518 research outputs found
Next-to-leading order QCD evolution of transversity fragmentation functions
We derive the next-to-leading order splitting kernels for the scale evolution
of fragmentation functions for transversely polarized quarks into transversely
polarized hadrons.Comment: 9 pages, LaTe
Isospin influences on particle emission and critical phenomenon in nuclear dissociation
Features of particle emission and critical point behavior are investigated as
functions of the isospin of disassembling sources and temperature at a moderate
freeze-out density for medium-size Xe isotopes in the framework of isospin
dependent lattice gas model. Multiplicities of emitted light particles,
isotopic and isobaric ratios of light particles show the strong dependence on
the isospin of the dissociation source, but double ratios of light isotope
pairs and the critical temperature determined by the extreme values of some
critical observables are insensitive to the isospin of the systems. Values of
the power law parameter of cluster mass distribution, mean multiplicity of
intermediate mass fragments (), information entropy () and Campi's
second moment () also show a minor dependence on the isospin of Xe
isotopes at the critical point. In addition, the slopes of the average
multiplicites of the neutrons (), protons (), charged particles
(), and IMFs (), slopes of the largest fragment mass number
(), and the excitation energy per nucleon of the disassembling source
() to temperature are investigated as well as variances of the
distributions of , , , , and . It
is found that they can be taken as additional judgements to the critical
phenomena.Comment: 9 Pages, 8 figure
Statistical nature of cluster emission in nuclear liquid-vapour phase coexistence
The emission of nuclear clusters is investigated within the framework of
isospin dependent lattice gas model and classical molecular dynamics model. It
is found that the emission of individual cluster which is heavier than proton
is almost Poissonian except near the transition temperature at which the system
is leaving the liquid-vapor phase coexistence and the thermal scaling is
observed by the linear Arrhenius plots which is made from the average
multiplicity of each cluster versus the inverse of temperature in the liquid
vapor phase coexistence. The slopes of the Arrhenius plots, {\it i.e.} the
"emission barriers", are extracted as a function of the mass or charge number
and fitted by the formula embodied with the contributions of the surface energy
and Coulomb interaction. The good agreements are obtained in comparison with
the data for low energy conditional barriers. In addition, the possible
influences of the source size, Coulomb interaction and "freeze-out" density and
related physical implications are discussed
The initial mass function of the rich young cluster NGC 1818 in the Large Magellanic Cloud
We use deep Hubble Space Telescope photometry of the rich, young (~20-45
Myr-old) star cluster NGC 1818 in the Large Magellanic Cloud to derive its
stellar mass function (MF) down to ~0.15 Msun. This represents the deepest
robust MF thus far obtained for a stellar system in an extragalactic,
low-metallicity ([Fe/H]~-0.4 dex) environment. Combining our results with the
published MF for masses above 1.0 Msun, we obtain a complete present-day MF.
This is a good representation of the cluster's initial MF (IMF), particularly
at low masses, because our observations are centred on the cluster's uncrowded
half-mass radius. Therefore, stellar and dynamical evolution of the cluster
will not have affected the low-mass stars significantly. The NGC 1818 IMF is
well described by both a lognormal and a broken power-law distribution with
slopes of Gamma=0.46+/-0.10 and Gamma~-1.35 (Salpeter-like) for masses in the
range from 0.15 to 0.8 Msun and greater than 0.8 Msun, respectively. Within the
uncertainties, the NGC 1818 IMF is fully consistent with both the Kroupa
solar-neighbourhood and the Chabrier lognormal mass distributions.Comment: 11 pages, 9 figures, accepted by MNRA
The emerging landscape of health research based on biobanks linked to electronic health records: Existing resources, statistical challenges, and potential opportunities
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154448/1/sim8445_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154448/2/sim8445.pd
Dynamics of the Galactic Bulge using Planetary Nebulae
Evidence for a bar at the center of the Milky Way triggered a renewed
enthusiasm for dynamical modelling of the Galactic bar-bulge. Our goal is to
compare the kinematics of a sample of tracers, planetary nebulae, widely
distributed over the bulge with the corresponding kinematics for a range of
models of the inner Galaxy. Three of these models are N-body barred systems
arising from the instabilities of a stellar disk (Sellwood, Fux and Kalnajs),
and one is a Schwarzschild system constructed to represent the 3D distribution
of the COBE/DIRBE near-IR light and then evolved as an N-body system for a few
dynamical times (Zhao). For the comparison of our data with the models, we use
a new technique developed by Saha (1998). The procedure finds the parameters of
each model, i.e. the solar galactocentric distance R_o in model units, the
orientation angle phi, the velocity scale (in km/s per model unit), and the
solar tangential velocity which best fit the data.Comment: 48 pages (Latex), 30 figures (PS), accepted for pub. in A
ARES. III. Unveiling the Two Faces of KELT-7 b with HST WFC3*
We present the analysis of the hot-Jupiter KELT-7 b using transmission and emission spectroscopy from the Hubble Space Telescope, both taken with the Wide Field Camera 3. Our study uncovers a rich transmission spectrum that is consistent with a cloud-free atmosphere and suggests the presence of H_{2}O and H^{â}. In contrast, the extracted emission spectrum does not contain strong absorption features and, although it is not consistent with a simple blackbody, it can be explained by a varying temperatureâpressure profile, collision induced absorption, and H^{-}. KELT-7 b had also been studied with other space-based instruments and we explore the effects of introducing these additional data sets. Further observations with Hubble, or the next generation of space-based telescopes, are needed to allow for the optical opacity source in transmission to be confirmed and for molecular features to be disentangled in emission
The low-mass stellar mass functions of rich, compact clusters in the Large Magellanic Cloud
Context. We use Hubble Space Telescope photometry of six rich, compact star
clusters in the Large Magellanic Cloud (LMC), with ages ranging from 0.01 to
1.0 Gyr, to derive the clusters' stellar mass functions (MFs) at their
half-mass radii.
Aims. The LMC is an ideal environment to study stellar MFs, because it
contains a large population of compact clusters at different evolutionary
stages. We aim to obtain constraints on the initial MFs (IMFs) of our sample
clusters on the basis of their present-day MFs, combined with our understanding
of their dynamical and photometric evolution.
Methods. We derive the clusters' present-day MFs below 1.0 Msun using deep
observations with the Space Telescope Imaging Spectrograph and updated stellar
population synthesis models.
Results. Since the relaxation timescales of low-mass stars are very long,
dynamical evolution will not have affected the MFs below 1.0 Msun
significantly, so that - within the uncertainties - the derived MFs are
consistent with the solar-neighbourhood IMF, at least for the younger clusters.
Conclusions. The IMF in the low-density, low-metallicity environment of the
LMC disk is not significantly different from that in the solar neighbourhood.Comment: 9 pages, 8 figures, 3 tables, accepted for publication in A&
A chemical survey of exoplanets with ARIEL
Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planetâs birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25â7.8 ÎŒm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10â100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed â using conservative estimates of mission performance and a full model of all significant noise sources in the measurement â using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL â in line with the stated mission objectives â will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.Peer reviewedFinal Published versio
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