 |
| la science |
| les supernovae |
| les novae |
| les objects compacts |
| les sursauts solaires |
| retour |
|
The most prominent tracer isotopes in novae are 13N, 18F, 7Be and 22Na. Novae also synthesise 26Al, but this isotope cannot be detected in individual events, because of its too long lifetime (t = 106 yr). The decays of 13N (t= 862 s) and of 18F (t = 158 min) do not directly produce g-rays but lead to emission of positrons. The subsequent electron-positron annihilation produces a 511 keV line (with some blueshift and width related to the expansion of the envelope) and a continuum at lower energies. Such a continuum comes both from the positronium continuum and from the comptonization of 511 keV photons. The decay of 7Be (t = 77 d), of 22Na (t = 3.75 y) and of 26Al (t = 106 y) produces respectively photons of 478 keV, 1.275 and 1.809 MeV, plus positron emission for 22Na and 26Al.
The possible nucleosynthesis of 7Be in nova explosions has some interesting implications on the chemical evolution of the Universe. The abundance of 7Li produced by standard big bang models, a mass fraction of ~10-9, is compatible with that observed in Population II dwarf stars (Kawano et al. 1988). However, Population I material obeys a 10 times larger mass fraction, ~10-8, requiring either a stellar 7Li production channel or some 7Li destruction mechanism in Population II dwarfs. 7Be production in classical novae, which would subsequently decay to 7Li, presents an interesting nucleosynthesis channel to resolve this puzzle. Gamma-ray observations of the 478 keV line will allow to verify this hypothesis.
Predicted gamma-ray emission from individual novae : Detailed
studies of the g-ray emission of novae and its relationship with particular
models of nova explosions require the use of realistic profiles of densities,
velocities and chemical abundances, which are crucial for the determination
of the g-ray spectrum at different epochs (Hernanz et al. 1997a,b; Gómez-Gomar
et al. 1998b; Hernanz et al. 1999). A good knowledge of all the nuclear
reaction rates involved in the synthesis of radioactive nuclei is also
crucial for the computation of reliable yields in classical novae explosions
(José, Coc, & Hernanz 1999). For a mass of 1.15 Mo,
CO nova models predict typically 10-10 Mo of 7Be,
4 10-12 Mo of 22Na and 6 10-10 Mo
of 26Al, while ONe nova models predict typically 10-11 M§ of 7Be,
7 10-9 Mo of 22Na and 2 10-8 Mo
of 26Al. Finally, similar amounts of the short-lived isotopes 13N and 18F
are synthesised in both CO and ONe novae: typically, one hour after peak
temperature, the models predict ejected masses of 10-8 Mo
for
13N and 10-9 Mo for 18F.
Gamma-ray spectra for CO and ONe novae at different epochs after the
explosion (defined as the time of peak temperature) feature two different
types of line emission: one at 511 keV, with very short duration, and one
at 478 keV and 1.275 MeV, with moderate and long duration, as well as a
continuum between 30 keV and 511 keV. Concerning the 511 keV line and the
continuum (related to 13N and 18F decays), CO and ONe novae display a similar
behaviour, although ONe novae emit larger fluxes. For the 478 keV and 1.275
MeV lines (induced respectively by the decay of 7Be and 22Na), the g-ray
signatures of CO and ONe novae are different, because of the different
yields of radioactive elements: spectra of CO novae will show the 7Be line
whereas those of ONe novae will show the 22Na line.
Current observational status The observation of the g-ray lines from classical novae would be an important step towards the understanding of these explosions, which occur quite often in the Galaxy (~ 20-40 yr-1). Up to now, no positive detection of either the 478 keV emission (Harris, Leising, & Share 1991), or the 1.275 MeV emission (Iyudin et al. 1995) has been obtained, although many novae have been observed by the instruments on CGRO. The 1.809 MeV line associated with Galactic 26Al has been detected some years ago by the High Energy Astrophysics Observatory-3 (Mahoney et al. 1982), but observations made with COMPTEL seem to indicate that the 26Al emission is better related to a young population of massive stars, yet a small contribution from classical novae cannot be ruled out (Diehl et al. 1995; Prantzos & Diehl 1996).
Observation of classical novae by MAX : Classical novae explosions synthesise
many radioactive nuclei (13N, 18F, 7Be, 22Na and 26Al), which emit g-rays
when decaying. Two main types of emission are produced, lines and continuum,
with duration and intensity depending on the type of emission and on the
particular nova which produces them. MAX would be perfectly suited for
the detection of g-ray lines from classical novae, which up to now is still
a challenge for current and even future (as e.g. INTEGRAL) g-ray missions.
For example, the 7Be gamma-ray line at 478 keV may be detected by MAX out
to distances of 2 kpc, considerably larger than the ~ 0.5 kpc that are
accessible to SPI aboard INTEGRAL. Within this distance range, it is expected
that MAX can observe 7Be from about 3 CO novae during a mission lifetime
of 5 years. Of course, statistical fluctuations with such small numbers
are large: for instance, there have been 2 novae at 2 kpc in the sole year
1999!
Additionally, 511 keV line emission from positron annihilation could
be seen with MAX to distances as large as 9 kpc. The main problem for the
detection of this early emission is that it appears well before the maximum
in visual magnitude, i.e. well before the visual discovery of the nova.
Therefore, its detectibility by MAX would require an alert trigger from
wide field-of-view instruments monitoring continuously the Galactic plane,
where most novae should explode.
The detection of gamma-ray line emission from novae would be a great
discovery. It would provide invaluable information about novae explosions:
the confirmation of the thermonuclear runaway model, the direct determination
of the synthesis of radioisotopes and crucial information about the conditions
in the expanding envelope. All together, this information would help not
only to understand the nova phenomenon, but also to determine their ejected
masses and their composition, helping to clarify the role played by novae
in the synthesis of some galactic isotopes and, in particular, in the explanation
of some isotopic anomalies observed in meteorites.
recent viewgraphs by M. Hernanz (2004)
mise à jour : mars 2004
questions et commentaires :Peter
von Ballmoos