1,187 research outputs found
Long tails on thermonuclear X-ray bursts from neutron stars: a signature of inward heating?
We report the discovery of one-hour long tails on the few-minutes long X-ray
bursts from the `clocked burster' GS 1826-24. We propose that the tails are due
to enduring thermal radiation from the neutron star envelope. The enduring
emission can be explained by cooling of deeper NS layers which were heated up
through inward conduction of heat produced in the thermonuclear shell flash
responsible for the burst. Similar, though somewhat shorter, tails are seen in
bursts from EXO 0748-676 and 4U 1728-34. Only a small amount of cooling is
detected in all these tails. This is either due to compton up scattering of the
tail photons or, more likely, to a NS that is already fairly hot due to other,
stable, nuclear processes.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 14
figure
Why a Single-Star Model Cannot Explain the Bipolar Nebula of Eta Carinae
I examine the angular momentum evolution during the 1837-1856 Great Eruption
of the massive star Eta Carinae. I find that the new estimate of the mass blown
during that eruption implies that the envelope of Eta Car substantially
spun-down during the 20 years eruption. Single-star models, most of which
require the envelope to rotate close to the break-up velocity, cannot account
for the bipolar nebula (the Homunculus) formed from matter expelled in that
eruption. The kinetic energy and momentum of the Homunculus further constrains
single-star models. I discuss how Eta Car can fit into a unified model for the
formation of bipolar lobes where two oppositely ejected jets inflate two lobes
(or bubbles). These jets are blown by an accretion disk, which requires stellar
companions in the case of bipolar nebulae around stellar objects.Comment: ApJ, in press. New references and segments were adde
Dependence of X-Ray Burst Models on Nuclear Reaction Rates
X-ray bursts are thermonuclear flashes on the surface of accreting neutron
stars and reliable burst models are needed to interpret observations in terms
of properties of the neutron star and the binary system. We investigate the
dependence of X-ray burst models on uncertainties in (p,),
(,), and (,p) nuclear reaction rates using fully
self-consistent burst models that account for the feedbacks between changes in
nuclear energy generation and changes in astrophysical conditions. A two-step
approach first identified sensitive nuclear reaction rates in a single-zone
model with ignition conditions chosen to match calculations with a
state-of-the-art 1D multi-zone model based on the {\Kepler} stellar evolution
code. All relevant reaction rates on neutron deficient isotopes up to mass 106
were individually varied by a factor of 100 up and down. Calculations of the 84
highest impact reaction rate changes were then repeated in the 1D multi-zone
model. We find a number of uncertain reaction rates that affect predictions of
light curves and burst ashes significantly. The results provide insights into
the nuclear processes that shape X-ray burst observables and guidance for
future nuclear physics work to reduce nuclear uncertainties in X-ray burst
models.Comment: 24 pages, 13 figures, 4 tables, submitte
Presupernova Evolution of Differentially Rotating Massive Stars Including Magnetic Fields
As a massive star evolves through multiple stages of nuclear burning on its
way to becoming a supernova, a complex, differentially rotating structure is
set up. Angular momentum is transported by a variety of classic instabilities,
and also by magnetic torques from fields generated by the differential
rotation. We present the first stellar evolution calculations to follow the
evolution of rotating massive stars including, at least approximately, all
these effects, magnetic and non-magnetic, from the zero-age main sequence until
the onset of iron-core collapse. The evolution and action of the magnetic
fields is as described by Spruit 2002 and a range of uncertain parameters is
explored. In general, we find that magnetic torques decrease the final rotation
rate of the collapsing iron core by about a factor of 30 to 50 when compared
with the non-magnetic counterparts. Angular momentum in that part of the
presupernova star destined to become a neutron star is an increasing function
of main sequence mass. That is, pulsars derived from more massive stars will
rotate faster and rotation will play a more dominant role in the star's
explosion. The final angular momentum of the core is determined - to within a
factor of two - by the time the star ignites carbon burning. For the lighter
stars studied, around 15 solar masses, we predict pulsar periods at birth near
15 ms, though a factor of two range is easily tolerated by the uncertainties.
Several mechanisms for additional braking in a young neutron star, especially
by fall back, are also explored.Comment: 32 pages, 3 figures (8 eps files), submitted to Ap
On the stability of very massive primordial stars
The stability of metal-free very massive stars ( = 0; M = 120 - 500
\msol) is analyzed and compared with metal-enriched stars. Such zero-metal
stars are unstable to nuclear-powered radial pulsations on the main sequence,
but the growth time scale for these instabilities is much longer than for their
metal-rich counterparts. Since they stabilize quickly after evolving off the
ZAMS, the pulsation may not have sufficient time to drive appreciable mass loss
in Z = 0 stars. For reasonable assumptions regarding the efficiency of
converting pulsational energy into mass loss, we find that, even for the larger
masses considered, the star may die without losing a large fraction of its
mass. We find a transition between the - and -mechanisms for
pulsational instability at Z\sim 2\E{-4} - 2\E{-3}. For the most metal-rich
stars, the -mechanism yields much shorter -folding times, indicating
the presence of a strong instability. We thus stress the fundamental difference
of the stability and late stages of evolution between very massive stars born
in the early universe and those that might be born today.Comment: 7 pages, 5 figures. Minor changes, more results given in Table 1,
accepted for publication in Ap
Astrophysical S factor for the radiative capture 12N(p,gamma)13O determined from the 14N(12N,13O)13C proton transfer reaction
The cross section of the radiative proton capture reaction on the drip line
nucleus 12N was investigated using the Asymptotic Normalization Coefficient
(ANC) method. We have used the 14N(12N,13O)13C proton transfer reaction at 12
MeV/nucleon to extract the ANC for 13O -> 12N + p and calculate from it the
direct component of the astrophysical S factor of the 12N(p,gamma)13O reaction.
The optical potentials used and the DWBA analysis of the proton transfer
reaction are discussed. For the entrance channel, the optical potential was
inferred from an elastic scattering measurement carried out at the same time
with the transfer measurement. From the transfer, we determined the square of
the ANC, C^2(13Og.s.) = 2.53 +/- 0.30 fm-1, and hence a value of 0.33(4) keVb
was obtained for the direct astrophysical S factor at zero energy. Constructive
interference at low energies between the direct and resonant captures leads to
an enhancement of Stotal(0) = 0.42(5) keVb. The 12N(p,gamma)13O reaction was
investigated in relation to the evolution of hydrogen-rich massive Population
III stars, for the role that it may play in the hot pp-chain nuclear burning
processes, possibly occurring in such objects.Comment: 15 pages, 10 figures, 3 tables submitted to Phys. Rev.
White dwarf spins from low mass stellar evolution models
The prediction of the spins of the compact remnants is a fundamental goal of
the theory of stellar evolution. Here, we confront the predictions for white
dwarf spins from evolutionary models including rotation with observational
constraints. We perform stellar evolution calculations for stars in the mass
range 1... 3\mso, including the physics of rotation, from the zero age main
sequence into the TP-AGB stage. We calculate two sets of model sequences, with
and without inclusion of magnetic fields. From the final computed models of
each sequence, we deduce the angular momenta and rotational velocities of the
emerging white dwarfs. While models including magnetic torques predict white
dwarf rotational velocities between 2 and 10 km s, those from the
non-magnetic sequences are found to be one to two orders of magnitude larger,
well above empirical upper limits. We find the situation analogous to that in
the neutron star progenitor mass range, and conclude that magnetic torques may
be required in order to understand the slow rotation of compact stellar
remnants in general.Comment: Accepted for A&A Letter
EC-SNe from super-AGB progenitors: theoretical models vs. observations
Using a parametric approach, we determine the configuration of super-AGB
stars at the explosion as a function of the initial mass and metallicity, in
order to verify if the EC-SN scenario involving a super-AGB star is compatible
with the observations regarding SN2008ha and SN2008S. The results show that
both the SNe can be explained in terms of EC-SNe from super-AGB progenitors
having a different configuration at the collapse. The impact of these results
on the interpretation of other sub-luminous SNe is also discussed.Comment: Accepted for publication in ApJ
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