White dwarfs are the last stages of stellar evolution of stars with masses smaller than 10 Msun approximately. When isolated they cool-down to invisibility, but when they are in close binary systems they can be "rejuvenated" by mass accretion from their companion star, and eventually explode as novae or thermonuclear supernovae. A nova explosion is a hydrogen thermonuclear runaway on top of the white dwarf, made of either carbon and oxygen (CO) or oxygen and neon (ONe). Mass is ejected at high velocities, but the white dwarf is not disrupted and a new explosion is expected to occur after thousands (103-105) of years.
Novae contribute to the chemical evolution of the Galaxy, through the ejection of large amounts of some nuclei, like 7Li (which is the daughter nucleus of the radioactive 7Be), 13C, 17O. Lithium is of special interest, since its origin in the Galaxy is still far from being completely understood; novae contribution can help to understand both its amount and its time evolution in our Galaxy.
It is worth noticing that some of the nuclei synthesized and ejected by nova explosions are radioactive, thus emitting gamma-rays when they decay (with energies around 1 MeV); in particular, some of them emit positrons (antimatter) when they decay, which annihilate with electrons and also produce gamma-rays, e.g., at 511 keV. Therefore, using gamma-ray observatories is of the maximum interest, and that's why we are leading proposals to observe novae in the MeV range with the ESA INTEGRAL satellite (launched in 2002) and also working in the definition of future more sensitive instruments, like e-ASTROGAM. Relatively recently, in 2007, it was predicted by us for the first time that novae can be particle accelerators (for RS Oph in its 2006 explosion), and thus play a role on the origin of cosmic rays; accelerated protons and electrons will subsequently emit high-energy gamma rays (with energies larger than 100 MeV), through inverse Compton and / or pion decay processes. The Fermi satellite, launched in 2008, has in fact detected already several novae at such energies, confirming our theoretical expectations.
Observations in X-rays are very crucial to understand novae:
A) Before the explosion, X-rays reveal the mass accretion process, strongly dependent on the mass and magnetic properties of the white dwarf, and also on the mass accretion rate, related to the binary system class (cataclysmic variable or symbiotic binary)
B) During the explosion itself and early afterwards, mass ejection produces internal and external shocks that are traced with X-rays (and high-energy gamma rays if they are strong enough and accelerate particles). In addition, the turn-off of H-nuclear burning on top of the white dwarf is revealed through super soft X-ray emission (energies below 1 keV), once the expanding ejecta are transparent enough. The duration of this phase of hot white dwarf photospheric emission (black body-like) indicates how much mass remained on the white dwarf after the explosion.
C) The post-outburst phase leads to the reestablishment of accretion onto the white dwarf, revealed again in X-rays, as in the pre-explosion phase.
The study of extragalactic novae is of the maximum interest, in particular in M31, because the distance is well established and also there's less absorption of X-rays (especially the softer ones) than for novae in the center of our Galaxy. This leads to the detection of several novae and the possibility to do population studies, not possible in our own Galaxy.
- Models of novae including detailed nucleosynthesis and gamma ray emission; prediction of spectra in the MeV range, to be observed with current (INTEGRAL) and future gamma-ray satellites (e-ASTROGAM). We lead large international collaborations responsible for ToO (Target of Opportunity) INTEGRAL observation proposals.
- Origin of Galactic 7Li and the nova contribution; approach through modeling and observations: 7Li in optical, 7Be in UV (co-Is of ESO proposals) and 7Be in 478 keV gamma-rays (PIs of INTEGRAL proposals)
- Participation in observations of novae in outburst, searching for supersoft emission revealing residual nuclear burning on top of the white dwarf, with XMM-Newton, Swift/XRT, Chandra X-ray satellites. Same for post-outburst novae, to understand how accretion is reestablished after the explosion.
- Particle acceleration in novae and its understanding through X-ray early observations (application to RS Oph and V745 Sco recurrent novae)
- Study of extragalactic novae, especially in the Andromeda galaxy (M31). The large data base (optical and in X-rays) includes exciting objects, like M31-2008-12a, the nova with the shortest recurrence period (about 1 year), hosting an extremely massive white dwarf and thus being the best-known candidate to explode as a thermonuclear supernova.
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Senior Institute members involved
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, J. Isern