898 research outputs found
A Near-Infrared L Band Survey of the Young Embedded Cluster NGC 2024
We present the results of the first sensitive L band (3.4 micron) imaging
study of the nearby young embedded cluster NGC 2024. Two separate surveys of
the cluster were acquired in order to obtain a census of the circumstellar disk
fraction in the cluster. From an analysis of the JHKL colors of all sources in
our largest area, we find an infrared excess fraction of > 86%. The JHKL colors
suggest that the infrared excesses arise in circumstellar disks, indicating
that the majority of the sources which formed in the NGC 2024 cluster are
currently surrounded by, and likely formed with circumstellar disks. The excess
fractions remain very high, within the errors, even at the faintest L
magnitudes from our deeper surveys suggesting that disks form around the
majority of the stars in very young clusters such as NGC 2024 independent of
mass. From comparison with published JHKL observations of Taurus, we find the K
- L excess fraction in NGC 2024 to be consistent with a high initial incidence
of circumstellar disks in both NGC 2024 and Taurus. Because NGC 2024 represents
a region of much higher stellar density than Taurus, this suggests that disks
may form around most of the YSOs in star forming regions independent of
environment. We find a relatively constant JHKL excess fraction with increasing
cluster radius, indicating that the disk fraction is independent of location in
the cluster. In contrast, the JHK excess fraction increases rapidly toward the
central region of the cluster, and is most likely due to contamination of the K
band measurements by bright nebulosity in the central regions of the cluster.
We identify 45 candidate protostellar sources in the central regions of the NGC
2024 cluster, and find a lower limit on the protostellar phase of early stellar
evolution of 0.4 - 1.4 X 10^5 yr, similar to that in Taurus.Comment: 37 pages, 8 figures, 3 tables, To appear in the Astronomical Journa
Application of pressure-sensitive paints to unsteady and high-speed flows
The Pressure-Sensitive Paint (PSP) technique allows the global pressure mapping of surfaces under aerodynamic conditions. The present study involves the application of Tris- Bathophenanthroline Ruthenium Perchlorate based PSP, developed in-house, to two different cases; a) the flow through a sonic nozzle, and b) the examination of the effect of dimples on glancing shock wave turbulent boundary layer interactions at transonic speeds
A comprehensive set of simulations studying the influence of gas expulsion on star cluster evolution
We have carried out a large set of N-body simulations studying the effect of
residual-gas expulsion on the survival rate and final properties of star
clusters.
We have varied the star formation efficiency, gas expulsion timescale and
strength of the external tidal field, obtaining a three-dimensional grid of
models which can be used to predict the evolution of individual star clusters
or whole star cluster systems by interpolating between our runs. The complete
data of these simulations is made available on the Internet.
Our simulations show that cluster sizes, bound mass fraction and velocity
profile are strongly influenced by the details of the gas expulsion. Although
star clusters can survive star formation efficiencies as low as 10% if the
tidal field is weak and the gas is removed only slowly, our simulations
indicate that most star clusters are destroyed or suffer dramatic loss of stars
during the gas removal phase. Surviving clusters have typically expanded by a
factor 3 or 4 due to gas removal, implying that star clusters formed more
concentrated than as we see them today. Maximum expansion factors seen in our
runs are around 10. If gas is removed on timescales smaller than the initial
crossing time, star clusters acquire strongly radially anisotropic velocity
dispersions outside their half-mass radii. Observed velocity profiles of star
clusters can therefore be used as a constraint on the physics of cluster
formation.Comment: 12 pages, 9 figures, MNRAS accepte
The nature of AFGL 2591 and its associated molecular outflow: Infrared and millimeter-wave observations
The results of infrared photometry from 2 to 160 microns of AFGL and CO(12) observations of its associated molecular cloud and high velocity molecular outflow are presented and discussed. The observed solar luminosity is 6.7 x 10(4) at a distance of 2 kpc. The spectrum of AFGL 2591 is interpreted in the context of a model in which a single embedded object is the dominant source of the infrared luminosity. This object is determined to be surrounded by a compact, optically thick dust shell with a temperature in excess of several hundred degrees kelvin. The extinction to this source is estimated to be between 26 and 50 visual magnitudes. The absolute position of the infrared sources at 10 microns was determined to an accuracy of + or in. This indicates for the first time that the IR source and H2O source are not coincident. The CO(12) observations show the high-velocity molecular flow near AFGL 2591 to be extended, bipolar and roughly centered on the infrared emission. The observations suggest that the red-shifted flow component extends beyond the boundary of the ambient cloud within which AFGL 2591 is embedded. The CO(12) observations also show that AFGL 2591 is embedded in a molecular cloud with an LSR velocity of -5 km/s
X Marks the Spot: Nexus of Filaments, Cores, and Outflows in a Young Star-Forming Region
We present a multiwavelength investigation of a region of a nearby giant
molecular cloud that is distinguished by a minimal level of star formation
activity. With our new 12CO(J=2-1) and 13CO(J=2-1) observations of a remote
region within the middle of the California molecular cloud, we aim to
investigate the relationship between filaments, cores, and a molecular outflow
in a relatively pristine environment. An extinction map of the region from
Herschel Space Observatory observations reveals the presence of two 2-pc-long
filaments radiating from a high-extinction clump. Using the 13CO observations,
we show that the filaments have coherent velocity gradients and that their
mass-per-unit-lengths may exceed the critical value above which filaments are
gravitationally unstable. The region exhibits structure with eight cores, at
least one of which is a starless, prestellar core. We identify a low-velocity,
low-mass molecular outflow that may be driven by a flat spectrum protostar. The
outflow does not appear to be responsible for driving the turbulence in the
core with which it is associated, nor does it provide significant support
against gravitational collapse.Comment: Accepted for publication in the Astrophysical Journa
Studi Parametrik Pengaruh Variasi Pengekangan Terhadap Nilai Kapasitas dan Daktilitas Penampang Kolom Beton Bertulang Bentuk T
Column has a function as a successor of the entire building load for foundation with a cross-sectional shape which generally a square. In its development, the shape of column section has begun to vary, namely the 'L', 'T' and 'Plus (+)' shaped sections. The purpose of this study was to determine the effect of restraint parameters such as diameter of stirrups, distance between stirrups, diameter and configuration of longitudinal reinforcement also concrete quality on the capacity and ductility of T-shaped non-square column using Response-2000, XTRACT and SAP2000 programs to ensure the percentage comparison of the analysis result data is not more than 5%. The results of this study for each type of cross-section, namely each increase in the percentage of concrete quality parameters ranging from 25-35 MPa, the diameter of longitudinal reinforcement and diameter of stirrups will increase the value of capacity and decrease the value of ductility, while for each increase in the percentage of parameters the distance between stirrups will decrease the value of capacity and the ductility value. Meanwhile, the variation of longitudinal reinforcement configuration produces T2-1 section as cross section with the largest capacity value and T1-2 section as cross section with the greatest ductility value
The energetics and mass structure of regions of star formation: S201
Theoretical predictions about dust and gas in star forming regions are tested by observing a 4 arcmin region surrounding the radio continuum source in 5201. The object was mapped in two far infrared wavelengths and found to show significant extended emission. Under the assumption that the molecular gas is heated solely via thermal coupling with the dust, the volume density was mapped in 5201. The ratios of infrared optical depth to CO column density were calculated for a number of positions in the source. Near the center of the cloud the values are found to be in good agreement with other determinations for regions with lower column density. In addition, the observations suggest significant molecular destruction in the outer parts of the object. Current models of gas heating were used to calculate a strong limit for the radius of the far infrared emitting grains, equal to or less than 0.15 micron. Grains of about this size are required by the observation of high temperature (T equal to or greater than 20 K) gas in many sources
Modeling the Near-Infrared Luminosity Functions of Young Stellar Clusters
We present the results of numerical experiments designed to evaluate the
usefulness of near-infrared luminosity functions for constraining the Initial
Mass Function (IMF) of young stellar populations. From this numerical modeling,
we find that the luminosity function of a young stellar population is
considerably more sensitive to variations in the underlying initial mass
function than to either variations in the star forming history or assumed
pre-main-sequence (PMS) mass-to-luminosity relation. To illustrate the
potential effectiveness of using the KLF of a young cluster to constrain its
IMF, we model the observed K band luminosity function of the nearby Trapezium
cluster. Our derived mass function for the Trapezium spans two orders of
magnitude in stellar mass (5 Msun to 0.02 Msun), has a peak near the hydrogen
burning limit, and has an IMF for Brown Dwarfs which steadily decreases with
decreasing mass.Comment: To appear in ApJ (1 April 2000). 37 pages including 11 figures, AAS:
ver 5.
A FLAMINGOS Deep Near Infrared Imaging Survey of the Rosette Complex I: Identification and Distribution of the Embedded Population
We present the results of a deep near-infrared imaging survey of the Rosette
Complex. We studied the distribution of young embedded sources using a
variation of the Nearest Neighbor Method applied to a carefully selected sample
of near-infrared excess (NIRX) stars which trace the latest episode of star
formation in the complex. Our analysis confirmed the existence of seven
clusters previously detected in the molecular cloud, and identified four more
clusters across the complex. We determined that 60% of the young stars in the
complex and 86% of the stars within the molecular cloud are contained in
clusters, implying that the majority of stars in the Rosette formed in embedded
clusters. We compare the sizes, infrared excess fractions and average
extinction towards individual clusters to investigate their early evolution and
expansion. We found that the average infrared excess fraction of clusters
increases as a function of distance from NGC 2244, implying a temporal sequence
of star formation across the complex. This sequence appears to be primordial,
possibly resulting from the formation and evolution of the molecular cloud and
not from the interaction with the HII region.Comment: Accepted by Astrophysical Journa
Advances in the management of gout: Critical appraisal of febuxostat in the control of hyperuricemia
Gout recently passed rheumatoid arthritis to become the most common inflammatory arthritis in the United States (US). However, epidemiologic studies indicate that the quality of gout management is suboptimal owing to both patient and physician issues. Only three options for urate-lowering therapy are currently available in the US: allopurinol, probenecid, and recently, febuxostat. Probenecid is generally safe except for the occurrence of urolithiasis, but is only effective for the subset of patients with better kidney function. Allopurinol use is limited due to its side effects, potential toxicity of uncertain magnitude in patients with renal disease, and failure to achieve targeted serum urate levels. In part this failure may be due to the necessity for it to be titrated for optimal therapeutic effect. Febuxostat is a new medication that may offer several advantages and can be given as an alternative to allopurinol. We review the basic biology and clinical performance of febuxostat, and consider the potential utility of this agent in comparison to the older, better-established gout therapeutics
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