448 research outputs found
A Quarter-Century of Observations of Comet 10P/Tempel 2 at Lowell Observatory: Continued Spin-Down, Coma Morphology, Production Rates, and Numerical Modeling
We report on photometry and imaging of Comet 10P/Tempel 2 obtained at Lowell
Observatory from 1983 through 2011. We measured a nucleus rotation period of
8.950 +/- 0.002 hr from 2010 September to 2011 January. This rotation period is
longer than the period we previously measured in 1999, which was itself longer
than the period measured in 1988. A nearly linear jet was observed which varied
little during a rotation cycle in both R and CN images acquired during the 1999
and 2010 apparitions. We measured the projected direction of this jet
throughout the two apparitions and, under the assumption that the source region
of the jet was near the comet's pole, determined a rotational pole direction of
RA/Dec = 151deg/+59deg from CN measurements and RA/Dec = 173deg/+57deg from
dust measurements (we estimate a circular uncertainty of 3deg for CN and 4deg
for dust). Different combinations of effects likely bias both gas and dust
solutions and we elected to average these solutions for a final pole of RA/Dec
= 162 +/- 11deg/+58 +/- 1deg. Photoelectric photometry was acquired in 1983,
1988, 1999/2000, and 2010/2011. The activity exhibited a steep turn-on ~3
months prior to perihelion (the exact timing of which varies) and a relatively
smooth decline after perihelion. The activity during the 1999 and 2010
apparitions was similar; limited data in 1983 and 1988 were systematically
higher and the difference cannot be explained entirely by the smaller
perihelion distance. We measured a "typical" composition, in agreement with
previous investigators. Monte Carlo numerical modeling with our pole solution
best replicated the observed coma morphology for a source region located near a
comet latitude of +80deg and having a radius of ~10deg. Our model reproduced
the seasonal changes in activity, suggesting that the majority of Tempel 2's
activity originates from a small active region located near the pole.Comment: Accepted by AJ; 29 pages of text (preprint style), 8 tables, 7
figure
Testing metallicity indicators at z~1.4 with the gravitationally lensed galaxy CASSOWARY 20
We present X-shooter observations of CASSOWARY 20 (CSWA 20), a star-forming
(SFR ~6 Msol/yr) galaxy at z=1.433, magnified by a factor of 11.5 by the
gravitational lensing produced by a massive foreground galaxy at z=0.741. We
analysed the integrated physical properties of the HII regions of CSWA 20 using
temperature- and density-sensitive emission lines. We find the abundance of
oxygen to be ~1/7 of solar, while carbon is ~50 times less abundant than in the
Sun. The unusually low C/O ratio may be an indication of a particularly rapid
timescale of chemical enrichment. The wide wavelength coverage of X-shooter
gives us access to five different methods for determining the metallicity of
CSWA 20, three based on emission lines from HII regions and two on absorption
features formed in the atmospheres of massive stars. All five estimates are in
agreement, within the factor of ~2 uncertainty of each method. The interstellar
medium of CSWA 20 only partially covers the star-forming region as viewed from
our direction; in particular, absorption lines from neutrals and first ions are
exceptionally weak. We find evidence for large-scale outflows of the
interstellar medium (ISM) with speeds of up 750 km/s, similar to the values
measured in other high-z galaxies sustaining much higher rates of star
formation.Comment: 18 pages, 11 figures, accepted for publication in MNRA
Faint dwarfs as a test of DM models: WDM vs. CDM
We use high resolution HydroN-Body cosmological simulations to compare the
assembly and evolution of a small field dwarf (stellar mass ~ 10
M, total mass 10 M in dominated CDM and 2keV WDM
cosmologies. We find that star formation (SF) in the WDM model is reduced and
delayed by 1-2 Gyr relative to the CDM model, independently of the details of
SF and feedback. Independent of the DM model, but proportionally to the SF
efficiency, gas outflows lower the central mass density through `dynamical
heating', such that all realizations have circular velocities 20kms at
500pc, in agreement with local kinematic constraints. As a result of
dynamical heating, older stars are less centrally concentrated than younger
stars, similar to stellar population gradients observed in nearby dwarf
galaxies. Introducing an important diagnostic of SF and feedback models, we
translate our simulations into artificial color-magnitude diagrams and star
formation histories in order to directly compare to available observations. The
simulated galaxies formed most of their stars in many 10 Myr long bursts.
The CDM galaxy has a global SFH, HI abundance and Fe/H and alpha-elements
distribution well matched to current observations of dwarf galaxies. These
results highlight the importance of directly including `baryon physics' in
simulations when 1) comparing predictions of galaxy formation models with the
kinematics and number density of local dwarf galaxies and 2) differentiating
between CDM and non-standard models with different DM or power spectra.Comment: 13 pages including Appendix on Color Magnitude Diagrams. Accepted by
MNRAS. Added one plot and details on ChaNGa implementation. Reduced number of
citations after editorial reques
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Realism of Lagrangian Large Eddy Simulations Driven by Renalysis Meteorology: Tracking a Pocket of Open Cells Under a Biomass Burning Aerosol Layer
An approach to drive Lagrangian large eddy simulation (LES) of boundary layer clouds with reanalysis data is presented and evaluated using satellite (Spinning Enhanced Visible and Infrared Imager, SEVIRI) and aircraft (CloudâAerosolâRadiation Interactions and Forcing, CLARIFY) measurements. The simulations follow trajectories of the boundary layer flow. They track the formation and evolution of a pocket of open cells (POC) underneath a biomass burning aerosol layer in the free troposphere. The simulations reproduce the evolution of observed stratocumulus cloud morphology, cloud optical depth, and cloud drop effective radius, and capture the timing of the cloud state transition from closed to open cells seen in the satellite imagery on the three considered trajectories. They reproduce a biomass burning aerosol layer identified by the inâsitu aircraft measurements above the inversion of the POC. Entrainment of aerosol from the biomass burning layer into the POC is limited to the extent of having no impact on cloudâ or boundary layer properties, in agreement with the CLARIFY observations. The twoâmoment bin microphysics scheme used in the simulations reproduces the inâsitu cloud microphysical properties reasonably well. A twoâmoment bulk microphysics scheme reproduces the satellite observations in the nonâprecipitating closedâcell state, but overestimates liquid water path and cloud optical depth in the precipitating openâcell state due to insufficient surface precipitation. A boundary layer cold and dry bias occurring in LES can be counteracted by reducing the grid aspect ratio and by tightening the large scale wind speed nudging towards the surface.
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Rapid reduction of central line infections in hospitalized pediatric oncology patients through simple quality improvement methods
Background Pediatric hematologyâoncology (PHO) patients are at significant risk for developing central lineâassociated bloodstream infections (CLAâBSIs) due to their prolonged dependence on such catheters. Effective strategies to eliminate these preventable infections are urgently needed. In this study, we investigated the implementation of bundled central line maintenance practices and their effect on hospitalâacquired CLAâBSIs. Materials and Methods CLAâBSI rates were analyzed within a singleâinstitution's PHO unit between January 2005 and June 2011. In May 2008, a multidisciplinary quality improvement team developed techniques to improve the PHO unit's safety culture and implemented the use of catheter maintenance practices tailored to PHO patients. Data analysis was performed using timeâseries methods to evaluate the preâ and postâintervention effect of the practice changes. Results The preâintervention CLAâBSI incidence was 2.92 per 1,000âpatient days (PD) and coagulaseânegative Staphylococcus was the most prevalent pathogen (29%). In the postâintervention period, the CLAâBSI rate decreased substantially (45%) to 1.61 per 1,000âPD ( P â<â0.004). Early on, blood and marrow transplant (BMT) patients had a threefold higher CLAâBSI rate compared to nonâBMT patients ( P â<â0.033). With additional infection control countermeasures added to the bundled practices, BMT patients experienced a larger CLAâBSI rate reduction such that BMT and nonâBMT CLAâBSI rates were not significantly different postâintervention. Conclusions By adopting and effectively implementing uniform maintenance catheter care practices, learning multidisciplinary teamwork, and promoting a culture of patient safety, the CLAâBSI incidence in our study population was significantly reduced and maintained. Pediatr Blood Cancer 2013;60:262â269. © 2012 Wiley Periodicals, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95163/1/24187_ftp.pd
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