1,636 research outputs found
Studies on the dissolution rate of sparingly soluble calcium citrate in water
The mass transfer coefficient of calcium citrate-water system was determined by the conductivity measurement. The range of the measured coefficient is 2.65 × 10-6m/s ± 35% at 25.5°C, independent of particle size and rpm under a full solid suspension condition. The prediction of the effects of particle size and agitation on the coefficient based on the classic mass trafer theories is also discussed
Two Populations of Molecular Clouds in the Antennae Galaxies
Super star clusters --- extremely massive clusters found predominately in
starburst environments --- are essential building blocks in the formation of
galaxies and thought to dominate star formation in the high-redshift universe.
However, the transformation from molecular gas into these ultra-compact star
clusters is not well understood. To study this process, we used the
Submillimeter Array and the Plateau de Bure Interferometer to obtain high
angular resolution (~1.5" or 160 pc) images of the Antennae overlap region in
CO(2--1) to search for the molecular progenitors of the super star clusters. We
resolve the molecular gas distribution into a large number of clouds, extending
the differential cloud mass function down to a 5\sigma completeness limit of
3.8x10^5 M_sun. We identify a distinct break in the mass function around log
M_mol/M_sun ~ 6.5, which separates the molecular clouds into two distinct
populations. The smaller, less massive clouds reside in more quiescent areas in
the region, while the larger, more massive clouds cluster around regions of
intense star formation. A broken power-law fit to the mass function yields
slopes of \alpha = -1.39+/-0.10 and \alpha = -1.44+/-0.14 for the low- and
high-mass cloud population, well-matched to the mass function found for super
star clusters in the Antennae galaxies. We find large velocity gradients and
velocity dispersions at the locations of intense star formation, suggestive of
compressive shocks. It is likely that these environmental factors contribute to
the formation of the observed massive molecular clouds and super star clusters
in the Antennae galaxies.Comment: 20 pages, 13 figures, accepted by Ap
The molecular basis of T cell acute lymphoblastic leukemia
T cell acute lymphoblastic leukemias (T-ALLs) arise from the malignant transformation of hematopoietic progenitors primed toward T cell development, as result of a multistep oncogenic process involving constitutive activation of NOTCH signaling and genetic alterations in transcription factors, signaling oncogenes, and tumor suppressors. Notably, these genetic alterations define distinct molecular groups of T-ALL with specific gene expression signatures and clinicobiological features. This review summarizes recent advances in our understanding of the molecular genetics of T-ALL
Spatially Resolving Substructures within the Massive Envelope around an Intermediate-mass Protostar: MMS 6/OMC-3
With the Submillimeter Array, the brightest (sub)millimeter continuum source
in the OMC-2/3 region, MMS 6, has been observed in the 850 um continuum
emission with approximately 10 times better angular resolution than previous
studies (~0.3"; ~120 AU at Orion). The deconvolved size, the mass, and the
column density of MMS 6-main are estimated to be 0.32"x0.29" (132 AUx120 AU),
0.29 Mo, and 2.1x10^{25} cm^{-2}, respectively. The estimated extremely high
mean number density, 1.5x10^{10} cm^{-3}, suggests that MMS 6-main is likely
optically thick at 850 um. We compare our observational data with three
theoretical core models: prestellar core, protostellar core + disk-like
structure, and first adiabatic core. These comparisons clearly show that the
observational data cannot be modeled as a simple prestellar core with a gas
temperature of 20 K. A self-luminous source is necessary to explain the
observed flux density in the (sub)millimeter wavelengths. Our recent detection
of a very compact and energetic outflow in the CO (3-2) and HCN (4-3) lines,
supports the presence of a protostar. We suggest that MMS 6 is one of the first
cases of an intermediate mass protostellar core at an extremely young stage. In
addition to the MMS 6-main peak, we have also spatially resolved a number of
spiky structures and sub-clumps, distributed over the central 1000 AU. The
masses of these sub-clumps are estimated to be 0.066-0.073 Mo, which are on the
order of brown dwarf masses. Higher angular resolution and higher sensitivity
observations with ALMA and EVLA will reveal the origin and nature of these
structures such as whether they are originated from fragmentations, spiral
arms, or inhomogeneity within the disk-like structures/envelope.Comment: Accepted to Ap
Dynamical Masses of Young Star Clusters in NGC 4038/4039
In order to estimate the masses of the compact, young star clusters in the
merging galaxy pair, NGC 4038/4039 (``the Antennae''), we have obtained medium
and high resolution spectroscopy using ISAAC on VLT-UT1 and UVES on VLT-UT2 of
five such clusters. The velocity dispersions were estimated using the stellar
absorption features of CO at 2.29 microns and metal absorption lines at around
8500 \AA, including lines of the Calcium Triplet. The size scales and light
profiles were measured from HST images. From these data and assuming Virial
equilibrium, we estimated the masses of five clusters. The resulting masses
range from 6.5 x 10^5 to 4.7 x 10^6 M_sun. These masses are large, factor of a
few to more than 10 larger than the typical mass of a globular cluster in the
Milky Way. The mass-to-light ratios for these clusters in the V- and K-bands in
comparison with stellar synthesis models suggest that to first order the IMF
slopes are approximately consistent with Salpeter for a mass range of 0.1 to
100 M_sun. However, the clusters show a significant range of possible IMF
slopes or lower mass cut-offs and that these variations may correlate with the
interstellar environment of the cluster. Comparison with the results of
Fokker-Planck simulations of compact clusters with properties similar to the
clusters studied here, suggest that they are likely to be long-lived and may
lose a substantial fraction of their total mass. This mass loss would make the
star clusters obtain masses which are comparable to the typical mass of a
globular cluster.Comment: 16 pages, 12 figures, A&A accepte
The Globular Cluster System of M60 (NGC 4649). II. Kinematics of the Globular Cluster System
We present a kinematic analysis of the globular cluster (GC) system in the
giant elliptical galaxy (gE) M60 in the Virgo cluster. Using the photometric
and spectroscopic database of 121 GCs (83 blue GCs and 38 red GCs), we have
investigated the kinematics of the GC system. We have found that the M60 GC
system shows a significant overall rotation. The rotation amplitude of the blue
GCs is slightly smaller than or similar to that of the red GCs, and their
angles of rotation axes are similar. The velocity dispersions about the mean
velocity and about the best fit rotation curve for the red GCs are marginally
larger than those for the blue GCs. Comparison of observed stellar and GC
velocity dispersion profiles with those calculated from the stellar mass
profile shows that the mass-to-light ratio should be increased as the
galactocentric distance increases, indicating the existence of an extended dark
matter halo. The entire sample of GCs in M60 is found to have a tangentially
biased velocity ellipsoid unlike the GC systems in other gEs. Two subsamples
appear to have different velocity ellipsoids. The blue GC system has a modest
tangentially biased velocity ellipsoid, while the red GC system has a modest
radially biased or an isotropic velocity ellipsoid. From the comparison of the
kinematic properties of the M60 GC system to those of other gEs (M87, M49, NGC
1399, NGC 5128, and NGC 4636), it is found that the velocity dispersion of the
blue GC system is similar to or larger than that of the red GC system except
for M60, and the rotation of the GC system is not negligible. The entire sample
of each GC system shows an isotropic velocity ellipsoid except for M60, while
the subsamples show diverse velocity ellipsoids. We discuss the implication of
these results for the formation models of the GC system in gEs.Comment: 48 pages, 16 figures. To appear in Ap
The Origin of OB Clusters: From 10 pc to 0.1 pc
We observe the 1.2 mm continuum emission around the OB cluster forming region
G10.6-0.4, using the IRAM 30m telescope MAMBO-2 bolometer array and the
Submillimeter array. Comparison of the Spitzer 24 m and 8 m images
with our 1.2 mm continuum maps reveals the ionization front of an HII region,
the photon-dominated layer, and several 5 pc scale filaments following the
outer edge of the photon-dominated layer. The filaments, which are resolved in
the MAMBO-2 observations, show regularly spaced parsec-scale molecular clumps,
embedded with a cluster of submillimeter molecular cores as shown in the SMA
0.87 mm observations. Toward the center of the G10.6-0.4 region, the combined
SMA+IRAM 30m continuum image reveals several, parsec-scale protrusions. They
may continue down to within 0.1 pc of the geometric center of a dense 3 pc size
structure, where a 200 M OB cluster resides. The observed filaments
may facilitate mass accretion onto the central cluster--forming region in the
presence of strong radiative and mechanical stellar feedbacks. Their
filamentary geometry may also facilitate fragmentation. We did not detect any
significant polarized emission at 0.87 mm in the inner 1 pc region with the
SMA.Comment: 32 pages, 10 figures, Accepted by ApJ on 2011.October
Evolutionary Status of Brightest and Youngest Source in the Orion Molecular Cloud-3 Region
The brightest continuum source in the Orion Molecular Cloud-3 region (OMC-3),
MMS 6, was observed with the Very Large Array (VLA), the Nobeyama Millimeter
Array (NMA), and the Submillimeter Array (SMA). Our data were supplemented by
near- to mid-infrared archival data taken by Spitzer Space Telescope. The
compact continuum source, MMS 6-main, was detected with an H_2 mass of 3.0 Msun
with a size of 510 AU. Despite its compact and well condensed appearance,
neither clear CO outflow, radio jet, nor infrared sources (at a wave-length
shorter than 8 um) were detected at MMS 6-main even with the present
high-spatial resolution and high-sensitivity observations. The derived H_2
column density, 2.6x10^25 cm^-2, corresponds to a visual extinction of
A_v~15000 mag., and the derived number density is at least two orders of
magnitude higher than for the other OMC-2/3 continuum sources. The volume
density profile of the source was estimated to have a power-law index of 2 or
steeper down to a radius of ~450 AU. The time scale to form a protostar at the
center or the time scale elapsed after its formation is estimated to be 830 to
7600 yr. This is much shorter than the typical lifetime of the Class 0/I
protostars, which is ~10^(4-5) yr, suggesting that MMS 6-main is probably in
either the earliest stage of the proto-stellar core or in the latest stage of
the pre-stellar phase.Comment: 34 pages, 7 figures, accepted to Ap
Correlating Bayesian date estimates with climatic events and domestication using a bovine case study
The tribe Bovini contains a number of commercially and culturally important species, such as cattle. Understanding their evolutionary time scale is important for distinguishing between post-glacial and domestication-associated population expansions, but estimates of bovine divergence times have been hindered by a lack of reliable calibration points. We present a Bayesian phylogenetic analysis of 481 mitochondrial D-loop sequences, including 228 radiocarbon-dated ancient DNA sequences, using a multi-demographic coalescent model. By employing the radiocarbon dates as internal calibrations, we co-estimate the bovine phylogeny and divergence times in a relaxed-clock framework. The analysis yields evidence for significant population expansions in both taurine and zebu cattle, European aurochs and yak clades. The divergence age estimates support domestication-associated expansion times (less than 12kyr) for the major haplogroups of cattle. We compare the molecular and palaeontological estimates for the Bison-Bos divergence
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Longâterm Cardiovascular Risks Associated With an Elevated Heart Rate: The Framingham Heart Study
Background: Higher heart rate has been associated with an adverse prognosis, but most prior studies focused on individuals with known cardiovascular disease or examined a limited number of outcomes. We sought to examine the association of baseline heart rate with both fatal and nonfatal outcomes during 2 decades of followâup. Methods and Results: Our study included 4058 Framingham Heart Study participants (mean age 55 years, 56% women). Cox models were performed with multivariable adjustment for clinical risk factors and physical activity. A total of 708 participants developed incident cardiovascular disease (303 heart failure, 343 coronary heart disease, and 216 stroke events), 48 received a permanent pacemaker, and 1186 died. Baseline heart rate was associated with incident cardiovascular disease (hazard ratio [HR] 1.15 per 1 SD [11 bpm] increase in heart rate, 95% CI 1.07 to 1.24, P=0.0002), particularly heart failure (HR 1.32, 95% CI 1.18 to 1.48, P<0.0001). Higher heart rate was also associated with higher allâcause (HR 1.17, 95% CI 1.11 to 1.24, P<0.0001) and cardiovascular mortality (HR 1.18, 95% CI 1.04 to 1.33, P=0.01). Spline analyses did not suggest a lower threshold beyond which the benefit of a lower heart rate abated or increased. In contrast, individuals with a higher heart rate had a lower risk of requiring permanent pacemaker placement (HR 0.55, 95% CI 0.38 to 0.79, P=0.001). Conclusions: Individuals with a higher heart rate are at elevated longâterm risk for cardiovascular events, in particular, heart failure, and allâcause death. On the other hand, a higher heart rate is associated with a lower risk of future permanent pacemaker implantation
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