The steady vision of a starry night is challenged by the unexpected appearance of cosmic beacons, stellar explosions that shed light and matter to space. Among the rich zoo of stellar explosions, classical novae have captivated the interest of astronomers for decades. They are produced in stellar binary systems consisting of two stars, each having the mass of our Sun but separated only by a distance of the order of the Earth-Moon orbit. In such conditions, mass transfer episodes ensue, with streams of matter flowing towards the most compact component of the system: a white dwarf star, an object of planetary size. The piling up of matter on top of the white dwarf turns out to be unstable and a thermonuclear-driven explosion takes place. In the event, 10-5 - 10-4 solar mass of nuclear processed material
are expelled. Astronomers have spectroscopically analyzed the composition of these ejected nova shells, unveiling an inhomogeneous distribution of chemical species whose origin has remained a puzzle for nearly half a century.

Now, an international team of researchers, from UPC-Barcelona (J. Casanova, J. Jose, E. Garcia-Berro), U Pisa (S.N. Shore) and Stony Brook U (A.C. Calder) have for the first time performed 3-D simulations of the mixing processes taking place at the core-envelope interface during classical novae. The simulations, published in Nature in october 2011, show that hydrodynamic (Kelvin-Helmholtz) instabilities are the likely origin of the inhomogeneous distribution of chemical species in the ejecta.

Snapshots of the development of Kelvin–Helmholtz instabilities at t = 151 s (a), 193 s (b), 296 s (c) and 379 s (d), shown in terms of the 12C mass fraction on a logarithmic scale

Application of hydrostatic pressure under adiabatic conditions causes a change in temperature in any substance. This effect is known as the barocaloric effect and the vast majority of materials heat up when adiabatically squeezed, and they cool down when pressure is released (conventional barocaloric effect). There are, however, materials exhibiting an inverse barocaloric effect: they cool when pressure is applied, and they warm when it is released. Materials exhibiting the inverse barocaloric effect are rather uncommon. Here we report an inverse barocaloric effect in the intermetallic compound La-Fe-Co-Si, which is one of the most promising candidates for magnetic refrigeration through its giant magnetocaloric effect. We have found that application of a pressure of only 1 kbar causes a temperature change of about 1.5 K. This value is larger than the magnetocaloric effect in this compound for magnetic fields that are available with permanent magnets.