Physics of white dwarfs
and related phenomena

Unravelling the mystery of white dwarfs

This group is focused on the study of white dwarfs for testing physics under extreme conditions and to obtain information about the evolution of the galaxy.

Physics of the explosion

White dwarfs are the final remnants of low and intermediate mass stars. Their evolution is essentially a cooling process that lasts for ∼10 Gyr. Therefore, it is important to identify all the relevant sources of energy as well as the mechanisms that control its flow into space. This, together with the relative simplicity of their structure, make them an important laboratory for testing Physics under extreme conditions that are impossible to obtain in terrestrial laboratories. Furthermore, since their luminosity is a function of time, they retain information about the original properties of their parent population and they can be used as a ”forensic” tool to obtain information about the evolution of the Galaxy.

The tool that allows the comparison between theoretical models and observations is their luminosity function, that is, the number of stars per unit volume and luminosity interval. The shape of the bright branch of this function is only sensitive to the average cooling rate and, thus, it is possible to use it to check for the possible existence of additional non standard sources or sinks of energy able to modify the expected ’normal evolution’. For instance, their properties can be used to bound possible changes of the constants of Nature, like the gravitational constant, G, the properties of axions and light bosons or the properties of Coulomb plasmas just to cite few applications.

Focus

Type Ia supernovae (SNIa) are thought to be the outcome of the thermonuclear explosion of a carbon/oxygen white dwarf in a close binary system. Their optical light curve is powered by thermalised gamma-rays produced by the radioactive decay of 56Ni, the most abundant isotope present in the debris. Gamma-rays escaping the ejecta can be used as a diagnostic tool for studying the structure of the exploding star and the characteristics of the explosion.

ICE-CSIC has been proposing since the launch of the INTEGRAL Gamma Ray Observatory in 2002 the observation of SNIa at such energies. Finally, after the attempt of detecting SN2011fe that failed as a consequence of the distance, it has been possible to detect the gamma emission of SN2014J in M82, the brightest SNIa event since the epoch of the historical Tycho and Kepler supernovae. This first detection has allowed us to prove that effectively SNIa shine thanks to the decay of 56Ni and 56Co but to test the so-called Arnett’s rule that connects the bolometric magnitude at maximum with the total amount of 56Ni synthesised. Furthermore, the detection of gamma rays around the time of the maximum of the optical light curve strongly suggests the presence of plumes of 56Ni in the outermost layers moving at high velocities. If this interpretation is correct, it could have important consequences on our current understanding of the physics of the explosion and on the nature of the systems that explode.