46 research outputs found
Radiative non-isothermal Bondi accretion onto a massive black hole
In this paper, we present the classical Bondi accretion theory for the case
of non-isothermal accretion processes onto a supermassive black hole (SMBH),
including the effects of X-ray heating and the radiation force due to electron
scattering and spectral lines. The radiation field is calculated by considering
an optically thick, geometrically thin, standard accretion disk as the emitter
of UV photons and a spherical central object as a source of X-ray emission. In
the present analysis, the UV emission from the accretion disk is assumed to
have an angular dependence, while the X-ray/central object radiation is assumed
to be isotropic. This allows us to build streamlines in any angular direction
we need to. The influence of both types of radiation is evaluated for different
flux fractions of the X-ray and UV emissions with and without the effects of
spectral line driving. We find that the radiation emitted near the SMBH
interacts with the infalling matter and modifies the accretion dynamics. In the
presence of line driving, a transition resembles from pure type 1 & 2 to type 5
solutions (see Fig2.1 of Frank etal. 2002), which takes place regardless of
whether or not the UV emission dominates over the X-ray emission. We compute
the radiative factors at which this transition occurs, and discard type 5
solution from all our models. Estimated values of the accretion radius and
accretion rate in terms of the classical Bondi values are also given. The
results are useful for the construction of proper initial conditions for
time-dependent hydrodynamical simulations of accretion flows onto SMBH at the
centre of galaxies.Comment: 10 pages, 10 figures, Accepted to be published in A&
Tunability experiments at the FERMI@Elettra free-electron laser
FERMI@Elettra is a free electron-laser (FEL)-based user facility that, after two years of commissioning, started preliminary users' dedicated runs in 2011. At variance with other FEL user facilities, FERMI@Elettra has been designed to deliver improved spectral stability and longitudinal coherence. The adopted scheme, which uses an external laser to initiate the FEL process, has been demonstrated to be capable of generating FEL pulses close to the Fourier transform limit. We report on the first instance of FEL wavelength tuning, both in a narrow and in a large spectral range (fine- and coarse-tuning). We also report on two different experiments that have been performed exploiting such FEL tuning. We used fine-tuning to scan across the 1s–4p resonance in He atoms, at ≈23.74 eV (52.2 nm), detecting both UV–visible fluorescence (4p–2s, 400 nm) and EUV fluorescence (4p–1s, 52.2 nm). We used coarse-tuning to scan the M4,5 absorption edge of Ge (∼29.5 eV) in the wavelength region 30–60 nm, measured in transmission geometry with a thermopile positioned on the rear side of a Ge thin foil
Breast cancer stem cells: implications for therapy of breast cancer
The concept of cancer stem cells responsible for tumour origin, maintenance, and resistance to treatment has gained prominence in the field of breast cancer research. The therapeutic targeting of these cells has the potential to eliminate residual disease and may become an important component of a multimodality treatment. Recent improvements in immunotherapy targeting of tumour-associated antigens have advanced the prospect of targeting breast cancer stem cells, an approach that might lead to more meaningful clinical remissions. Here, we review the role of stem cells in the healthy breast, the role of breast cancer stem cells in disease, and the potential to target these cells
Protostellar collapse models of prolate molecular cloud cores
The continued detection of binary systems among pre-main-sequence stars suggests
that fragmentation is a very frequent process during the early stages of star
formation. However, the fragmentation hypothesis rests only upon the results of
three-dimensional hydrodynamics code calculations. The validity of isothermal
fragmentation calculations was questioned by the results of Truelove et al. (1997), and more recently, of Boss et al. (2000), who found, working at very high
spatial resolution, that a particular Gaussian cloud model collapsed isothermally
to form a singular filament rather than a binary or quadruple protostellar system
as predicted by previous calculations. Sufficiently high spatial resolution is
necessary to resolve the Jeans length and hence avoid artificial fragmentation
in isothermal collapse calculations. Here we use an adaptive, spherical-coordinate
hydrodynamics code based on the "zooming" coordinates to investigate the
isothermal collapse of centrally condensed (Gaussian), prolate (2:1 axial ratio)
cloud core models, with thermal energy and varied
rotational energy (), to discern whether
they will still undergo fragmentation into a protostellar binary system, as
found in most previous prolate cloud collapse calculations, or condense all
the way into a thin filament, as suggested by the linear analysis of Inutsuka &
Miyama (1992) and the findings of Truelove et al. and Boss et al. for the spherical,
Gaussian cloud model. The prolate clouds all collapsed self-similarly to produce
an intermediate barlike core, which then shrank indefinetely into a singular filament
without fragmenting. Collapse of the bar into a thin filament also occurred
self-similarly, with the forming filaments being much longer than the Jeans length.
Since the filaments form at maximum densities that are typical of the transition
from the isothermal to the nonisothermal phase, gradual heating may retard
the collapse and allow fragmentation of the filament into a binary or multiple
protostellar core, as required to explain the high frequency of binary stars
Dynamics of solar coronal loops
Observations in X-ray and EUV of the solar corona reveal the existence of
very complex and dynamic structures made of plasma magnetically confined in
loops. These structures can be studied by means of one-dimensional hydrodynamical
loop models. Here we use a Lagrangian-remap code to simulate the dynamics of solar
coronal loops with the purpose of quantifying the effects of varying the initial
distribution of energy along the loop, the amount of input heating (h0),
the total loop length () and including/excluding the solar gravity
term. In particular, the model calculations with no gravity are compared with the
results obtained from previous isobaric, time-dependent models. Using a heat
function that depends on distance along the loop and temperatures at
the base of the loop typical of the solar corona, we find that in the
non-gravity cases the plasma is allowed to cool down to chromospheric temperatures
only when the decay length of the heating is below a certain critical value
(). For the same initial parameters, the inclusion of
gravity produces final equilibrium states which are considerably hotter than
those obtained when gravity is neglected and lowers the critical value of
the decay length of the heating for which a cool condensation forms. In all
cases, the outcome of the evolution can be predicted by a diagnostic diagram which
describes the location of possible solutions for thermal equilibrium models
Gravitational collapse of nonsingular logatropic spheres
We present the results of high-resolved, hydrodynamic calculations
of the spherical gravitational collapse and subsequent accretion of nonsingular
subcritical and critical logatropes, starting with initial configurations
close to hydrostatic equilibrium. Two sequences of models with varying masses and
the same central temperature  K are defined, which differ only in the
fiducial value of the truncation pressure ( cm-3 K and  cm-3 K). In all cases, we follow the calculations
until the central protostar has accreted 99% of the total available mass. Thus,
the models may be indicative of early evolution from the Class 0 to the Class I
protostellar phase. We find that the approach to the singular density profile is
never entirely subsonic. In the lower ps sequence, about 6% of the mass
collapses supersonically in a sphere, while only ∼0.02% behaves
this way in a critical (92.05Â ) logatrope. In the high ps
sequence the same trend is observed, with ∼0.7% of the mass now infalling
supersonically at the time of singularity formation in a sphere.
Immediately after singularity formation, the accretion rate rises steeply in all
cases, reaching a maximum value when the central protostar has accreted ∼40%
of its final mass. Thereafter, it decreases monotonically for the remainder
of the evolution. Our models predict peak values of as
high as ∼
 yr-1 for logatropes close to the
critical mass. In contrast, a subcritical logatrope reaches a maximum
value of ∼
yr-1 for the lower ps sequence
compared to ~ yr-1 for the higher ps case.
The results also imply that the accretion lifetimes are longer in logatropes with
lower ps, consistent with the observational evidence that star formation
in clumped regions occurs on shorter timescales compared to more isolated
environments