10,342 research outputs found
Quantum states and specific heat of low-density He gas adsorbed within the carbon nanotube interstitial channels: Band structure effects and potential dependence
We calculate the energy-band structure of a He atom trapped within the
interstitial channel between close-packed nanotubes within a bundle and its
influence on the specific heat of the adsorbed gas. A robust prediction of our
calculations is that the contribution of the low-density adsorbed gas to the
specific heat of the nanotube material shows pronounced nonmonotonic variations
with temperature. These variations are shown to be closely related to the band
gaps in the adsorbate density of states
Is NGC 3108 transforming itself from an early to late type galaxy -- an astronomical hermaphrodite?
A common feature of hierarchical galaxy formation models is the process of
"inverse" morphological transformation: a bulge dominated galaxy accretes a gas
disk, dramatically reducing the system's bulge-to-disk mass ratio. During their
formation, present day galaxies may execute many such cycles across the Hubble
diagram. A good candidate for such a "hermaphrodite" galaxy is NGC 3108: a
dust-lane early-type galaxy which has a large amount of HI gas distributed in a
large scale disk. We present narrow band H_alpha and R-band imaging, and
compare the results with the HI distribution. The emission is in two
components: a nuclear bar and an extended disk component which coincides with
the HI distribution. This suggests that a stellar disk is currently being
formed out of the HI gas. The spatial distributions of the H_alpha and HI
emission and the HII regions are consistent with a barred spiral structure,
extending some 20 kpc in radius. We measure an extinction- corrected SFR of
0.42 Msun/yr. The luminosity function of the HII regions is similar to other
spiral galaxies, with a power law index of -2.1, suggesting that the star
formation mechanism is similar to other spiral galaxies. We measured the
current disk mass and find that it is too massive to have been formed by the
current SFR over the last few Gyr. It is likely that the SFR in NGC 3108 was
higher in the past. With the current SFR, the disk in NGC 3108 will grow to be
~6.2x10^9 Msun in stellar mass within the next 5.5 Gyr. While this is
substantial, the disk will be insignificant compared with the large bulge mass:
the final stellar mass disk-to-bulge ratio will be ~0.02. NGC 3108 will fail to
transform into anything resembling a spiral without a boost in the SFR and
additional supply of gas.Comment: 9 pages, 3 figures, accepted for publication in MNRA
A Hybrid Approach to Parallel Pattern Discovery in C++
Funding: EU Horizon 2020 project, TeamPlay, grant number 779882, and UK EPSRC Discovery, grant number EP/P020631/1.Parallel pattern libraries offer a strong combination of abstraction and performance. However, discovering places in sequential code where parallel patterns should be introduced is still highly non-trivial, often requiring expert manual analysis and profiling. We present a hybrid discovery technique to detect instances of parallel patterns in sequential code. This employs both static and dynamic trace-based analysis, together with hotspot detection. We evaluate our pattern discovery mechanism on a number of representative benchmarks. We evaluate the performance of the resulting parallelised benchmarks on a 24-core parallel machine.Postprin
Multichannel fiber laser Doppler vibrometer studies of low momentum and hypervelocity impacts
A multichannel optical fiber laser Doppler vibrometer was demonstrated with the capability of making simultaneous non-contact measurements of impacts at 3 different locations. Two sets of measurements were performed, firstly using small ball bearings (1 mmâ5.5 mm) falling under gravity and secondly using small projectiles (1 mm) fired from an extremely high velocity light gas gun (LGG) with speeds in the range 1 km/sâ8 km/s. Determination of impact damage is important for industries such as aerospace, military and rail, where the effect of an impact on the structure can result in a major structural damage. To our knowledge the research reported here demonstrates the first trials of a multichannel fiber laser Doppler vibrometer being used to detect hypervelocity impacts
Radial Velocity and Metallicity of the Globular Cluster IC4499 Obtained with AAOmega
We present radial velocity and metallicity measurements for the far-southern
Galactic globular cluster IC4499. We selected several hundred target red giant
stars in and around the cluster from the 2MASS point source catalog, and
obtained spectra at the near-infrared calcium triplet using the AAOmega
spectrograph. Observations of giants in globular clusters M4, M22, and M68 were
taken to provide radial velocity and metallicity comparison objects. Based on
velocity data we conclude that 43 of our targets are cluster members, by far
the largest sample of IC4499 giants spectroscopically studied. We determine the
mean heliocentric radial velocity of the cluster to be 31.5 plus or minus 0.4
km/s, and find the most likely central velocity dispersion to be 2.5 plus or
minus 0.5 km/s. This leads to a dynamical mass estimate for the cluster of 93
plus or minus 37 thousand solar masses. We are sensitive to cluster rotation
down to an amplitude of about 1 km/s, but no evidence for cluster rotation is
seen. The cluster metallicity is found to be [Fe/H] = -1.52 plus or minus 0.12
on the Carretta-Gratton scale. The radial velocity of the cluster, previously
highly uncertain, is consistent with membership in the Monoceros tidal stream,
but also with a halo origin. The horizontal branch morphology of the cluster is
slightly redder than average for its metallicity, but it is likely not
unusually young compared to other clusters of the halo. The new constraints on
the cluster kinematics and metallicity may give insight into its extremely high
specific frequency of RR Lyrae stars.Comment: Accepted to MNRAS, 13 pages, 9 figure
Mass-Varying Neutrinos from a Variable Cosmological Constant
We consider, in a completely model-independent way, the transfer of energy
between the components of the dark energy sector consisting of the cosmological
constant (CC) and that of relic neutrinos. We show that such a cosmological
setup may promote neutrinos to mass-varying particles, thus resembling a
recently proposed scenario of Fardon, Nelson, and Weiner (FNW), but now without
introducing any acceleronlike scalar fields. Although a formal similarity of
the FNW scenario with the variable CC one can be easily established, one
nevertheless finds different laws for neutrino mass variation in each scenario.
We show that as long as the neutrino number density dilutes canonically, only a
very slow variation of the neutrino mass is possible. For neutrino masses to
vary significantly (as in the FNW scenario), a considerable deviation from the
canonical dilution of the neutrino number density is also needed. We note that
the present `coincidence' between the dark energy density and the neutrino
energy density can be obtained in our scenario even for static neutrino masses.Comment: 8 pages, minor corrections, two references added, to apear in JCA
Central GIP signaling stimulates peripheral GIP release and promotes insulin and pancreatic polypeptide secretion in nonhuman primates
Glucose-dependent insulinotropic polypeptide (GIP) has important actions on whole body metabolic function. GIP and its receptor are also present in the central nervous system and have been linked to neurotrophic actions. Metabolic effects of central nervous system GIP signaling have not been reported. We investigated whether centrally administered GIP could increase peripheral plasma GIP concentrations and influence the metabolic response to a mixed macronutrient meal in nonhuman primates. An infusion and sampling system was developed to enable continuous intracerebroventricular (ICV) infusions with serial venous sampling in conscious nonhuman primates. Male baboons (Papio sp.) that were healthy and had normal body weights (28.9 ± 2.1 kg) were studied (n = 3). Animals were randomized to receive continuous ICV infusions of GIP (20 pmol·kgâ1·hâ1) or vehicle before and over the course of a 300-min mixed meal test (15 kcal/kg, 1.5g glucose/kg) on two occasions. A significant increase in plasma GIP concentration was observed under ICV GIP infusion (66.5 ± 8.0 vs. 680.6 ± 412.8 pg/ml, P = 0.04) before administration of the mixed meal. Increases in postprandial, but not fasted, insulin (P = 0.01) and pancreatic polypeptide (P = 0.04) were also observed under ICV GIP. Effects of ICV GIP on fasted or postprandial glucagon, glucose, triglyceride, and free fatty acids were not observed. Our data demonstrate that central GIP signaling can promote increased plasma GIP concentrations independent of nutrient stimulation and increase insulin and pancreatic polypeptide responses to a mixed meal
The Young Stellar Population of IC1613. I. A New Catalogue of OB Associations
Context: Determining the parameters of massive stars is crucial to understand
many processes in galaxies and the Universe, since these objects are important
sources of ionization, chemical enrichment and momentum. 10m class telescopes
enable us to perform detailed quantitative spectroscopic analyses of massive
stars in other galaxies, sampling areas of different metallicity. Relating the
stars to their environment is crucial to understand the physical processes
ruling their formation and evolution.
Aims: In preparation for the GTC, our goal is to build a catalogue of massive
star candidates in the metal-poor irregular galaxy IC1613 with high astrometric
accuracy, apt for the current generation of multi-object spectrographs. A
census of OB associations in this galaxy is also needed, to provide important
additional information about age and environment of the candidate OB stars.
Methods: From INT-WFC observations, we have built an astrometric and
photometric catalogue of stars in IC1613. Candidate blue massive stars are
preselected from their colors. A friends-of-friends algorithm is developed to
find their clustering in the galaxy. While a common physical origin for all the
members of the associations cannot be ensured, this is a necessary first step
to place candidate OB stars in a population context.
Results: We have produced a deep catalogue of targets in IC1613 that covers a
large field of view. To achieve high astrometric accuracy a new astrometric
procedure is developed for the INT-WFC data. We have also built a catalogue of
OB associations in IC1613. We have found that they concentrate in the central
regions, specially in the HII bubbles. The study of extinction confirms that it
is patchy, with local values of color-excess above the foreground value.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages +
appendix (14 pages); 20 figure
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