123 research outputs found
Ground State Bands of the E(5) and X(5) Critical Symmetries Obtained from Davidson Potentials through a Variational Procedure
Davidson potentials of the form , when used in
the original Bohr Hamiltonian for -independent potentials bridge the
U(5) and O(6) symmetries. Using a variational procedure, we determine for each
value of angular momentum the value of at which the derivative of
the energy ratio with respect to has a sharp maximum,
the collection of values at these points forming a band which practically
coincides with the ground state band of the E(5) model, corresponding to the
critical point in the shape phase transition from U(5) to O(6). The same
potentials, when used in the Bohr Hamiltonian after separating variables as in
the X(5) model, bridge the U(5) and SU(3) symmetries, the same variational
procedure leading to a band which practically coincides with the ground state
band of the X(5) model, corresponding to the critical point of the U(5) to
SU(3) shape phase transition. A new derivation of the Holmberg-Lipas formula
for nuclear energy spectra is obtained as a by-product.Comment: LaTeX, 12 pages, 4 postscript figure
The effects of Population III stars and variable IMF on the chemical evolution of the Galaxy
We studied the effects of a hypothetical initial stellar generation (PopIII)
of only massive and very massive stars (VMS) on the chemical evolution of the
Galaxy. We adopted the two-infall chemical evolution model of Chiappini et al.
and tested several sets of yields for primordial VMS (Pair-Creation SNe), which
produce different amounts of heavy elements than lower mass stars. We focused
on the evolution of alpha-elements, C, N, Fe. The effects of PopIII stars on
the Galactic evolution of these elements is negligible if a few generations of
such stars occurred, whereas they produce different results from the standard
models if they formed for a longer period. Also the effects of a more strongly
variable IMF were discussed, making use of suggestions appeared in the
literature to explain the lack of metal-poor stars in the Galactic halo with
respect to model predictions. The predicted variations in abundances, SN rates,
G-dwarf [Fe/H] distribution are here more dramatic and in contrast with
observations; we concluded that a constant or slightly varying IMF is the best
solution. Our main conclusion is that if VMS existed they must have formed only
for a very short period of time (until the halo gas reached the threshold
metallicity for the formation of very massive objects); in this case, their
effects on the evolution of the studied elements was negligible also in the
earliest phases. We thus cannot prove or disprove the existence of such stars
on the basis of the available data. Due to their large metal production and
short lives, primordial VMS should have enriched the halo gas beyond the
metallicity of the most metal poor stars known in a few Myrs. This constrains
the number of Pair-Creation SNe: we find that a number of 2-20 of such SNe
occurred in our Galaxy depending on the stellar yields.Comment: 30 pages, 10 figures, accepted for publication in New Astronom
Heating of the Intergalactic Medium by Primordial Miniquasars
A simple analytical model is used to calculate the X-ray heating of the IGM
for a range of black hole masses. This process is efficient enough to decouple
the spin temperature of the intergalactic medium from the cosmic microwave
background (CMB) temperature and produce a differential brightness temperature
of the order of out to distances as large as a few
co-moving Mpc, depending on the redshift, black hole mass and lifetime. We
explore the influence of two types of black holes, those with and without
ionising UV radiation. The results of the simple analytical model are compared
to those of a full spherically symmetric radiative transfer code. Two simple
scenarios are proposed for the formation and evolution of black hole mass
density in the Universe. The first considers an intermediate mass black hole
that form as an end-product of Population III stars, whereas the second
considers super-massive black holes that form directly through the collapse of
massive halos with low spin parameter. These scenarios are shown not to violate
any of the observational constraints, yet produce enough X-ray photons to
decouple the spin-temperature from that of the CMB. This is an important issue
for future high redshift 21 cm observations.Comment: Replaced with a revised version to match the MNRAS accepted versio
A past capture event at Sagittarius A* inferred from the fluorescent X-ray emission of Sagittarius B clouds
The fluorescent X-ray emission from neutral iron in the molecular clouds (Sgr
B) indicates that the clouds are being irradiated by an external X-ray source.
The source is probably associated with the Galactic central black hole (Sgr
A*), which triggered a bright outburst one hundred years ago. We suggest that
such an outburst could be due to a partial capture of a star by Sgr A*, during
which a jet was generated. By constraining the observed flux and the time
variability ( 10 years) of the Sgr B's fluorescent emission, we find that
the shock produced by the interaction of the jet with the dense interstellar
medium represents a plausible candidate for the X-ray source emission.Comment: 7 pages, 1 figure, accepted for publication in MNRA
A New, Efficient Stellar Evolution Code for Calculating Complete Evolutionary Tracks
We present a new stellar evolution code and a set of results, demonstrating
its capability at calculating full evolutionary tracks for a wide range of
masses and metallicities. The code is fast and efficient, and is capable of
following through all evolutionary phases, without interruption or human
intervention. It is meant to be used also in the context of modeling the
evolution of dense stellar systems, for performing live calculations for both
normal star models and merger-products.
The code is based on a fully implicit, adaptive-grid numerical scheme that
solves simultaneously for structure, mesh and chemical composition. Full
details are given for the treatment of convection, equation of state, opacity,
nuclear reactions and mass loss.
Results of evolutionary calculations are shown for a solar model that matches
the characteristics of the present sun to an accuracy of better than 1%; a 1
Msun model for a wide range of metallicities; a series of models of stellar
populations I and II, for the mass range 0.25 to 64 Msun, followed from
pre-main-sequence to a cool white dwarf or core collapse. An initial final-mass
relationship is derived and compared with previous studies. Finally, we briefly
address the evolution of non-canonical configurations, merger-products of
low-mass main-sequence parents.Comment: MNRAS, in press; several sections and figures revise
Analytic descriptions for transitional nuclei near the critical point
Exact solutions of the Bohr Hamiltonian with a five-dimensional square well
potential, in isolation or coupled to a fermion by the five-dimensional
spin-orbit interaction, are considered as examples of a new class of dynamical
symmetry or Bose-Fermi dynamical symmetry. The solutions provide baselines for
experimental studies of even-even [E(5)] and odd-mass [E(5|4)] nuclei near the
critical point of the spherical to deformed gamma-unstable phase transition.Comment: LaTeX (elsart), 53 pages; typographical correction to (3.15
Pair-Instability Supernovae at the Epoch of Reionization
Pristine stars with masses between ~140 and 260 M_sun are theoretically
predicted to die as pair-instability supernovae. These very massive progenitors
could come from Pop III stars in the early universe. We model the light curves
and spectra of pair-instability supernovae over a range of masses and envelope
structures. At redshifts of reionization z >= 6, we calculate the rates and
detectability of pair-instability and core collapse supernovae, and show that
with the James Webb Space Telescope, it is possible to determine the
contribution of Pop III and Pop II stars toward reionization by constraining
the stellar initial mass function at that epoch using these supernovae. We also
find the rates of Type Ia supernovae, and show that they are not rare during
reionization, and can be used to probe the mass function at 4-8 M_sun. If the
budget of ionizing photons was dominated by contributions from top-heavy Pop
III stars, we predict that the bright end of the galaxy luminosity function
will be contaminated by pair-instability supernovae.Comment: 12 pages, 11 figures. Matches MNRAS accepted versio
On the Progenitors of Core-Collapse Supernovae
Theory holds that a star born with an initial mass between about 8 and 140
times the mass of the Sun will end its life through the catastrophic
gravitational collapse of its iron core to a neutron star or black hole. This
core collapse process is thought to usually be accompanied by the ejection of
the star's envelope as a supernova. This established theory is now being tested
observationally, with over three dozen core-collapse supernovae having had the
properties of their progenitor stars directly measured through the examination
of high-resolution images taken prior to the explosion. Here I review what has
been learned from these studies and briefly examine the potential impact on
stellar evolution theory, the existence of "failed supernovae", and our
understanding of the core-collapse explosion mechanism.Comment: 7 Pages, invited review accepted for publication by Astrophysics and
Space Science (special HEDLA 2010 issue
Supernova 2007bi as a pair-instability explosion
Stars with initial masses 10 M_{solar} < M_{initial} < 100 M_{solar} fuse
progressively heavier elements in their centres, up to inert iron. The core
then gravitationally collapses to a neutron star or a black hole, leading to an
explosion -- an iron-core-collapse supernova (SN). In contrast, extremely
massive stars (M_{initial} > 140 M_{solar}), if such exist, have oxygen cores
which exceed M_{core} = 50 M_{solar}. There, high temperatures are reached at
relatively low densities. Conversion of energetic, pressure-supporting photons
into electron-positron pairs occurs prior to oxygen ignition, and leads to a
violent contraction that triggers a catastrophic nuclear explosion. Tremendous
energies (>~ 10^{52} erg) are released, completely unbinding the star in a
pair-instability SN (PISN), with no compact remnant. Transitional objects with
100 M_{solar} < M_{initial} < 140 M_{solar}, which end up as iron-core-collapse
supernovae following violent mass ejections, perhaps due to short instances of
the pair instability, may have been identified. However, genuine PISNe, perhaps
common in the early Universe, have not been observed to date. Here, we present
our discovery of SN 2007bi, a luminous, slowly evolving supernova located
within a dwarf galaxy (~1% the size of the Milky Way). We measure the exploding
core mass to be likely ~100 M_{solar}, in which case theory unambiguously
predicts a PISN outcome. We show that >3 M_{solar} of radioactive 56Ni were
synthesized, and that our observations are well fit by PISN models. A PISN
explosion in the local Universe indicates that nearby dwarf galaxies probably
host extremely massive stars, above the apparent Galactic limit, perhaps
resulting from star formation processes similar to those that created the first
stars in the Universe.Comment: Accepted version of the paper appearing in Nature, 462, 624 (2009),
including all supplementary informatio
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