303 research outputs found
Temperature Dependence of the Optical Response of Small Sodium Clusters
We present an analysis of the temperature dependence of the optical response
of small sodium clusters in a temperature range bracketing the melting phase
transition. When the temperature increases, the mean excitation energy
undergoes a red shift and the plasmon is significantly broadened, in agreement
with recent experimental data. We show that the single--particle levels acquire
a prominent width and the HOMO--LUMO gap as well as the width of the occupied
band are reduced due to large thermal cluster size and shape fluctuations. This
results in a sharp increase of the static polarizability with temperature.Comment: 9 pages, Revtex, 3 uuencoded postscript figure
Forming circumnuclear disks and rings in galactic nuclei: a competition between supermassive black hole and nuclear star cluster
We investigate the formation of circumnuclear gas structures from the tidal
disruption of molecular clouds in galactic nuclei, by means of smoothed
particle hydrodynamics simulations. We model galactic nuclei as composed of a
supermassive black hole (SMBH) and a nuclear star cluster (NSC) and consider
different mass ratios between the two components. We find that the relative
masses of the SMBH and the NSC have a deep impact on the morphology of the
circumnuclear gas. Extended disks form only inside the sphere of influence of
the SMBH. In contrast, compact rings naturally form outside the SMBH's sphere
of influence, where the gravity is dominated by the NSC. This result is in
agreement with the properties of the Milky Way's circumnuclear ring, which
orbits outside the SMBH sphere of influence. Our results indicate that compact
circumnuclear rings can naturally form outside the SMBH sphere of influence.Comment: Accepted for publication in ApJ. 12 pages, 6 figures, 3 tables.
Comments welcom
Simulating the pericentre passage of the Galactic centre star S2
The so-called S2 star reached its closest approach to the massive black hole
(BH) at around 1500 in May 2018. It has been proposed that the
interaction of its stellar wind with the high-density accretion flow at this
distance from Sgr A* will lead to a detectable, month-long X-ray flare. Our
goal is to verify whether or not the S2 star wind can be used as a diagnostic
tool to infer the properties of the accretion flow towards Sgr A* at its
pericentre (an unprobed distance regime), putting important constraints on BH
accretion flow models. We run a series of three-dimensional adaptive mesh
refinement simulations with the help of the Ramses code which include the
realistic treatment of the interaction of S2's stellar wind with the accretion
flow along its orbit and - apart from hydrodynamical and thermodynamical
effects - include the tidal interaction with the massive BH. These are
post-processed to derive the X-ray emission in the observable 2-10 keV window.
No significant excess of X-ray emission from Sgr A* is found for typical
accretion flow models. A measurable excess is produced for a significantly
increased density of the accretion flow. This can, however, be ruled out for
standard power-law accretion flow models as in this case the thermal X-ray
emission without the S2 wind interaction would already exceed the observed
quiescent luminosity. Only a significant change of the wind parameters
(increased mass loss rate and decreased wind velocity) might lead to an
(marginally) observable X-ray flaring event. Even the detection of an
(month-long) X-ray flare during the pericentre passage of the S2 star would not
allow for strict constraints to be put on the accretion flow around Sgr A* due
to the degeneracy caused by the dependence on multiple parameters (of the
accretion flow model as well as the stellar wind).Comment: accepted by A&A Letter
Hydrodynamical simulations of a compact source scenario for G2
The origin of the dense gas cloud G2 discovered in the Galactic Center
(Gillessen et al. 2012) is still a debated puzzle. G2 might be a diffuse cloud
or the result of an outflow from an invisible star embedded in it. We present
here detailed simulations of the evolution of winds on G2's orbit. We find that
the hydrodynamic interaction with the hot atmosphere present in the Galactic
Center and the extreme gravitational field of the supermassive black hole must
be taken in account when modeling such a source scenario. We find that the
hydrodynamic interaction with the hot atmosphere present in the Galactic Center
and the extreme gravitational field of the supermassive black hole must be
taken in account when modeling such a source scenario. We also find that in
this scenario most of the Br\gamma\ luminosity is expected to come from the
highly filamentary densest shocked wind material. G2's observational properties
can be used to constrain the properties of the outflow and our best model has a
mass outflow rate of Mdot,w=8.8 x 10^{-8} Msun/yr and a wind velocity of vw =
50 km/s. These values are compatible with those of a young TTauri star wind, as
already suggested by Scoville & Burkert (2013).Comment: 4 pages, 3 figures; Proceeding of the IAU 303: "The GC: Feeding and
Feedback in a Normal Galactic Nucleus" / September 30 - October 4, 2013,
Santa Fe, New Mexico (USA
Thermoresponsive Ionic Liquid/Water Mixtures: From Nanostructuring to Phase Separation
The thermodynamics, structures, and applications of thermoresponsive systems, consisting primarily of water solutions of organic salts, are reviewed. The focus is on organic salts of low melting temperatures, belonging to the ionic liquid (IL) family. The thermo-responsiveness is represented by a temperature driven transition between a homogeneous liquid state and a biphasic state, comprising an IL-rich phase and a solvent-rich phase, divided by a relatively sharp interface. Demixing occurs either with decreasing temperatures, developing from an upper critical solution temperature (UCST), or, less often, with increasing temperatures, arising from a lower critical solution temperature (LCST). In the former case, the enthalpy and entropy of mixing are both positive, and enthalpy prevails at low T. In the latter case, the enthalpy and entropy of mixing are both negative, and entropy drives the demixing with increasing T. Experiments and computer simulations highlight the contiguity of these phase separations with the nanoscale inhomogeneity (nanostructuring), displayed by several ILs and IL solutions. Current applications in extraction, separation, and catalysis are briefly reviewed. Moreover, future applications in forward osmosis desalination, low-enthalpy thermal storage, and water harvesting from the atmosphere are discussed in more detail
3D AMR hydrosimulations of a compact source scenario for the Galactic Centre cloud G2
The nature of the gaseous and dusty cloud G2 in the Galactic Centre is still
under debate. We present three-dimensional hydrodynamical adaptive mesh
refinement (AMR) simulations of G2, modeled as an outflow from a "compact
source" moving on the observed orbit. The construction of mock
position-velocity (PV) diagrams enables a direct comparison with observations
and allow us to conclude that the observational properties of the gaseous
component of G2 could be matched by a massive () and slow ()
outflow, as observed for T Tauri stars. In order for this to be true, only the
material at larger () distances from the source must be
actually emitting, otherwise G2 would appear too compact compared to the
observed PV diagrams. On the other hand, the presence of a central dusty source
might be able to explain the compactness of G2's dust component. In the present
scenario, 5-10 years after pericentre the compact source should decouple from
the previously ejected material, due to the hydrodynamic interaction of the
latter with the surrounding hot and dense atmosphere. In this case, a new
outflow should form, ahead of the previous one, which would be the smoking gun
evidence for an outflow scenario.Comment: resubmitted to MNRAS after referee report, 16 pages, 11 figure
Seasonal Occurrence and Distribution on Grapevine Roots of Eurhizococcus brasiliensis (Wille) (Hemiptera: Margarodidae) in Brazil
The ground pearl, Eurhizococcus brasiliensis (Wille) (Hemiptera: Margarodidae), is the most importantgrapevine pest in Brazil. Its seasonal occurrence and distribution on the roots of the different developmentstages were determined to allow better monitoring of this insect and better targeting of its vulnerable lifestages. Yellow cysts (after the first nymphal moult) showed the lowest density in October, followed by agradual increase towards August. White cysts (cysts with enclosed pre-pupal males or females) occurredfrom August to December, with a peak in November. Mobile females (adult females emerging from thewhite cysts) were found from August to December, with a peak in August. Parthenogenetic females thatremain in the ruptured white cysts for egg laying were present from August to April, with a peak inNovember. Mobile nymphs (first instar) were also found from August to April, with a peak in December.Yellow cysts were most abundant at depths of 0 to 25 cm. The horizontal survey showed that cysts occurredmostly on the trunk below the ground (trunk of the rootstock), and that almost all occurred in an area of20 cm width around the trunk. These results provide important information for better monitoring of thispest and to develop better methods for and timing of chemical control
Evolution of fractality and rotation in embedded star clusters
More and more observations indicate that young star clusters could retain imprints of their formation process. In particular, the degree of substructuring and rotation are possibly the direct result of the collapse of the parent molecular cloud from which these systems form. Such properties can, in principle, be washed-out, but they are also expected to have an impact on the relaxation of these systems. We ran and analysed a set of 10 hydrodynamical simulations of the formation of embedded star clusters through the collapse of turbulent massive molecular clouds. We systematically studied the fractality of our star clusters, showing that they are all extremely substructured (fractal dimension D = 1.0-1.8). We also found that fractality is slowly reduced, with time, on small scales, while it persists on large scales on longer time-scales. Signatures of rotation are found in different simulations at every time of the evolution, even for slightly supervirial substructures, proving that the parent molecular gas transfers part of its angular momentum to the new stellar systems
Formation of black holes in the pair-instability mass gap: Hydrodynamical simulation of a massive star collision
The detection of GW190521 by the LIGO-Virgo collaboration proved the
existence of black holes in the theoretically predicted pair-instability gap of
their mass spectrum. Some recent studies suggest that such massive black holes
could be produced by the collision of an evolved star with a carbon-oxygen core
and a main sequence star. Such a post-coalescence star could end its life
avoiding the pair-instability regime and with a direct collapse of its very
massive envelope. It is still not clear, however, how the collision shapes the
structure of the newly produced star and how much mass is actually lost in the
impact. We investigated this issue by means of hydrodynamical simulations with
the smoothed particle hydrodynamics code StarSmasher, finding that the
collision can remove up to 12% of the initial mass of the colliding stars. This
is a non-negligible percentage of the initial mass and could affect the further
evolution of the stellar remnant, particularly in terms of the final mass of a
possibly forming black hole, if the core avoids the pair-instability regime. We
also found that the main sequence star can plunge down to the outer boundary of
the carbon-oxygen core of the primary, changing the inner chemical composition
of the remnant. The collision expels the outer layers of the primary, leaving a
remnant with an helium-enriched envelope (reaching He fractions of about 0.4 at
the surface). These more complex abundance profiles can be directly used in
stellar evolution simulations of the collision product.Comment: 7 pages, 5 figures. Comments welcome
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