179 research outputs found
On the Unruh effect in de Sitter space
We give an interpretation of the temperature in de Sitter universe in terms
of a dynamical Unruh effect associated with the Hubble sphere. As with the
quantum noise perceived by a uniformly accelerated observer in static
space-times, observers endowed with a proper motion can in principle detect the
effect. In particular, we study a "Kodama observer" as a two-field Unruh
detector for which we show the effect is approximately thermal. We also
estimate the back-reaction of the emitted radiation and find trajectories
associated with the Kodama vector fields are stable.Comment: 8 pages; corrected typos; sections structure revise
Hamilton-Jacobi Method and Gravitation
Studying the behaviour of a quantum field in a classical, curved, spacetime
is an extraordinary task which nobody is able to take on at present time.
Independently by the fact that such problem is not likely to be solved soon,
still we possess the instruments to perform exact predictions in special,
highly symmetric, conditions. Aim of the present contribution is to show how it
is possible to extract quantitative information about a variety of physical
phenomena in very general situations by virtue of the so-called Hamilton-Jacobi
method. In particular, we shall prove the agreement of such semi-classical
method with exact results of quantum field theoretic calculations.Comment: To appear in the proceedings of "Cosmology, the Quantum Vacuum, and
Zeta Functions": A workshop with a celebration of Emilio Elizalde's Sixtieth
birthday, Bellaterra, Barcelona, Spain, 8-10 Mar 201
On tunneling across horizons
The tunneling method for stationary black holes in the Hamilton-Jacobi
variant is reconsidered in the light of various critiques that have been moved
against. It is shown that once the tunneling trajectories have been correctly
identified the method isfree from internal inconsistencies, it is manifestly
covariant, it allows for the extension to spinning particles and it can even be
used without solving the Hamilton-Jacobi equation. These conclusions borrow
support on a simple analytic continuation of the classical action of a
pointlike particle, made possible by the unique assumption that it should be
analytic in complexified Schwarzschild or Kerr-Newman spacetimes. A more
general version of the Parikh-Wilczek method will also be proposed along these
lines.Comment: Latex Document, 5 pages, 2 figures, title changed, abstract changed,
added references, results unchange
AGB stars in the SMC: evolution and dust properties based on Spitzer observations
We study the population of asymptotic giant branch (AGB) stars in the Small
Magellanic Cloud (SMC) by means of full evolutionary models of stars of mass
1Msun < M < 8Msun, evolved through the thermally pulsing phase. The models also
account for dust production in the circumstellar envelope. We compare Spitzer
infrared colours with results from theoretical modelling. We show that ~75% of
the AGB population of the SMC is composed by scarcely obscured objects, mainly
stars of mass M < 2.5Msun at various metallicity, formed between 700 Myr and 5
Gyr ago; ~ 70% of these sources are oxygen--rich stars, while ~ 30% are
C-stars. The sample of the most obscured AGB stars, accounting for ~ 25% of the
total sample, is composed almost entirely by carbon stars. The distribution in
the colour-colour ([3.6]-[4.5], [5.8]-[8.0]) and colour-magnitude ([3.6]-[8.0],
[8.0]) diagrams of these C-rich objects, with a large infrared emission, traces
an obscuration sequence, according to the amount of carbonaceous dust in their
surroundings. The overall population of C-rich AGB stars descends from
1.5-2Msun stars of metallicity Z=0.004, formed between 700 Myr and 2 Gyr ago,
and from lower metallicity objects, of mass below 1.5Msun, 2-5 Gyr old. We also
identify obscured oxygen-rich stars (M ~ 4-6Msun) experiencing hot bottom
burning. The differences between the AGB populations of the SMC and LMC are
also commented.Comment: 18, pages, 11 figures, accepted for publication on MNRA
Dust from AGBs: relevant factors and modelling uncertainties
The dust formation process in the winds of Asymptotic Giant Branch stars is
discussed, based on full evolutionary models of stars with mass in the range
MMM, and metallicities .
Dust grains are assumed to form in an isotropically expanding wind, by growth
of pre--existing seed nuclei. Convection, for what concerns the treatment of
convective borders and the efficiency of the schematization adopted, turns out
to be the physical ingredient used to calculate the evolutionary sequences with
the highest impact on the results obtained. Low--mass stars with MM produce carbon type dust with also traces of silicon carbide. The
mass of solid carbon formed, fairly independently of metallicity, ranges from a
few M, for stars of initial mass M, to
M for MM; the size of dust
particles is in the range mm. On the contrary,
the production of silicon carbide (SiC) depends on metallicity. For the size of SiC grains varies in the range m, while the mass of SiC formed is
. Models of
higher mass experience Hot Bottom Burning, which prevents the formation of
carbon stars, and favours the formation of silicates and corundum. In this case
the results scale with metallicity, owing to the larger silicon and aluminium
contained in higher--Z models. At Z= we find that the most
massive stars produce dust masses M, whereas models of
smaller mass produce a dust mass ten times smaller. The main component of dust
are silicates, although corundum is also formed, in not negligible quantities
().Comment: Paper accepted for publication in Monthly Notices of the Royal
Astronomical Society Main Journal (2014 January 4
On the alumina dust production in the winds of O-rich Asymptotic Giant Branch stars
The O-rich Asymptotic Giant Branch (AGB) stars experience strong mass loss
with efficient dust condensation and they are major sources of dust in the
interstellar medium. Alumina dust (AlO) is an important dust component
in O-rich circumstellar shells and it is expected to be fairly abundant in the
winds of the more massive and O-rich AGB stars. By coupling AGB stellar
nucleosynthesis and dust formation, we present a self-consistent exploration on
the AlO production in the winds of AGB stars with progenitor masses
between 3 and 7 M and metallicities in the range 0.0003 Z
0.018. We find that AlO particles form at radial distances from
the centre between and 4 R (depending on metallicity), which is in
agreement with recent interferometric observations of Galactic O-rich AGB
stars. The mass of AlO dust is found to scale almost linearly with
metallicity, with solar metallicity AGBs producing the highest amount (about
10 M) of alumina dust. The AlO grain size decreases
with decreasing metallicity (and initial stellar mass) and the maximum size of
the AlO grains is 0.075 for the solar metallicity models.
Interestingly, the strong depletion of gaseous Al observed in the
low-metallicity HBB AGB star HV 2576 seems to be consistent with the formation
of AlO dust as predicted by our models. We suggest that the content of
Al may be used as a mass (and evolutionary stage) indicator in AGB stars
experiencing HBB.Comment: 13 pages, 8 figures, accepted for publication in MNRA
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