331 research outputs found
Circinus X-1: survivor of a highly asymmetric supernova
We have analyzed the kinematical parameters of Cir X-1 to constrain the
nature of its companion star, the eccentricity of the binary and the
pre-supernova parameter space. We argue that the companion is most likely to be
a low-mass (< 2.0 M_sun) unevolved star and that the eccentricity of the orbit
is 0.94 +/- 0.04. We have evaluated the dynamical effects of the supernova
explosion and we find it must have been asymmetric. On average, we find that a
kick of 740 km/s is needed to account for the recently measured radial velocity
of +430 km/s (Johnston, Fender & Wu) for this extreme system. The corresponding
minimum kick velocity is 500 km/s. This is the largest kick needed to explain
the motion of any observed binary system. If Cir X-1 is associated with the
supernova remnant G321.9-0.3 then we find a limiting minimum age of this
remnant of 60000 yr. Furthermore, we predict that the companion star has lost
10% of its mass as a result of stripping and ablation from the impact of the
supernova shell shortly after the explosion.Comment: 6 pages, 3 figues, 2 tables, accepted for publication in MNRA
Carbon-Oxygen White Dwarfs Accreting CO-Rich Matter I: A Comparison Between Rotating and Non-Rotating Models
We investigate the lifting effect of rotation on the thermal evolution of CO
WDs accreting CO-rich matter. We find that rotation induces the cooling of the
accreting star so that the delivered gravitational energy causes a greater
expansion with respect to the standard non-rotating case. The increase in the
surface radius produces a decrease in the surface value of the critical angular
velocity and, therefore, the accreting WD becomes gravitationally unbound
(Roche instability). This occurrence is due to an increase in the total angular
momentum of the accreting WD and depends critically on the amount of specific
angular momentum deposited by the accreted matter. If the specific angular
momentum of the accreted matter is equal to that of the outer layers of the
accreting structure, the Roche instability occurs well before the accreting WD
can attain the physical conditions for C-burning. If the values of both initial
angular velocity and accretion rate are small, we find that the accreting WD
undergoes a secular instability when its total mass approaches 1.4 Msun. At
this stage, the ratio between the rotational and the gravitational binding
energy of the WD becomes of the order of 0.1, so that the star must deform by
adopting an elliptical shape. In this case, since the angular velocity of the
WD is as large as 1 rad/s, the anisotropic mass distribution induces the loss
of rotational energy and angular momentum via GWR. We find that, independent of
the braking efficiency, the WD contracts and achieves the physical conditions
suitable for explosive C-burning at the center so that a type Ia supernova
event is produced.Comment: 39 pages, 22 eps-figures; accepted for publication in Astrophysical
Journa
Abundances in intermediate-mass AGB stars undergoing third dredge-up and hot-bottom burning
High dispersion near-infrared spectra have been taken of seven
highly-evolved, variable, intermediate-mass (4-6 Msun) AGB stars in the LMC and
SMC in order to look for C, N and O variations that are expected to arise from
third dredge-up and hot-bottom burning. The pulsation of the objects has been
modelled, yielding stellar masses, and spectral synthesis calculations have
been performed in order to derive abundances from the observed spectra. For two
stars, abundances of C, N, O, Na, Al, Ti, Sc and Fe were derived and compared
with the abundances predicted by detailed AGB models. Both stars show very
large N enhancements and C deficiencies. These results provide the first
observational confirmation of the long-predicted production of primary nitrogen
by the combination of third dredge-up and hot-bottom burning in
intermediate-mass AGB stars. It was not possible to derive abundances for the
remaining five stars: three were too cool to model, while another two had
strong shocks in their atmospheres which caused strong emission to fill the
line cores and made abundance determination impossible. The latter occurrence
allows us to predict the pulsation phase interval during which observations
should be made if successful abundance analysis is to be possible.Comment: Accepted for publication in MNRA
Reaction rates for Neutron Capture Reactions to C-, N- and O-isotopes to the neutron rich side of stability
The reaction rates of neutron capture reactions on light nuclei are important
for reliably simulating nucleosynthesis in a variety of stellar scenarios.
Neutron capture reaction rates on neutron-rich C-, N-, and O-isotopes are
calculated in the framework of a hybrid compound and direct capture model. The
results are tabulated and compared with the results of previous calculations as
well as with experimental results.Comment: 33 pages (uses revtex) and 9 postscript figures, accepted for
publication in Phys. Rev.
Supersoft X-ray Sources. Basic Parameters
The parameters of ten supersoft X-ray sources (RX J0439.8-6809, RX
J0513.9-6951, RX J0527.8-6954, CAL 87, CAL 83, 1E 0035.4-7230, RX J0048.4-7332,
1E 0056.8-7154, RX J0019.8 +2156, RX J0925.7-4758) observed by ROSAT obtained
using blanketing LTE model atmospheres are analyzed. The consistency of the
resulting parameters with a model with stable/recurrent burning on the surface
of the white dwarf is studied. The luminosity and sizes of seven of the sources
are in good agreement with this model. The masses of the white dwarfs in these
sources are estimated. A formula that can be used to estimate the masses of
white dwarfs in classical supersoft sources based on their effective
temperatures is presented.Comment: 8 pages, 2 tables, 4 figure
Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with M~0.9 M_sun
Type Ia supernovae (SNe Ia) are thought to result from thermonuclear
explosions of carbon-oxygen white dwarf stars. Existing models generally
explain the observed properties, with the exception of the sub-luminous
1991-bg-like supernovae. It has long been suspected that the merger of two
white dwarfs could give rise to a type Ia event, but hitherto simulations have
failed to produce an explosion. Here we report a simulation of the merger of
two equal-mass white dwarfs that leads to an underluminous explosion, though at
the expense of requiring a single common-envelope phase, and component masses
of ~0.9 M_sun. The light curve is too broad, but the synthesized spectra, red
colour and low expansion velocities are all close to what is observed for
sub-luminous 1991bg-like events. While mass ratios can be slightly less than
one and still produce an underluminous event, the masses have to be in the
range 0.83-0.9 M_sun.Comment: Accepted to Natur
Cluster-based density-functional approach to quantum transport through molecular and atomic contacts
We present a cluster-based density-functional approach to model charge
transport through molecular and atomic contacts. The electronic structure of
the contacts is determined in the framework of density functional theory, and
the parameters needed to describe transport are extracted from finite clusters.
A similar procedure, restricted to nearest-neighbor interactions in the
electrodes, has been presented by Damle et al. [Chem. Phys. 281, 171 (2002)].
Here, we show how to systematically improve the description of the electrodes
by extracting bulk parameters from sufficiently large metal clusters. In this
way we avoid problems arising from the use of nonorthogonal basis functions.
For demonstration we apply our method to electron transport through Au contacts
with various atomic-chain configurations and to a single-atom contact of Al.Comment: 18 pages, 13 figure
Discovery of the progenitor of the type Ia supernova 2007on
Type Ia supernovae are exploding stars that are used to measure the
accelerated expansion of the Universe and are responsible for most of the iron
ever produced. Although there is general agreement that the exploding star is a
white dwarf in a binary system, the exact configuration and trigger of the
explosion is unclear, which could hamper their use for precision cosmology. Two
families of progenitor models have been proposed. In the first, a white dwarf
accretes material from a companion until it exceeds the Chandrasekhar mass,
collapses and explodes. Alternatively, two white dwarfs merge, again causing
catastrophic collapse and an explosion. It has hitherto been impossible to
determine if either model is correct. Here we report the discovery of an object
in pre-supernova archival X-ray images at the position of the recent type Ia
supernova (2007on) in the elliptical galaxy NGC 1404. Deep optical images (also
archival) show no sign of this object. From this we conclude that the X-ray
source is the progenitor of the supernova, which favours the accretion model
for this supernova, although the host galaxy is older (6-9 Gyr) than the age at
which the explosions are predicted in the accreting models.Comment: Published in Nature See also the two follow-up papers: Roelofs,
Bassa, Voss, Nelemans Nelemans, Voss, Roelofs, Bassa both on astro-ph
02/15/0
On the origin of the Boson peak in globular proteins
We study the Boson Peak phenomenology experimentally observed in globular
proteins by means of elastic network models. These models are suitable for an
analytic treatment in the framework of Euclidean Random Matrix theory, whose
predictions can be numerically tested on real proteins structures. We find that
the emergence of the Boson Peak is strictly related to an intrinsic mechanical
instability of the protein, in close similarity to what is thought to happen in
glasses. The biological implications of this conclusion are also discussed by
focusing on a representative case study.Comment: Proceedings of the X International Workshop on Disordered Systems,
Molveno (2006
Asteroseismological constraints on the coolest GW Vir variable star (PG 1159-type)PG 0122+200
We present an asteroseismological study on PG 0122+200, the coolest known
pulsating PG1159 (GW Vir) star. Our results are based on an augmented set of
the full PG1159 evolutionary models recently presented by Miller Bertolami &
Althaus (2006). We perform extensive computations of adiabatic g-mode pulsation
periods on PG1159 evolutionary models with stellar masses ranging from 0.530 to
0.741 Msun. We derive a stellar mass of 0.626 Msun from a comparison between
the observed period spacing and the computed asymptotic period spacing, and a
stellar mass of 0.567 Msun by comparing the observed period spacing with the
average of the computed period spacing. We also find, on the basis of a
period-fit procedure, an asteroseismological model representative of PG
0122+200 which is able to reproduce the observed period pattern with an average
of the period differences of 0.88 s. The model has an effective temperature of
81500 K, a stellar mass of 0.556 Msun, a surface gravity log g= 7.65, a stellar
luminosity and radius of log(L/Lsun)= 1.14 and log(R/Rsun)= -1.73,
respectively, and a He-rich envelope thickness of Menv= 0.019 Msun. We derive a
seismic distance of about 614 pc and a parallax of about 1.6 mas. The results
of the period-fit analysis carried out in this work suggest that the
asteroseismological mass of PG 0122+200 could be 6-20 % lower than thought
hitherto and in closer agreement (to within 5 %) with the spectroscopic mass.
This result suggests that a reasonable consistency between the stellar mass
values obtained from spectroscopy and asteroseismology can be expected when
detailed PG1159 evolutionary models are considered.Comment: 10 pages, 6 figures. To be published in Astronomy & Astrophysic
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