1,144 research outputs found
A minimum hypothesis explanation for an IMF with a lognormal body and power law tail
We present a minimum hypothesis model for an IMF that resembles a lognormal
distribution at low masses but has a distinct power-law tail. Even if the
central limit theorem ensures a lognormal distribution of condensation masses
at birth, a power-law tail in the distribution arises due to accretion from the
ambient cloud, coupled with a non-uniform (exponential) distribution of
accretion times.Comment: 2 pages, 1 figure, to appear in IMF@50, eds. E. Corbelli, F. Palla,
and H. Zinnecker, Kluwer, Astrophysics and Space Science Librar
The stability of the spectator, Dirac, and Salpeter equations for mesons
Mesons are made of quark-antiquark pairs held together by the strong force.
The one channel spectator, Dirac, and Salpeter equations can each be used to
model this pairing. We look at cases where the relativistic kernel of these
equations corresponds to a time-like vector exchange, a scalar exchange, or a
linear combination of the two. Since the model used in this paper describes
mesons which cannot decay physically, the equations must describe stable
states. We find that this requirement is not always satisfied, and give a
complete discussion of the conditions under which the various equations give
unphysical, unstable solutions
Nuclear fusion in a dense plasma
The standard theory of nuclear fusion rates in a strongly interacting plasma
can be (correctly) derived only when the energy release, Q, is large compared
to other energies in the problem. We exhibit a result for rates that provides a
basis for calculating the finite Q corrections. Crude estimates indicate a
significant defect in the conventional results for some regions of high density
and strong plasma coupling. We also lay some groundwork for a path integral
calculation of the new effects.Comment: Final version to appear in Phys. Rev. Lett
A Variational Approach to the Spinless Relativistic Coulomb Problem
By application of a straightforward variational procedure we derive a simple,
analytic upper bound on the ground-state energy eigenvalue of a
semirelativistic Hamiltonian for (one or two) spinless particles which
experience some Coulomb-type interaction.Comment: 7 pages, HEPHY-PUB 606/9
Embedded Clusters and the IMF
Despite valiant efforts over nearly five decades, attempts to determine the
IMF over a complete mass range for galactic field stars and in open clusters
have proved difficult. Infrared imaging observations of extremely young
embedded clusters coupled with Monte Carlo modeling of their luminosity
functions are improving this situation and providing important new
contributions to our fundamental knowledge of the IMF and its universality in
both space and time.Comment: 6 pages, 2 figures to appear in "The IMF@50", Kluwer Academic Press,
eds. C. Corbelli, F. Palla, & Hans Zinnecke
Normalization of the covariant three-body bound state vertex function
The normalization condition for the relativistic three nucleon Bethe-Salpeter
and Gross bound state vertex functions is derived, for the first time, directly
from the three body wave equations. It is also shown that the relativistic
normalization condition for the two body Gross bound state vertex function is
identical to the requirement that the bound state charge be conserved, proving
that charge is automatically conserved by this equation.Comment: 24 pages, 9 figures, published version, minor typos correcte
Mg II Absorber Number Density at z~0.05: Implications for Omega_DLA Evolution
An unbiased sample of 147 quasar/AGN spectra, obtained with the FOS/HST, has
been searched for intervening MgII absorbers over the redshift range 0<z<0.15.
The total redshift path searched is 18.8, with the survey being 80% complete to
a 5-sigma rest-frame equivalent width, W_r(2796), of 0.6 Ang. Main results of
this work are: [1] Four systems were found, with a mean redshift of =0.06,
yielding a redshift number density dN/dz=0.22(+0.12)(-0.09) for absorbers with
W_r(2796)>0.6 Ang. This is consistent with the value expected if these systems
do not evolve from higher redshifts (z=2.2). [2] No systems with W_r(2796)<0.6
Ang were found. It is a 2-sigma result to have a null detection of smaller
W_r(2796) systems. If this implies a turnover in the low W_r(2796) region of
the equivalent width distribution at z~0, then there is at least a 25%
reduction in the average galaxy gas cross section from z<0.2 galaxies. [3]
These systems have strong FeII absorption and are good candidates for damped
Ly-alpha absorbers DLAs (see Rao & Turnshek 2000, ApJS, 130, 1). This
translates to a redshift number density of dN/dz=0.08(+0.09)(-0.05) for DLAs at
z~0. In tandem with the data analyzed by Rao & Turnshek, these results indicate
that the redshift number density of DLAs does not evolve from z~4 to z~0. If
the HI mass function does not evolve from z~0.5 to z~0, then the cosmological
HI mass density is also deduced to not evolve from z~4 to z~0. These z~0
results for MgII absorption-selected DLAs are at odds with those based upon
21-cm emission from HI galaxies by a factor of five to six.Comment: 23 pages, 7 Figures, accepted to ApJ. Replaced version includes
additional figures and tables and substantial modifications to the tex
Are Neutron-Rich Elements Produced in the Collapse of Strange Dwarfs ?
The structure of strange dwarfs and that of hybrid stars with same baryonic
number is compared. There is a critical mass (M~0.24M_sun) in the strange dwarf
branch, below which configurations with the same baryonic number in the hybrid
star branch are more stable. If a transition occurs between both branches, the
collapse releases an energy of about of 3x10^{50} erg, mostly under the form of
neutrinos resulting from the conversion of hadronic matter onto strange quark
matter. Only a fraction (~4%) is required to expel the outer neutron-rich
layers. These events may contribute significantly to the chemical yield of
nuclides with A>80 in the Galaxy, if their frequency is of about one per 1500
years.Comment: Accepted for publication in IJMP
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