Physics of white dwarfs

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 to the space. This, together with the relative simplicity of their structure, make them an important laboratory for testing Physics under extreme conditions 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 gravitation constant, G, the properties of axions and light bosons or the properties of Coulomb plasmas just to cite few applications.
Isolated magnetic white dwarfs have field strengths ranging from k-gauss to G-gauss. However, the origin of the magnetic field has not been hitherto elucidated. Whether these fields are fossil, hence the remnants of original weak magnetic fields amplified during the course of the evolution of their progenitor stars, or are the result of binary interactions or, finally, they are produced by other internal physical mechanisms during the cooling of the white dwarf itself, remains a mystery. At sufficiently low temperatures white dwarfs crystallize. Upon solidification, phase separation of its main constituents, 12C, 16O, and of the impurities left by previous evolution occurs. This process leads to the formation of a Rayleigh-Taylor unstable liquid mantle on top of a solid core. This convective region, as it occurs in Solar System planets like the Earth, can produce a dynamo able to yield magnetic fields of strengths of up to 0.1~MG, thus providing a mechanism that could explain the existence of magnetism in single white dwarfs.


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 thermalized 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.  The ICE 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 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 synthesized. Furthermore, the detection of gamma rays around the time of the maximum of the optical light curve strongly suggest 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.

Selected publications
- Isern, J., et al (3 more author) 2017. A common origin of magnetism from planets to white dwarfs. ApJ Letters 836, L28.
- Isern, J., et al (21 more authors) 2016. Gamma-ray emission from SN2014J near the maximum optical light. A&A 588, 671.
- Miller Bertolami et al. (3 more authors, JI last author) 2014. Revisiting the axion bounds from the Galactic white dwarf luminosity function. JCAP 10, 069.
- Churajov, E., Sunyaev, R., Isern, J., et al (8 authors more) 2014. Cobalt-56 gamma-ray emission lines from the type Ia supernovae 2014J. Nature 512, 406 .
- Dreiner, H.K., Fortin, J.F, Isern, J., Ubaldi, L. 2013, White dwarfs constrain dark forces. PRD 88d3517D

Senior Institute members involved
J. Isern, M. Hernanz, A. Serenelli
Institute of Space Sciences (IEEC-CSIC)

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An institute of the Consejo Superior de Investigaciones Científicas

An institute of the Consejo Superior de Investigaciones Científicas
Affiliated with the Institut d'Estudis Espacials de Catalunya

Affiliated with the Institut d'Estudis Espacials de Catalunya