arXiv subjects

J. R. Fuentes

Publications and source records attributed to J. R. Fuentes.

26 records · Page 2Linked to original sources

arXiv2023

Heat transport and convective velocities in compositionally-driven convection in neutron star and white dwarf interiors

We investigate heat transport associated with compositionally-driven convection driven by crystallization at the ocean-crust interface in accreting neutron stars, or growth of the solid core in cooling white dwarfs. We study the effect of thermal diffusion and rapid rotation on the convective heat transport, using both mixing length theory and numerical simulations of Boussinesq convection. We determine the heat flux, composition gradient and Péclet number, $\mathrm{Pe}$ (the ratio of thermal diffusion time to convective turnover time) as a function of the composition flux. We find two regimes of convection with a rapid transition between them as the composition flux increases. At small Pe, the ratio between the heat flux and composition flux is independent of Pe,, because the loss of heat from convecting fluid elements due to thermal diffusion is offset by the smaller composition gradient needed to overcome the reduced thermal buoyancy. At large Pe, the temperature gradient approaches the adiabatic gradient, saturating the heat flux. We discuss the implications for neutron star and white dwarf cooling. Convection in neutron stars spans both regimes. We find rapid mixing of neutron star oceans, with a convective turnover time of order weeks to minutes depending on rotation. Except during the early stages of core crystallization, white dwarf convection is in the thermal-diffusion-dominated fingering regime. We find convective velocities much smaller than recent estimates for crystallization-driven dynamos. The small fraction of energy carried as kinetic energy calls into question the effectiveness of crystallization-driven dynamos as an explanation for observed white dwarf magnetic fields.

astro-ph.SR↗

arXiv2022

Layer formation in a stably-stratified fluid cooled from above. Towards an analog for Jupiter and other gas giants

In 1D evolution models of gas giant planets, an outer convection zone advances into the interior as the surface cools, and multiple convective layers form beneath that convective front. To study layer formation below an outer convection zone in a similar scenario, we investigate the evolution of a stably-stratified fluid with a linear composition gradient that is constantly being cooled from above. We use the Boussinesq approximation in a series of 2D simulations at low and high Prandtl numbers ($\mathrm{Pr} = 0.5$ and 7), initialized with constant temperature everywhere, and cooled at different rates. We find that multiple convective layers form at $\mathrm{Pr} = 7$, {as the result of an instability in the} diffusive thermal boundary layer below the outer convection zone. At low Pr, layers do not form because the temperature gradient within the boundary layer is much smaller than at large Pr and, consequently, is not large enough to overcome the stabilizing effect of the composition gradient. For the stratification used in this study, on the long-term the composition gradient is an ineffective barrier against the propagation of the outer convection zone and the entire fluid becomes fully-mixed, whether layers form or not. Our results challenge 1D evolutionary models of gas giant planets, which predict that layers are long-lived and that the outer convective envelope stops advancing inwards. We discuss what is needed for future work to build more realistic models.

astro-ph.EP↗

arXiv2022

Establishing Dust Rings and Forming Planets Within Them

Radio images of protoplanetary disks demonstrate that dust grains tend to organize themselves into rings. These rings may be a consequence of dust trapping within gas pressure maxima wherein the local high dust-to-gas ratio is expected to trigger the formation of planetesimals and eventually planets. We revisit the behavior of dust near gas pressure perturbations enforced by a planet in two-dimensional, shearing box simulations. While dust grains collect into generally long-lived rings, particles with small Stokes parameter $τ_s < 0.1$ tend to advect out of the ring within a few drift timescales. Scaled to the properties of ALMA disks, we find that rings composed of larger particles ($τ_s \geq 0.1$) can nucleate a dust clump massive enough to trigger pebble accretion which proceeds to ingest the entire dust ring well within $\sim$1 Myr. To ensure the survival of the dust rings, we favor a non-planetary origin and typical grain size $τ_s \lesssim 0.05$--0.1. Planet-driven rings may still be possible but if so we would expect the orbital distance of the dust rings to be larger for older systems.

astro-ph.EP↗

arXiv2022

Schwarzschild and Ledoux are equivalent on evolutionary timescales

Stellar evolution models calculate convective boundaries using either the Schwarzschild or Ledoux criterion, but confusion remains regarding which criterion to use. Here we present a 3D hydrodynamical simulation of a convection zone and adjacent radiative zone, including both thermal and compositional buoyancy forces. As expected, regions which are unstable according to the Ledoux criterion are convective. Initially, the radiative zone adjacent to the convection zone is Schwarzschild-unstable but Ledoux-stable due to a composition gradient. Over many convective overturn timescales the convection zone grows via entrainment. The convection zone saturates at the size originally predicted by the Schwarzschild criterion, although in this final state the Schwarzschild and Ledoux criteria agree. Therefore, the Schwarzschild criterion should be used to determine the size of stellar convection zones, except possibly during short-lived evolutionary stages in which entrainment persists.

astro-ph.SR↗

arXiv2021

Shear flows and their suppression at large aspect ratio. Two-dimensional simulations of a growing convection zone

We investigate the onset and evolution of zonal flows in a growing convective layer when a stably-stratified fluid with a composition gradient is cooled from above. This configuration allows the study of zonal flows for a wide range of values of the Rayleigh number, $Ra$, and aspect ratio of the convection zone within a given simulation. We perform a series of 2D simulations using the Boussinesq approximation, with aspect ratio of the computational domain between $1$ and $5$, and Prandtl number $Pr = 0.1$, 0.5, 1, and $7$. We find that for square domains zonal flows appear when the aspect ratio of the convective layer is smaller than two, and the evolution of the system depends on the Prandtl number. For $Pr\leq 1$, the fluid experiences bursts of convective transport with negligible convective transport between bursts. The magnitude and frequency of the bursts are smaller at low $Pr$, which suggests that the bursting regime is stronger in a narrow range around $Pr=1$, as observed in previous studies of thermal convection. For $Pr=7$, the structure of the flow consists of tilted convective plumes, and the convective transport is sustained at all times. In wider domains, the aspect ratio of the convective zone is always much larger than two and zonal flows do not appear. These results confirm and extend to fluids with stable composition gradients previous findings on thermal convection. The fact that zonal flows can be avoided by using computational domains with large aspect ratios opens up the possibility of 2D studies of convective overshoot, layer formation and transport properties across diffusive interfaces.

physics.flu-dyn↗

arXiv2021

Penetration of a cooling convective layer into a stably-stratified composition gradient: entrainment at low Prandtl number

We study the formation and evolution of a convective layer when a stably-stratified fluid with a composition gradient is cooled from above. We perform a series of 2D simulations using the Bousinessq approximation with Prandtl number ranging from Pr = 0.1 to 7, extending previous work on salty water to low Pr. We show that the evolution of the convection zone is well-described by an entrainment prescription in which a fixed fraction of the kinetic energy of convective motions is used to mix fluid at the interface with the stable layer. We measure the entrainment efficiency and find that it grows with decreasing Prandtl number or increased applied heat flux. The kinetic energy flux that determines the entrainment rate is a small fraction of the total convective luminosity. In this time-dependent situation, the density ratio at the interface is driven to a narrow range that depends on the value of Pr, and with low enough values that advection dominates the interfacial transport. We characterize the interfacial flux ratio and how it depends on the interface stability. We present an analytic model that accounts for the growth of the convective layer with two parameters, the entrainment efficiency and the interfacial heat transport, both of which can be measure from the simulations.

astro-ph.SR↗

arXiv2019

Glitch time series and size distributions in eight prolific pulsars

Glitches are rare spin-up events that punctuate the smooth slow-down of the rotation of pulsars. For the Vela pulsar and PSR J0537-6910, the glitch sizes and the times between consecutive events have clear preferred scales, contrary to the handful of other pulsars with enough glitches for such a study. Moreover, PSR J0537-6910 is the only pulsar showing a strong positive correlation between the size of each glitch and the waiting time until the following one. We attempt to understand this behavior through a detailed study of the distributions and correlations of glitch properties for the eight pulsars with at least ten detected glitches. We model the distributions of glitch sizes and times between consecutive glitches for this sample and perform Monte Carlo simulations to explore two hypotheses that could explain why the correlation is so much weaker in other pulsars than in PSR J0537-6910. We confirm the above results for the Vela pulsar and PSR J0537-6910, and verify that the latter is the only pulsar with a strong correlation between glitch size and waiting time to the following glitch. For the remaining six pulsars, the waiting time distributions are best fitted by exponentials, and the size distributions either by power laws, exponentials, or log-normal functions. Some pulsars in the sample yield significant Pearson and Spearman coefficients ($r_p$ and $r_s$) for the aforementioned correlation. Moreover, for all except the Crab, both coefficients are positive. Our simulations show that the weaker correlations in pulsars other than PSR J0537$-$6910 cannot be due to missing glitches too small to be detected. We also tested the hypothesis that each pulsar may have two kinds of glitches, namely large, correlated ones and small, uncorrelated ones. We found for all pulsars that their glitches are not consistent with a pure uncorrelated population either.

astro-ph.HE↗

arXiv2018

The glitch activity of neutron stars

We present a statistical study of the glitch population and the behaviour of the glitch activity across the known population of neutron stars. An unbiased glitch database was put together based on systematic searches of radio timing data of 898 rotation-powered pulsars obtained with the Jodrell Bank and Parkes observatories. Glitches identified in similar searches of 5 magnetars were also included. The database contains 384 glitches found in the rotation of 141 of these neutron stars. We confirm that the glitch size distribution is at least bimodal, with one sharp peak at approximately $20\, \rm{μ\,Hz}$, which we call large glitches, and a broader distribution of smaller glitches. We also explored how the glitch activity $\dotν_{\rm{g}}$, defined as the mean frequency increment per unit of time due to glitches, correlates with the spin frequency $ν$, spin-down rate $|\dotν|$, and various combinations of these, such as energy loss rate, magnetic field, and spin-down age. It is found that the activity is insensitive to the magnetic field and that it correlates strongly with the energy loss rate, though magnetars deviate from the trend defined by the rotation-powered pulsars. However, we find that a constant ratio $\dotν_{\rm{g}}/|\dotν| = 0.010 \pm 0.001$ is consistent with the behaviour of all rotation-powered pulsars and magnetars. This relation is dominated by large glitches, which occur at a rate directly proportional to $|\dotν|$. The only exception are the rotation-powered pulsars with the highest values of $|\dotν|$, such as the Crab pulsar and PSR B0540$-$69, which exhibit a much smaller glitch activity, intrinsically different from each other and from the rest of the population. The activity due to small glitches also shows an increasing trend with $|\dotν|$, but this relation is biased by selection effects.

astro-ph.HE↗