349 research outputs found
Nucleosynthesis imprints from different Type Ia Supernova explosion scenarios and implications for galactic chemical evolution
We analyze the nucleosynthesis yields of various Type Ia supernova explosion
simulations including pure detonations in sub- Chandrasekhar mass white dwarfs,
double detonations and pure helium detonations of sub-Chandrasekhar mass white
dwarfs with an accreted helium envelope, a violent merger model of two white
dwarfs and deflagrations as well as delayed detonations in Chandrasekhar mass
white dwarfs. We focus on the iron peak elements Mn, Zn and Cu. To this end, we
also briefly review the different burning regimes and production sites of these
elements as well as the results of abundance measurements and several galactic
chemical evolution studies. We find that super-solar values of [Mn/Fe] are not
restricted to Chandrasekhar mass explosion models. Scenarios including a helium
detonation can significantly contribute to the production of Mn, in particular
the models proposed for calcium-rich transients. Although Type Ia supernovae
are often not accounted for as production sites of Zn and Cu, our models
involving helium shell detonations can produce these elements in super-solar
ratios relative to Fe. Our results suggest a re-consideration of Type Ia
supernova yields in galactic chemical evolution models. A detailed comparison
with observations can provide new insight into the progenitor and explosion
channels of these events.Comment: 15 paged, 8 figures, accepted for publication in A&
An atlas of 2.4 to 4.1 microns ISO/SWS spectra of early-type stars
We present an atlas of spectra of O- and B-type stars, obtained with the
Short Wavelength Spectrometer (SWS) during the Post-Helium program of the
Infrared Space Observatory (ISO). This program is aimed at extending the Morgan
& Keenan classification scheme into the near-infrared. Later type stars will be
discussed in a seperate publication. The observations consist of 57 SWS
Post-Helium spectra from 2.4 to 4.1 microns, supplemented with 10 spectra
acquired during the nominal mission with a similar observational setting. For
B-type stars, this sample provides ample spectral converage in terms of subtype
and luminosity class. For O-type stars,the ISO sample is coarse and therefore
is complemented with 8 UKIRT L'-band observations. In terms of the presence of
diagnostic lines, the L'-band is likely the most promising of the near-infrared
atmospheric windows for the study of the physical properties of B stars.
Specifically, this wavelength interval contains the Brackett alpha, Pfund
gamma, and other Pfund lines which are probes of spectral type, luminosity
class and mass loss. Here, we present simple empirical methods based on the
lines present in the 2.4 to 4.1 microns interval that allow the determination
of: the spectral type of B dwarfs and giants to within two subtypes; the
luminosity class of B stars to within two classes; the mass-loss rate of O
stars and B supergiants to within 0.25 dex.Comment: 19 pages, 11 Postscript figures, accepted by A&
Static Properties of Trapped Bose-Fermi Mixed Condensate of Alkali Atoms
Static properties of a bose-fermi mixture of trapped potassium atoms are
studied in terms of coupled Gross-Pitaevskii and Thomas-Fermi equations for
both repulsive and attractive bose-fermi interatomic potentials. Qualitative
estimates are given for solutions of the coupled equations, and the parameter
regions are obtained analytically for the boson-density profile change and for
the boson/fermion phase separation. Especially, the parameter ratio
is found that discriminates the region of the large boson-profile change. These
estimates are applied for numerical results for the potassium atoms and checked
their consistency. It is suggested that a small fraction of fermions could be
trapped without an external potential for the system with an attractive
boson-fermion interaction.Comment: 8 pages,5 figure
Zero-temperature phase diagram of binary boson-fermion mixtures
We calculate the phase diagram for dilute mixtures of bosons and fermions at
zero temperature. The linear stability conditions are derived and related to
the effective boson-induced interaction between the fermions. We show that in
equilibrium there are three possibilities: a) a single uniform phase, b) a
purely fermionic phase coexisting with a purely bosonic one and c) a purely
fermionic phase coexisting with a mixed phase.Comment: 8 pages, revtex, 3 postscript figures; NORDITA-1999/71 C
Orientation of the Stripe Formed by the Two-Dimensional Electrons in Higher Landau Levels
Effect of periodic potential on the stripe phase realized in the higher
Landau levels is investigated by the Hartree-Fock approximation. The period of
the potential is chosen to be two to six times of the fundamental period of the
stripe phase. It is found that the stripe aligns perpendicularly to the
external potential in contrast to a naive expectation and hydrodynamic theory.
Charge modulation towards the Wigner crystallization along the stripe is
essential for the present unexpected new result.Comment: 5 pages, RevTex, two figures included in the tex
Sliding motion of a two-dimensional Wigner crystal in a strong magnetic field
We study the sliding state of a two-dimensional Wigner crystal in a strong
magnetic field and a random impurity potential. Using a high-velocity
perturbation theory, we compute the nonlinear conductivity, various correlation
functions, and the interference effects arising in combined AC + DC electric
effects, including the Shapiro anomaly and the linear response to an AC field.
Disorder is found to induce mainly transverse distortions in the sliding state
of the lattice. The Hall resistivity retains its classical value. We find that,
within the large velocity perturbation theory, free carriers which affect the
longitudinal phonon modes of the Wigner crystal do not change the form of the
nonlinear conductivity. We compare the present sliding Wigner crystal in a
strong magnetic field to the conventional sliding charge-density wave systems.
Our result for the nonlinear conductivity agrees well with the
characteristics measured in some experiments at low temperatures or large
depinning fields, for the insulating phases near filling factor = 1/5. We
summarize the available experimental data, and point out the differences among
them.Comment: appeared in RPB vol. 50, 4600 (1994); LaTex file; 3 figures available
from [email protected]
Globular cluster system and Milky Way properties revisited
Updated data of the 153 Galactic globular clusters are used to readdress
fundamental parameters of the Milky Way. We build a reduced sample,
decontaminated of the clusters younger than 10Gyr, those with retrograde orbits
and/or evidence of relation to dwarf galaxies. The 33 metal-rich globular
clusters of the reduced sample extend basically to the Solar circle and
distribute over a region with projected axial-ratios typical of an oblate
spheroidal, . The 81
metal-poor globular clusters span a nearly spherical region of axial-ratios
extending from the central parts to the outer halo. A new
estimate of the Sun's distance to the Galactic center is provided, . The metal-rich and metal-poor radial-density distributions
flatten for and are well represented both by a power-law
with a core-like term and S\'ersic's law; at large distances they fall off as
. Both metallicity components appear to have a common origin,
which is different from that of the dark matter halo. Structural similarities
of the metal-rich and metal-poor radial distributions with the stellar halo are
consistent with a scenario where part of the reduced sample was formed in the
primordial collapse, and part was accreted in an early period of merging. This
applies to the bulge as well, suggesting an early merger affecting the central
parts of the Galaxy. We estimate that the present globular cluster population
corresponds to of the original one. The fact that the
volume-density radial distributions of the metal-rich and metal-poor globular
clusters of the reduced sample follow both a core-like power-law and S\'ersic's
law indicates that we are dealing with spheroidal subsystems in all scales.Comment: 14 pages and 6 figures. Astronomy & Astrophysics, accepted on NOv. 2
GHASP : an H alpha kinematic survey of spiral and irregular galaxies. V. Dark matter distribution in 36 nearby spiral galaxies
The results obtained from a study of the mass distribution of 36 spiral
galaxies are presented. The galaxies were observed using Fabry-Perot
interferometry as part of the GHASP survey. The main aim of obtaining high
resolution H alpha 2D velocity fields is to define more accurately the rising
part of the rotation curves which should allow to better constrain the
parameters of the mass distribution. The H alpha velocities were combined with
low resolution HI data from the literature, when available. Combining the
kinematical data with photometric data, mass models were derived from these
rotation curves using two different functional forms for the halo: an
isothermal sphere and an NFW profile. For the galaxies already modeled by other
authors, the results tend to agree. Our results point at the existence of a
constant density core in the center of the dark matter halos rather than a
cuspy core, whatever the type of the galaxy from Sab to Im. This extends to all
types the result already obtained by other authors studying dwarf and LSB
galaxies but would necessitate a larger sample of galaxies to conclude more
strongly. Whatever model is used (ISO or NFW), small core radius halos have
higher central densities, again for all morphological types. We confirm
different halo scaling laws, such as the correlations between the core radius
and the central density of the halo with the absolute magnitude of a galaxy:
low luminosity galaxies have small core radius and high central density. We
find that the product of the central density with the core radius of the dark
matter halo is nearly constant, whatever the model and whatever the absolute
magnitude of the galaxy. This suggests that the halo surface density is
independent from the galaxy type.Comment: 21 pages, 14 figures. MNRAS (accepted october 3rd 2007
The composition of circumstellar gas and dust in 51 Oph
We analyze ISO archive data of the nearby bright emission-line star 51 Oph,
previously classified as a proto-planetary system similar to beta Pic. The
infrared spectrum reveals the presence of gas-phase emission bands of hot
(approx 850 K) CO, CO_2, H_2O and NO. In addition to this, partially
crystalline silicate dust is present. The solid-state bands and the energy
distribution are indicative of dust that has formed recently, rather than of
debris dust. The presence of hot molecular gas and the composition of the
circumstellar dust are highly unusual for a young star and are reminiscent of
what is found around evolved (AGB) stars, although we exclude the possibility
of 51 Oph belonging to this group. We suggest several explanations for the
nature of 51 Oph, including a recent episode of mass loss from a Be star, and
the recent destruction of a planet-sized body around a young star.Comment: 4 pages, 3 figures, accepted by A&A (letters
Random-phase approximation study of collective excitations in the Bose-Fermi mixed condensate of alkali-metal gases
We perform Random Phase Approximation (RPA) study of collective excitations
in the bose-fermi mixed degenerate gas of Alkali-metal atoms at T=0. The
calculation is done by diagonalization in a model space composed of
particle-hole type excitations from the ground state, the latter being obtained
from the coupled Gross-Pitaevskii and Thomas-Fermi equations. We investigate
strength distributions for different combinations of bose and fermi multipole
() operators with . Transition densities and dynamical structure
factors are calculated for collective excitations. Comparison with the sum rule
prediction for the collective frequency is given. Time dependent behavior of
the system after an external impulse is studied.Comment: 28 pages, 13 figures, submitted to Phys. Rev.
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