935 research outputs found
The SDSS Damped Lya Survey: Data Release 1
We present the results from an automated search for damped Lya (DLA) systems
in the quasar spectra of Data Release 1 from the Sloan Digital Sky Survey
(SDSS-DR1). At z~2.5, this homogeneous dataset has greater statistical
significance than the previous two decades of research. We derive a statistical
sample of 71 damped Lya systems (>50 previously unpublished) at z>2.1 and
measure HI column densities directly from the SDSS spectra. The number of DLA
systems per unit redshift is consistent with previous measurements and we
expect our survey has >95% completeness. We examine the cosmological baryonic
mass density of neutral gas Omega_g inferred from the damped Lya systems from
the SDSS-DR1 survey and a combined sample drawn from the literature. Contrary
to previous results, the Omega_g values do not require a significant correction
from Lyman limit systems at any redshift. We also find that the Omega_g values
for the SDSS-DR1 sample do not decline at high redshift and the combined sample
shows a (statistically insignificant) decrease only at z>4. Future data
releases from SDSS will provide the definitive survey of DLA systems at z~2.5
and will significantly reduce the uncertainty in Omega_g at higher redshift.Comment: 12 pages, includes color figures. Accepted to PASP, April 20 200
On the Incidence of C IV Absorbers Along the Sightlines to Gamma-Ray Bursts
We report on the statistics of strong (W_r > 0.15 A) C IV absorbers at
z=1.5-3.5 toward high-redshift gamma-ray bursts (GRBs). In contrast with a
recent survey for strong Mg II absorption systems at z < 2, we find that the
number of C IV absorbers per unit redshift dN/dz does not show a significant
deviation from previous surveys using quasi-stellar objects (QSOs) as
background sources. We find that the number density of C IV toward GRBs is
dN/dz(z~1.5)= 2.2 +2.8/-1.4, dN/dz(z~2.5)= 2.3 +1.8/-1.1 and dN/dz(z~3.5)= 1.1
+2.6/-0.9. These numbers are consistent with previous C IV surveys using QSO
spectra. Binning the entire dataset, we set a 95% c.l. upper limit to the
excess of C IV absorbers along GRB sightlines at twice the incidence observed
along QSO sightlines. Furthermore, the distribution of equivalent widths of the
GRB and QSO samples are consistent with being drawn from the same parent
population. Although the results for Mg II and C IV absorbers along GRB
sightlines appear to contradict one another, we note that the surveys are
nearly disjoint: the C IV survey corresponds to higher redshift and more highly
ionized gas than the Mg II survey. Nevertheless, analysis on larger statistical
samples may constrain properties of the galaxies hosting these metals (e.g.
mass, dust content) and/or the coherence-length of the gas giving rise to the
metal-line absorption.Comment: Accepted version (for publication in ApJ), results unchanged, 18
pages, 3 tables, 5 figure
Formation et désexcitation des noyaux chauds dans les réactions induites par des faisceaux de protons (475 MeV et 2 GeV) et d'He (2 GeV)
We are studying the formation and the de-excitation of hot nuclei created in reactions induced by light high energy projectiles. These reactions, described in a two step model: an intranuclear cascade followed by an evaporation phase, produce nuclei in which the collective modes (compression, rotation, deformation) are weakly excited. By measuring the neutron multiplicities, event by event with ORION, and the light charged particle energies and multiplicities one can evaluate the excitation energy distribution of the nuclei. At the same time, theoretical simulations are carried out using the intranuclear cascade code developed by J. Cugnon and the statistical de-excitation code GEMINI. The good agreement with experimental results indicate that 10% of the p-nucleus interactions lead to temperatures greater than 5 MeV. The observation of the fission of a nucleus with a temperature close to 5 MeV shows that the nucleus behaves as a set of bound nucleons and, that the temperature stability limit is not yet reached. The observed decline of fission probability at high excitation energies is most likely to be correlated to the appearance of an other de-excitation process (evaporation residues emission or multifragmentation) which could not be experimentally detected. Finally, in the last chapter, we briefly present the principle of transmutation for long-lived nuclear waste with a proton accelerator and underline the interest of the present work in such studies.Nous étudions la formation et la désexcitation des noyaux chauds issus de réactionsinduites par des projectiles légers de grande énergie. Ces réactions, décrites par unprocessus en deux étapes: une cascade intranucléaire suivie d'une phase d'évaporation,produisent des noyaux dont les modes collectifs (compression, rotation, déformation)sont peu excités. La multiplicité totale de neutrons mesurée événement par événementavec le détecteur de neutrons, ORION, d'une part, les spectres énergétiques et lesmultiplicités de particules chargées légères d'autre part, permettent d'évaluer ladistribution d'énergie d'excitation des noyaux formés. En parallèle des simulationsthéoriques sont réalisées en utilisant le code de cascade intranucléaire développé parJ. Cugnon et le programme de désexcitation statistique GEMINI. Le bon accord avec lesrésultats expérimentaux nous indique que 10% des interactions p-noyau conduisent àdes températures supérieures à 5 MeV. L'observation de la fission d'un noyau detempérature voisine de 5 MeV indique que le noyau reste un ensemble de nucléons liésrépondant collectivement et que la limite de stabilité en température de ce noyau n'estdonc pas encore atteinte. Le déclin de la probabilité de fission à haute énergied'excitation doit être corrélé à l'apparition d'un autre processus de désexcitation(émission de résidus d'évaporation ou multifragmentation) qui n'a pu être mis enévidence expérimentalement. Enfin dans le dernier chapitre nous évoquons brièvementle principe de la transmutation des déchets nucléaires de longue période à l'aide d'unaccélérateur de protons et soulignons l'intérêt que peut présenter ce travail pour de tellesétudes
Ionized Gas in Damped Lyman Alpha Protogalaxies: I. Model-Independent Inferences From Kinematic Data
We investigate the kinematics of ionized and neutral gas in a sample of 35
damped Lyman alpha protogalaxies (DLAs) with HIRES on the Keck I 10 m
telescope. Velocity profiles with resolution of ~ 8 km/s are obtained for high
ions such as C IV and Si IV, and for intermediate ions such as Al III.
Combining these profiles with accurate low-ion (e.g., Fe II) profiles, we
investigate the kinematic state of DLAs in the redshift range 1.8 < z < 4.4 by
comparisons between data for various ion pairs.
We find the DLAs comprise distinct kinematic subsystems: a low ion subsystem
in which the low ions are physically associated with intermediate ions, and a
high subsystem containing neither low nor intermediate ions. The evidence for
two subsystems stems from (a) differences between the widths of the velocity
profiles, (b) misalignment in velocity space between the narrow components
comprising the profiles in each subsystem, and (c) significant dissimilarities
between the mean velocities of the high ion and low ion velocity profiles. In
every case we find that test statistics such as velocity width and various
asymmetry parameters distribute differently for low ions than for high ions.
Despite misalignment in velocity space, the low and high ion kinematic
subsystems are interrelated. This is indicated by detection of a statistically
significant C IV versus low-ion cross correlation function, and by a systematic
effect where the C IV velocity widths are greater than or equal to the low ion
velocity widths in 29 out of 32 systems. These phenomena are consistent with
the location of the two subsystems in the same potential well.Comment: 15 pages, 9 figures, Accepted for publication in Dec. 20 issue of the
Astrophysical Journa
Spallation Neutron Production by 0.8, 1.2 and 1.6 GeV Protons on various Targets
Spallation neutron production in proton induced reactions on Al, Fe, Zr, W,
Pb and Th targets at 1.2 GeV and on Fe and Pb at 0.8, and 1.6 GeV measured at
the SATURNE accelerator in Saclay is reported. The experimental
double-differential cross-sections are compared with calculations performed
with different intra-nuclear cascade models implemented in high energy
transport codes. The broad angular coverage also allowed the determination of
average neutron multiplicities above 2 MeV. Deficiencies in some of the models
commonly used for applications are pointed out.Comment: 20 pages, 32 figures, revised version, accepted fpr publication in
Phys. Rev.
Cognitive appraisal of environmental stimuli induces emotion-like states in fish
The occurrence of emotions in non-human animals has been the focus of debate over the years. Recently, an interest in expanding this debate to non-tetrapod vertebrates and to invertebrates has emerged. Within vertebrates, the study of emotion in teleosts is particularly interesting since they represent a divergent evolutionary radiation from that of tetrapods, and thus they provide an insight into the evolution of the biological mechanisms of emotion. We report that Sea Bream exposed to stimuli that vary according to valence (positive, negative) and salience (predictable, unpredictable) exhibit different behavioural, physiological and neuromolecular states. Since according to the dimensional theory of emotion valence and salience define a two-dimensional affective space, our data can be interpreted as evidence for the occurrence of distinctive affective states in fish corresponding to each the four quadrants of the core affective space. Moreover, the fact that the same stimuli presented in a predictable vs. unpredictable way elicited different behavioural, physiological and neuromolecular states, suggests that stimulus appraisal by the individual, rather than an intrinsic characteristic of the stimulus, has triggered the observed responses. Therefore, our data supports the occurrence of emotion-like states in fish that are regulated by the individual's perception of environmental stimuli.European Commission [265957 Copewell]; Fundacao para a Ciencia e Tecnologia [SFRH/BD/80029/2011, SFRH/BPD/72952/2010]info:eu-repo/semantics/publishedVersio
The Photophysics of the Carrier of Extended Red Emission
Interstellar dust contains a component which reveals its presence by emitting
a broad, unstructured band of light in the 540 to 950 nm wavelength range,
referred to as Extended Red Emission (ERE). The presence of interstellar dust
and ultraviolet photons are two necessary conditions for ERE to occur. This is
the basis for suggestions which attribute ERE to an interstellar dust component
capable of photoluminescence. In this study, we have collected all published
ERE observations with absolute-calibrated spectra for interstellar
environments, where the density of ultraviolet photons can be estimated
reliably. In each case, we determined the band-integrated ERE intensity, the
wavelength of peak emission in the ERE band, and the efficiency with which
absorbed ultraviolet photons are contributing to the ERE. The data show that
radiation is not only driving the ERE, as expected for a photoluminescence
process, but is modifying the ERE carrier as manifested by a systematic
increase in the ERE band's peak wavelength and a general decrease in the photon
conversion efficiency with increasing densities of the prevailing exciting
radiation. The overall spectral characteristics of the ERE and the observed
high quantum efficiency of the ERE process are currently best matched by the
recently proposed silicon nanoparticle (SNP) model. Using the experimentally
established fact that ionization of semiconductor nanoparticles quenches their
photoluminescence, we proceeded to test the SNP model by developing a
quantitative model for the excitation and ionization equilibrium of SNPs under
interstellar conditions for a wide range of radiation field densities.Comment: 42 p., incl. 8 fig. Accepted for publication by Ap
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