2,907,928 research outputs found
The Solid-on-Solid Surface Width Around the Roughening Transition
We investigate the surface width of solid-on-solid surfaces in the
vicinity of the roughening temperature . Above , is expected to
diverge with the system size like . However, close to a clean
behavior can only be seen on extremely large lattices. Starting from
the Kosterlitz-Thouless renormalization group, we derive an improved formula
that describes the small behavior on both sides of . For the Discrete
Gaussian model, we used the valleys-to-mountains-reflections cluster algorithm
in order to simulate the fluctuating solid-on-solid surface. The base plane
above which the surface is defined is an square lattice. In the
simulation we took . The improved formula fits the numerical
results very well. {}From the analysis, we estimate the roughening temperature
to be .Comment: 9 pages, LaTeX (no figures), FSU-SCRI-93-67, CERN-TH.6893/9
On the asymptotic giant branch star origin of peculiar spinel grain OC2
Microscopic presolar grains extracted from primitive meteorites have
extremely anomalous isotopic compositions revealing the stellar origin of these
grains. The composition of presolar spinel grain OC2 is different from that of
all other presolar spinel grains. Large excesses of the heavy Mg isotopes are
present and thus an origin from an intermediate-mass (IM) asymptotic giant
branch (AGB) star was previously proposed for this grain. We discuss the
isotopic compositions of presolar spinel grain OC2 and compare them to
theoretical predictions. We show that the isotopic composition of O, Mg and Al
in OC2 could be the signature of an AGB star of IM and metallicity close to
solar experiencing hot bottom burning, or of an AGB star of low mass (LM) and
low metallicity suffering very efficient cool bottom processing. Large
measurement uncertainty in the Fe isotopic composition prevents us from
discriminating which model better represents the parent star of OC2. However,
the Cr isotopic composition of the grain favors an origin in an IM-AGB star of
metallicity close to solar. Our IM-AGB models produce a self-consistent
solution to match the composition of OC2 within the uncertainties related to
reaction rates. Within this solution we predict that the 16O(p,g)17F and the
17O(p,a)14N reaction rates should be close to their lower and upper limits,
respectively. By finding more grains like OC2 and by precisely measuring their
Fe and Cr isotopic compositions, it may be possible in the future to derive
constraints on massive AGB models from the study of presolar grains.Comment: 10 pages, 8 figures, accepted for publication on Astronomy &
Astrophysic
The Effect of Composition on Nova Ignitions
The accretion of hydrogen-rich matter onto C/O and O/Ne white dwarfs in
binary systems leads to unstable thermonuclear ignition of the accreted
envelope, triggering a convective thermonuclear runaway and a subsequent
classical, recurrent, or symbiotic nova. Prompted by uncertainties in the
composition at the base of the accreted envelope at the onset of convection, as
well as the range of abundances detected in nova ejecta, we examine the effects
of varying the composition of the accreted material. For high accretion rates
and carbon mass fractions < 0.002, we find that carbon, which is usually
assumed to trigger the runaway via proton captures, is instead depleted and
converted to 14N. Additionally, we quantify the importance of 3He, finding that
convection is triggered by 3He+3He reactions for 3He mass fractions > 0.002.
These different triggering mechanisms, which occur for critical abundances
relevant to many nova systems, alter the amount of mass that is accreted prior
to a nova, causing the nova rate to depend on accreted composition. Upcoming
deep optical surveys such as Pan-STARRS-1, Pan-STARRS-4, and the Large Synoptic
Survey Telescope may allow us to detect the dependence of nova rates on
accreted composition. Furthermore, the burning and depletion of 3He with a mass
fraction of 0.001, which is lower than necessary for triggering convection,
still has an observable effect, resulting in a pre-outburst brightening in disk
quiescence to > Lsun and an increase in effective temperature to 6.5e4 K for a
1.0 Msun white dwarf accreting at 1e-8 Msun/yr.Comment: Submitted to The Astrophysical Journal, 11 pages, 11 figure
The supersoft X-ray source in V5116 Sgr I. The high resolution spectra
Classical novae occur on the surface of an accreting white dwarf in a binary
system. After ejection of a fraction of the envelope and when the expanding
shell becomes optically thin to X-rays, a bright source of supersoft X-rays
arises, powered by residual H burning on the surface of the white dwarf. While
the general picture of the nova event is well established, the details and
balance of accretion and ejection processes in classical novae are still full
of unknowns. The long-term balance of accreted matter is of special interest
for massive accreting white dwarfs, which may be promising supernova Ia
progenitor candidates. V5116 Sgr was observed as a bright and variable
supersoft X-ray source by XMM-Newton 610~days after outburst. The light curve
showed a periodicity consistent with the orbital period. During one third of
the orbit the luminosity was a factor of seven brighter than during the other
two thirds of the orbital period. In the present work we aim to disentangle the
X-ray spectral components of V5116 Sgr and their variability. We present the
high resolution spectra obtained with XMM-Newton RGS and Chandra LETGS/HRC-S in
March and August 2007. The grating spectrum during the periods of high-flux
shows a typical hot white dwarf atmosphere dominated by absorption lines of N
VI and N VII. During the low-flux periods, the spectrum is dominated by an
atmosphere with the same temperature as during the high-flux period, but with
several emission features superimposed. Some of the emission lines are well
modeled with an optically thin plasma in collisional equilibrium, rich in C and
N, which also explains some excess in the spectra of the high-flux period. No
velocity shifts are observed in the absorption lines, with an upper limit set
by the spectral resolution of 500 km/s, consistent with the expectation of a
non-expanding atmosphere so late in the evolution of the post-nova.Comment: 12 pages, 15 figures, 4 tables; accepted for publication in Astronomy
and Astrophysic
The prompt gamma-ray emission of novae
Classical novae are potential gamma-ray emitters, because of the
disintegration of some radioactive nuclei synthesized during the explosion.
Some short-lived isotopes (such as 13N and 18F), as well as the medium-lived
22Na, decay emitting positrons, which annihilate with electrons and thus are
responsible for the prompt emission of gamma-rays from novae. This emission
consists of a 511 keV line plus a continuum between 20 and 511 keV, and is
released before the maximum in visual luminosity, i.e., before the discovery of
the nova. The main characteristics of this prompt emission, together with the
related uncertainties (both of nuclear and hydrodynamical origin, with a
particular emphasis on the influence of the envelope properties) and prospects
for detectability are analyzed in this paper.Comment: 6 pages, 3 figures (6 PS files), to appear in New Astronomy Reviews
(Proceedings of the Ringberg Workshop "Astronomy with Radioactivities III"
HR4049: signature of nova nucleosynthesis ?
The post-Asymptotic Giant Branch (AGB) star HR4049 is in an eccentric binary
system with a relatively short period probably surrounded by a dusty
circumbinary disk. Extremely anomalous oxygen isotopic ratios, O16/O17 ~
O16/O18 ~ 7, have been measured from CO_2 molecules likely residing in the
disk. Such a composition cannot be explained in the framework of AGB and
post-AGB evolution while it can be qualitatively associated with the
nucleosynthesis occurring during nova outbursts. We discuss nova models, the
presence of a white dwarf companion to HR4049 and possible scenarios for the
dynamical evolution of this binary system. Circumbinary disks in which mixing
occurs between red-giant and nova material may also be invoked as the site of
formation of some rare types of meteoritic presolar grains.Comment: 4 pages, 2 figures, submitted for the proceedings of the 8th Nuclei
in the Cosmos symposium (Vancouver, Canada, 19-23 July 2004
Discovery, photometry, and astrometry of 49 classical nova candidates in M81 galaxy
This paper reports on a search for new classical nova candidates in the M81
galaxy based on archival, as well as recent, new images. We used images from
1999-2007 to search for optical transients in M81. The positions of the
identified classical nova candidates were used to study their spatial
distribution. Kolmogorov - Smirnov test (KS) and bottom-to-top (BTR) ratio
diagnostic were used to analyze the nova candidate distribution and
differentiate between the disk and the bulge populations. In total, 49
classical nova candidates were discovered. In this study, we present the
precise positions and photometry of these objects, plus the photometry of an
additional 9 classical nova candidates found by Neill and Shara (2004). With
our large sample, we find a different spatial distribution of classical nova
candidates when compared to the results of earlier studies. Also, an
extraordinarily bright nova was found and studied in detail.Comment: Accepted for publication in Astronomy & Astrophysics, 23 pages, 8
figure
Asymptotically free theories based on discrete subgroups
We study the critical behavior of discrete spin models related to the 2d O(3)
non-linear sigma model. Precise numerical results suggest that models with
sufficiently large discrete subgroups are in the same universality class as the
original sigma model. We observe that at least up to correlation lengths
the cut-off effects follow effectively an
behaviour both in the O(3) and in the dodecahedron model.Comment: 4 pages, 3 figures, Lattice 2000 (Spin Models
Asymptotically free models and discrete non-Abelian groups
We study the two-dimensional renormalization-group flow induced by
perturbations that reduce the global symmetry of the O(3) sigma-model to the
discrete symmetries of Platonic solids. We estimate the value of the
correlation length at which differences in the behaviour of the various models
should be expected. For the icosahedron model, we find xi > 200. We provide an
explanation for the recent numerical results of Patrascioiu and Seiler and of
Hasenfratz and Niedermayer.Comment: 12 pages, revised versio
Engineering superpositions of displaced number states of a trapped ion
We present a protocol that permits the generation of a subtle with
superposition with 2^(l+1) displaced number states on a circle in phase space
as target state for the center-of-mass motion of a trapped ion. Through a
sequence of 'l' cycles involving the application of laser pulses and
no-fluorescence measurements, explicit expressions for the total duration of
laser pulses employed in the sequence and probability of getting the ion in the
upper electronic state during the 'l' cycles are obtained and analyzed in
detail. Furthermore, assuming that the effective relaxation process of a
trapped ion can be described in the framework of the standard master equation
for the damped harmonic oscillator, we investigate the degradation of the
quantum interference effects inherent to superpositions via Wigner function.Comment: 14 pages, 10 figure
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