1,990 research outputs found
Is the dark matter halo of the Milky Way flattened?
We performed an extended analysis of the parameter space for the interaction
of the Magellanic System with the Milky Way (MW). The varied parameters cover
the phase space parameters, the masses, the structure, and the orientation of
both Magellanic Clouds, as well as the flattening of the dark matter halo of
the MW. The analysis was done by a specially adopted optimization code
searching for the best match between numerical models and the detailed HI map
of the Magellanic System by Bruens et al. (2005). The applied search algorithm
is a genetic algorithm combined with a code based on the fast, but
approximative restricted N-body method. By this, we were able to analyze more
than 10^6 models, which makes this study one of the most extended ones for the
Magellanic System. Here we focus on the flattening q of the axially symmetric
MW dark matter halo potential, that is studied within the range 0.74<=q<=1.20.
We show that creation of a trailing tail (Magellanic Stream) and a leading
stream (Leading Arm) is quite a common feature of the Magellanic System-MW
interaction, and such structures were modeled across the entire range of halo
flattening values. However, important differences exist between the models,
concerning density distribution and kinematics of HI, and also the dynamical
evolution of the Magellanic System. Detailed analysis of the overall agreement
between modeled and observed distribution of neutral hydrogen shows that the
models assuming an oblate (q<1.0) dark matter halo of the Galaxy allow for
better satisfaction of HI observations than models with other halo
configurations.Comment: 19 pages, 20 figures, 2 appendices, accepted for publication in A&
Search for cold and hot gas in the ram pressure stripped Virgo dwarf galaxy IC3418
We present IRAM 30m sensitive upper limits on CO emission in the ram pressure
stripped dwarf Virgo galaxy IC3418 and in a few positions covering HII regions
in its prominent 17 kpc UV/Ha gas-stripped tail. In the central few arcseconds
of the galaxy, we report a possible marginal detection of about 1x10^6 M_sun of
molecular gas (assuming a Galactic CO-to-H_2 conversion factor) that could
correspond to a surviving nuclear gas reservoir. We estimate that there is less
molecular gas in the main body of IC3418, by at least a factor of 20, than
would be expected from the pre-quenching UV-based star formation rate assuming
the typical gas depletion timescale of 2 Gyr. Given the lack of star formation
in the main body, we think the H_2-deficiency is real, although some of it may
also arise from a higher CO-to-H_2 factor typical in low-metallicity, low-mass
galaxies. The presence of HII regions in the tail of IC3418 suggests that there
must be some dense gas; however, only upper limits of < 1x10^6 M_sun were found
in the three observed points in the outer tail. This yields an upper limit on
the molecular gas content of the whole tail < 1x10^7 M_sun, which is an amount
similar to the estimates from the observed star formation rate over the tail.
We also present strong upper limits on the X-ray emission of the stripped gas
in IC3418 from a new Chandra observation. The measured X-ray luminosity of the
IC3418 tail is about 280 times lower than that of ESO 137-001, a spiral galaxy
in a more distant cluster with a prominent ram pressure stripped tail.
Non-detection of any diffuse X-ray emission in the IC3418 tail may be due to a
low gas content in the tail associated with its advanced evolutionary state
and/or due to a rather low thermal pressure of the surrounding intra-cluster
medium.Comment: 15 pages, 11 figures, A&A accepte
Spatially resolved femtosecond pump-probe study of topological insulator Bi2Se3
Carrier and phonon dynamics in Bi2Se3 crystals are studied by a spatially
resolved ultrafast pump-probe technique. Pronounced oscillations in
differential reflection are observed with two distinct frequencies, and are
attributed to coherent optical and acoustic phonons, respectively. The rising
time of the signal indicates that the thermalization and energy relaxation of
hot carriers are both sub-ps in this material. We found that the thermalization
and relaxation time decreases with the carrier density. The expansion of the
differential reflection profile allows us to estimate an ambipolar carrier
diffusion coefficient on the order of 500 square centimeters per second. A
long-term slow expansion of the profile shows a thermal diffusion coefficient
of 1.2 square centimeters per second.Comment: 8 pages, 6 figure
Competing interactions in arrested states of colloidal clays
Using experiments, theory and simulations, we show that the arrested state
observed in a colloidal clay at intermediate concentrations is stabilized by
the screened Coulomb repulsion (Wigner glass). Dilution experiments allow us to
distinguish this high-concentration disconnected state, which melts upon
addition of water, from a low-concentration gel state, which does not melt.
Theoretical modelling and simulations reproduce the measured Small Angle X-Ray
Scattering static structure factors and confirm the long-range electrostatic
nature of the arrested structure. These findings are attributed to the
different timescales controlling the competing attractive and repulsive
interactions.Comment: Accepted for publication in Physical Review Letter
Meniscus dynamics in bubble formation : a parametric study
This study brings new experimental data on the bubble formation process with a particular focus on the behaviour of the gas–liquid interface (meniscus) inside a transparent orifice on a perforated plate. The meniscus plays an important role by coupling the gas and liquid sides. The goal was to find the effect of five important control parameters on the bubble production and the meniscus behaviour (orifice diameter, plate thickness, gas chamber size, liquid height, and liquid viscosity). The gas input was also varied. Three physical signals were taken in the measurements to extract the information needed (gas pressure in plenum, acoustic pressure in liquid, and video record of meniscus motion). Several relevant quantities were measured to characterize the bubbling process (bubbling period, active orifice time, and periods of meniscus oscillations). The data obtained show how these quantities depend on the control parameters. The recognition of the role of the meniscus dynamics in the gas dispersing process with perforated plates offers a deeper insight into this important and complicated problem
Hot carrier diffusion in graphene
We report an optical study of charge transport in graphene. Diffusion of hot
carriers in epitaxial graphene and reduced graphene oxide samples are studied
using an ultrafast pump-probe technique with a high spatial resolution.
Spatiotemporal dynamics of hot carriers after a point-like excitation are
monitored. Carrier diffusion coefficients of 11,000 and 5,500 squared
centimeters per second are measured in epitaxial graphene and reduced graphene
oxide samples, respectively, with a carrier temperature on the order of 3,600
K. The demonstrated optical techniques can be used for non-contact and
non-invasive in-situ detection of transport properties of graphene.Comment: 5 pages, 3 figure
Effect of viscosity on homogeneous–heterogeneous flow regime transition in bubble columns
Experiments were performed in a cylindrical 0.14m diameter bubble column with a metal perforated plate. Air and aqueous solutions
of glycerol with viscosity 1–22mPa s were the phases. Gas holdup was measured and plotted against the gas flow rate. The critical
point where the homogeneous–heterogeneous regime transition begins was determined by the drift-flux plot of the primary data. The
homogeneous regime stability was expressed by the critical values of the gas holdup and gas flow rate. The results show that moderate
viscosity (3–22 mPa s) destabilizes the homogeneous regime and advance the transition. The results indicate that low viscosity (1–3 mPa s)
could stabilize the homogeneous regime. The destabilizing effect of the column height proved previously for air–water system applies also
to viscous batches.Comissão Europeia (CE) - Marie Curie Training Site Fellowship Contract Number HPMT-CT-2000-00074.Grantová agentura České republiky (GA CR)
- Grant No. 104/01/0547
Effect of solids on flow regime transition in three-phase bubble columns
In bubble column reactors there are two principal flow regimes [1,2,3], the
homogeneous (HoR) and the heterogeneous (HeR). These reactors have different behaviour in
HoR and HeR, thus the dependences of the rates of mass, heat and momentum transfer on the
design and operating parameters (such as reactor geometry, gas and liquid flow rates and
properties of the contacting phases) are also very different. Therefore, for rational reactor
design and operation it is of crucial importance to know the range of parameters over which a
certain regime prevails and the regime transition conditions [4].
The effect of the presence of solids on gas holdup in bubble columns has been
extensively investigated. Most of the published work reports that the gas holdup decreases
with increasing solid concentration [5-14]. However, a favourable effect of solids on gas
holdup has been also observed by Douek et al. [15]. In spite of all these efforts, information
on the effect of solids on homogeneous-heterogeneous regime transition is very scarce.
Krishna et al. [16] found that the transition gas holdup was significantly reduced due to the
presence of silica particles. Xie et al. [17] reported that as pulp consistency is increased, the
regime transition is delayed.
The goal of this work was to examine the influence of solid particles on homogenous
regime stability and regime transition in a three phase bubble column. For that, two studies
were done, one focused on the regime transition and the other on visualization of bubbleparticle
interaction.
The regime transition experiments were performed in a cylindrical bubble column of
0.14 m diameter. Air, distilled water and calcium alginate beads (deq=2.1mm) at
concentrations up to 30%(v/v) were the phases. The dependence of the gas holdup (e) on the
gas flow rate (q) was measured. At low solid loading, the experimental data e(q) show a slight
increase of gas holdup with solid concentration, which is not normally observed. At higher
solid loading, a significant reduction of gas holdup with solid concentration is observed. This
suggests that solid concentration could play a dual role in the homogeneous regime stability:
low concentrations stabilize, while high concentrations destabilize. The critical point, where
the HoR loses stability and the transition begins, was evaluated using the drift flux plot. The
critical values of gas flow rate and voidage were the measures of the homogeneous regime
stability. For low solid volume fraction, a stabilizing effect of the presence of solids is shown
by an increase in the critical values, while for higher solids volume fraction ( φ
s>3%) the
critical values decrease, witnessing a destabilizing effect of the solids.
The visualization study was performed in a 0.07 m diameter cylindrical bubble
column, using a standard and high-speed cameras. This work shows important phenomena
that result from the bubble-particle interactions, which partly explains the regime transition
results. At low solid loading, bubble-particle collisions lead to reduction of bubble rise
velocity that results in higher gas holdup and a delay of the transition. At higher solid loading,
the bubble coalescence rate increases with the solid content that decreases the gas holdup and
advances the transition.Commission of the European
Communities for the Marie Curie Training Site - contract No. HPMT-CT-2000-0074
Effect of solids on homogeneous–heterogeneous flowregime transition in bubble columns
Experiments were conducted to study the effect of the presence of the solid phase on the homogeneous–heterogeneous flow regime transition in a bubble column 0.14m diameter. Air, distilled water and calcium alginate beads (2.1 mm, 1023 kg/m3) at concentrations c=0–30% (vol.) were the phases. The basic data were the voidage–gas flow rate dependences. The critical point, where the homogeneous
regime loses stability and the transition begins, was evaluated by the drift flux model. The critical values of voidage and gas flow rate were the quantitative measures of the homogeneous regime stability. These were plotted against the solid phase concentration. It was found, that both the voidage and the critical values increased with the solid content at low solid loading, approx. c=0–3%, and decreased at higher loading, c>3%. The homogeneous regime was thus first stabilized and then destabilized. To explain this dual effect, possible physicalmechanisms of the solid phase influence on the uniform bubble bed were discussed
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