arXiv subjects

F. J. Sanchez-Salcedo

Publications and source records attributed to F. J. Sanchez-Salcedo.

At least 19 recordsLinked to original sources

arXiv2025

Multipolar magnetic configuration: a trace of post-mergers events in circumstellar disks around FS CMa Stars

Context: Observations suggest that magnetic fields of disk-bearing stars may have non-dipolar configurations. However, the influence of these configurations on magnetospheric accretion remains poorly understood. Aims: We aim to simulate magnetospheric accretion incorporating non-dipolar and strong magnetic field. Our model is informed by observations of IRAS 17449+2320, a post-merger belonging to the group of FS CMa stars, which indicate a dominant dipolar magnetic field with an additional quadrupole component. Methods: Using the PLUTO code, we conduct 2.5-D non-ideal viscous-resistive magnetohydrodynamical (MHD) simulations of star-disk magnetospheric interactions. We consider a thin accretion disk and strong stellar magnetic field ($B_\star= 6.2\mathrm{kG}$) under four configurations: pure dipole, pure quadrupole, dipole plus quadrupole, and dipole plus octupole. In the latter two cases, different magnetic polar strength ratios are explored. Results: For asymmetric magnetic field configurations, we find that accretion exhibits funnel streams below the midplane, indicating the dominance of the quadrupolar and octupolar components. In contrast, in dipolar configurations, we observe the formation of two symmetrical funnels with respect to the midplane. However, in the quadrupolar configuration, accretion is entirely confined to the disk midplane forming a cone-like pattern that leads to disk widening. Remarkably, the presence of a quadrupolar component gives rise to highly asymmetric substructures in the corona region. Conclusions: Multipolar stellar magnetic fields drive non-uniform accretion and lead to asymmetric density distributions in both the disk and corona. These results resemble observed features of some FS CMa post-mergers and Herbig Ae/Be stars, highlighting the critical role of magnetic field complexity in shaping circumstellar environments.

astro-ph.SR↗

arXiv2025

Structure of gaps induced by retrograde satellites embedded in accretion discs

Using 2D simulations, we investigate how a non-accreting satellite on a fixed retrograde circular orbit affects the structure of the accretion disc in which it is embedded. We vary the satellite-to-primary mass ratio $q$, the disc viscosity $ν$, and the inner boundary conditions. A viscous criterion for gap opening is derived, which is broadly consistent with the simulations. We find a scaling relation of the gap depth with $q$ and $ν$. Unlike the prograde case, the satellite is located at the gap's inner edge, resulting in a surface density at the satellite's orbital radius up to $20$ times higher than at the gap's minimum. As the viscosity decreases, the gap depth increases, while the radial shift of the gap and the satellite's orbital radius decreases. Gap-opening satellites may drive radial motions in the disc, producing eccentric gaps. Positioned at the gap edge, satellites experience a rapidly fluctuating environment. Migrating satellites can develop orbital eccentricities comparable to the disc's aspect ratio. In a 3D simulation with $q=0.01$, the flow velocity exhibits a notorious vertical component in the gap's inner edge. A comparison between 2D and 3D simulations reveals a slight radial offset in gap position, resulting in a lower surface density at the perturber's orbital radius in the 3D simulation.

astro-ph.GA↗

arXiv2024

A close pair of orbiters embedded in a gaseous disk: the repulsive effect

We develop a theoretical framework and use two-dimensional hydrodynamical simulations to study the repulsive effect between two close orbiters embedded in an accretion disk. We consider orbiters on fixed Keplerian orbits with masses low enough to open shallow gaps. The simulations indicate that the repulsion is larger for more massive orbiters and decreases with the orbital separation and the disk's viscosity. We use two different assumptions to derive theoretical scaling relations for the repulsion. A first scenario assumes that each orbiter absorbs the angular momentum deposited in its horseshoe region by the companion's wake. A second scenario assumes that the corotation torques of the orbiters are modified because the companion changes the underlying radial gradient of the disk surface density. We find a substantial difference between the predictions of these two scenarios. The first one fails to reproduce the scaling of the repulsion with the disk viscosity and generally overestimates the strength of the repulsion. The second scenario, however, gives results that are broadly consistent with those obtained in the simulations.

astro-ph.EP↗

arXiv2022

Estimating the depth of gaps opened by planets in eccentric orbit

Planets can carve gaps in the surface density of protoplanetary discs. The formation of these gaps can reduce the corotation torques acting on the planets. In addition, gaps can halt the accretion of solids onto the planets as dust and pebbles can be trapped at the edge of the gap. This accumulation of dust could explain the origin of the ring-like dust structures observed using high-resolution interferometry. In this work we provide an empirical scaling relation for the depth of the gap cleared by a planet on an eccentric orbit as a function of the planet-to-star mass ratio $q$, the disc aspect ratio $h$, Shakura-Sunyaev viscosity parameter $α$, and planetary eccentricity $e$. We construct the scaling relation using a heuristic approach: we calibrate a toy model based on the impulse approximation with 2D hydrodynamical simulations. The scaling reproduces the gap depth for moderate eccentricities ($e\leq 4h$) and when the surface density contrast outside and inside the gap is $\leq 10^{2}$. Our framework can be used as the basis of more sophisticated models aiming to predict the radial gap profile for eccentric planets.

astro-ph.EP↗

arXiv2022

The spatial distribution of globular clusters in dwarf spheroidal galaxies and the timing problem

The dynamical friction timescale of massive globular clusters (GCs) in the inner regions of cuspy dark haloes in dwarf spheroidal (dSph) galaxies can be much shorter than the Hubble time. This implies that a small fraction of the GCs is expected to be caught close to the centre of these galaxies. We compare the radial distribution of GCs predicted in simple Monte Carlo models with that of a sample of $38$ spectroscopically confirmed GCs plus 17 GC candidates, associated mainly to low-luminosity dSph galaxies. If dark matter haloes follow an NFW profile, the observed number of off-center GCs at projected distances less than one half the galaxy effective radius is significantly higher than models predict. This timing problem can be viewed as a fine-tuning of the starting GC distances. As a result of the short sinking timescale for GCs in the central regions, the radial distribution of GCs is expected to evolve significantly during the next 1-2 Gyr. However, dark matter haloes with cores of size comparable to the galaxy effective radii can lead to a slow orbital in-spiral of GCs in the central regions of these galaxies, providing a simple solution to the timing problem. We also examine any indication of mass segregation in the summed distribution of our sample of GCs.

astro-ph.GA↗

arXiv2020

Orbital evolution of gas-driven inspirals with extreme mass-ratios: retrograde eccentric orbits

Using two-dimensional simulations, we compute the torque and rate of work (power) on a low-mass gravitational body, with softening length $R_{\rm soft}$, embedded in a gaseous disk when its orbit is eccentric and retrograde with respect to the disk. We explore orbital eccentricities $e$ between $0$ and $0.6$. We find that the power has its maximum at $e\simeq 0.25(h/0.05)^{2/3}$, where $h$ is the aspect ratio of the disk. We show that the power and the torque converge to the values predicted in the local (non-resonant) approximation of the dynamical friction (DF) when $R_{\rm soft}$ tends to zero. For retrograde inspirals with mass ratios $\lesssim 5\times 10^{-4}$ embedded in disks with $h\geq 0.025$, our simulations suggest that (i) the rate of inspiral barely depends on the orbital eccentricity and (ii) the local approximation provides the value of this inspiral rate within a factor of $1.5$. The implications of the results for the orbital evolution of extreme mass-ratio inspirals are discussed.

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arXiv2019

Orbital evolution of eccentric low-mass companions embedded in gaseous disks: testing the local approximation

We study the tidal interaction between a low-mass companion (e.g., a protoplanet or a black hole) in orbit about a central mass, and the accretion disk within which it is submerged. We present results for a companion on a coplanar orbit with eccentricity e between 0.1 and 0.6. For these eccentricities, dynamical friction arguments in its local approximation, that is, ignoring differential rotation and the curvature of the orbit, provide simple analytical expressions for the rates of energy and angular momentum exchange between the disk and the companion. We examine the range of validity of the dynamical friction approach by conducting a series of hydrodynamical simulations of a perturber with softening radius R_soft embedded in a two-dimensional disk. We find close agreement between predictions and the values in simulations provided that R_soft is chosen sufficiently small, below a threshold value Rtilde_soft, which depends on the disk parameters and on eccentricity. We give Rtilde_soft for both razor-thin disks and disks with a finite scaleheight. For point-like perturbers, the local approximation is valid if the accretion radius is smaller than Rtilde_soft. This condition imposes an upper value on the mass of the perturber.

astro-ph.EP↗

arXiv2018

Gaseous wakes and dynamical friction: mass-losing and mass-gaining perturbers

An extended gravitational object embedded in a parent system comprised of gas and collisionless particles may undergo both dynamical friction (DF) and mass loss by tidal forces. If the object is compact enough, it can increase its mass through accretion of material from the surrounding medium. We extend the classical linear analysis of DF on a constant-mass body in a gaseous medium to the case where its mass changes with time. We show that the structure of the wake may differ significantly from the constant-mass case. For instance, the front-back symmetry of density about subsonic constant-mass perturbers is broken down for variable-mass perturbers. The density wake keeps a memory of the past mass history of the perturber. For dissolving perturbers, the density field is more dense than expected using the instantaneous mass of the perturber in the classical formula. As a consequence, the instantaneous-mass approximation underestimates the drag force for mass-losing perturbers and overestimates it for mass-gaining perturbers. We present cases in which the percentage error in the drag force using the instantaneous-mass approximation is greater than 50%.

astro-ph.GA↗

arXiv2018

Torques on low-mass bodies in retrograde orbit in gaseous disks

We evaluate the torque acting on a gravitational perturber on a retrograde circular orbit in the midplane of a gaseous disk. We assume that the mass of this satellite is so low it weakly disturbs the disk (type I migration). The perturber may represent the companion of a binary system with a small mass ratio. We compare the results of hydrodynamical simulations with analytic predictions. Our two-dimensional (2D) simulations indicate that the torque acting on a perturber with softening radius $R_{\rm soft}$ can be accounted for by a scattering approach if $R_{\rm soft}<0.3H$, where $H$ is defined as the ratio between the sound speed and the angular velocity at the orbital radius of the perturber. For $R_{\rm soft}>0.3H$, the torque may present large and persistent oscillations, but the resultant time-averaged torque decreases rapidly with increasing $R_{\rm soft}/H$, in agreement with previous analytical studies. We then focus on the torque acting on small-size perturbers embedded in full three-dimensional (3D) disks and argue that the density waves propagating at distances $\lesssim H$ from the perturber contribute significantly to the torque because they transport angular momentum. We find a good agreement between the torque found in 3D simulations and analytical estimates based on ballistic orbits. We compare the radial migration timescales of prograde versus retrograde perturbers. For a certain range of the perturber's mass and aspect ratio of the disk, the radial migration timescale in the retrograde case may be appreciably shorter than in the prograde case. We also provide the smoothing length required in 2D simulations in order to account for 3D effects.

astro-ph.GA↗

arXiv2017

Bondi-Hoyle-Lyttleton accretion in the presence of small rigid bodies around a black hole

We study the relativistic Bondi-Hoyle-Lyttleton accretion onto a Schwarzschild black hole (BH), which is surrounded by rigid and small, randomly distributed, bodies. These bodies are idealized representations of substructure like stars passing close to the BH, bubbles created by stellar winds or cold clumps.We explore cases where the filling factor of these bodies is small. The flow is assumed to be adiabatic and move supersonically towards the black hole. The interaction with these rigid obstacles transforms ram pressure of the flow into thermal pressure through bow shocks, slowing down the flow and making the accreting gas turbulent. As a consequence, although the flow reaches a statistically-steady state, the accretion rate presents some variability. For a flow Mach number at infinity of 4, a few of these objects (5 - 10) are enough to increase the accretion rate about 50% over the accretion rate without bodies, even though the gas is adiabatic and the filling factor of the obstacles is small.

astro-ph.HE↗

arXiv2017

Gap formation by inclined massive planets in locally isothermal three-dimensional discs

We study gap formation in gaseous protoplanetary discs by a Jupiter mass planet. The planet's orbit is circular and inclined relative to the midplane of the disc. We use the impulse approximation to estimate the gravitational tidal torque between the planet and the disc, and infer the gap profile. For low-mass discs, we provide a criterion for gap opening when the orbital inclination is $\leq 30^{\circ}$. Using the FARGO3D code, we simulate the disc response to an inclined massive planet. The dependence of the depth and width of the gap obtained in the simulations on the inclination of the planet is broadly consistent with the scaling laws derived in the impulse approximation. Although we mainly focus on planets kept on fixed orbits, the formalism permits to infer the temporal evolution of the gap profile in cases where the inclination of the planet changes with time. This study may be useful to understand the migration of massive planets on inclined orbit, because the strength of the interaction with the disc depends on whether a gap is opened or not.

astro-ph.EP↗

arXiv2016

Low-mass disc galaxies and the issue of stability: MOND vs dark matter

We analyse the rotation curves and gravitational stability of a sample of six bulgeless galaxies for which detailed images reveal no evidence for strong bars. We explore two scenarios: Newtonian dark matter models and MOdified Newtonian Dynamics (MOND). By adjusting the stellar mass-to-light ratio, dark matter models can match simultaneously both the rotation curve and bar-stability requirements in these galaxies. To be consistent with stability constraints, in two of these galaxies, the stellar mass-to-light ratio is a factor of ~1.5-2 lower than the values suggested from galaxy colours. In contrast, MOND fits to the rotation curves are poor in three galaxies, perhaps because the gas tracer contains noncircular motions. The bar stability analysis provides a new observational test to MOND. We find that most of the galaxies under study require abnormally-high levels of random stellar motions to be bar stable in MOND. In particular, for the only galaxy in the sample for which the line-of-sight stellar velocity dispersion has been measured (NGC 6503), the observed velocity dispersion is not consistent with MOND predictions because it is far below the required value to guarantee bar stability. Precise measurements of mass-weighted velocity dispersions in (unbarred and bulgeless) spiral galaxies are crucial to test the consistency of MOND.

astro-ph.GA↗

arXiv2015

Jeans analysis of the Galactic thick disk and the local dark matter density

Dynamical estimates of the mass surface density at the solar radius can be made up to a height of 4 kpc using thick disk stars as tracers of the potential. We investigate why different Jeans estimators of the local surface density lead to puzzling and conflicting results. Using the Jeans equations, we compute the vertical (F_z) and radial (F_R) components of the gravitational force, as well as Gamma(z), defined as the radial derivative of V_c^2, with V_c^{2}= -RF_R. If we assume that the thick disk does not flare and that all the components of the velocity dispersion tensor of the thick disk have a uniform radial scalelength of 3.5 kpc, Gamma takes implausibly large negative values, when using the currently available kinematical data of the thick disk. This implies that the input parameters or the model assumptions must be revised. We have explored, using a simulated thick disk, the impact of the assumption that the scale lengths of the density and velocity dispersions do not depend on the vertical height z above the midplane. In the lack of any information about how these scale radii depend on z, we define a different strategy. By using a parameterized Galactic potential, we find that acceptable fits to F_z, F_R and Gamma are obtained for a flaring thick disk and a spherical dark matter halo with a local density larger than 0.0064 M_sun pc^{-3}. Disk-like dark matter distributions might be also compatible with the current data of the thick disk. A precise measurement of Gamma at the midplane could be very useful to discriminate between models.

astro-ph.GA↗

arXiv2015

Evolution of a dwarf satellite galaxy embedded in a scalar field dark matter halo

The cold dark matter (CDM) model has two unsolved issues: simulations overpredict the satellite abundance around the Milky Way (MW) and it disagrees with observations of the central densities of dwarf galaxies which prefer constant density (core) profiles.One alternative explanation known as the scalar field dark matter (SFDM) model, assumes that the dark matter is a scalar field of mass($\sim 10^{-22}$ eV/$c^2$); this model can reduce the overabundance issue due to the lack of halo formation below a mass scale of $\sim 10^8$M$_{\odot}$ and successfully fits the density distribution in dwarfs. One of the attractive features of the model is predicting core profiles in halos, although the determination of the core sizes is set by fitting the observational data. We perform \textit{N}-body simulations to explore the influence of tidal forces over a stellar distribution embedded in a SFDM halo orbiting a MW-like SFDM host halo with a disk. Our simulations intend to test the viability of SFDM as an alternative model by comparing the tidal effects that result in this paradigm with those obtained in CDM for similar mass halos. We found that galaxies in subhalos with core profiles and high central densities survive for 10 Gyr. The same occurs for galaxies in low density subhalos located far from the host disk influence, whereas satellites in low density DM halos and in tight orbits can eventually be stripped of stars. We conclude that SFDM shows consistency with results from CDM for dwarf galaxies, but naturally offer a possibility to solve the missing satellite problem.

astro-ph.GA↗

arXiv2014

Binaries traveling through a gaseous medium: Dynamical drag forces and internal torques

Using time-dependent linear theory, we investigate the morphology of the gravitational wake induced by a binary, whose center of mass moves at velocity Vcm against a uniform background of gas. For simplicity, we assume that the binary's components are on circular orbits about their common center of mass. The consequences of dynamical friction is twofold. First, gas dynamical friction may drag the binary's center of mass and cause the binary to migrate. Second, drag forces also induce a braking torque, which causes the orbits of the binary components to shrink. We compute the drag forces acting on one component of the binary due to the gravitational interaction with its own wake. We show that the dynamical friction force responsible to decelerate the binary's center of mass is smaller than it is in the point-mass case because of the loss of gravitational focusing. We show that the braking internal torque depends on the Mach numbers of each binary component about their center of mass, and also on the Mach number of the center of mass of the binary. In general, the internal torque decreases with increasing the velocity of the binary relative to the ambient gas cloud. However, this is not always the case. We also mention the relevance of our results on the period distribution of binaries.

astro-ph.GA↗

arXiv2014

The inclination of the dwarf irregular galaxy Holmberg II

We provide constraints on the inclination angle of the H\,{\sc i} disk of the dwarf irregular galaxy Holmberg II (Ho II) from stability analysis of the outer gaseous disk. We point out that a mean inclination angle of 27 degrees and thus a flat circular velocity of ~60 km/s, is required to have a level of gravitational stability similar to that found in other galaxies. Adopting this inclination angle, we find that Ho II lies on the right location in the baryonic Tully-Fisher relation. Moreover, for this inclination, its rotation curve is consistent with MOND. However, the corresponding analysis of the stability under MOND indicates that this galaxy could be problematic for MOND because its outer parts are marginally unstable in this gravity theory. We urge MOND simulators to study numerically the non-linear stability of gas-rich dwarf galaxies since it may provide a new key test for MOND.

astro-ph.GA↗

arXiv2013

Sextans' cold substructures as a dynamical judge: Core, Cusp or MOND?

The cold dark matter model predicts cuspy dark matter halos. However, it has been found that, in some low-mass galaxies, cored dark halos provide a better description of their internal dynamics. Here we give constraints on the dark halo profile in the Sextans dwarf spheroidal galaxy by studying the longevity of two cold kinematic substructures detected in this galaxy. We perform N-body simulations of a stellar clump in the Sextans dwarf galaxy, including a live dark matter halo and the main stellar component. We find that, if the dark halo is cuspy, stellar clumps orbiting with semi-major axis ~400 pc are disrupted in ~5 Gyr, even if the clump is initially as compact stellar cluster with a radius of r_c=5 pc. Stellar clusters in an initial orbit with semi-major axis \leq 250 pc may survive to dissolution but their orbits decay towards the center by dynamical friction. In contrast, the stellar clumps can persist for a Hubble time within a cored dark matter halo, even if the initial clump's radius is as extended as r_c=80 pc. We also study the evolution of the clump in the MONDian context. In this scenario, we find that even an extended stellar clump with radius r_c=80 pc survives for a Hubble time, but an unrealistic value for the stellar mass-to-light ratio of 9.2 is needed.

astro-ph.CO↗

arXiv2013

The gravitational drag force on an extended object moving in a gas

Using axisymmetrical numerical simulations, we revisit the gravitational drag felt by a gravitational Plummer sphere with mass M and core radius Rs, moving at constant velocity V0 through a background homogeneous medium of adiabatic gas. Since the potential is non-diverging, there is no gas removal due to accretion. When Rs is larger than the Bondi radius RB, the perturbation is linear at every point and the drag force is well fitted by the time-dependent Ostriker's formula with r_{min}= 2.25Rs, where r_{min} is the minimum impact parameter in the Coulomb logarithm. In the deep nonlinear supersonic regime (Rs<< RB), the minimum radius is no longer related with Rs but with RB. We find r_min=3.3mach^{-2.5}RB, for Mach numbers of the perturber between $1.5$ and $4$, although r_{min} = 2\mach^{-2}RB=2GM/V0^{2} also provides a good fit at mach>2. As a consequence, the drag force does not depend sensitively on the nonlinearity parameter RB/Rs, for RB/Rs-values larger than a certain critical value. We show that our generalized Ostriker's formula for the drag force is more accurate than the formula suggested by Kim & Kim (2009).

astro-ph.CO↗