arXiv subjects

Andrew Cumming

Publications and source records attributed to Andrew Cumming.

At least 37 records · Page 2Linked to original sources

arXiv2020

The Challenge of Forming a Fuzzy Core in Jupiter

Recent structure models of Jupiter that match Juno gravity data suggest that the planet harbours an extended region in its deep interior that is enriched in heavy elements, a so-called dilute/fuzzy core. This finding raises the question of what possible formation pathways would lead to such a structure. We model Jupiter's formation and long-term evolution, starting at late-stage formation before runaway gas accretion. The formation scenarios we consider include both primordial composition gradients, as well as gradients that are built as proto-Jupiter rapidly acquires its gaseous envelope. We then follow Jupiter's evolution as it cools down and contracts, with a particular focus on the energy and material transport in the interior. We find that none of the scenarios we consider lead to a fuzzy core that is compatible with interior structure models. In all the cases, most of Jupiter's envelope becomes convective and fully mixed after at most a few million years. This is true even when we consider a case where the gas accretion leads to a cold planet, and large amounts of heavy elements are accreted. It is therefore concluded that it is very challenging to explain Jupiter's dilute core from standard formation models. We suggest that future work should consider more complex formation pathways as well as the modelling of additional physical processes that could lead to Jupiter's current-state internal structure.

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arXiv2020

The Imprint of the Protoplanetary Disk in the Accretion of Super-Earth Envelopes

Super-Earths are by far the most dominant type of exoplanet, yet their formation is still not well understood. In particular, planet formation models predict that many of them should have accreted enough gas to become gas giants. Here we examine the role of the protoplanetary disk in the cooling and contraction of the protoplanetary envelope. In particular, we investigate the effects of 1) the thermal state of the disk as set by the relative size of heating by accretion or irradiation, and whether its energy is transported by radiation or convection, and 2) advection of entropy into the outer envelope by disk flows that penetrate the Hill sphere, as found in 3D global simulations. We find that, at 5 and 1 AU, this flow at the level reported in the non-isothermal simulations where it penetrates only to ~ 0.3 times the Hill radius has little effect on the cooling rate since most of the envelope mass is concentrated close to the core, and far from the flow. On the other hand, at 0.1 AU, the envelope quickly becomes fully-radiative, nearly isothermal, and thus cannot cool down, stalling gas accretion. This effect is significantly more pronounced in convective disks, leading to envelope mass orders of magnitude lower. Entropy advection at 0.1 AU in either radiative or convective disks could therefore explain why super-Earths failed to undergo runaway accretion. These results highlight the importance of the conditions and energy transport in the protoplanetary disk for the accretion of planetary envelopes.

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arXiv2020

The effect of diffusive nuclear burning in neutron star envelopes on cooling in accreting systems

Valuable information about the neutron star interior can be obtained by comparing observations of thermal radiation from a cooling neutron star crust with theoretical models. Nuclear burning of lighter elements that diffuse to deeper layers of the envelope can alter the relation between surface and interior temperatures and can change the chemical composition over time. We calculate new temperature relations and consider two effects of diffusive nuclear burning (DNB) for H-C envelopes. First, we consider the effect of a changing envelope composition and find that hydrogen is consumed on short timescales and our temperature evolution simulations correspond to those of a hydrogen-poor envelope within ~100 days. The transition from a hydrogen-rich to a hydrogen-poor envelope is potentially observable in accreting NS systems as an additional initial decline in surface temperature at early times after the outburst. Second, we find that DNB can produce a non-negligible heat flux, such that the total luminosity can be dominated by DNB in the envelope rather than heat from the deep interior. However, without continual accretion, heating by DNB in H-C envelopes is only relevant for <1-80 days after the end of an accretion outburst, as the amount of light elements is rapidly depleted. Comparison to crust cooling data shows that DNB does not remove the need for an additional shallow heating source. We conclude that solving the time-dependent equations of the burning region in the envelope self-consistently in thermal evolution models instead of using static temperature relations would be valuable in future cooling studies.

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arXiv2019

LRP2020: The cosmic origin and evolution of the elements

The origin of many elements of the periodic table remains an unsolved problem. While many nucleosynthetic channels are broadly understood, significant uncertainties remain regarding certain groups of elements such as the intermediate and rapid neutron-capture processes, the p-process, or the origin of odd-Z elements in the most metal-poor stars. Canada has a long tradition of leadership in nuclear astrophysics, dating back to the work of Alastair Cameron in the 1950s. Recent faculty hires have further boosted activity in the field, including transient observation and theory, survey science on galactic nucleosynthesis, and nuclear experiments. This white paper contains a brief overview of recent activity in the community, highlighting strengths in each sub-field, and provides recommendations to improve interdisciplinary collaboration. Sustaining Canadian leadership in the next decade will require, on the observational side, access to transient and non-transient surveys like LSST, SKA, or MSE, support for target-of-opportunity observing in current and future Canadian telescopes, and participation in next-generation X-ray telescopes such as ATHENA. State-of-the-art theoretical predictions will require an ambitious succession plan for the Niagara supercomputer to support large parallel jobs. We propose a funding instrument for postdoctoral training that reflects the interdisciplinary nature of nuclear astrophysics research, and the creation of a national collaborative funding program that allows for joint projects and workshop organization.

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arXiv2018

Observatory science with eXTP

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.

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arXiv2018

Glitch rises as a test for rapid superfluid coupling in neutron stars

Pulsar glitches provide a unique way to study neutron star microphysics because short post-glitch dynamics are directly linked to strong frictional processes on small scales. To illustrate this connection between macroscopic observables and microphysics, we review calculations of vortex interactions focusing on Kelvin wave excitations and determine the corresponding mutual friction strength for realistic microscopic parameters in the inner crust. These density-dependent crustal coupling profiles are combined with a simplified treatment of the core coupling and implemented in a three-component neutron star model to construct a predictive framework for glitch rises. As a result of the density-dependent dynamics, we find the superfluid to transfer angular momentum to different parts of the crust and the core on different timescales. This can cause the spin frequency change to become non-monotonic in time, allowing for a maximum value much larger than the measured glitch size, as well as a delay in the recovery. The exact shape of the calculated glitch rise is strongly dependent on the relative strength between the crust and core mutual friction, providing the means to probe not only the crustal superfluid but also the deeper neutron star interior. To demonstrate the potential of this approach, we compare our predictive model with the first pulse-to-pulse observations recorded during the December 2016 glitch of the Vela pulsar. Our analysis suggests that the glitch rise behavior is relatively insensitive to the crustal mutual friction strength as long as $\mathcal{B} \gtrsim 10^{-3}$, while being strongly dependent on the core coupling strength, which we find to be in the range $3 \times 10^{-5} \lesssim \mathcal{B}_{\rm core} \lesssim 10^{-4}$.

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arXiv2018

Deep crustal heating by neutrinos from the surface of accreting neutron stars

We present a new mechanism for deep crustal heating in accreting neutron stars. Charged pions ($π^+$) are produced in nuclear collisions on the neutron star surface during active accretion and upon decay they provide a flux of neutrinos into the neutron star crust. For massive and/or compact neutron stars, neutrinos deposit $\approx 1\textrm{--} 2 \, \mathrm{MeV}$ of heat per accreted nucleon into the inner crust. The strength of neutrino heating is comparable to the previously known sources of deep crustal heating, such as from pycnonuclear fusion reactions, and is relevant for studies of cooling neutron stars. We model the thermal evolution of a transient neutron star in a low-mass X-ray binary, and in the particular case of the neutron star MXB~1659-29 we show that additional deep crustal heating requires a higher thermal conductivity for the neutron star inner crust. A better knowledge of pion production cross sections near threshold would improve the accuracy of our predictions.

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arXiv2018

Polycrystalline Crusts in Accreting Neutron Stars

The crust of accreting neutron stars plays a central role in many different observational phenomena. In these stars, heavy elements produced by H-He burning in the rapid proton capture (rp-) process continually freeze to form new crust. In this paper, we explore the expected composition of the solid phase. We first demonstrate using molecular dynamics that two distinct types of chemical separation occur, depending on the composition of the rp-process ashes. We then calculate phase diagrams for three-component mixtures and use them to determine the allowed crust compositions. We show that, for the large range of atomic numbers produced in the rp-process ($Z\sim 10$--$50$), the solid that forms has only a small number of available compositions. We conclude that accreting neutron star crusts should be polycrystalline, with domains of distinct composition. Our results motivate further work on the size of the compositional domains, and have implications for crust physics and accreting neutron star phenomenology.

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arXiv2018

The Primordial Entropy of Jupiter

The formation history of giant planets determines their primordial structure and consequent evolution. We simulate various formation paths of Jupiter to determine its primordial entropy, and find that a common outcome is for proto-Jupiter to have non-convective regions in its interior. We use planet formation models to calculate how the entropy and post-formation luminosity depend on model properties such as the solid accretion rate and opacity, and show that the gas accretion rate and its time evolution play a key role in determining the entropy profile. The predicted luminosity of Jupiter shortly after formation varies by a factor of $2$--$3$ for different choices of model parameters. We find that entropy gradients inside Jupiter persist for $\sim 10\ {\rm Myr}$ after formation. We suggest that these gradients should be considered together with heavy-element composition gradients when modeling Jupiter's evolution and internal structure.

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arXiv2018

Flux Relaxation after two outbursts of the magnetar SGR 1627$-$41 and possible hard X-ray emission

We report on the long-term flux relaxation of the magnetar SGR 1627-41 after its 2008 outburst, and evidence for hard X-ray excess measured with NuSTAR. We use new observations made with Chandra and XMM-Newton, and an archival NuSTAR observation which add flux measurements at ~2000 days into quiescence after the 2008 outburst. We find that the source flux has further declined since the last measurement made in 2011, ~1000 days after the outburst in 2008. This trend is similar to the relaxation after the source's 1998 outburst. We use crustal cooling models to reproduce the flux relaxation; if the whole surface of the star is heated in the outbursts, the modeling suggests that the 2008 outburst of SGR 1627-41 deposited energy into the inner crust and that the core temperature of SGR 1627-41 is low (T_c<10^8 K) as previously suggested. On the other hand, if only a small fraction of the surface is heated or the temperature in the crust reached the melting temperature, relaxation at early times requires another emission mechanism. Finally, we report on evidence for hard X-ray emission in SGR 1627-41 which follows the observational correlation suggested by Kaspi & Boydstun (2010) in magnetars.

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arXiv2018

The California-Kepler Survey V. Peas in a Pod: Planets in a Kepler Multi-planet System are Similar in Size and Regularly Spaced

We have established precise planet radii, semimajor axes, incident stellar fluxes, and stellar masses for 909 planets in 355 multi-planet systems discovered by Kepler. In this sample, we find that planets within a single multi-planet system have correlated sizes: each planet is more likely to be the size of its neighbor than a size drawn at random from the distribution of observed planet sizes. In systems with three or more planets, the planets tend to have a regular spacing: the orbital period ratios of adjacent pairs of planets are correlated. Furthermore, the orbital period ratios are smaller in systems with smaller planets, suggesting that the patterns in planet sizes and spacing are linked through formation and/or subsequent orbital dynamics. Yet, we find that essentially no planets have orbital period ratios smaller than $1.2$, regardless of planet size. Using empirical mass-radius relationships, we estimate the mutual Hill separations of planet pairs. We find that $93\%$ of the planet pairs are at least 10 mutual Hill radii apart, and that a spacing of $\sim20$ mutual Hill radii is most common. We also find that when comparing planet sizes, the outer planet is larger in $65 \pm 0.4\%$ of cases, and the typical ratio of the outer to inner planet size is positively correlated with the temperature difference between the planets. This could be the result of photo-evaporation.

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arXiv2017

Rapid neutrino cooling in the neutron star MXB 1659-29

We show that the neutron star in the transient system MXB~1659-29 has a core neutrino luminosity that substantially exceeds that of the modified Urca reactions (i.e., $n+n\to n+p+e^{-}+\barν_{e}$ and inverse) and is consistent with the direct Urca reactions ($n\to p+e^{-}+\barν_{e}$ and inverse) occurring in a small fraction of the core. Observations of the thermal relaxation of the neutron star crust following 2.5 years of accretion allow us to measure the energy deposited into the core during accretion, which is then reradiated as neutrinos, and infer the core temperature. For a nucleonic core, this requires that the nucleons are unpaired and that the proton fraction exceed a critical value to allow the direct Urca reaction to proceed. The neutron star in MXB~1659-29 is the first with a firmly detected thermal component in its X-ray spectrum that needs a fast neutrino cooling process. Measurements of the temperature variation of the neutron star core during quiescence would place an upper limit on the core specific heat and serve as a check on the fraction of the neutron star core in which nucleons are unpaired.

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arXiv2017

Predictions of planet detections with near infrared radial velocities in the up-coming SPIRou Legacy Survey-Planet Search

The SPIRou near infrared spectro-polarimeter is destined to begin science operations at the Canada-France-Hawaii Telescope in mid-2018. One of the instrument's primary science goals is to discover the closest exoplanets to the Solar System by conducting a 3-5 year long radial velocity survey of nearby M dwarfs at an expected precision of $\sim 1$ m s$^{-1}$; the SPIRou Legacy Survey-Planet Search (SLS-PS). In this study we conduct a detailed Monte-Carlo simulation of the SLS-PS using our current understanding of the occurrence rate of M dwarf planetary systems and physical models of stellar activity. From simultaneous modelling of planetary signals and activity, we predict the population of planets detected in the SLS-PS. With our fiducial survey strategy and expected instrument performance over a nominal survey length of $\sim 3$ years, we expect SPIRou to detect $85.3^{+29.3}_{-12.4}$ planets including $20.0^{+16.8}_{-7.2}$ habitable zone planets and $8.1^{+7.6}_{-3.2}$ Earth-like planets from a sample of 100 M1-M8.5 dwarfs out to 11 pc. By studying mid-to-late M dwarfs previously inaccessible to existing optical velocimeters, SPIRou will put meaningful constraints on the occurrence rate of planets around those stars including the value of $η_{\oplus}$ at an expected level of precision of $\lesssim 45$%. We also predict a subset of $46.7^{+16.0}_{-6.0}$ planets may be accessible with dedicated high-contrast imagers on the next generation of ELTs including $4.9^{+4.7}_{-2.0}$ potentially imagable Earth-like planets. Lastly, we compare the results of our fiducial survey strategy to other foreseeable survey versions to quantify which strategy is optimized to reach the SLS-PS science goals. The results of our simulations are made available to the community on github.

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arXiv2017

Hot Start Giant Planets Form With Radiative Interiors

In the hot-start core accretion formation model for gas giants, the interior of a planet is usually assumed to be fully convective. By calculating the detailed internal evolution of a planet assuming hot start outer boundary conditions, we show that such a planet will in fact form with a radially increasing internal entropy profile, so that its interior will be radiative instead of convective. For a hot outer boundary, there is a minimum value for the entropy of the internal adiabat $S_{min}$ below which the accreting envelope does not match smoothly onto the interior, but instead deposits high entropy material onto the growing interior. One implication of this would be to at least temporarily halt the mixing of heavy elements within the planet, which are deposited by planetesimals accreted during formation. The compositional gradient this would impose could subsequently disrupt convection during post-accretion cooling, which would alter the observed cooling curve of the planet. However even with a homogeneous composition, for which convection develops as the planet cools, the difference in cooling timescale will change the inferred mass of directly-imaged gas giants.

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arXiv2016

Evolution of Gas Giant Entropy During Formation by Runaway Accretion

We calculate the evolution of gas giant planets during the runaway gas accretion phase of formation, to understand how the luminosity of young giant planets depends on the accretion conditions. We construct steady-state envelope models, and run time-dependent simulations of accreting planets with the Modules for Experiments in Stellar Astrophysics (MESA) code. We show that the evolution of the internal entropy depends on the contrast between the internal adiabat and the entropy of the accreted material, parametrized by the shock temperature $T_0$ and pressure $P_0$. At low temperatures ($T_0\lesssim 300$--$1000\ {\rm K}$, depending on model parameters), the accreted material has a lower entropy than the interior. The convection zone extends to the surface and can drive a large luminosity, leading to rapid cooling and cold starts. For higher temperatures, the accreted material has a larger entropy than the interior, giving a radiative zone that stalls cooling. For $T_0\gtrsim 2000\ {\rm K}$, the surface--interior entropy contrast cannot be accommodated by the radiative envelope, and the accreted matter accumulates with high entropy, forming a hot start. The final state of the planet depends on the shock temperature, accretion rate, and starting entropy at the onset of runaway accretion. Cold starts with $L\lesssim 5\times 10^{-6}\ L_\odot$ require low accretion rates and starting entropy, and that the temperature of the accreting material is maintained close to the nebula temperature. If instead the temperature is near the value required to radiate the accretion luminosity, $4πR^2σT_0^4\sim (GM\dot M/R)$, as suggested by previous work on radiative shocks in the context of star formation, gas giant planets form in a hot start with $L\sim 10^{-4}\ L_\odot$.

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arXiv2016

Urca cooling pairs in the neutron star ocean and their effect on superbursts

An accretion outburst onto a neutron star deposits hydrogen-rich and/or helium-rich material into the neutron star's envelope. Thermonuclear burning of accreted material robustly produces Urca pairs\textrm{---}pairs of nuclei that undergo cycles of $e^-$-capture and $β^-$-decay. The strong $T^5$ dependence of the Urca cooling neutrino luminosity means that Urca pairs in the neutron star interior potentially remove heat from accretion-driven nuclear reactions. In this study, we identify Urca pairs in the neutron star's ocean \textrm{---} a plasma of ions and electrons overlaying the neutron star crust \textrm{---} and demonstrate that Urca cooling occurs at all depths in the ocean. We find that Urca pairs in the ocean and crust lower the ocean's steady-state temperature during an accretion outburst and that unstable carbon ignition, which is thought to trigger superbursts, occurs deeper than it would otherwise. Cooling superburst light curves, however, are only marginally impacted by cooling from Urca pairs because the superburst peak radiative luminosity $L_{\rm peak}$ is always much greater than the Urca pair neutrino luminosity $L_ν$ in the hot post-superburst ocean.

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arXiv2016

The thermal state of KS 1731-260 after 14.5 years in quiescence

Crustal cooling of accretion-heated neutron stars provides insight into the stellar interior of neutron stars. The neutron star X-ray transient, KS~1731$-$260, was in outburst for 12.5 years before returning to quiescence in 2001. We have monitored the cooling of this source since then through {\it Chandra} and {\it XMM-Newton} observations. Here, we present a 150 ks {\it Chandra} observation of KS~1731$-$260 taken in August 2015, about 14.5 years into quiescence, and 6 years after the previous observation. We find that the neutron star surface temperature is consistent with the previous observation, suggesting that crustal cooling has likely stopped and the crust has reached thermal equilibrium with the core. Using a theoretical crust thermal evolution code, we fit the observed cooling curves and constrain the core temperature (T$_c = 9.35\pm0.25\times10^7$ K), composition (Q$_{imp} = 4.4^{+2.2}_{-0.5}$) and level of extra shallow heating required (Q$_{sh} = 1.36\pm0.18$ MeV/nucleon). We find that the presence of a low thermal conductivity layer, as expected from nuclear pasta, is not required to fit the cooling curve well, but cannot be excluded either.

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arXiv2016

Late time cooling of neutron star transients and the physics of the inner crust

An accretion outburst onto a neutron star transient heats the neutron star's crust out of thermal equilibrium with the core. After the outburst the crust thermally relaxes toward equilibrium with the neutron star core and the surface thermal emission powers the quiescent X-ray light curve. Crust cooling models predict that thermal equilibrium of the crust will be established $\approx 1000 \, \mathrm{d}$ into quiescence. Recent observations of the cooling neutron star transient MXB 1659-29, however, suggest that the crust did not reach thermal equilibrium with the core on the predicted timescale and continued to cool after $\approx 2500 \, \mathrm{d}$ into quiescence. Because the quiescent light curve reveals successively deeper layers of the crust, the observed late time cooling of MXB 1659-29 depends on the thermal transport in the inner crust. In particular, the observed late time cooling is consistent with a low thermal conductivity layer near the depth predicted for nuclear pasta that maintains a temperature gradient between the neutron star's inner crust and core for thousands of days into quiescence. As a result, the temperature near the crust-core boundary remains above the critical temperature for neutron superfluidity and a layer of normal neutrons forms in the inner crust. We find that the late time cooling of MXB 1659-29 is consistent with heat release from a normal neutron layer near the crust-core boundary with a long thermal time.

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