922 research outputs found
The depletion in Bose Einstein condensates using Quantum Field Theory in curved space
Using methods developed in Quantum Field Theory in curved space we can
estimate the effects of the inhomogeneities and of a non vanishing velocity on
the depletion of a Bose Einstein condensate within the hydrodynamical
approximation.Comment: 4 pages, no figure. Discussion extended and references adde
Hawking radiation in dispersive theories, the two regimes
We compute the black hole radiation spectrum in the presence of
high-frequency dispersion in a large set of situations. In all cases, the
spectrum diverges like the inverse of the Killing frequency. When studying the
low-frequency spectrum, we find only two regimes: an adiabatic one where the
corrections with respect to the standard temperature are small, and an abrupt
one regulated by dispersion, in which the near-horizon metric can be replaced
by step functions. The transition from one regime to the other is governed by a
single parameter which also governs the net redshift undergone by dispersive
modes. These results can be used to characterize the quasiparticles spectrum of
recent and future experiments aiming to detect the analogue Hawking radiation.
They also apply to theories of quantum gravity which violate Lorentz
invariance.Comment: 11 pages, 9 figure
Hawking radiation of massive modes and undulations
We compute the analogue Hawking radiation for modes which posses a small wave
vector perpendicular to the horizon. For low frequencies, the resulting mass
term induces a total reflection. This generates an extra mode mixing that
occurs in the supersonic region, which cancels out the infrared divergence of
the near horizon spectrum. As a result, the amplitude of the undulation
(0-frequency wave with macroscopic amplitude) emitted in white hole flows now
saturates at the linear level, unlike what was recently found in the massless
case. In addition, we point out that the mass introduces a new type of
undulation which is produced in black hole flows, and which is well described
in the hydrodynamical regime.Comment: 37 pages, 8 figures, published versio
Anomaly-Induced Effective Action and Inflation
In the early Universe matter can be described as a conformal invariant
ultra-relativistic perfect fluid, which does not contribute, on classical
level, to the evolution of the isotropic and homogeneous metric. If we suppose
that there is some desert in the particle spectrum just below the Planck mass,
then the effect of conformal trace anomaly is dominating at the corresponding
energies. With some additional constraints on the particle content of the
underlying gauge model (which favor extended or supersymmetric versions of the
Standard Model rather than the minimal one), one arrives at the stable
inflation. We review the model and report about the calculation of the
gravitational waves on the background of the anomaly-induced inflation. The
result for the perturbation spectrum is close to the one for the conventional
inflaton model, and is in agreement with the existing Cobe data (see also
[hep-th/0009197]).Comment: 4 pages, LaTeX. Contribution to the Proceedings of the EuroConference
on Frontiers in Particle Astrophysics and Cosmology, 30 September - 5 October
2000. San Feliu, Spai
Two-dimensional black holes in accelerated frames: quantum aspects
By considering charged black hole solutions of a one parameter family of two
dimensional dilaton gravity theories, one finds the existence of quantum
mechanically stable gravitational kinks with a simple mass to charge relation.
Unlike their Einsteinian counterpart (i.e. extreme Reissner-Nordstr\"om), these
have nonvanishing horizon surface gravity.Comment: 18 pages, harvmac, 2 figure
Evidence for C and Mg variations in the GD-1 stellar stream
Dynamically cold stellar streams are the relics left over from globular cluster dissolution. These relics offer a unique insight into a now fully disrupted population of ancient clusters in our Galaxy. Using a combination of Gaia eDR3 proper motions, optical and near-UV colours, we select a sample of likely Red Giant Branch stars from the GD-1 stream for medium-low resolution spectroscopic follow-up. Based on radial velocity and metallicity, we are able to find 14 new members of GD-1, 5 of which are associated with the spur and blob/cocoon off-stream features. We measured C-abundances to probe for abundance variations known to exist in globular clusters. These variations are expected to manifest in a subtle way in globular clusters with such low masses (similar to 10(4) M-circle dot) and metallicities ([Fe/H] similar to -2.1 dex). We find that the C-abundances of the stars in our sample display a small but significant (3 sigma level) spread. Furthermore, we find similar to 3 sigma variation in Mg-abundances among the stars in our sample that have been observed by APOGEE. These abundance patterns match the ones found in Galactic globular clusters of similar metallicity. Our results suggest that GD-1 represents another fully disrupted low-mass globular cluster where light-element abundance spreads have been found
Bridge over troubled gas: clusters and associations under the SMC and LMC tidal stresses
We obtained SOAR telescope B and V photometry of 14 star clusters and 2
associations in the Bridge tidal structure connecting the LMC and SMC. These
objects are used to study the formation and evolution of star clusters and
associations under tidal stresses from the Clouds. Typical star clusters in the
Bridge are not richly populated and have in general relatively large diameters
(~30-35 pc), being larger than Galactic counterparts of similar age. Ages and
other fundamental parameters are determined with field-star decontaminated
photometry. A self-consistent approach is used to derive parameters for the
most-populated sample cluster NGC 796 and two young CMD templates built with
the remaining Bridge clusters. We find that the clusters are not coeval in the
Bridge. They range from approximately a few Myr (still related to optical HII
regions and WISE and Spitzer dust emission measurements) to about 100-200 Myr.
The derived distance moduli for the Bridge objects suggests that the Bridge is
a structure connecting the LMC far-side in the East to the foreground of the
SMC to the West. Most of the present clusters are part of the tidal dwarf
candidate D 1, which is associated with an H I overdensity. We find further
evidence that the studied part of the Bridge is evolving into a tidal dwarf
galaxy, decoupling from the Bridge.Comment: 15 pages, 15 figures, MNRAS, Accepted 2015 July 2
Black hole lasers, a mode analysis
We show that the black hole laser effect discovered by Corley & Jacobson
should be described in terms of frequency eigenmodes that are spatially bound.
The spectrum contains a discrete and finite set of complex frequency modes
which appear in pairs and which encode the laser effect. In addition, it
contains real frequency modes that form a continuous set when space is
infinite, and which are only elastically scattered, i.e., not subject to any
Bogoliubov transformation. The quantization is straightforward, but the
calculation of the asymptotic fluxes is rather involved. When the number of
complex frequency modes is small, our expressions differ from those given
earlier. In particular, when the region between the horizons shrinks, there is
a minimal distance under which no complex frequency mode exists, and no
radiation is emitted. Finally, we relate this effect to other dynamical
instabilities found for rotating black holes and in electric fields, and we
give the conditions to get this type of instability.Comment: 19 pages, 3 figures, main changes: new figure and new Sec.6
`conditions for having a laser effect', final version accepted in PR
The globular cluster NGC 6642: Evidence for a depleted mass function in a very old cluster
We present photometry for the globular cluster NGC 6642 using the F606W and
F814W filters with the ACS/WFC third generation camera on board of Hubble Space
Telescope. The Colour Magnitude Diagram shows sources reaching ~ 6 mags below
the turn-off in m_F606W. A theoretical isochrone fitting was performed and
evolutionary parameters were obtained, such as the metallicity [Fe/H] = -1.80
+/- 0.2 and age log(Age) = 10.14 +/- 0.05. We confirm that NGC 6642 is located
in the Galactic bulge, with a distance to the Sun d_{\odot} = 8.05 +/- 0.66 ~
kpc$ and the reddening E(B-V) = 0.46 +/- 0.02. These values are in general
agreement with those of previous authors. About 30 blue stragglers were found
within the central 1.6 pc of NGC 6642. They are strongly concentrated to the
very central regions. The cluster displays a well-developed horizontal branch,
with a much redder morphology than that of typical old halo globular clusters
of similar metallicity. Completeness corrected luminosity and mass functions
were obtained for different annuli centred on NGC 6642. Their spatial variation
indicates the existence of mass segregation and depletion of low mass stars.
Most striking is the inverted shape of the mass function itself, with an
increase in number as a function of increasing mass. This has been previously
observed in other globular clusters and is also the result of N-body
simulations of stellar systems which have undergone ~ 90% of their lifetime and
which are subjected to strong tidal effects. We also analysed the density
profile and concluded that NGC 6642 has a collapsed core, provided completeness
effects are correctly accounted for. We thus conclude from independent means
that NGC 6642 is a very old, highly-evolved, core-collapsed globular cluster
with an atypical HB morphology.Comment: Paper, contains 8 figures, 1 table and 8 page
Back-reaction effects in acoustic black holes
Acoustic black holes are very interesting non-gravitational objects which can
be described by the geometrical formalism of General Relativity. These models
can be useful to experimentally test effects otherwise undetectable, as for
example the Hawking radiation. The back-reaction effects on the background
quantities induced by the analogue Hawking radiation could be the key to
indirectly observe it. We briefly show how this analogy works and derive the
backreaction equations for the linearized quantum fluctuations in the
background of an acoustic black hole. A first order in hbar solution is given
in the near horizon region. It indicates that acoustic black holes, unlike
Schwarzschild ones, get cooler as they radiate phonons. They show remarkable
analogies with near-extremal Reissner-Nordstrom black holes.Comment: 10 pages, 1 figure; Talk given at the conference ``Constrained
Dynamics and Quantum Gravity (QG05)", Cala Gonone (Italy), September 200
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