216 research outputs found
Brightest Cluster Galaxies and Core Gas Density in REXCESS Clusters
We investigate the relationship between brightest cluster galaxies (BCGs) and
their host clusters using a sample of nearby galaxy clusters from the
Representative XMM Cluster Structure Survey (REXCESS). The sample was imaged
with the Southern Observatory for Astrophysical Research (SOAR) in R band to
investigate the mass of the old stellar population. Using a metric radius of
12h^-1 kpc, we found that the BCG luminosity depends weakly on overall cluster
mass as L_BCG \propto M_cl^0.18+-0.07, consistent with previous work. We found
that 90% of the BCGs are located within 0.035 r_500 of the peak of the X-ray
emission, including all of the cool core (CC) clusters. We also found an
unexpected correlation between the BCG metric luminosity and the core gas
density for non-cool core (non-CC) clusters, following a power law of n_e
\propto L_BCG^2.7+-0.4 (where n_e is measured at 0.008 r_500). The correlation
is not easily explained by star formation (which is weak in non-CC clusters) or
overall cluster mass (which is not correlated with core gas density). The trend
persists even when the BCG is not located near the peak of the X-ray emission,
so proximity is not necessary. We suggest that, for non-CC clusters, this
correlation implies that the same process that sets the central entropy of the
cluster gas also determines the central stellar density of the BCG, and that
this underlying physical process is likely to be mergers.Comment: 16 pages, 8 figures, accepted Astrophysical Journa
Green Cooling of High Performance Micro Processors: Parametric Study between Flow Boiling and Water Cooling
Due to the increase in energy prices and spiralling consumption, there is a need to greatly reduce the cost of electricity within data centers, where it makes up 50% of the total cost of the IT infrastructure. A technological solution to this is using on-chip cooling with a single-phase or evaporating liquid to replace energy intensive air-cooling. The energy carried away by the liquid or vapour can also potentially be used in district heating, as an example. Thus, the important issue here is âwhat is the most energy efficient heat removal process?â As an answer, this paper presents a direct comparison of single-phase water, a 50% water ethylene glycol mixture and several two-phase refrigerants, including the new fourth generation refrigerants HFO1234yf and HFO1234ze. Two-phase cooling using HFC134a had an average junction temperature 9 to 15ËC lower than for single-phase cooling, while the required pumping power for the CPU cooling element for single-phase cooling was on the order of 20-130 times higher to achieve the same junction temperature uniformity. Hot-spot simulations also showed that two-phase refrigerant cooling was able to adjust to local hot-spots because of flow boiling's dependency on the local heat flux, with junction temperatures being 20 to 30ËC lower when compared to water and the 50% water-ethylene glycol mixture, respectively. An exergy analysis was developed considering a cooling cycle composed by a pump, a condenser and a multi-microchannel cooler. The focus was to show the exergetic efficiency of each component and of the entire cycle when the subject energy recovery is considered. Water and HFC134a were the working fluids evaluated in such analysis. The overall exergetic efficiency was higher when using HFC134a (about 2%) and the exergy destroyed, i.e. irreversibilities, showed that the cooling cycle proposed still have a huge potential to increase the thermodynamic performance
X-ray and UV emission from the recurrent nova RS Ophiuchi in quiescence: Signatures of accretion and shocked gas
RS Ophiuchi is a recurrent nova system that experiences outbursts every ~20
years, implying accretion at a high rate onto a massive white dwarf. However,
previous X-ray observations of the system in quiescence have detected only
faint emission that is difficult to reconcile with the high accretion rate
predicted by nova theory for such frequent outbursts. Here, we use new Chandra
and XMM-Newton observations obtained 537 and 744 days after the 2006 outburst
to constrain both the accretion rate onto the white dwarf and the properties of
the nova ejecta at these times. We detect low level UV variability with the
XMM-Newton Optical Monitor on day 744 that is consistent with accretion disk
flickering, and use this to place a lower limit on the accretion rate. The
X-ray spectra in both observations are well described by a two component
thermal plasma model. The first component originates in the nova shell, which
can emit X-rays for up to a decade after the outburst. The other component
likely arises in the accretion disk boundary layer, and can be equally well fit
by a single temperature plasma or a cooling flow model. Although the flux of
the single temperature model implies an accretion rate that is 40 times lower
than theoretical predictions for RS Oph, the best fit cooling flow model
implies Mdot < 1.2x10^-8 M_sol/yr 537 days after the outburst, which is within
a factor of 2 of the theoretical accretion rate required to power an outburst
every 20 years. Furthermore, we place an upper limit on the accretion rate
through an optically thick region of the boundary layer of 2.0x10^-8 M_sol/yr.
Thus, the X-ray emission in quiescence is consistent with the accretion rate
expectations of nova theory. Finally, we discuss the possible origins of the
low temperature associated with the accretion component, which is a factor of
10 lower than in T CrB, an otherwise similar recurrent nova.Comment: 16 pages, 6 figures, accepted for publication in Ap
X-ray Emission from the Binary Central Stars of the Planetary Nebulae HFG 1, DS 1, and LoTr 5
Close binary systems undergoing mass transfer or common envelope interactions
can account for the morphological properties of some planetary nebulae. The
search for close binary companions in planetary nebulae is hindered by the
difficulty of detecting cool, late-type, main sequence companions in binary
systems with hot pre-white dwarf primaries. However, models of binary PN
progenitor systems predict that mass accretion or tidal interactions can induce
rapid rotation in the companion, leading to X-ray-emitting coronae. To test
such models, we have searched for, and detected, X-ray emission from three
binary central stars within planetary nebulae: the post-common envelope close
binaries in HFG 1 and DS 1 consisting of O-type subdwarfs with late-type, main
sequence companions, and the binary system in LoTr 5 consisting of O-type
subdwarf and rapidly rotating, late-type giant companion. The X-ray emission in
each case is best characterized by spectral models consisting of two
optically-thin thermal plasma components with characteristic temperatures of
about 10 MK and 15-40 MK, and total X-ray luminosities about 10^30 erg/s. We
consider the possible origin of the X-ray emission from these binary systems
and conclude that the most likely origin is, in each case, a corona around the
late-type companion, as predicted by models of interacting binaries.Comment: tell your friend
Developing the next generation of renewable energy technologies:an overview of low-TRL EU-funded research projects
A cluster of eleven research and innovation projects, funded under the same call of the EUâs H2020 programme, are developing breakthrough and game-changing renewable energy technologies that will form the backbone of the energy system by 2030 and 2050 are, at present, at an early stage of development. These projects have joined forces at a collaborative workshop, entitled â Low-TRL Renewable Energy Technologiesâ, at the 10th Sustainable Places Conference (SP2022), to share their insights, present their projectsâ progress and achievements to date, and expose their approach for exploitation and market uptake of their solutions.</p
Developing the next generation of renewable energy technologies:an overview of low-TRL EU-funded research projects
A cluster of eleven research and innovation projects, funded under the same call of the EUâs H2020 programme, are developing breakthrough and game-changing renewable energy technologies that will form the backbone of the energy system by 2030 and 2050 are, at present, at an early stage of development. These projects have joined forces at a collaborative workshop, entitled â Low-TRL Renewable Energy Technologiesâ, at the 10th Sustainable Places Conference (SP2022), to share their insights, present their projectsâ progress and achievements to date, and expose their approach for exploitation and market uptake of their solutions.</p
The complete set of ASCA X-ray observations of non-magnetic cataclysmic variables
We present the complete set of thirty four ASCA observations of non-magnetic
cataclysmic variables. Timing analysis reveals large X-ray flux variations in
dwarf novae in outburst (Z Cam, SS Cyg and SU UMa) and orbital modulation in
high inclination systems (including OY Car, HT Cas, U Gem, T Leo). We also
found episodes of unusually low accretion rate during quiescence (VW Hyi and SS
Cyg). Spectral analysis reveals broad temperature distributions in individual
systems, with emission weighted to lower temperatures in dwarf novae in
outburst. Absorption in excess of interstellar values is required in dwarf
novae in outburst, but not in quiescence. We also find evidence for sub-solar
abundances and X-ray reflection in the brightest systems. LS Peg, V426 Oph and
EI UMa have X-ray spectra that are distinct from the rest of the sample and all
three exhibit candidate X-ray periodicities. We argue that they should be
reclassified as intermediate polars. In the case of V345 Pav we found that the
X-ray source had been previously misidentified.Comment: 19 pages, 24 figures. Published in MNRAS, Volume 357, Issue 2, Page
626-644, February 200
Caractérisation des transferts de chaleur convectifs d'un écoulement refroidi de dioxyde de carbone supercritique
Study on convective heat transfer of supercritical carbon dioxyde flow have been performed. An experimental apparatus was build, allowing an experimental investigation of heat transfer and pressure drop during cooling of supercritical CO2 turbulent flows. Horizontal and vertical flows were considered.Experimental results on vertical flows show that heat transfer coefficient reach a peak at a temperature close to the pseudo-critical one and that the peak value is decreasing with increasing pressure. A strong influence of mixed convection has to be taken into account too. Results were presented on a classical form used in mixed convection studies in heating condition, leading to the conclusion that mixed convection behaviors are quite similar in heating and cooling conditions.A numerical model for supercritical fuids flows was developped too, in heating conditions.Une Ă©tude des transferts de chaleur convectifs de CO2 supercritique a Ă©tĂ© menĂ©e. Une installation expĂ©rimentale a Ă©tĂ© construite pour la thĂšse et a permis une investigation expĂ©rimentale des transferts de chaleur et des pertes de pressions lors du refroidissement d'Ă©coulements turbulents de CO2 supercritique. Des configurations verticales et horizontales ont Ă©tĂ© considĂ©rĂ©es.Les campagnes d'essais en Ă©coulement vertical ont montrĂ© que le cĆfficient d'Ă©change montrait une forme particuliĂšre caractĂ©risĂ©e par un pic au voisinage de la tempĂ©rature pseudo-critique, la valeur au pic diminue Ă mesure que la pression augmente. Il apparaĂźt Ă©galement qu'une forte influence de la convection mixte est Ă considĂ©rĂ©e. Les rĂ©sultats ont Ă©tĂ© prĂ©sentĂ©s sous une forme caractĂ©ristique des Ă©tudes sur la convection mixte en chauffage, confirmant ainsi que les comportements de la convection mixte sont analogues en chauffage et en refroidissement.ParallĂšlement, un modĂšle numĂ©rique pour la simulation d'Ă©coulement turbulents de fluides supercritiques a Ă©tĂ© dĂ©veloppĂ©
Caractérisation des transferts de chaleur convectifs d'un écoulement refroidi de dioxyde de carbone supercritique
L'objectif de ce travail est la caractérisation du comportement thermique d'un écoulement refroidi de CO2 supercritique, en configurations verticales et horizontales. Le travail s'appuie plus particuliÚrement sur une installation expérimentale conçue durant la thÚse et dédiée à l'étude du refroidissement de CO2 supercritique. Une campagne expérimentale réalisée sur des écoulements verticaux a montré la forte influence des forces de flottabilité sur les régimes thermiques. En écoulement montant (convection mixte turbulente opposée), les échanges sont améliorés avec l'apparition et le développement de convection mixte. En écoulement descendant (convection turbulente mixte aidée), une forte dégradation est observée dans la région de convection mixte. Les résultats ont été présentés sous une forme adimensionnelle caractéristique de la convection mixte et ont ainsi permis de généraliser et compléter les données de la littérature au cas particulier du refroidissement. ParallÚlement, un modÚle numérique a été développé pour les régimes turbulents et validés sur les données expérimentales issues de la littérature en chauffage.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF
Experimental investigation of the effect of flow direction on heat transfer of super critical carbon dioxide flowing in cooled vertical tube
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