972 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&
Collapse and Fragmentation Models of Prolate Molecular Cloud Cores. I. Initial Uniform Rotation
Studies of the distribution of young stars in well-known regions of star formation indicate the existence of a characteristic length scale (~0.04 pc), separating the regime of self-similar clustering from that of binary and multiple systems. The evidence that this length scale is comparable to the size of typical molecular cloud clumps, along with the observed high frequency of companions to pre-main-sequence stars, suggest that stars may ultimately form through fragmentation of collapsing molecular cloud cores. Here we use a new hydrodynamic code to investigate the gravitational collapse and fragmentation of protostellar condensations, starting from moderately centrally condensed (Gaussian), prolate configurations with axial ratios of 2:1 and 4:1 and varying thermal energy (α). All the models start with uniform rotation and ratios of the rotational to the gravitational energy β ≈ 0.036. The results indicate that these condensations collapse all the way to form a narrow cylindrical core that subsequently fragments into two or more clumps, even if they are initially close to virial equilibrium (α + β ≈ -->½). The 2:1 clouds formed triple systems for α 0.36 and a binary system for α ≈ 0.27, while the 4:1 clouds all formed binary systems independently of α. The mass and separation of the binary fragments increase with increasing the cloud elongation. The widest binaries formed from clouds with α ≈ 0.36, and starting from this value, the binary separation decreases with either increasing or decreasing α. In all cases, fragmentation did not result in a net loss of the a/m ratio (specific spin angular momentum per unit mass), as expected from stellar observations. The fragments that formed possess low values of α (~0.06) and are appreciably elongated, and so they could subfragment before becoming true first protostellar cores
Feedback-controlled transport in an interacting colloidal system
Based on dynamical density functional theory (DDFT) we consider a
non-equilibrium system of interacting colloidal particles driven by a constant
tilting force through a periodic, symmetric "washboard" potential. We
demonstrate that, despite of pronounced spatio-temporal correlations, the
particle current can be reversed by adding suitable feedback control terms to
the DDFT equation of motion. We explore two distinct control protocols with
time delay, focussing on either the particle positions or the density profile.
Our study shows that the DDFT is an appropriate framework to implement
time-delayed feedback control strategies widely used in other fields of
nonlinear physicsComment: 6 pages, 5 figure
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