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D. Molteni

Publications and source records attributed to D. Molteni.

At least 19 recordsLinked to original sources
arXiv2021

Measuring close proximity interactions in summer camps during the COVID-19 pandemic

Policy makers have implemented multiple non-pharmaceutical strategies to mitigate the COVID-19 worldwide crisis. Interventions had the aim of reducing close proximity interactions, which drive the spread of the disease. A deeper knowledge of human physical interactions has revealed necessary, especially in all settings involving children, whose education and gathering activities should be preserved. Despite their relevance, almost no data are available on close proximity contacts among children in schools or other educational settings during the pandemic. Contact data are usually gathered via Bluetooth, which nonetheless offers a low temporal and spatial resolution. Recently, ultra-wideband (UWB) radios emerged as a more accurate alternative that nonetheless exhibits a significantly higher energy consumption, limiting in-field studies. In this paper, we leverage a novel approach, embodied by the Janus system that combines these radios by exploiting their complementary benefits. The very accurate proximity data gathered in-field by Janus, once augmented with several metadata, unlocks unprecedented levels of information, enabling the development of novel multi-level risk analyses. By means of this technology, we have collected real contact data of children and educators in three summer camps during summer 2020 in the province of Trento, Italy. The wide variety of performed daily activities induced multiple individual behaviors, allowing a rich investigation of social environments from the contagion risk perspective. We consider risk based on duration and proximity of contacts and classify interactions according to different risk levels. We can then evaluate the summer camps' organization, observe the effect of partition in small groups, or social bubbles, and identify the organized activities that mitigate the riskier behaviors. [...]

cs.NI↗
arXiv2008

Smoothed Particle Hydrodynamics in Thermal Phases of a One Dimensional Molecular Cloud

We present an investigation on effect of the ion-neutral (or ambipolar) diffusion heating rate on thermal phases of a molecular cloud. We use the modeling of ambipolar diffusion with two-fluid smoothed particle hydrodynamics, as discussed by Nejad-Asghar & Molteni. We take into account the ambipolar drift heating rate on the net cooling function of the molecular clouds, and we investigate the thermal phases in a self-gravitating magnetized one dimensional slab. The results show that the isobaric thermal instability criterion is satisfied in the outer parts of the cloud, thus, these regions are thermally unstable while the inner part is stable. This feature may be responsible for the planet formation in the outer parts of a collapsing molecular cloud and/or may also be relevant for the formation of star forming dense cores in the clumps.

astro-ph↗
arXiv2008

Modeling fragmentation of the self-gravitating molecular layer by smoothed particle hydrodynamics

We revisit the modeling of ion-neutral (or ambipolar) diffusion with two fluid smoothed particle hydrodynamics, as discussed by Hosking & Whitworth. Some parts of the technique are optimized to testify the pioneer works on behavior of the ambipolar diffusion in an isothermal self-gravitating layer. The frictional heating by ambipolar diffusion is examined, and its effect on fragmentation of the layer is studied. The results are compared to the thermal phases of instability as obtained by Nejad-Asghar.

astro-ph↗
arXiv2007

Shock oscillation model for QPOs in stellar-mass and supermassive black holes

We numerically examine centrifugally supported shock waves in 2D rotating accretion flows around a stellar-mass (10M_sun) and a supermassive (10^6M_sun) black holes over a wide range of input accretion rates of 10^7 >\dot M/\dot M_E>10^{-4}. The resultant 2D-shocks are unstable with time and the luminosities show quasi-periodic oscillations (QPOs) with modulations of a factor of 2-3 andwith periods of a tenth seconds to several hours, depending on the black hole masses. The shock oscillation model may explain the intermediate frequency QPOs with 1-10 Hz observed in the stellar-mass black hole candidates and also suggest the existence of QPOs with the period of hours in AGNs. When the accretion rate is low, the luminosity increases in proportion to the accretion rate. However, when the accretion rate exceeds greatly the Eddington critical rate \dot M_E, the luminosity is insensitive to the accretion rate and is kept constantly around 3 L_E. On the other hand, the mass-outflow rate increases in proportion to the accretion rate and it amounts to about a few percent of theinput mass-flow rate.

astro-ph↗
arXiv2006

A simple model of radiative emission in M87

We present a simple physical model of the central source emission in the M87 galaxy. It is well known that the observed X-ray luminosity from this galactic nucleus is much lower than the predicted one, if a standard radiative efficiency is assumed. Up to now the main model invoked to explain such a luminosity is the ADAF (Advection-Dominated-Accretion-Flow) model. Our approach supposes only a simple axis-symmetric adiabatic accretion with a low angular momentum together with the bremsstrahlung emission process in the accreting gas. With no other special hypothesis on the dynamics of the system, this model agrees well enough with the luminosity value measured by Chandra.

astro-ph↗
arXiv2005

Steady shocks around black holes produced by sub-keplerian flows with negative energy

We discuss a special case of formation of axisymmetric shocks in the accretion flow of ideal gas onto a Schwarzschild black hole: when the total energy of the flow is negative. The result of our analysis enlarges the parameter space for which these steady shocks are exhibited in the accretion of gas rotating around relativistic stellar objects. Since keplerian disks have negative total energy, we guess that, in this energy range, the production of the shock phenomenon might be easier than in the case of positive energy. So our outcome reinforces the view that sub-keplerian flows of matter may significantly affect the physics of the high energy radiation emission from black hole candidates. We give a simple procedure to obtain analytically the position of the shocks. The comparison of the analytical results with the data of 1D and 2D axisymmetric numerical simulations confirms that the shocks form and are stable.

astro-ph↗
arXiv2005

3D SPH Simulations of Shocks in Accretion Flows around black holes

We present the simulation of 3D time dependent flow of rotating ideal gas falling into a Schwarzschild black hole. It is shown that also in the 3D case steady shocks are formed in a wide range of parameters (initial angular momentum and thermal energy). We therefore highlight the stability of the phenomenon of shock formation in sub keplerian flows onto black holes, and reenforce the role of the shocks in the high luminosity emission from black hole candidates. The simulations have been performed using a parallelized code based on the Smoothed Particles Hydrodynamics method (SPH). We also discuss some properties of the shock problem that allow its use as a quantitative test of the accuracy of the used numerical method. This shows that the accuracy of SPH is acceptable although not excellent.

astro-ph↗
arXiv2004

Black-Hole Accretion Discs and Jets at Super-Eddington Luminosity

Super-Eddington accretion discs with 3 and 15 dot M_E around black holes with mass 10 M_sun are examined by two-dimensional radiation hydrodynamical calculations extending from the inner disc edge to 5*10^4 r_g and lasting up to \sim 10^6 r_g/c. The dominant radiation-pressure force in the inner region of the disc accelerates the gas vertically to the disc plane, and jets with 0.2 -- 0.4$c$ are formed along the rotational axis. In the case of the lower accretion rate, the initially anisotropic high-velocity jet expands outward and becomes gradually isotropic flow in the distant region. The mass-outflow rate from the outer boundary is as large as \sim 10^{19} -- 10^{23} g s^{-1}, but it is variable and intermittent with time; that is, the outflow switches occasionally to inflow in the distant region. The luminosity also varies as \sim 10^{40} -- 10^{42} erg s^{-1} on a long time-scale. On the other hand, the jet in the case of the higher accretion rate maintains its initial anisotropic shape even after it goes far away. The mass-outflow rate and the luminosity attain to steady values of 3*10^{19} g s^{-1} and 1.3*10^{40} erg s^{-1}, respectively. In accordance with the local analysis of the slim accretion disc model, the disc is thermally unstable in the case of 3 \dot M_E} but stable in the case of 15 \dot M_E. The super-Eddington model with 15 \dot M_E is promising to explain a small collimation degree of the jet and a large mass-outflow rate observed in the X-ray source SS 433.

astro-ph↗
arXiv2004

Radiative Shocks in Rotating Accretion Flows around Black Holes

It is well known that the rotating inviscid accretion flows with adequate injection parameters around black holes could form shock waves close to the black holes, after the flow passes through the outer sonic point and can be virtually stopped by the centrifugal force. We examine numerically such shock waves in 1D and 2D accretion flows, taking account of cooling and heating of the gas and radiation transport. The numerical results show that the shock location shifts outward compared with that in the adiabatic solutions and that the more rarefied ambient density leads to the more outward shock location. In the 2D-flow, we find an intermediate frequency QPO behavior of the shock location as is observed in the black hole candidate GRS 1915+105.

astro-ph↗
arXiv2004

Ab initio simulations of accretion disks instability

We show that accretion disks, both in the subcritical and supercritical accretion rate regime, may exhibit significant amplitude luminosity oscillations. The luminosity time behavior has been obtained by performing a set of time-dependent 2D SPH simulations of accretion disks with different values of alpha and accretion rate. In this study, to avoid any influence of the initial disk configuration, we produced the disks injecting matter from an outer edge far from the central object. The period of oscillations is 2 - 50 s respectively for the two cases, and the variation amplitude of the disc luminosity is 10^38 - 10^39 erg/s. An explanation of this luminosity behavior is proposed in terms of limit cycle instability: the disk oscillates between a radiation pressure dominated configuration (with a high luminosity value) and a gas pressure dominated one (with a low luminosity value). The origin of this instability is the difference between the heat produced by viscosity and the energy emitted as radiation from the disk surface (the well-known thermal instability mechanism). We support this hypothesis showing that the limit cycle behavior produces a sequence of collapsing and refilling states of the innermost disk region.

astro-ph↗
arXiv2004

The effect of cooling on time dependent behaviour of accretion flows around black holes

We present the results of several numerical simulations of two dimensional axi-symmetric accretion flows around black holes using Smoothed Particle Hydrodynamics (SPH) in the presence of cooling effects. We consider both stellar black holes and super-massive black holes. We observe that due to {\it both radial and vertical oscillation} of shock waves in the accretion flow, the luminosity and average thermal energy content of the inner disk exhibit very interesting behaviour. When power density spectra are taken, quasi-periodic variabilities are seen at a few Hz and also occasionally at hundreds of Hz for stellar black holes. For super-massive black holes, the time scale of the oscillations ranges from hours to weeks. The power density spectra have a flat top behavior with average {\it rms} amplitude of a few percent and a broken power-law behavior. The break frequency is generally found to be close to the frequency where the shock oscillates.

astro-ph↗
arXiv2003

SPH simulations of Shakura-Sunyaev instability at intermediate accretion rates

We show that a standard Shakura-Sunyaev accretion disc around a black hole with an accretion rate lower than the critical Eddington limit does show the instability in the radiation pressure dominated zone. We obtain this result performing time-dependent simulations of accretion disks for a set of values of the viscosity parameter and accretion rate. In particular we always find the occurrence of the collapse of the disc: the instability develops always towards a collapsed gas pressure dominated disc and not towards the expansion. This result is valid for all initial configurations we tested. We find significant convective heat flux that increases the instability development time, but is not strong enough to inhibit the disc collapse. A physical explanation of the lack of the expansion phase is proposed considering the role of the radial heat advection. Our finding is relevant since it excludes the formation of the hot comptonizing corona -often suggested to be present- around the central object by the mechanism of the Shakura-Sunyaev instability. We also show that, in the parameters range we simulated, accretion disks are crossed by significant amplitude acoustic waves.

astro-ph↗
arXiv2002

Quasi-Periodic Oscillations in Numerical Simulation of Accretion Flows Around Black Holes

We present results of several numerical simulations of two dimensional axi-symmetric accretion flows around black holes using the Smoothed Particle Hydrodynamics (SPH). We consider both stellar black holes and as well as super-massive black holes. We assume bremsstrahlung to be the only source of cooling as it is simpler to implement in numerical simulations. We observe that due to both radial and vertical oscillation of shock waves in the accretion flow, the luminosity and average thermal energy content of the inner disk exhibit very interesting behaviors. When power density spectra are taken, quasi-periodic oscillations are seen at a few Hz and also occasionally at hundreds of Hz for stellar black holes. For super-massive black holes, the time-scale of the oscillations ranges from hours to weeks. The power density spectra have usual flat top behavior with average {\it rms} amplitude a few percent and a broken power-law behavior. The break frequency is generally found to be close to the QPO frequency as seen in the observed power spectra of black holes.

astro-ph↗
arXiv2001

Bending Instability of an Accretion Disc Around a Black Hole

We demonstrate that generically an accretion disk around a compact object could have a new type of instability in that the accretion flow need not be symmetric with respect to the equatorial plane even when matter is supplied symmetrically farther out. We find that this behaviour is mainly due to interaction of outgoing matter bounced off the centrifugal barrier and the incoming accretion. We believe that X-ray variability could be developed by this instability.

astro-ph↗
arXiv2001

Why Canonical Disks Cannot Produce Advection Dominated Flows

Using simple arguments we show that the canonical thin keplerian accretion disks cannot smoothly match any plain advection dominated flow (ADAF) model. By 'plain' ADAF model we mean the ones with zero cooling. The existence of sonic points in exact solutions is critical and imposes constraints that cannot be surpassed adopting 'reasonable' physical conditions at the hypothetical match point. Only the occurrence of new critical physical phenomena may produce a transition. We propose that exact advection models are a class of solutions which don't necessarily involve the standard thin cool disks and suggest a different scenario in which good ADAF solutions could eventually occur.

astro-ph↗
arXiv1999

Comparison of 2D and 3D models of flow structure in semidetached binaries

We present the results of systematic comparison of 2D and 3D numerical models of mass transfer in semidetached binaries. It is shown that only for case of γ\sim 1 (near-isothermal case) the obtained 2D and 3D solutions are qualitatively similar. For higher value of γthe 3D flow structure is drastically changed and for γ=1.2 the accretion disk is not formed in the system. Numerical results show that for case of γ=1.2 the 2D and 3D solutions are different. We discuss the spiral-shaped shock waves obtained in numerical models. It is shown that these shocks are not intrinsic spirals and are caused by collisions of the gaseous flows in the system.

astro-ph↗
arXiv1998

3D numerical simulation of gaseous flows structure in semidetached binaries

The results of 3D hydrodynamic simulation of mass transfer in semidetached binaries of different types (cataclysmic variables and low-mass X-ray binaries) are presented. We find that taking into account of a circumbinary envelope leads to significant changes in the stream-disc morphology. In particular, the obtained steady-state self-consistent solutions show an absence of impact between gas stream from the inner Lagrangian point L1 and forming accretion disc. The stream deviates under the action of gas of circumbinary envelope, and does not cause the shock perturbation of the disc boundary (traditional `hotspot'). At the same time, the gas of circumbinary envelope interacts with the stream and causes the formation of an extended shock wave, located on the stream edge. We discuss the implication of this model without `hotspot' (but with a shock wave located outside the disc) for interpretation of observations. The comparison of synthetic light curves with observations proves the validity of the discussed hydrodynamic model without `hotspot'. We also consider the influence of a circumbinary envelope on the mass transfer rate in semidetached binaries. The obtained features of flow structure in the vicinity of L1 show that the gas of circumbinary envelope plays an important role in the flow dynamics, and that it leads to significant (in order of magnitude) increasing of the mass transfer rate. The comparison of gaseous flows structure obtained in 2D and 3D approaches is presented. We discuss the common features of the flow structures and the possible reasons of revealed differences.

astro-ph↗