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Andrew Cumming

Publications and source records attributed to Andrew Cumming.

At least 19 recordsLinked to original sources
arXiv2025

Post-Supernova Accretion of Light Elements onto a New-Born Neutron Star and NS 1987A

We model early accretion of light elements, He, C, and O, onto a new-born neutron star using the public stellar evolution code MESA, simulating what may happen during the first few years of its life. We find that, under the appropriate conditions, significant amounts of these elements can be accreted up to densities of 10^9 g/cc without triggering a nuclear explosion that would convert them into heavy elements. These results help to understand observations that favor light elements in the atmospheres of young cooling neutron stars, as the one found in the supernova remnant Cassiopeia A, and also add support to the recent indications for the presence of a neutron star, NS 1987A, in the remnant of SN 1987A.

astro-ph.SR↗
arXiv2025

3D Simulations of Convective Entrainment in Gas Giants: Rotation and Decreasing Luminosity as Barriers to Mixing

Observations from Juno and Cassini suggest that Jupiter and Saturn may possess fuzzy cores -- central regions where the abundance of heavy elements varies smoothly with depth. Such gradients pose a longstanding puzzle for models of planetary evolution and formation, which predict that vigorous convection would homogenize the interior of gas giants within the first $\sim 10^6$--$10^8~\mathrm{years}$ of cooling. Previous 3D simulations and analytic predictions for the propagation of a convection zone into a stable region have demonstrated that the rapid rotation of gas giants can significantly slow convective mixing, but not enough to stop it. Another piece of the puzzle is luminosity. Gas giants cool as they age, and with that comes a declining heat flux over time. Recent ideas suggest that when this declining luminosity is combined with rotational effects, convection may stall. We explore this possibility using 3D hydrodynamic simulations that include both rotation and a surface cooling flux that decreases as $1/t$. Our results demonstrate that, even without rotation, a declining luminosity can suppress mixing sufficiently to preserve an initial compositional gradient in the deep interior of gas giants. If confirmed by more realistic simulations, this may help to explain the long-term survival of fuzzy cores.

astro-ph.EP↗
arXiv2025

Three-component Phase Separation for Ultramassive White Dwarf Models

We investigate phase separation in oxygen-neon (O/Ne) ultramassive white dwarfs (UMWDs). Current stellar evolution codes, such as MESA, only account for $\mathrm{^{16}O/^{20}Ne}$ separation and do not include other minor species. To improve this, we implement ternary phase diagrams into MESA. We construct UMWD models with O/Ne/sodium (Na) and O/Ne/magnesium (Mg) cores to test our implementation. We also assess the effect of including $\mathrm{^{22}Ne}$ in the current two-species framework. Our results show that incorporating additional components into the phase separation significantly alters the chemical evolution of UMWDs. Heavier elements preferentially enrich the solid core, enhancing mixing in the overlying liquid. We compute the buoyancy flux driven by compositional instabilities during crystallization. As in previous studies, we find two convective regimes: an early, fast overturning convection, lasting less than a million years, followed by inefficient (thermohaline) convection. The fast convective regime lasts up to 100 times longer with three-species separation compared to the standard $\mathrm{^{16}O/^{20}Ne}$ case. We find that neutron-rich species can have a significant contribution to the buoyancy flux despite their small mass fraction (<10\%). We compute the amount of cooling delay induced by phase separation in UMWDs, and find that the three-species phase separation produces a delay up to $\sim10$ times larger than the simplest case of fractionation, although still less than 1 Gyr. We predict that the change in the composition profile in the liquid region when three components are included should change the frequency of gravity modes that can propagate in the interior of pulsating UMWDs.

astro-ph.SR↗
arXiv2025

Weak Bursts on Slowly Accreting Neutron Stars

Nearly all observed thermonuclear X-ray bursts are thought to be triggered by the thermally unstable triple-alpha process. Unlike in accreting white dwarfs, the $\gtrsim10^{8} \mathrm{K}$ envelope temperature forces hydrogen burning to proceed via the $\beta$-limited, thermally stable hot CNO cycle. Recent observations of weak X-ray bursts from SAX J1808.4-3658 occurring within 1-3 days of the onset of an accretion outburst have raised the question that these bursts were triggered by thermally unstable hydrogen ignition, analogously to classical novae. Using the stellar evolution code MESA, we explore the unstable ignition of hydrogen on slowly accreting neutron stars. For solar metallicities, the burning is insufficiently vigorous to launch convection and the burst rise is on the envelope thermal timescale, with the hydrogen being consumed over hours. For elevated metallicities ($Z\gtrsim0.06$), however, the initial proton captures onto CNO nuclei heat the envelope enough to drive convection and produce a sharp peak in the luminosity. This peak can match that of the SAX J1808.4-3658 burst, for sufficient enrichment. Following this peak, the hydrogen burning stabilizes at a rate set solely by the ignition depth and CNO abundance in the accreted matter. This quasi-steady-state burning produces an extended tail of elevated emission that lasts until the hydrogen is exhausted. Observations of the tail's luminosity and duration measure the hydrogen and metallicity abundance of the accreted material, while the distance-independent ratio of peak-to-tail luminosities would suggest the presence of significant metallicity gradients prior to convection.

astro-ph.HE↗
arXiv2024

Hot Spot Offset Variability from Magnetohydrodynamical Thermoresistive Instability in Hot Jupiters

Hot Jupiter atmospheres are possibly subject to a thermoresistive instability. Such an instability may develop as the ohmic heating increases the electrical conductivity in a positive feedback loop, which ultimately leads to a runaway of the atmospheric temperature. We extend our previous axisymmetric one-dimensional radial model, by representing the temperature and magnetic diffusivity as a first order Fourier expansion in longitude. This allows us to predict the hot spot offset during the unfolding of the thermoresistive instability and following Alfv\'enic oscillations. We show a representative simulation undergoing the thermoresistive instability, in which the peak flux offset varies between approximately $\pm 60^{\circ}$ on timescales of a few days with potentially observable brightness variations. Therefore, this thermoresistive instability could be an observable feature of hot Jupiters, given the right timing of observation and transit and the right planetary parameters.

astro-ph.EP↗
arXiv2024

Magnetic field evolution for crystallization-driven dynamos in C/O white dwarfs

We investigate the evolution of magnetic fields generated by the crystallization-driven dynamo in carbon-oxygen white dwarfs (WDs) with masses $\lesssim1.05\ M_{\odot}$. We use scalings for the dynamo to demonstrate that the initial magnetic field strength ($B_{0}$) has an upper limit that depends on the initial convection zone size ($R_{\mathrm{out},0}$) and the WD mass. We solve the induction equation to follow the magnetic field evolution after the dynamo phase ends. We show that the predicted surface magnetic field strength ($B_{\mathrm{surf}}$) differs from $B_{0}$ by at least a factor of $\sim$0.3. This reduction depends on $R_{\mathrm{out},0}$, where values smaller than half of the star radius give $B_{\mathrm{surf}}\lesssim0.01\ B_{0}$. We implement electrical conductivities that account for the solid phase effect on the Ohmic diffusion. We observe that the conductivity increases as the solid core grows, freezing in the magnetic field at a certain point of the evolution and slowing its outwards transport. We study the effect of turbulent magnetic diffusivity induced by the convection and find that for a small $R_{\mathrm{out},0}$, $B_{\mathrm{surf}}$ is stronger than the non-turbulent diffusion cases because of the more rapid transport, but still orders of magnitude smaller than $B_{0}$. Given these limitations, the crystallization-driven dynamo theory could explain only magnetic C/O WDs with field strengths less than a few MG for the mass range 0.45-1.05 $M_{\odot}$. Our results also suggest that a buried fossil field must be at least 100 times stronger than observed surface fields if crystallization-driven convection is responsible for its transport to the surface.

astro-ph.SR↗
arXiv2024

Hydrodynamical simulations of proton ingestion flashes in Type I X-ray Bursts

We perform the first multidimensional fluid simulations of thermonuclear helium ignition underneath a hydrogen-rich shell. This situation is relevant to Type I X-ray bursts on neutron stars that accrete from a hydrogen-rich companion. Using the low Mach number fluid code MAESTROeX, we investigate the growth of the convection zone due to nuclear burning, and the evolution of the chemical abundances in the atmosphere of the star. We also examine the convective boundary mixing processes that cause the evolution to differ significantly from previous one-dimensional simulations that rely on mixing-length theory. We find that the convection zone grows outwards as penetrating fluid elements cool the overlying radiative layer, rather than directly from the increasing entropy of the convection zone itself. Simultaneously, these flows efficiently mix composition, carrying carbon out of, and protons into the convection zone even before contact with the hydrogen shell. We discuss the implications of these effects for future modeling of these events and observations.

astro-ph.HE↗
arXiv2024

A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs

The origin of large magnetic fields ($\gtrsim 10^6~\mathrm{G}$) in isolated white dwarfs is not clear. One possible explanation is that crystallization of the star's core drives compositional convection, which when combined with the star's rotation, can drive a dynamo. However, whether convection is efficient enough to explain the large intensity of the observed magnetic fields is still under debate. Recent work has shown that convection in cooling white dwarfs spans two regimes: efficient convection at the onset of crystallization, and thermohaline convection during most of the star's cooling history. Here, we calculate the properties of crystallization-driven convection for cooling models of several white dwarfs of different masses. We combine mixing-length theory with scalings from magneto-rotational convection to estimate the typical magnitude of the convective velocity and induced magnetic field for both scenarios. In the thermohaline regime, we find velocities $\sim 10^{-6}$--$10^{-5}~\mathrm{cm~s^{-1}}$, with fields restricted to $\lesssim~100~\mathrm{G}$. However, when convection is efficient, the flow velocity can reach magnitudes of $\sim 10^2$--$10^3~\mathrm{cm~s^{-1}}$, with fields of $\sim 10^6$--$10^8~\mathrm{G}$, independent of the star's rotation rate. Thus, dynamos driven at the onset of crystallization could explain the large intensity magnetic fields measured for single white dwarfs.

astro-ph.SR↗
arXiv2024

Fast and Slow Crystallization-driven Convection in White Dwarfs

We investigate crystallization-driven convection in carbon-oxygen white dwarfs. We present a version of the mixing length theory (MLT) that self-consistently includes the effects of thermal diffusion and composition gradients, and provides solutions for the convective parameters based on the local heat and composition fluxes. Our formulation smoothly transitions between the regimes of fast adiabatic convection at large Peclet number and slow thermohaline convection at low Peclet number. It also allows for both thermally-driven and compositionally-driven convection, including correctly accounting for the direction of heat transport for compositionally-driven convection in a thermally-stable background. We use the MESA stellar evolution code to calculate the composition and heat fluxes during crystallization in different models of cooling white dwarfs, and determine the regime of convection and the convective velocity. We find that convection occurs in the regime of slow thermohaline convection during most of the cooling history of the star. However, at the onset of crystallization, the composition flux is large enough to drive fast overturning convection for a short time ($\sim 10\ \mathrm{Myr}$). We estimate the convective velocities in both of these phases and discuss the implications for explaining observed white dwarf magnetic fields with crystallization-driven dynamos.

astro-ph.SR↗
arXiv2023

Expanded atmospheres and winds in Type I X-ray bursts from accreting neutron stars

We calculate steady-state models of radiation-driven super-Eddington winds and static expanded envelopes of neutron stars caused by high luminosities in type I X-ray bursts. We use flux-limited diffusion to model the transition from optically thick to optically thin, and include effects of general relativity, allowing us to study the photospheric radius close to the star as the hydrostatic atmosphere evolves into a wind. We find that the photospheric radius evolves monotonically from static envelopes ($r_{\rm ph}\lesssim 50-70$ km) to winds ($r_{\rm ph}\approx 100-1000$ km). Photospheric radii of less than $100$ km, as observed in most photospheric radius expansion bursts, can be explained by static envelopes, but only in a narrow range of luminosity. In most bursts, we would expect the luminosity to increase further, leading to a wind with photospheric radius $\gtrsim 100$ km. In the contraction phase, the expanded envelope solutions show that the photosphere is still $\approx 1$ km above the surface when the effective temperature is only $3\%$ away from its maximum value. This is a possible systematic uncertainty when interpreting the measured Eddington fluxes from bursts at touchdown. We also discuss the applicability of steady-state models to describe the dynamics of bursts. In particular, we show that the sub to super-Eddington transition during the burst rise is rapid enough that static models are not appropriate. Finally, we analyze the strength of spectral shifts in our models. Expected shifts at the photosphere are dominated by gravitational redshift, and are therefore predicted to be less than a few percent.

astro-ph.HE↗
arXiv2023

The imprint of convection on Type I X-ray bursts: Pauses in photospheric radius expansion lightcurves

Motivated by the recent observation by NICER of a type I X-ray burst from SAX J1808.4-3658 with a distinct "pause" feature during its rise (Bult et al. 2019), we show that bursts which ignite in a helium layer underneath a hydrogen-rich shell naturally give rise to such pauses, as long as enough energy is produced to eject the outer layers of the envelope by super-Eddington winds. The length of the pause is determined by the extent of the convection generated after ignition, while the rate of change of luminosity following the pause is set by the hydrogen gradient left behind by convection. Using the MESA stellar evolution code, we simulate the accumulation, nuclear burning and convective mixing prior to and throughout the ignition of the burst, followed by the hydrodynamic wind. We show that the results are sensitive to the treatment of convection adopted within the code. In particular, the efficiency of mixing at the H/He interface plays a key role in determining the shape of the lightcurve. The data from SAX J1808.4-3658 favors strong mixing scenarios. Multidimensional simulations will be needed to properly model the interaction between convection and nuclear burning during these bursts, which will then enable a new way to use X-ray burst lightcurves to study neutron star surfaces.

astro-ph.HE↗
arXiv2023

A catalogue of unusually long thermonuclear bursts on neutron stars

Rare, energetic (long) thermonuclear (Type I) X-ray bursts are classified either as intermediate-duration or superbursts, based on their duration. Intermediate-duration bursts lasting a few to tens of minutes are thought to arise from the thermonuclear runaway of a relatively thick (10^10 g/cm2) helium layer, while superbursts lasting hours are attributed to the detonation of an underlying carbon layer. We present a catalogue of 84 long thermonuclear bursts from 40 low-mass X-ray binaries, and defined from a new set of criteria distinguishing them from the more frequent short bursts. The three criteria are: (1) a total energy release larger than 10^40 erg, (2) a photospheric radius expansion phase longer than 10 s, and (3) a burst time-scale longer than 70 s. This work is based on a comprehensive systematic analysis of 70 bursts found with INTEGRAL, RXTE, Swift, BeppoSAX, MAXI, and NICER, as well as 14 long bursts from the literature that were detected with earlier generations of X-ray instruments. For each burst, we measure its peak flux and fluence, which eventually allows us to confirm the distinction between intermediate-duration bursts and superbursts. Additionally, we list 18 bursts that only partially meet the above inclusion criteria, possibly bridging the gap between normal and intermediate-duration bursts. With this catalogue, we significantly increase the number of long-duration bursts included in the MINBAR and thereby provide a substantial sample of these rare X-ray bursts for further study.

astro-ph.HE↗
arXiv2023

Magnetohydrodynamical torsional oscillations from thermo-resistive instability in hot jupiters

Hot jupiter atmospheres may be subject to a thermo-resistive instability where an increase in the electrical conductivity due to ohmic heating results in runaway of the atmospheric temperature. We introduce a simplified one-dimensional model of the equatorial sub-stellar region of a hot jupiter which includes the temperature-dependence and time-dependence of the electrical conductivity, as well as the dynamical back-reaction of the magnetic field on the flow. This model extends our previous one-zone model to include the radial structure of the atmosphere. Spatial gradients of electrical conductivity strongly modify the radial profile of Alfvén oscillations, leading to steepening and downwards transport of magnetic field, enhancing dissipation at depth. We find unstable solutions that lead to self-sustained oscillations for equilibrium temperatures in the range $T_\mathrm{eq}\approx 1000$--$1200$~K, and magnetic field in the range $\approx 10$--$100$~G. For a given set of parameters, self-sustained oscillations occur in a narrow range of equilibrium temperatures which allow the magnetic Reynolds number to alternate between large and small values during an oscillation cycle. Outside of this temperature window, the system reaches a steady state in which the effect of the magnetic field can be approximated as a magnetic drag term. Our results show that thermo-resistive instability is a possible source of variability in magnetized hot jupiters at colder temperatures, and emphasize the importance of including the temperature-dependence of electrical conductivity in models of atmospheric dynamics.

astro-ph.EP↗
arXiv2023

Rotation reduces convective mixing in Jupiter and other gas giants

Recent measurements of Jupiter's gravitational moments by the Juno spacecraft and seismology of Saturn's rings suggest that the primordial composition gradients in the deep interior of these planets have persisted since their formation. One possible explanation is the presence of a double-diffusive staircase below the planet's outer convection zone, which inhibits mixing across the deeper layers. However, hydrodynamic simulations have shown that these staircases are not long-lasting and can be disrupted by overshooting convection. In this paper we suggests that planetary rotation could be another factor for the longevity of primordial composition gradients. Using rotational mixing-length theory and 3D hydrodynamic simulations, we demonstrate that rotation significantly reduces both the convective velocity and the mixing of primordial composition gradients. In particular, for Jovian conditions at $t\sim10^{8}~\mathrm{yrs}$ after formation, rotation reduces the convective velocity by a factor of 6, and in turn, the kinetic energy flux available for mixing gets reduced by a factor of $6^3\sim 200$. This leads to an entrainment timescale that is more than two orders of magnitude longer than without rotation. We encourage future hydrodynamic models of Jupiter and other gas giants to include rapid rotation, because the decrease in the mixing efficiency could explain why Jupiter and Saturn are not fully mixed.

astro-ph.EP↗
arXiv2022

Variability from thermo-resistive instability in the atmospheres of hot jupiters

The atmosphere of a hot jupiter may be subject to a thermo-resistive instability, in which the increasing electrical conductivity with temperature leads to runaway Ohmic heating. We introduce a simplified model of the local dynamics in the equatorial region of a hot jupiter that incorporates the back reaction on the atmospheric flow as the increasing electrical conductivity leads to flux freezing, which in turn quenches the flow and therefore the Ohmic heating. We demonstrate a new time-dependent solution that emerges for a temperature-dependent electrical conductivity (whereas a temperature-independent conductivity always evolves to a steady-state). The periodic cycle consists of bursts of Alfven oscillations separated by quiescent intervals, with the magnetic Reynolds number alternating between values smaller than and larger than unity, maintaining the oscillation. We investigate the regions of pressure and temperature in which the instability operates. For the typical equatorial accelerations seen in atmospheric models, we find instability at pressures $\sim 0.1$--$1\ {\rm bar}$ and temperatures $\approx 1300$--$1800\ {\rm K}$ for magnetic fields $\sim 10\ {\rm G}$. Unlike previous studies based on a constant wind velocity, we find that the instability is stronger for weaker magnetic fields. Our results add support to the idea that variability should be a feature of magnetized hot jupiter atmospheres, particularly at intermediate temperatures. The temperature-dependence of the electrical conductivity is an important ingredient that should be included in MHD models of hot jupiter atmospheric dynamics.

astro-ph.EP↗
arXiv2022

Fast neutrino cooling in the accreting neutron star MXB 1659-29

Modelling of crust heating and cooling across multiple accretion outbursts of the low mass X-ray binary MXB 1659-29 indicates that the neutrino luminosity of the neutron star core is consistent with direct Urca reactions occurring in $\sim 1\%$ of the core volume. We investigate this scenario with neutron star models that include a detailed equation of state parametrized by the slope of the nuclear symmetry energy $L$, and a range of neutron and proton superfluid gaps. We find that the predicted neutron star mass depends sensitively on $L$ and the assumed gaps. We discuss which combinations of superfluid gaps reproduce the inferred neutrino luminosity. Larger values of $L\gtrsim 80\ {\rm MeV}$ require superfluidity to suppress dUrca reactions in low mass neutron stars, i.e. that the proton or neutron gap is sufficiently strong and extends to high enough density. However, the largest gaps give masses near the maximum mass, making it difficult to accommodate colder neutron stars. We consider models with reduced dUrca normalization as an approximation of alternative, less efficient, fast cooling processes in exotic cores. We find solutions with a larger emitting volume, providing a more natural explanation for the observed neutrino luminosity, provided the fast cooling process is within a factor of $\sim 1000$ of dUrca. The heat capacities of our models span the range from fully-paired to fully-unpaired nucleons meaning that long term observations of core cooling could distinguish between models. We discuss the impact of future constraints on neutron star mass, radius and the density dependence of the symmetry energy.

astro-ph.HE↗
arXiv2021

Probing dense matter physics with transiently-accreting neutron stars: the case of source MXB 1659-29

Recent observational data on transiently-accreting neutron stars has unequivocally shown fast-cooling sources, such as in the case of neutron star MXB 1659-29. Previous calculations have estimated its total neutrino luminosity and heat capacity, as well as suggested that direct Urca reactions take place in $1 \%$ of the volume of the core. In this paper, we reproduce the inferred luminosity of this source with detailed models of equations of state (EOS) and nuclear pairing gaps. We show that three superfluidity gap models are inconsistent with data for all EOS and another three are disfavoured because of fine tuning arguments. We also calculate the total heat capacity for all constructed stars and show that independent observations of mass and luminosity could set constraints on the core superfluidity of a source as well as the density slope of the symmetry energy, L. This is an important step towards defining a universal equation of state for neutron stars and therefore, towards a better understanding of the phase diagram of asymmetric matter at high densities.

nucl-th↗
arXiv2021

The effect of late giant collisions on the atmospheres of protoplanets and the formation of cold sub-Saturns

We investigate the origins of cold sub-Saturns (CSS), an exoplanetary population inferred from microlensing surveys. If confirmed, these planets would rebut a theorised gap in planets' mass distribution between those of Neptune and Jupiter caused by the rapid runaway accretion of super-critical cores. In an attempt to resolve this theoretical-observational disparity, we examine the outcomes of giant collisions between sub-critical protoplanets. Due to the secular interaction among protoplanets, these events may occur in rapidly depleting discs. We show that impactors ~ 5% the mass of near-runaway envelopes around massive cores can efficiently remove these envelopes entirely via a thermally-driven super-Eddington wind emanating from the core itself, in contrast with the stellar Parker winds usually considered. After a brief cooling phase, the merged cores resume accretion. But, the evolution timescale of transitional discs is too brief for the cores to acquire sufficiently massive envelopes to undergo runaway accretion despite their large combined masses. Consequently, these events lead to the emergence of CSS without their transformation into gas giants. We show that these results are robust for a wide range of disc densities, grain opacities and silicate abundance in the envelope. Our fiducial case reproduces CSS with heavy (>= 30 M_Earth) cores and less massive (a few M_Earth) sub-critical envelopes. We also investigate the other limiting cases, where continuous mergers of comparable-mass cores yield CSS with wider ranges of core-to-envelope mass ratios and envelope opacities. Our results indicate that it is possible for CSS and Uranus and Neptune to emerge within the framework of well studied processes and they may be more common than previously postulated.

astro-ph.EP↗