arXiv subjects

Jorge Cuadra

Publications and source records attributed to Jorge Cuadra.

87 records · Page 5Linked to original sources

arXiv2015

Formation of discs around super-massive black hole binaries

We model numerically the evolution of $10^4M_\odot$ turbulent molecular clouds in near-radial infall onto $10^6M_\odot$, equal-mass super-massive black hole binaries, using a modified version of the SPH code GADGET-3. We investigate the different gas structures formed depending on the relative inclination between the binary and the cloud orbits. Our first results indicate that an aligned orbit produces mini-discs around each black hole, almost aligned with the binary; a perpendicular orbit produces misaligned mini-discs; and a counter-aligned orbit produces a circumbinary, counter-rotating ring.

astro-ph.HE↗

arXiv2014

Tidal disruptions in circumbinary disks. II: Observational signatures in the reverberation spectra

Supermassive Binary Black Holes (SMBBHs) with sub-pc separations form in the course of galaxy mergers, if both galaxies harbour massive black holes. Clear observational evidence for them however still eludes us. We propose a novel method of identifying these systems by means of reverberation mapping their circumbinary disk after a tidal disruption event has ionized it. The tidal disruption of a star at the secondary leads to strong asymmetries in the disk response. We model the shape of the velocity--delay maps for various toy disk models and more realistic gas distributions obtained by SPH simulations. The emissivity of the ionized disk is calculated with {\em Cloudy}. We find peculiar asymmetries in the maps for off center ionizing sources that may help us constrain geometrical parameters of a circumbinary disk such as semi-major axis and orbital phase of the secondary, as well as help strengthen the observational evidence for sub-parsec SMBBHs as such.

astro-ph.GA↗

arXiv2013

On the role of initial and boundary conditions in numerical simulations of accretion flows

We study the effects of initial and boundary conditions, taking two-dimensional hydrodynamical numerical simulations of hot accretion flow as an example. The initial conditions considered include a rotating torus, a solution expanded from the one-dimensional global solution of hot accretion flows, injected gas with various angular momentum distributions, and the gas from a large-scale numerical simulation. Special attention is paid to the radial profiles of the mass accretion rate and density. Both can be described by a power-law function, $\dot{M}\propto r^s$ and $ρ\propto r^{-p}$. We find that if the angular momentum is not very low, the value of $s$ is not sensitive to the initial condition and lies within a narrow range, $0.47\la s \la 0.55$. However, the value of $p$ is more sensitive to the initial condition and lies in the range $0.48\la p \la 0.8$. The diversity of the density profile is because different initial conditions give different radial profiles of radial velocity due to the different angular momentum of the initial conditions. When the angular momentum of the accretion flow is very low, the inflow rate is constant with radius. Taking the torus model as an example, we have also investigated the effects of inner and outer boundary conditions by considering the widely adopted "outflow" boundary condition and the "mass flux conservation" condition. We find that the results are not sensitive to these two boundary conditions.

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arXiv2012

Tidal disruptions in circumbinary discs (I): Star formation, dynamics, and binary evolution

In our current interpretation of the hierarchical structure of the universe it is well established that galaxies collide and merge with each other during their lifetime. If massive black holes (MBHs) reside in galactic centres, we expect them to form binaries in galactic nuclei surrounded by a circumbinary disc. If cooling is efficient enough, the gas in the disc will clump and trigger stellar formation in situ. In this first paper we address the evolution of the binary under the influence of the newly formed stars, which form individually and also clustered. We use SPH techniques to evolve the gas in the circumbinary disc and to study the phase of star formation. When the amount of gas in the disc is negligible, we further evolve the system with a high-accurate direct-summation $N-$body code to follow the evolution of the stars, the innermost binary and tidal disruption events (TDEs). For this, we modify the direct N-body code to (i) include treatment of TDEs and to (ii) include "gas cloud particles" that mimic the gas, so that the stellar clusters do not disolve when we follow their infall on to the MBHs. We find that the amount of stars disrupted by either infalling stellar clusters or individual stars is as large as 10^{-4}/yr per binary, higher than expected for typical galaxies.

astro-ph.CO↗

arXiv2012

Evolution of binary black holes in self gravitating discs: dissecting the torques

We study the interplay between gas accretion and gravity torques in changing a binary elements and its total angular momentum (L) budget. Especially, we analyse the physical origin of the gravity torques (T_g) and their location within the disc. We analyse 3D SPH simulations of the evolution of initially quasi-circular massive black hole binaries (BHBs) residing in the central hollow of massive self-gravitating circumbinary discs. We use different thermodynamics within the cavity and for the numerical size of the black holes to show that (i) the BHB eccentricity growth found previously is a general result, independent of the accretion and the adopted thermodynamics; (ii) the semi-major axis decay depends both on the T_g and on the interplay with the disc-binary L-transfer due to accretion; (iii) the spectral structure of the T_g is predominately caused by disc edge overdensities and spiral arms developing in the body of the disc and, in general, does not reflect directly the period of the binary; (iv) the net T_g changes sign across the BHB corotation radius. We quantify the relative importance of the two, which appear to depend on the thermodynamical properties of the instreaming gas, and which is crucial in assessing the disc-binary L-transfer; (v) the net torque manifests as a purely kinematic (non-resonant) effect as it stems from the cavity, where the material flows in and out in highly eccentric orbits. Both accretion onto the black holes and the interaction with gas streams inside the cavity must be taken into account to assess the fate of the BHB. Moreover, the total torque exerted by the disc affects L(BHB) by changing all the elements (mass, mass ratio, eccentricity, semimajor axis) of the BHB. Common prescriptions equating tidal torque to semi-major axis shrinking might therefore be poor approximations for real astrophysical systems.

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arXiv2011

The butterfly effect in the extreme-mass ratio inspiral problem

Measurements of gravitational waves from the inspiral of a stellar-mass compact object into a massive black hole are unique probes to test General Relativity (GR) and MBH properties, as well as the stellar distribution about these holes in galactic nuclei. Current data analysis techniques can provide us with parameter estimation with very narrow errors. However, an EMRI is not a two-body problem, since other stellar bodies orbiting nearby will influence the capture orbit. Any deviation from the isolated inspiral will induce a small, though observable deviation from the idealised waveform which could be misinterpreted as a failure of GR. Based on conservative analysis of mass segregation in a Milky Way like nucleus, we estimate that the possibility that another star has a semi-major axis comparable to that of the EMRI is non-negligible, although probably very small. This star introduces an observable perturbation in the orbit in the case in which we consider only loss of energy via gravitational radiation. When considering the two first-order non-dissipative post-Newtonian contributions (the periapsis shift of the orbit), the evolution of the orbital elements of the EMRI turns out to be chaotic in nature. The implications of this study are twofold. From the one side, the application to testing GR and measuring MBHs parameters with the detection of EMRIs in galactic nuclei with a millihertz mission will be even more challenging than believed. From the other side, this behaviour could in principle be used as a signature of mass segregation in galactic nuclei.

astro-ph.CO↗

arXiv2011

Limiting eccentricity of sub-parsec massive black hole binaries surrounded by self-gravitating gas discs

We study the dynamics of supermassive black hole binaries embedded in circumbinary gaseous discs, with the SPH code Gadget-2. The sub-parsec binary (of total mass M and mass ratio q=1/3) has excavated a gap and transfers its angular momentum to the self--gravitating disc (M_disc=0.2 M). We explore the changes of the binary eccentricity e, by simulating a sequence of binary models that differ in the initial eccentricity e_0, only. In initially low-eccentric binaries, the eccentricity increases with time, while in high-eccentric binaries e declines, indicating the existence of a limiting eccentricity e_crit that is found to fall in the interval [0.6,0.8]. We also present an analytical interpretation for this saturation limit. An important consequence of the existence of e_crit is the detectability of a significant residual eccentricity e_LISA} by the proposed gravitational wave detector LISA. It is found that at the moment of entering the LISA frequency domain e_LISA ~ 10^{-3}-10^{-2}; a signature of its earlier coupling with the massive circumbinary disc. We also observe large periodic inflows across the gap, occurring on the binary and disc dynamical time scales rather than on the viscous time. These periodic changes in the accretion rate (with amplitudes up to ~100%, depending on the binary eccentricity) can be considered a fingerprint of eccentric sub-parsec binaries migrating inside a circumbinary disc.

astro-ph.CO↗

arXiv2008

Stellar dynamical evidence against a cold disc origin for stars in the Galactic Centre

Observations of massive stars within the central parsec of the Galaxy show that, while most stars orbit within a well-defined disc, a significant fraction have large eccentricities and / or inclinations with respect to the disc plane. Here, we investigate whether this dynamically hot component could have arisen via scattering from an initially cold disc -- the expected initial condition if the stars formed from the fragmentation of an accretion disc. Using N-body methods, we evolve a variety of flat, cold, stellar systems, and study the effects of initial disc eccentricity, primordial binaries, very massive stars and intermediate mass black holes. We find, consistent with previous results, that a circular disc does not become eccentric enough unless there is a significant population of undetected 100--1000 Msun objects. However, since fragmentation of an eccentric disc can readily yield eccentric stellar orbits, the strongest constraints come from inclinations. We show that_none_ of our initial conditions yield the observed large inclinations, regardless of the initial disc eccentricity or the presence of massive objects. These results imply that the orbits of the young massive stars in the Galactic Centre are largely primordial, and that the stars are unlikely to have formed as a dynamically cold disc.

astro-ph↗

arXiv2007

Variable accretion and emission from the stellar winds in the Galactic centre

We present numerical simulations of stellar wind dynamics in the central parsec of the Galactic centre, studying in particular the accretion of gas on to Sgr A*, the super-massive black hole. Unlike our previous work, here we use state-of-the-art observational data on orbits and wind properties of individual wind-producing stars. Since wind velocities were revised upwards and non-zero eccentricities were considered, our new simulations show fewer clumps of cold gas and no conspicuous disc-like structure. The accretion rate is dominated by a few close `slow wind stars' v_w < 750 km/s, and is consistent with the Bondi estimate, but variable on time-scales of tens to hundreds of years. This variability is due to the stochastic in-fall of cold clumps of gas, as in earlier simulations, and to the eccentric orbits of stars. The present models fail to explain the high luminosity of Sgr A* a few hundred years ago implied by Integral observations, but we argue that the accretion of a cold clump with a small impact parameter could have caused it. Finally, we show the possibility of constraining the total mass-loss rate of the `slow wind stars' using near infra-red observations of gas in the central few arcseconds.

astro-ph↗

arXiv2007

Simulations of star formation in a gaseous disc around sgr A* - a failed AGN

We numerically model fragmentation of a gravitationally unstable gaseous disc under conditions that may be appropriate for the formation of the young massive stars observed in the central parsec of our Galaxy. In this study, we adopt a simple prescription with a locally constant cooling time. We find that, for cooling times just short enough to induce disc fragmentation, stars form with a top-heavy Initial Mass Function (IMF), as observed in the Galactic Centre (GC). For shorter cooling times, the disc fragments much more vigorously, leading to lower average stellar masses. Thermal feedback associated with gas accretion onto protostars slows down disc fragmentation, as predicted by some analytical models. We also simulate the fragmentation of a gas stream on an eccentric orbit in a combined Sgr A* plus stellar cusp gravitational potential. The stream precesses, self-collides and forms stars with a top-heavy IMF. None of our models produces large enough co-moving groups of stars that could account for the observed ``mini star cluster'' IRS13E in the GC. In all of the gravitationally unstable disc models that we explored, star formation takes place too fast to allow any gas accretion onto the central super-massive black hole. While this can help to explain the quiescence of `failed AGN' such as Sgr A*, it poses a challenge for understanding the high gas accretion rates infered for many quasars.

astro-ph↗

arXiv2006

Clumpy winds and the obscuration of Active Galactic Nuclei

The role of star-formation driven outflows in the obscuration of the central source in the Active Galactic Nuclei (AGN) is discussed. The outflow from a sub-parsec scale accretion disc is numerically modelled for parameters appropriate to the Galactic Centre. The resulting obscuration pattern is very patchy, with some lines of sight becoming optically thick to Thomson scattering. A fixed observer would see column depth changing by factors of many over time scales of order months to hundreds of years, depending on the physical size of the outflow region. Such winds may be relevant for obscuration of some AGN and especially "changing look AGN". However, averaged over the sky as seen from the central source, these winds are always optically thin unless wind outflow rates are super-Eddington. A simple scaling argument shows that this is true not only for stellar-driven winds but for any AGN winds. We therefore conclude that AGN winds are unable to account for the vast majority of optically thick obscured AGN (a significant fraction of all AGN). We suggest that the most likely source of optically thick obscuration in AGN is a warped parsec scale accretion disc.

astro-ph↗

arXiv2005

Weighing the young stellar discs around Sgr A*

It is believed that young massive stars orbiting Sgr A* in two stellar discs on scales of 0.1-0.2 parsecs were formed either farther out in the Galaxy and then quickly migrated inward, or in situ in a massive self-gravitating disc. Comparing N-body evolution of stellar orbits with observational constraints, we set upper limits on the masses of the two stellar systems. These masses turn out to be few times lower than the expected total stellar mass estimated from the observed young high-mass stellar population and the standard galactic IMF. If these stars were formed in situ, in a massive self-gravitating disc, our results suggest that the formation of low-mass stars was suppressed by a factor of at least a few, requiring a top-heavy initial mass function (IMF) for stars formed near sgr A*.

astro-ph↗

arXiv2004

Self-gravitating accretion disk in Sgr A* few million years ago: was Sgr A* a failed quasar?

Sgr A* is extra-ordinarily dim in all wavelengths requiring a very low accretion rate at the present time. However, at a radial distance of a fraction of a parsec from Sgr A*, two rings populated by young massive stars suggest a recent burst of star formation in a rather hostile environment. Here we explore two ways of creating such young stellar rings with a gaseous accretion disk: by self-gravity in a massive disk, and by capturing "old" low mass stars and growing them via gas accretion in a disk. The minimum disk mass is above 10^4 Msun for the first mechanism and is few tens times larger for the second one. The observed relatively small velocity dispersion of the stars rules out disks more massive than around 10^5 Msun: heavier stellar or gas disks would warp each other by orbital precession in an axisymmetric potential too strongly. The capture of "old" stars by a disk is thus unlikely as the origin of the young stellar disks. The absence of a massive nuclear gas disk in Sgr A* now implies that the disk was either accreted by the SMBH, which would then imply almost a quasar-like luminosity for Sgr A*, or was consumed in the star formation episode. The latter possibility appears to be more likely on theoretical grounds. We also consider whether accretion disk plane changes, expected to occur due to fluctuations in the angular momentum of gas infalling into the central parsec of a galaxy, would dislodge the embedded stars from the disk midplane. We find that the stars leave the disk midplane only if the disk orientation changes on time scales much shorter than the disk viscous time.

astro-ph↗

arXiv2003

Bright stars and an inactive disk in Sgr A* and other dormant galaxy centers. I. The optically thick disk

Cold inactive disks are believed to exist in Low Luminosity AGN (LLAGN). They may also exist in the nuclei of inactive galaxies and in the center of our own Galaxy. These disks would then be embedded in the observed dense nuclear stellar clusters. Making the simplest assumption of an optically thick disk, we explore several ways to detect the disk presence through its interaction with the cluster. First of these is the eclipses of close bright stars by the disk. The second is the increase in the infrared flux of the disk due to illumination of its surface by such stars during close passages. Finally the surface brightness of the star cluster should show an anisotropy that depends on the inclination angle of the disk. We apply the first two of the methods to Sgr A*, the super massive black hole in our Galactic Center. Using the orbital parameters of the close star S2, we strongly rule out a disk optically thick in the near infrared unless it has a relatively large inner hole. For disks with no inner holes, we estimate that the data permit a disk with infrared optical depth no larger than about 0.01. Such a disk could also be responsible for the detected 3.8 microns excess in the spectrum of S2. The constraints on the disk that we obtain here can be reconciled with the disk parameters needed to explain the observed X-ray flares if dust particles in the disk have sizes greater than ~ 30 microns. The destruction of small dust particles by strong UV heating and shocks from star passages through the disk, and grain growth during "quiescent" times, are mentioned as possible mechanisms of creating the unusual grain size distribution. (...)

astro-ph↗

arXiv2003

X-ray flares from Sgr A*: star-disk interactions?

Sgr A*, the putative black hole in our Galactic Center (GC), is extraordinary dim in all frequencies. Apparently the black hole is unable to accrete at the Bondi accretion rate for some reason. Another mystery of Sgr A* is the recently discovered large magnitude and short duration X-ray flares (Baganoff et al. 2001). Here we point out that X-ray flares should be expected from star passages through an inactive (i.e. formerly accreting) disk. There are thousands of stars in Sgr A* stellar cluster, and each star will pass through the disk twice per orbit. A shock hot enough to emit X-rays is formed around the star when the latter is inside the disk. We develop a simple yet somewhat detailed model for the X-ray emission from the shock. The duration of the flares, their X-ray spectra, frequency of events, and weakness of emission in the radio and near infra-red appear to be consistent with the available observations of X-ray flares. We therefore suggest that at least a fraction of the observed flares is due to the star-disk passages. Such star-disk flares are also of interest in the nuclei of nearby inactive galaxies, especially in connection with perspective Constellation-X and XEUS X-ray missions.

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