780 research outputs found
Classical and Quantum Shell Dynamics, and Vacuum Decay
Following a minisuperspace approach to the dynamics of a spherically
symmetric shell, a reduced Lagrangian for the radial degree of freedom is
derived directly from the Einstein-Hilbert action. The key feature of this new
Lagrangian is its invariance under time reparametrization. Indeed, all
classical and quantum dynamics is encoded in the Hamiltonian constraint that
follows from that invariance. Thus, at the classical level, we show that the
Hamiltonian constraint reproduces, in a simple gauge, Israel's matching
condition which governs the evolution of the shell. In the quantum case, the
vanishing of the Hamiltonian (in a weak sense), is interpreted as the
Wheeler-DeWitt equation for the physical states, in analogy to the
corresponding case in quantum cosmology. Using this equation, quantum tunneling
through the classical barrier is then investigated in the WKB approximation,
and the connection to vacuum decay is elucidated.Comment: 36 pages, ReVTeX, 10 Figs. in postscript format, in print on Class.&
Quant.Gra
Effective dynamics of self-gravitating extended objects
We introduce an effective Lagrangian which describes the classical and
semiclassical dynamics of spherically symmetric, self-gravitating objects that
may populate the Universe at large and small (Planck) scale. These include
wormholes, black holes and inflationary bubbles. We speculate that such objects
represent some possible modes of fluctuation in the primordial spacetime foam
out of which our universe was born. Several results obtained by different
methods are encompassed and reinterpreted by our effective approach. As an
example, we discuss: i) the gravitational nucleation coefficient for a pair of
Minkowski bubbles, and ii) the nucleation coefficient of an inflationary vacuum
bubble in a Minkowski backgroundComment: 13 pages, no figures, ReVTe
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
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
Biases in the determination of dynamical parameters of star clusters: today and in the Gaia era
The structural and dynamical properties of star clusters are generally
derived by means of the comparison between steady-state analytic models and the
available observables. With the aim of studying the biases of this approach, we
fitted different analytic models to simulated observations obtained from a
suite of direct N-body simulations of star clusters in different stages of
their evolution and under different levels of tidal stress to derive mass, mass
function and degree of anisotropy. We find that masses can be
under/over-estimated up to 50% depending on the degree of relaxation reached by
the cluster, the available range of observed masses and distances of radial
velocity measures from the cluster center and the strength of the tidal field.
The mass function slope appears to be better constrainable and less sensitive
to model inadequacies unless strongly dynamically evolved clusters and a
non-optimal location of the measured luminosity function are considered. The
degree and the characteristics of the anisotropy developed in the N-body
simulations are not adequately reproduced by popular analytic models and can be
detected only if accurate proper motions are available. We show how to reduce
the uncertainties in the mass, mass-function and anisotropy estimation and
provide predictions for the improvements expected when Gaia proper motions will
be available in the near future.Comment: 14 pages, 8 figures, accepted for publication by MNRA
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
A model of radiating black hole in noncommutative geometry
The phenomenology of a radiating Schwarzschild black hole is analyzed in a
noncommutative spacetime. It is shown that noncommutativity does not depend on
the intensity of the curvature. Thus we legitimately introduce noncommutativity
in the weak field limit by a coordinate coherent state approach. The new
interesting results are the following: i) the existence of a minimal non-zero
mass to which black hole can shrink; ii) a finite maximum temperature that the
black hole can reach before cooling down to absolute zero; iii) the absence of
any curvature singularity. The proposed scenario offers a possible solution to
conventional difficulties when describing terminal phase of black hole
evaporation.Comment: 10 pages, 4 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
Casimir effect for scalar fields with Robin boundary conditions in Schwarzschild background
The stress tensor of a massless scalar field satisfying Robin boundary
conditions on two one-dimensional wall in two-dimensional Schwarzschild
background is calculated. We show that vacuum expectation value of stress
tensor can be obtained explicitly by Casimir effect, trace anomaly and Hawking
radiation.Comment: 10 pages, no figure
Quantum Gravity effects near the null black hole singularity
The structure of the Cauchy Horizon singularity of a black hole formed in a
generic collapse is studied by means of a renormalization group equation for
quantum gravity. It is shown that during the early evolution of the Cauchy
Horizon the increase of the mass function is damped when quantum fluctuations
of the metric are taken into account.Comment: 15 Pages, one figure. Minor changes in the presentation, to appear on
Phys.Rev.
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