41,614 research outputs found

### Galaxy And Mass Assembly (GAMA): linking star formation histories and stellar mass growth

We present evidence for stochastic star formation histories in low-mass (M* < 1010 MâŠ™) galaxies from observations within the Galaxy And Mass Assembly (GAMA) survey. For Ëœ73 000 galaxies between 0.05 < z < 0.32, we calculate star formation rates (SFR) and specific star formation rates (SSFR = SFR/M*) from spectroscopic HÎ± measurements and apply dust corrections derived from Balmer decrements. We find a dependence of SSFR on stellar mass, such that SSFRs decrease with increasing stellar mass for star-forming galaxies, and for the full sample, SSFRs decrease as a stronger function of stellar mass. We use simple parametrizations of exponentially declining star formation histories to investigate the dependence on stellar mass of the star formation time-scale and the formation redshift. We find that parametrizations previously fit to samples of z Ëœ 1 galaxies cannot recover the distributions of SSFRs and stellar masses observed in the GAMA sample between 0.05 < z < 0.32. In particular, a large number of low-mass (M* < 1010 MâŠ™) galaxies are observed to have much higher SSFRs than can be explained by these simple models over the redshift range of 0.05 < z < 0.32, even when invoking mass-dependent staged evolution. For such a large number of galaxies to maintain low stellar masses, yet harbour such high SSFRs, requires the late onset of a weak underlying exponentially declining star formation history with stochastic bursts of star formation superimposed

### Theoretical and experimental investigation of the physics of crystalline surfaces Quarterly report, 1 Feb. - 30 Apr. 1967

Theoretical and experimental investigation of physics of crystalline surface

### Microscopic Approach to Nucleon Spectra in Hypernuclear Non-Mesonic Weak Decay

A consistent microscopic diagrammatic approach is applied for the first time
to the calculation of the nucleon emission spectra in the non-mesonic weak
decay of Lambda-hypernuclei. We adopt a nuclear matter formalism extended to
finite nuclei via the local density approximation, a one--meson exchange weak
transition potential and a Bonn nucleon-nucleon strong potential. Ground state
correlations and final state interactions, at second order in the
nucleon--nucleon interaction, are introduced on the same footing for all the
isospin channels of one- and two-nucleon induced decays. Single and
double--coincidence nucleon spectra are predicted for 12_Lambda^C and compared
with recent KEK and FINUDA data. The key role played by quantum interference
terms allows us to improve the predictions obtained with intranuclear cascade
codes. Discrepancies with data remain for proton emission.Comment: 11 pages, 6 figures, 2 tables. To be published in Physics Letters

### On some derivatives of phenylethers, 2

Products and the synthesis of chloronitrobenzol with certain phenolates are discussed, as is the p-oxyphenylether occasionally produced. Yield, melting point, and physical description are given for each product. The products include 2,4'-dinitrophenylether; 2,2'-dinitrophenylether; p-nitrophenylether-p-oxybenzoic acid and its methylester; p-aminophenylether-p-oxybenzoic acid, its sulfate, and its barium salt; and p-oxypenylether

### Neutron matter under strong magnetic fields: a comparison of models

The equation of state of neutron matter is affected by the presence of a
magnetic field due to the intrinsic magnetic moment of the neutron. Here we
study the equilibrium configuration of this system for a wide range of
densities, temperatures and magnetic fields. Special attention is paid to the
behavior of the isothermal compressibility and the magnetic susceptibility. Our
calculation is performed using both microscopic and phenomenological approaches
of the neutron matter equation of state, namely the Brueckner--Hartree--Fock
(BHF) approach using the Argonne V18 nucleon-nucleon potential supplemented
with the Urbana IX three-nucleon force, the effective Skyrme model in a
Hartree--Fock description, and the Quantum Hadrodynamic formulation with a mean
field approximation. All these approaches predict a change from completely spin
polarized to partially polarized matter that leads to a continuous equation of
state. The compressibility and the magnetic susceptibility show characteristic
behaviors, which reflect that fact. Thermal effects tend to smear out the
sharpness found for these quantities at T=0. In most cases a thermal increase
of 10 MeV is enough to hide the signals of the change of polarization. The set
of densities and magnetic field intensities for which the system changes it
spin polarization is different for each model. However, there is an overall
agreement between the three theoretical descriptions.Comment: updated to correspond with the published versio

### On the role of ground state correlations in hypernuclear non-mesonic weak decay

The contribution of ground state correlations (GSC) to the non--mesonic weak
decay of $^{12}_\Lambda$C and other medium to heavy hypernuclei is studied
within a nuclear matter formalism implemented in a local density approximation.
We adopt a weak transition potential including the exchange of the complete
octets of pseudoscalar and vector mesons as well as a residual strong
interaction modeled on the Bonn potential. Leading GSC contributions, at first
order in the residual strong interaction, are introduced on the same footing
for all isospin channels of one-- and two--nucleon induced decays. Together
with fermion antisymmetrization, GSC turn out to be important for an accurate
determination of the decay widths. Besides opening the two--nucleon stimulated
decay channels, for $^{12}_\Lambda$C GSC are responsible for 14% of the rate
$\Gamma_1$ while increasing the $\Gamma_{n}/\Gamma_{p}$ ratio by 4%. Our final
results for $^{12}_\Lambda$C are: $\Gamma_{\rm NM}=0.98$,
$\Gamma_{n}/\Gamma_{p}=0.34$ and $\Gamma_2/\Gamma_{\rm NM}=0.26$. The
saturation property of $\Gamma_{\rm NM}$ with increasing hypernuclear mass
number is clearly observed. The agreement with data of our predictions for
$\Gamma_{\rm NM}$, $\Gamma_n/\Gamma_p$ and $\Gamma_2$ is rather good.Comment: 32 pages, 9 figure

### Diffusion of Monochromatic Classical Waves

We study the diffusion of monochromatic classical waves in a disordered
acoustic medium by scattering theory. In order to avoid artifacts associated
with mathematical point scatterers, we model the randomness by small but finite
insertions. We derive expressions for the configuration-averaged energy flux,
energy density, and intensity for one, two and three dimensional (1D, 2D and
3D) systems with an embedded monochromatic source using the ladder
approximation to the Bethe-Salpeter equation. We study the transition from
ballistic to diffusive wave propagation and obtain results for the
frequency-dependence of the medium properties such as mean free path and
diffusion coefficient as a function of the scattering parameters. We discover
characteristic differences of the diffusion in 2D as compared to the
conventional 3D case, such as an explicit dependence of the energy flux on the
mean free path and quite different expressions for the effective transport
velocity.Comment: 11 pages, 2 figure

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