In this paper, we explore the possibility that isolated carbon–oxygen-WDs (white dwarfs) with mass smaller than the Chandrasekhar limit may undergo nuclear runaway and supernova (SNa) explosion, triggered by the energy produced by underbarrier pycnonuclear reactions between carbon and light elements. Such reactions would be due to left over impurities of the light elements, which would remain inactive until the WDs transit from the liquid to the solid state. We devise a simple formulation for the Coulombian potential and the local density in an ionic lattice affected by impurities and introduce it in the known rates of pycnonuclear reactions for multicomponent plasmas. Our semi-analytical results indicate that the energy generated by these pycnonuclear reactions exceeds the WD luminosity and provides enough energy to elementary cells of matter to balance the energy cost for C-ignition at much younger ages than the age of the Universe, even for WDs with masses as low as ≃0.85 M_⊙. A thermonuclear runaway may thus be triggered in isolated WDs. The explosion would occur from few hundred thousand to a few million years after the WD formation in the mass interval 0.85–1.2 M_⊙.

We have employed state-of-the-art evolutionary models of low- and intermediate-mass asymptotic giant branch (AGB) stars and included the effect of circumstellar dust shells on the spectral energy distribution (SED) of AGB stars in order to revise the Padua library of isochrones, which covers an extended range of ages and initial chemical compositions. The major revision involves the thermally pulsing AGB phase, which is now taken from fully evolutionary calculations by Weiss & Ferguson. Two libraries of about 600 AGB dust-enshrouded SEDs each have also been calculated, one for oxygen-rich M stars and one for carbon-rich C stars. Each library accounts for different values of input parameters like the optical depth τ, dust composition and temperature of the inner boundary of the dust shell. These libraries of dusty AGB spectra have been implemented into a large composite library of theoretical stellar spectra, to cover all regions of the Hertzsprung–Russell diagram (HRD) crossed by the isochrones. With the aid of the above isochrones and libraries of stellar SEDs, we have calculated the spectrophotometric properties (SEDs, magnitudes and colours) of single-generation stellar populations (SSPs) for six metallicities, more than 50 ages (from ∼3 Myr to 15 Gyr) and nine choices of the initial mass function. The new isochrones and SSPs have been compared with the colour–magnitude diagrams (CMDs) of field populations in the Large and Small Magellanic Clouds, with particular emphasis on AGB stars, and the integrated colours of star clusters in the same galaxies, using data from the catalogue ‘Surveying the Agents of Galaxy Evolution’ (SAGE). We have also examined the integrated colours of a small sample of star clusters located on the outskirts of M31. The agreement between theory and observations is generally good. In particular, the new SSPs reproduce the red tails of the AGB star distribution in the CMDs of field stars in the Magellanic Clouds. Some discrepancies still exist and need to be investigated further.

Context. This is the second work dedicated to the observed parallelism between galaxy clusters (GCs) and early-type galaxies (ETGs). The focus is on the distribution of these systems in the scaling relations (SRs) observed when effective radii, effective surface brightness, total luminosities, and velocity dispersions are mutually correlated. Aims. Using the data of the Illustris simulation we speculate on the origin of the observed SRs. Methods. We compare the observational SRs extracted from the database of the WIde-field Nearby Galaxy-cluster Survey with the relevant parameters coming from the Illustris simulations. Then we use the simulated data at different redshift to infer the evolution of the SRs. Results. The comparison demonstrate that GCs at z ∼ 0 follow the same log(L)−​log(σ) relation of ETGs and that both in the log(⟨I⟩_e)−​log(R_e) and log(R_e)−​log(M^*) planes the distribution of GCs is along the sequence defined by the brightest and massive early-type galaxies (BCGs). The Illustris simulation reproduces the tails of the massive galaxies visible both in the log(⟨I⟩_e)−​log(R_e) and log(R_e)−​log(M^*) planes, but fails to give the correct estimate of the effective radii of the dwarf galaxies that appear too large and those of GCs that are too small. The evolution of the SRs up to z = 4 permits to reveal the complex evolutionary paths of galaxies in the SRs and indicate that the line marking the zone of exclusion, visible in the log(⟨I⟩_e)−​log(R_e) and the log(R_e)−​log(M^*) planes, is the trend followed by virialized and passively evolving systems. Conclusions. We speculate that the observed SRs originate from the intersection of the virial theorem and a relation L = L₀^′σ^β where the luminosities depend on the star formation history.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

We investigate the evolution of the cosmic star formation rate density (SFRD) from redshiftz = 20 toz = 0 and compare it with the observational one by Madau and Dickinson derived from recent compilations of ultraviolet (UV) and infrared (IR) data. The theoretical SFRD(z) and its evolution are obtained using a simple model that folds together the star formation histories of prototype galaxies that are designed to represent real objects of different morphological type along the Hubble sequence and the hierarchical growing of structures under the action of gravity from small perturbations to large-scale objects in Λ-CDM cosmogony, i.e., the number density of dark matter halos . Although the overall model is very simple and easy to set up, it provides results that mimic results obtained from highly complex large-scaleN-body simulations well. The simplicity of our approach allows us to test different assumptions for the star formation law in galaxies, the effects of energy feedback from stars to interstellar gas, the efficiency of galactic winds, and also the effect of . The result of our analysis is that in the framework of the hierarchical assembly of galaxies, the so-called time-delayed star formation under plain assumptions mainly for the energy feedback and galactic winds can reproduce the observational SFRD(z).

We present a new Galaxy population synthesis Model, GalMod. GalMod is a star-count model featuring an asymmetric bar/bulge as well as spiral arms and related extinction. The model, initially introduced in Pasetto et al., has been here completed with a central bar, a new bulge description, new disk vertical profiles, and several new bolometric corrections. The model can generate synthetic mock catalogs of visible portions of the Milky Way, external galaxies like M31, or N-body simulation initial conditions. At any given time, e.g., at a chosen age of the Galaxy, the model contains a sum of discrete stellar populations, namely the bulge/bar, disk, and halo. These populations are in turn the sum of different components: the disk is the sum of the spiral arms, thin disks, a thick disk, and various gas components, while the halo is the sum of a stellar component, a hot coronal gas, and a dark-matter component. The Galactic potential is computed from these population density profiles and used to generate detailed kinematics by considering up to the first four moments of the collisionless Boltzmann equation. The same density profiles are then used to define the observed color–magnitude diagrams in a user-defined field of view (FoV) from an arbitrary solar location. Several photometric systems have been included and made available online, and no limits on the size of the FoV are imposed thus allowing full-sky simulations, too. Finally, we model the extinction by adopting a dust model with advanced ray-tracing solutions. The model's Web page (and tutorial) can be accessed at www.GalMod.org and support is provided at Galaxy.Model@yahoo.com.