West Africa. Results indicate that functional composition and the overall number of species are less affected by degradation, while forest structure, soil carbon content and species abundance are seriously impacted, with resources distribution reflecting the protection level of the areas. Remote sensing analysis showed an increase in productivity in the last three decades, with higher resiliency to change in drier forest types, and stronger productivity correlation with solar radiation in the short dry season. The study region is affected by growing anthropogenic pressure on natural resources and by an increased climate variability: possible interactions of disturbance with climate are also discussed, together with the urgency to reduce degradation in order to preserve the full range of ecosystem functions

Microbial Resources and Innovation in the Wine Production Sector

Microbial starter cultures represent a fundamental level of innovation in the wine sector. Selected yeast strains are routinely used to achieve the needed biomass preparation to accelerate and steer alcoholic fermentation in grape must. The use of starter cultures to induce malolactic fermentation in wine relies on the characterisation and propagation of suitable strains of lactic acid bacteria. Furthermore, the selection of new strains, the renewal of management of microbial resources and new technologies allow continuous improvements in oenology, which may increase the beneficial aspects of wine. In this review, with the aim to stimulate microbial-driven, consumer-oriented advances in the oenological sector, we propose an overview of recent trends in this field that are reported by following the classical separation into 'product innovation' and 'process innovation'. Hence, we shall highlight i) the possible positive innovative impacts of microbial resources on the safety and the sensorial and functional properties of wine (product innovation) and ii) the potential microbial-based improvements allowing the reduction of time/costs and the environmental impacts associated with winemaking (process innovation)