117 research outputs found
The Discreteness-driven Relaxation of Collisionless Gravitating Systems: Entropy Evolution in External Potentials, N-dependence, and the Role of Chaos
We investigate the old problem of the fast relaxation of collisionless N-body systems that are collapsing or perturbed,
emphasizing the importance of (noncollisional) discreteness effects. We integrate orbit ensembles in fixed potentials,
estimating the entropy to analyze the time evolution of the distribution function. These estimates capture the correct
physical behavior expected from the second law of thermodynamics, without any spurious entropy production. For
self-consistent (i.e., stationary) samples, the entropy is conserved, while for non-self-consistent samples, it increases
within a few dynamical times, stabilizing at a maximum (even in integrable potentials). Our results make transparent
that the main ingredient for this fast collisionless relaxation is the discreteness (finite N) of gravitational systems in
any potential. Additionally, in nonintegrable potentials, the presence of chaotic orbits accelerates the entropy
production. Contrary to the traditional violent relaxation scenario, our results indicate that a time-dependent potential
is not necessary for this relaxation. For the first time, in connection with the Nyquist–Shannon theorem, we derive the
typical timescale T tcr » 0.1N 1 6 for this discreteness-driven relaxation, with slightly weaker N-dependencies for
nonintegrable potentials with substantial fractions of chaotic orbits. This timescale is much smaller than the
collisional relaxation time even for small-N systems such as open clusters and represents an upper limit for the
relaxation time of real N-body collisionless systems. Additionally, our results reinforce the conclusion of Beraldo e
Silva et al. that the Vlasov equation does not provide an adequate kinetic description of the fast relaxation of
collapsing collisionless N-body systems.MTM2017-82160-C2-2-P.
FAPESP (2009/54006-4) and the INCT-A.
FAPESP (2014/23751-4 and 2017-01421-0). W.dS.P. is
CNPq (308337/2017-4).
HST-AR-13890.001, NSF award AST-1515001,
NASA-ATP award NNX15AK79G.
FAPESP (2017/25620-2)
FAPESP (2017/22340-9),
by the Basque Government (IT641-13)
Tracing Birth Properties of Stars with Abundance Clustering
To understand the formation and evolution of the Milky Way disk, we must connect its current properties to its past. We explore hydrodynamical cosmological simulations to investigate how the chemical abundances of stars might be linked to their origins. Using hierarchical clustering of abundance measurements in two Milky Way-like simulations with distributed and steady star formation histories, we find that groups of chemically similar stars comprise different groups in birth place (R birth) and time (age). Simulating observational abundance errors (0.05 dex), we find that to trace distinct groups of (R birth, age) requires a large vector of abundances. Using 15 element abundances (Fe, O, Mg, S, Si, C, P, Mn, Ne, Al, N, V, Ba, Cr, Co), up to ≈10 groups can be defined with ≈25% overlap in (R birth, age). We build a simple model to show that in the context of these simulations, it is possible to infer a star's age and R birth from abundances with precisions of ±0.06 Gyr and ±1.17 kpc, respectively. We find that abundance clustering is ineffective for a third simulation, where low-α stars form distributed in the disk and early high-α stars form more rapidly in clumps that sink toward the Galactic center as their constituent stars evolve to enrich the interstellar medium. However, this formation path leads to large age dispersions across the [α/Fe]-[Fe/H] plane, which is inconsistent with the Milky Way's observed properties. We conclude that abundance clustering is a promising approach toward charting the history of our Galaxy
Characterisation of high velocity stars in the S-PLUS internal fourth data release
In general, the atypical high velocity of some stars in the Galaxy can only
be explained by invoking acceleration mechanisms related to extreme
astrophysical events in the Milky Way. Using astrometric data from Gaia and the
photometric information in 12 filters of the S-PLUS, we performed a kinematic,
dynamical, and chemical analysis of 64 stars with galactocentric velocities
higher than 400 . All the stars are gravitationally bound
to the Galaxy and exhibit halo kinematics. Some of the stars could be remnants
of structures such as the Sequoia and the Gaia-Sausage/Enceladus. Supported by
orbital and chemical analysis, we identified Gaia DR3 5401875170994688896 as a
star likely to be originated at the centre of the Galaxy. Application of a
machine learning technique to the S-PLUS photometric data allows us to obtain
very good estimates of magnesium abundances for this sample of high velocity
stars
DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor
We present DESI observations of the inner halo of M31, which reveal the
kinematics of a recent merger - a galactic immigration event - in exquisite
detail. Of the 11,416 sources studied in 3.75 hour of on-sky exposure time,
7,438 are M31 sources with well measured radial velocities. The observations
reveal intricate coherent kinematic structure in the positions and velocities
of individual stars: streams, wedges, and chevrons. While hints of coherent
structures have been previously detected in M31, this is the first time they
have been seen with such detail and clarity in a galaxy beyond the Milky Way.
We find clear kinematic evidence for shell structures in the Giant Stellar
Stream, the Northeast Shelf and Western Shelf regions. The kinematics are
remarkably similar to the predictions of dynamical models constructed to
explain the spatial morphology of the inner halo. The results are consistent
with the interpretation that much of the substructure in the inner halo of M31
is produced by a single galactic immigration event 1 - 2 Gyr ago. Significant
numbers of metal-rich stars ([Fe/H]) are present in all of the detected
substructures, suggesting that the immigrating galaxy had an extended star
formation history. We also investigate the ability of the shells and Giant
Stellar Stream to constrain the gravitational potential of M31, and estimate
the mass within a projected radius of 125 kpc to be . The results herald a
new era in our ability to study stars on a galactic scale and the immigration
histories of galaxies.Comment: 45 pages, 22 figures, 8 tables; Astrophysical Journal in press; Data
at https://zenodo.org/record/697749
DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor
Avaliação do potencial de utilização da madeira de Schizolobium amazonicum "Paricá" e Cecropia hololeuca "Embaúba" para produção de painéis cimento-madeira
Overview of the DESI Milky Way Survey
We describe the Milky Way Survey (MWS) that will be undertaken with the Dark Energy Spectroscopic Instrument (DESI) on the Mayall 4 m telescope at the Kitt Peak National Observatory. Over the next 5 yr DESI MWS will observe approximately seven million stars at Galactic latitudes ∣b∣ > 20°, with an inclusive target selection scheme focused on the thick disk and stellar halo. MWS will also include several high-completeness samples of rare stellar types, including white dwarfs, low-mass stars within 100 pc of the Sun, and horizontal branch stars. We summarize the potential of DESI to advance understanding of the Galactic structure and stellar evolution. We introduce the final definitions of the main MWS target classes and estimate the number of stars in each class that will be observed. We describe our pipelines for deriving radial velocities, atmospheric parameters, and chemical abundances. We use ≃500,000 spectra of unique stellar targets from the DESI Survey Validation program (SV) to demonstrate that our pipelines can measure radial velocities to ≃1 km s−1 and [Fe/H] accurate to ≃0.2 dex for typical stars in our main sample. We find the stellar parameter distributions from ≈100 deg2 of SV observations with ≳90% completeness on our main sample are in good agreement with expectations from mock catalogs and previous surveys
Geoeconomic variations in epidemiology, ventilation management, and outcomes in invasively ventilated intensive care unit patients without acute respiratory distress syndrome: a pooled analysis of four observational studies
Background: Geoeconomic variations in epidemiology, the practice of ventilation, and outcome in invasively ventilated intensive care unit (ICU) patients without acute respiratory distress syndrome (ARDS) remain unexplored. In this analysis we aim to address these gaps using individual patient data of four large observational studies. Methods: In this pooled analysis we harmonised individual patient data from the ERICC, LUNG SAFE, PRoVENT, and PRoVENT-iMiC prospective observational studies, which were conducted from June, 2011, to December, 2018, in 534 ICUs in 54 countries. We used the 2016 World Bank classification to define two geoeconomic regions: middle-income countries (MICs) and high-income countries (HICs). ARDS was defined according to the Berlin criteria. Descriptive statistics were used to compare patients in MICs versus HICs. The primary outcome was the use of low tidal volume ventilation (LTVV) for the first 3 days of mechanical ventilation. Secondary outcomes were key ventilation parameters (tidal volume size, positive end-expiratory pressure, fraction of inspired oxygen, peak pressure, plateau pressure, driving pressure, and respiratory rate), patient characteristics, the risk for and actual development of acute respiratory distress syndrome after the first day of ventilation, duration of ventilation, ICU length of stay, and ICU mortality. Findings: Of the 7608 patients included in the original studies, this analysis included 3852 patients without ARDS, of whom 2345 were from MICs and 1507 were from HICs. Patients in MICs were younger, shorter and with a slightly lower body-mass index, more often had diabetes and active cancer, but less often chronic obstructive pulmonary disease and heart failure than patients from HICs. Sequential organ failure assessment scores were similar in MICs and HICs. Use of LTVV in MICs and HICs was comparable (42\ub74% vs 44\ub72%; absolute difference \u20131\ub769 [\u20139\ub758 to 6\ub711] p=0\ub767; data available in 3174 [82%] of 3852 patients). The median applied positive end expiratory pressure was lower in MICs than in HICs (5 [IQR 5\u20138] vs 6 [5\u20138] cm H2O; p=0\ub70011). ICU mortality was higher in MICs than in HICs (30\ub75% vs 19\ub79%; p=0\ub70004; adjusted effect 16\ub741% [95% CI 9\ub752\u201323\ub752]; p<0\ub70001) and was inversely associated with gross domestic product (adjusted odds ratio for a US$10 000 increase per capita 0\ub780 [95% CI 0\ub775\u20130\ub786]; p<0\ub70001). Interpretation: Despite similar disease severity and ventilation management, ICU mortality in patients without ARDS is higher in MICs than in HICs, with a strong association with country-level economic status. Funding: No funding
Impactos dos subsÃdios agrÃcolas dos Estados Unidos na expansão do agronegócio brasileiro
Efetividade no diagnóstico da tuberculose em Foz do Iguaçu, trÃplice fronteira Brasil, Paraguai e Argentina
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