136 research outputs found
Models for the soft X-ray emission of post-outburst classical novae
A hydrostatic and stationary white dwarf envelope model has been developed
for the study of the post-outburst phases of classical novae and their soft
X-ray emission. Several white dwarf masses and chemical compositions typical
for classical novae have been considered. The results show that the luminosity,
maximum effective temperature and envelope masses depend on the white dwarf
mass and on the chemical composition. Envelope masses for which equilibrium
solutions exist are pretty small ~10^{-7}-10^{-6} Msun, thus leading to a short
duration of the soft X-ray emitting phase of classical novae, in agreement with
most of the observations. The models presented provide a useful tool for the
determination of the white dwarf properties from observable parameters in the
X-ray range.Comment: 15 pages, 11 figures, 2 tables, accepted for publication in Astronomy
& Astrophysic
Thermonuclear Runaways on Accreting White Dwarfs: Models of Classical Novae Explosions
The mechanism of classical novae explosions is explained, together with some
of their observational properties. The scarce but not null impact of novae in
the chemical evolution of the Milky Way is analyzed, as well as their relevance
for the radioactivity in the Galaxy. A special emphasis is given to the
predicted gamma-ray emission from novae and its relationship with the
thermonuclear model itself and its related nucleosynthesis.Comment: 10 pages, 2 figures, LaTex (aipproc.sty), review to appear in "Cosmic
Explosions", proceedings. of the 10th Annual October Maryland Astrophysics
Conference, eds. S. Holt and W. W. Zhang, AI
Nuclear uncertainties in the NeNa-MgAl cycles and synthesis of 22Na and 26Al in classical novae
Classical novae eject significant amounts of matter into the interstellar
medium, as a result of thermonuclear runaways. Nucleosynthesis associated with
nova outbursts includes products from explosive H burning, such as 17O, 15N and
13C, and also radioactive species like 7Be, 22Na and 26Al. In this paper we
report on new hydrodynamic calculations of nova outbursts, from the onset of
accretion to mass ejection. We stress the role played by the nuclear
uncertainties associated with key reactions of the NeNa-MgAl cycles on the
synthesis of 22Na and 26Al.Comment: 4 pages (to appear in "Nuclei in the Cosmos V" Conference
Proceedings
Evolutionary calculations of carbon dredge-up in helium envelope white dwarfs
We investigate the evolution of cooling helium atmosphere white dwarfs using
a full evolutionary code, specifically developed for following the effects of
element diffusion and gravitational settling on white dwarf cooling. The major
difference between this work and previous work is that we use more recent
opacity data from the OPAL project. Since, in general, these opacities are
higher than those available ten years ago, at a given effective temperature,
convection zones go deeper than in models with older opacity data. Thus
convective dredge-up of observationally detectable carbon in helium atmosphere
white dwarfs can occur for thicker helium layers than found by Pelletier et al
(1986). We find that the range of observed C to He ratios in different DQ white
dwarfs of similar effective temperature is well explained by a range of initial
helium layer mass between and , in good agreement
with stellar evolution theory, assuming a typical white dwarf mass of . We also predict that oxygen will be present in DQ white dwarf
atmospheres in detectable amounts if the helium layer mass is near the lower
limit compatible with stellar evolution theory. Determination of the oxygen
abundance has the potential of providing information on the profile of oxygen
in the core and hence on the important CO reaction
rate.Comment: Accepted for publication in MNRAS, 8 pages, 6 figure
V5116 Sgr, an Eclipsing Supersoft Post-Outburst Nova?
V5116 Sgr (Nova Sgr 2005 No. 2), discovered on 2005 July 4, was observed with
XMM-Newton in March 2007, 20 months after the optical outburst. The X-ray
spectrum shows that the nova had evolved to a pure supersoft X-ray source, with
no significant emission at energies above 1 keV. The X-ray light-curve shows
abrupt decreases and increases of the flux by a factor ~8. It is consistent
with a periodicity of 2.97 h, the orbital period suggested by Dobrotka et al.
(2007), although the observation lasted just a little more than a whole period.
We estimate the distance to V5116 Sgr to be 11+/-3 kpc. A simple blackbody
model does not fit correctly the EPIC spectra, with reduced chi^2>4. In
contrast, ONe rich white dwarf atmosphere models provide a good fit, with
nH=1.3(+/-0.1)e21 cm^-2, T=6.1(+/-0.1)e5 K, and L=3.9(+/-0.8)e37(D/10kpc)^2
erg/s (during the high-flux periods). This is consistent with residual hydrogen
burning in the white dwarf envelope. The white dwarf atmosphere temperature is
the same both in the low and the high flux periods, ruling out an intrinsic
variation of the X-ray source as the origin of the flux changes. We speculate
that the X-ray light-curve may result from a partial coverage by an asymmetric
accretion disk in a high inclination system.Comment: 2 figures, emulateapj, to appear in ApJ
Influence of new reaction rates on 18F production in novae
Gamma-ray emission from classical novae is dominated, during the first hours,
by positron annihilation resulting from the beta decay of radioactive nuclei.
The main contribution comes from the decay of 18F and hence is directly related
to 18F formation during the outburst. A good knowledge of the nuclear reaction
rates of production and destruction of 18F is required to study 18F synthesis
in novae and the resulting gamma-ray emission. The rates relevant for the main
mode of 18F destruction (i.e, through proton captures) have been the object of
many recent experiments. However, subsequent analyses were focused on providing
rates for X-ray burst nucleosynthesis not valid at nova temperatures (lower
than 3.5 10^8 K). Accordingly, it is crucial to propose and discuss new
reaction rates, incorporating all new experimental results, down to the domain
of nova nucleosynthesis. We show that in this temperature regime, the
18F(p,gamma) and (p,alpha) reaction rates remain uncertain and deserve further
experimental and theoretical efforts. Our hydrodynamic calculations including
the new nuclear rates demonstrate that their impact on 18F synthesis in nova
explosions is quite large and, consequently, the early gamma-ray emission from
classical novae is also affected.Comment: To appear in Astron. Astrophys., 14 pages with 9 figure
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