28,377 research outputs found
r-Process Nucleosynthesis in Shocked Surface Layers of O-Ne-Mg Cores
We demonstrate that rapid expansion of the shocked surface layers of an
O-Ne-Mg core following its collapse can result in r-process nucleosynthesis. As
the supernova shock accelerates through these layers, it makes them expand so
rapidly that free nucleons remain in disequilibrium with alpha-particles
throughout most of the expansion. This allows heavy r-process isotopes
including the actinides to form in spite of the very low initial neutron excess
of the matter. We estimate that yields of heavy r-process nuclei from this site
may be sufficient to explain the Galactic inventory of these isotopes.Comment: 11 pages, 1 figure, to appear in the Astrophysical Journal Letter
Evolution of O Abundance Relative to Fe
We present a three-component mixing model for the evolution of O abundance
relative to Fe, taking into account the contributions of the first very massive
(> 100 solar masses) stars formed from Big Bang debris. We show that the
observations of O and Fe abundances in metal-poor stars in the Galaxy by
Israelian et al. and Boesgaard et al. can be well represented both
qualitatively and quantitatively by this model. Under the assumption of an
initial Fe ([Fe/H] = -3) and O inventory due to the prompt production by the
first very massive stars, the data at -3 < [Fe/H] < -1 are interpreted to
result from the addition of O and Fe only from type II supernovae (SNII) to the
prompt inventory. At [Fe/H] = -1, SNII still contribute O while both SNII and
type Ia supernovae contribute Fe. During this later stage, (O/Fe) sharply drops
off to an asymptotic value of 0.8(O/Fe)_sun. The value of (O/Fe) for the prompt
inventory at [Fe/H] = -3 is found to be (O/Fe) = 20(O/Fe)_sun. This result
suggests that protogalaxies with low ``metallicities'' should exhibit high
values of (O/Fe). The C/O ratio produced by the first very massive stars is
expected to be much less than 1 so that all the C should be tied up as CO and
that C dust and hydrocarbon compounds should be quite rare at epochs
corresponding to [Fe/H] < -3.Comment: 25 pages, 8 postscript figures, to appear in Ap
High Fill-Out, Extreme Mass Ratio Overcontact Binary Systems. X. The new discovered binary XY Leonis Minoris
The new discovered short-period close binary star, XY LMi, was monitored
photometrically since 2006. It is shown that the light curves are typical
EW-type and show complete eclipses with an eclipse duration of about 80
minutes. By analyzing the complete B, V, R, and I light curves with the 2003
version of the W-D code, photometric solutions were determined. It is
discovered that XY LMi is a high fill-out, extreme mass ratio overcontact
binary system with a mass ratio of q=0.148 and a fill-out factor of f=74.1%,
suggesting that it is on the late evolutionary stage of late-type tidal-locked
binary stars. As observed in other overcontact binary stars, evidence for the
presence of two dark spots on both components are given. Based on our 19
epoches of eclipse times, it is found that the orbital period of the
overcontact binary is decreasing continuously at a rate of
dP/dt=-1.67\times10^{-7}\,days/year, which may be caused by the mass transfer
from the primary to the secondary or/and angular momentum loss via magnetic
stellar wind. The decrease of the orbital period may result in the increase of
the fill-out, and finally, it will evolve into a single rapid-rotation star
when the fluid surface reaching the outer critical Roche Lobe.Comment: 19 pages, 4 figures, 9 table
A Circumbinary Planet in Orbit Around the Short-Period White-Dwarf Eclipsing Binary RR Cae
By using six new determined mid-eclipse times together with those collected
from the literature, we found that the Observed-Calculated (O-C) curve of RR
Cae shows a cyclic change with a period of 11.9 years and an amplitude of
14.3s, while it undergoes an upward parabolic variation (revealing a long-term
period increase at a rate of dP/dt =+4.18(+-0.20)x10^(-12). The cyclic change
was analyzed for the light-travel time effect that arises from the
gravitational influence of a third companion. The mass of the third body was
determined to be M_3*sin i' = 4.2(+-0.4) M_{Jup} suggesting that it is a
circumbinary giant planet when its orbital inclination is larger than 17.6
degree. The orbital separation of the circumbinary planet from the central
eclipsing binary is about 5.3(+-0.6)AU. The period increase is opposite to the
changes caused by angular momentum loss via magnetic braking or/and
gravitational radiation, nor can it be explained by the mass transfer between
both components because of its detached configuration. These indicate that the
observed upward parabolic change is only a part of a long-period (longer than
26.3 years) cyclic variation, which may reveal the presence of another giant
circumbinary planet in a wide orbit.Comment: It will be published in the MNRA
Clustering Coefficients of Protein-Protein Interaction Networks
The properties of certain networks are determined by hidden variables that
are not explicitly measured. The conditional probability (propagator) that a
vertex with a given value of the hidden variable is connected to k of other
vertices determines all measurable properties. We study hidden variable models
and find an averaging approximation that enables us to obtain a general
analytical result for the propagator. Analytic results showing the validity of
the approximation are obtained. We apply hidden variable models to
protein-protein interaction networks (PINs) in which the hidden variable is the
association free-energy, determined by distributions that depend on
biochemistry and evolution. We compute degree distributions as well as
clustering coefficients of several PINs of different species; good agreement
with measured data is obtained. For the human interactome two different
parameter sets give the same degree distributions, but the computed clustering
coefficients differ by a factor of about two. This shows that degree
distributions are not sufficient to determine the properties of PINs.Comment: 16 pages, 3 figures, in Press PRE uses pdflate
Angle-resolved photoemission studies of the superconducting gap symmetry in Fe-based superconductors
The superconducting gap is the fundamental parameter that characterizes the
superconducting state, and its symmetry is a direct consequence of the
mechanism responsible for Cooper pairing. Here we discuss about angle-resolved
photoemission spectroscopy measurements of the superconducting gap in the
Fe-based high-temperature superconductors. We show that the superconducting gap
is Fermi surface dependent and nodeless with small anisotropy, or more
precisely, a function of momentum. We show that while this observation is
inconsistent with weak coupling approaches for superconductivity in these
materials, it is well supported by strong coupling models and global
superconducting gaps. We also suggest that the strong anisotropies measured by
other probes sensitive to the residual density of states are not related to the
pairing interaction itself, but rather emerge naturally from the smaller
lifetime of the superconducting Cooper pairs that is a direct consequence of
the momentum dependent interband scattering inherent to these materials.Comment: 7 pages, 5 figure
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