7,469 research outputs found
Essence and Cause: Making Something Be What It Is
Aristotle frequently describes essence as a âcauseâ or âexplanationâ, thus ascribing to essence some sort of causal or explanatory role. This explanatory role is often explicated by scholars in terms of essence âmaking the thing be what it isâ or âmaking it the very thing that it isâ. I argue that this is problematic, at least on the assumption that âmakingâ expresses an explanatory relation, since it violates certain formal features of explanation. I then consider whether Aristotle is vulnerable to this problem by examining the explanatory role of essence in Posterior Analytics and Metaphysics Z 17
Origin of the heavy elements in HD 140283. Measurement of europium abundance
HD 140283 is a nearby (V=7.7) subgiant metal-poor star, extensively analysed
in the literature. Although many spectra have been obtained for this star, none
showed a signal-to-noise (S/N) ratio high enough to enable a very accurate
derivation of abundances from weak lines. The detection of europium proves that
the neutron-capture elements in this star originate in the r-process, and not
in the s-process, as recently claimed in the literature. Based on the OSMARCS
1D LTE atmospheric model and with a consistent approach based on the spectrum
synthesis code Turbospectrum, we measured the europium lines at 4129 {\AA} and
4205 {\AA}, taking into account the hyperfine structure of the transitions. The
spectrum, obtained with a long exposure time of seven hours at the
Canada-France-Hawaii Telescope (CFHT), has a resolving power of 81000 and a S/N
ratio of 800 at 4100 {\AA}. We were able to determine the abundance A(Eu)=-2.35
dex, compatible with the value predicted for the europium from the r-process.
The abundance ratio [Eu/Ba]=+0.58 dex agrees with the trend observed in
metal-poor stars and is also compatible with a strong r-process contribution to
the origin of the neutron-capture elements in HD 140283.Comment: 10 pages, 7 figures. To be published in A\&
High-resolution abundance analysis of HD 140283
HD 140283 is a reference subgiant that is metal poor and confirmed to be a
very old star. The abundances of this type of old star can constrain the nature
and nucleosynthesis processes that occurred in its (even older) progenitors.
The present study may shed light on nucleosynthesis processes yielding heavy
elements early in the Galaxy. A detailed abundance analysis of a high-quality
spectrum is carried out, with the intent of providing a reference on stellar
lines and abundances of a very old, metal-poor subgiant. We aim to derive
abundances from most available and measurable spectral lines. The analysis is
carried out using high-resolution (R = 81 000) and high signal-to-noise ratio
(800 < S/N/pixel < 3400) spectrum, in the wavelength range 3700 - 10475,
obtained with a seven-hour exposure time, using the ESPaDOnS at the CFHT. The
calculations in LTE were performed with the OSMARCS 1D atmospheric model and
the spectrum synthesis code Turbospectrum, while the analysis in NLTE is based
on the MULTI code. We present LTE abundances for 26 elements, and NLTE
calculations for the species C I, O I, Na I, Mg I, Al I, K I, Ca I, Sr II, and
Ba II lines. The abundance analysis provided an extensive line list suitable
for metal-poor subgiant stars. The results for Li, CNO, alpha-, and iron peak
elements are in good agreement with literature. The newly NLTE Ba abundance,
along with a NLTE Eu correction and a 3D Ba correction from literature, leads
to [Eu/Ba] = +0.59 +/- 0.18. This result confirms a dominant r-process
contribution, possibly together with a very small contribution from the main
s-process, to the neutron-capture elements in HD 140283. Overabundances of the
lighter heavy elements and the high abundances derived for Ba, La, and Ce
favour the operation of the weak r-process in HD 140283.Comment: 34 pages, 27 figure
Vacuum Polarization for a Massless Spin-1/2 Field in the Global Monopole Spacetime at Nonzero Temperature
In this paper we present the effects produced by the temperature in the
renormalized vacuum expectation value of the zero-zero component of the
energy-momentum tensor associated with massless left-handed spinor field in the
pointlike global monopole spacetime. In order to develop this calculation we
had to obtain the Euclidean thermal Green function in this background. Because
the expression obtained for the thermal energy density cannot be expressed in a
closed form, its explicit dependence on the temperature is not completely
evident. So, in order to obtain concrete information about its thermal
behavior, we develop a numerical analysis of our result in the high-temperature
limit for specific values of the parameter which codify the presence
of the monopole.Comment: 22 pages, LaTex format, 5 figure
The statistical properties of the city transport in Cuernavaca (Mexico) and Random matrix ensembles
We analyze statistical properties of the city bus transport in Cuernavaca
(Mexico) and show that the bus arrivals display probability distributions
conforming those given by the Unitary Ensemble of random matrices.Comment: 4 pages, 3 figure
A Multiwavelength Study on the Fate of Ionizing Radiation in Local Starbursts
The fate of ionizing radiation is vital for understanding cosmic ionization,
energy budgets in the interstellar and intergalactic medium, and star formation
rate indicators. The low observed escape fractions of ionizing radiation have
not been adequately explained, and there is evidence that some starbursts have
high escape fractions. We examine the spectral energy distributions of a sample
of local star-forming galaxies, containing thirteen local starburst galaxies
and ten of their ordinary star-forming counterparts, to determine if there
exist significant differences in the fate of ionizing radiation in these
galaxies. We find that the galaxy-to-galaxy variations in the SEDs is much
larger than any systematic differences between starbursts and non-starbursts.
For example, we find no significant differences in the total absorption of
ionizing radiation by dust, traced by the 24um, 70um, and 160um MIPS bands of
the Spitzer Space Telescope, although the dust in starburst galaxies appears to
be hotter than that of non-starburst galaxies. We also observe no excess
ultraviolet flux in the GALEX bands that could indicate a high escape fraction
of ionizing photons in starburst galaxies. The small H-alpha fractions of the
diffuse, warm ionized medium in starburst galaxies are apparently due to
temporarily boosted H-alpha luminosity within the star-forming regions
themselves, with an independent, constant WIM luminosity. This independence of
the WIM and starburst luminosities contrasts with WIM behavior in non-starburst
galaxies and underscores our poor understanding of radiation transfer in both
ordinary and starburst galaxies.Comment: 10 pages, 8 figures, accepted to ApJ 10/11/1
Wave Scattering through Classically Chaotic Cavities in the Presence of Absorption: An Information-Theoretic Model
We propose an information-theoretic model for the transport of waves through
a chaotic cavity in the presence of absorption. The entropy of the S-matrix
statistical distribution is maximized, with the constraint : n is the dimensionality of S, and meaning complete (no) absorption. For strong absorption our result
agrees with a number of analytical calculations already given in the
literature. In that limit, the distribution of the individual (angular)
transmission and reflection coefficients becomes exponential -Rayleigh
statistics- even for n=1. For Rayleigh statistics is attained even
with no absorption; here we extend the study to . The model is
compared with random-matrix-theory numerical simulations: it describes the
problem very well for strong absorption, but fails for moderate and weak
absorptions. Thus, in the latter regime, some important physical constraint is
missing in the construction of the model.Comment: 4 pages, latex, 3 ps figure
Quantum and Boltzmann transport in the quasi-one-dimensional wire with rough edges
We study quantum transport in Q1D wires made of a 2D conductor of width W and
length L>>W. Our aim is to compare an impurity-free wire with rough edges with
a smooth wire with impurity disorder. We calculate the electron transmission
through the wires by the scattering-matrix method, and we find the Landauer
conductance for a large ensemble of disordered wires. We study the
impurity-free wire whose edges have a roughness correlation length comparable
with the Fermi wave length. The mean resistance and inverse mean
conductance 1/ are evaluated in dependence on L. For L -> 0 we observe the
quasi-ballistic dependence 1/ = = 1/N_c + \rho_{qb} L/W, where 1/N_c
is the fundamental contact resistance and \rho_{qb} is the quasi-ballistic
resistivity. As L increases, we observe crossover to the diffusive dependence
1/ = = 1/N^{eff}_c + \rho_{dif} L/W, where \rho_{dif} is the
resistivity and 1/N^{eff}_c is the effective contact resistance corresponding
to the N^{eff}_c open channels. We find the universal results
\rho_{qb}/\rho_{dif} = 0.6N_c and N^{eff}_c = 6 for N_c >> 1. As L exceeds the
localization length \xi, the resistance shows onset of localization while the
conductance shows the diffusive dependence 1/ = 1/N^{eff}_c + \rho_{dif} L/W
up to L = 2\xi and the localization for L > 2\xi only. On the contrary, for the
impurity disorder we find a standard diffusive behavior, namely 1/ =
= 1/N_c + \rho_{dif} L/W for L < \xi. We also derive the wire conductivity from
the semiclassical Boltzmann equation, and we compare the semiclassical electron
mean-free path with the mean free path obtained from the quantum resistivity
\rho_{dif}. They coincide for the impurity disorder, however, for the edge
roughness they strongly differ, i.e., the diffusive transport is not
semiclassical. It becomes semiclassical for the edge roughness with large
correlation length
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