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We present new nonlinear, time-dependent convective hydrodynamical models of RR Lyrae stars computed assuming a constant helium-to-metal enrichment ratio and a broad range in metal abundances (Z = 0.0001-0.02). The stellar masses and luminosities adopted to construct the pulsation models were fixed according to detailed central He burning Horizontal Branch evolutionary models. The pulsation models cover a broad range in stellar luminosity and effective temperatures and the modal stability is investigated for both fundamental and first overtones. We predict the topology of the instability strip as a function of the metal content and new analytical relations for the edges of the instability strip in the observational plane. Moreover, a new analytical relation to constrain the pulsation mass of double pulsators as a function of the period ratio and the metal content is provided. We derive new Period-Radius-Metallicity relations for fundamental and first-overtone pulsators. They agree quite well with similar empirical and theoretical relations in the literature. From the predicted bolometric light curves, transformed into optical (U BVRI) and near-infrared (JHK) bands, we compute the intensity-averaged mean magnitudes along the entire pulsation cycle and, in turn, new and homogenous metaldependent (RI JHK) Period-Luminosity relations. Moreover, we compute new dual and triple band optical, optical-NIR and NIR Period-Wesenheit-Metallicity relations. Interestingly, we find that the optical Period-W(V, B − V) is independent of the metal content and that the accuracy of individual distances is a balance between the adopted diagnostics and the precision of photometric and spectroscopic datasets.

Supernovae and their remnants are a central problem in astrophysics due to their role in the stellar evolution and nuclear synthesis. A supernova’s explosion is driven by a blast wave causing the development of Rayleigh–Taylor and Richtmyer–Meshkov instabilities and leading to intensive interfacial mixing of materials of a progenitor star. Rayleigh–Taylor and Richtmyer–Meshkov mixing breaks spherical symmetry of a star and provides conditions for synthesis of heavy mass elements in addition to light mass elements synthesized in the star before its explosion. By focusing on hydrodynamic aspects of the problem, we apply group theory analysis to identify the properties of Rayleigh–Taylor and Richtmyer–Meshkov dynamics with variable acceleration, discover subdiffusive character of the blast wave-induced interfacial mixing, and reveal the mechanism of energy accumulation and transport at small scales in supernovae.

The pulsation properties and modal stability of Cepheid models are investigated in both linear and nonlinear regimes. The linear survey is based on nonadiabatic, radiative models, whereas the nonlinear one relies on full-amplitude models that include a nonlocal and time-dependent treatment of stellar convection. To account for Cepheid pulsation characteristics over a substantial portion of the region in which they are expected to be pulsationally unstable, a wide range of stellar masses (5 ¹ M/M _ ¹ 11) and e †ective temperatures K) was adopted. The luminosity of each model was Ðxed (4000 ¹ T e ¹ 7000 according to the mass-luminosity (ML) relations predicted by evolutionary models that either neglect or take into account a mild convective core overshooting. Moreover, in order to estimate the e †ects of the helium and metal content on the limiting amplitude behavior of both Magellanic Clouds and Galactic Cepheids we adopted three di †erent chemical compositions, namely, Y \ 0.25, Z \ 0.004 ; Y \ 0.25, Z \ 0.008 ; and Y \ 0.28, Z \ 0.02. For each set of input parameters we investigated the modal stability of both fundamental and Ðrst-overtone modes.The results of recent linear investigations are conÐrmed by our Ðnding that linear observables such as periods and blue edges of the instability strip are only marginally a †ected by the chemical composition and that either an increase in metallicity or an increase in both the helium and metal content causes a mild shift of these edges toward lower e †ective temperatures.The approach to the nonlinear limit cycle stability, the physical structure, and the mechanisms that govern the pulsation instability are described in detail. The main results of this analysis are as follows :(1) At Ðxed chemical composition the width of the instability strip changes going from low-to high-mass Cepheids. (2) At Ðxed mass and luminosity an increase in metallicity shifts the instability strip toward lower e †ective temperatures. A thorough analysis of the total nonlinear work inside the instability strip points out that this e †ect is due to a decrease in the pulsation destabilization caused by the H ionization region. Therefore, the current theoretical scenario suggests that, at Ðxed period, metal-poor pulsators are brighter than metal-rich pulsators. (3) The dynamical structure of full-amplitude, Ðrst-overtone models supports the evidence that their nonlinear limit cycle behavior has been properly identiÐed.The variations over a full pulsation cycle of the convective structure of fundamental and Ðrst-overtone pulsators located close to the blue and the red edges of the instability strip are discussed by taking into account the changes of the convective quantities across the convectively unstable region. As expected, we Ðnd that the main e †ect of convection on the limit cycle behavior is either to reduce the local radiative driving of the destabilizing regions, thus reducing the Ðnal amplitudes, or to damp the oscillations toward lower e †ective temperatures. We also Ðnd that th...

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