arXiv subjects

Lucy O. McNeill

Publications and source records attributed to Lucy O. McNeill.

6 recordsLinked to original sources

arXiv2026

Angular momentum transport in the convection zone of a 3D MHD simulation of a rapidly rotating core-collapse progenitor

Rotation and magnetic fields in the cores of evolved massive stars in their final phase are thought to play an important role in the subsequent supernova explosion and the formation of a compact object, especially in hyperenergetic explosions. However, the interplay between rotation, magnetic fields, and convection up to the final collapse is a nonlinear, multidimensional effect that is difficult to capture with standard one-dimensional (1D) stellar evolution models. We quantify the magnetic angular momentum (AM) transport in the convective oxygen burning shell in a three-dimensional (3D) rotating core-collapse progenitor model. We find that the radial direction of magnetic AM transport is directly related to the Rossby number of the convective oxygen shell. We also analyze the magnetic energy, which sets the amplitude of the magnetic AM flux. The magnetic energy is determined both by rotation and the nuclear energy generation rate analogously to low-mass stars like the Sun. Based on these results, we construct a 1D model of magnetic AM transport in the convection zone for the first time in terms of properties of a given stellar evolution model. This model successfully reproduces the AM transport in the 3D model when the convective dynamo is in a quasi-steady state. Notably, our model for radial AM transport is the first to account for inward AM flux. This may result in interesting differences compared to the conventional treatment of magnetic AM transport in stellar evolution models, which assume AM is transported outward by a purely diffusive process.

astro-ph.SR↗

arXiv2026

Mass transfer stability for AM CVn binaries with white dwarf donors

Double white dwarf binaries (DWDBs) with Helium components are progenitors to AM CVn binary systems. Their Galactic production rate may be given by the number of stably mass transferring DWDBs in the Milky Way. The theoretical criteria for stable mass transfer in DWDBs is calculated assuming that component white dwarfs are completely cold and degenerate. Respective fractions of surviving AM CVn and DWDB which merge are then calculated by applying this criteria to population synthesis estimates for Galactic DWDB. However, emerging observations of the local DWDB population suggest that Helium white dwarf (He WD) components are typically hot, and only partially degenerate when they begin mass transferring. Using recent numerical simulations of He WD donors in DWDBs, we qualitatively describe a temperature dependent stable mass transfer criteria for Galactic DWDBs. Mass transfer is even more stable than previously thought, or equivalently, DWDB mergers are even rarer. Realistic finite temperature treatments will deepen the dearth in observed AM CVn binaries compared to DWDB merger products.

astro-ph.SR↗

arXiv2025

Tidal heating in detached double white dwarf binaries

Short--period ($P<$1 hr orbits) detached double white dwarf binary (DWDB) components identified with transient surveys (e.g. SDSS, ZTF) have hot surface temperatures ($>$10,000 K) and observed radii a factor two larger than completely degenerate white dwarfs. We formulate tidal heating in helium composition extremely low mass white dwarf (ELM WD) components of detached DWDBs which reach mass transfer within a Hubble time. We combine a mass radius relation which varies with surface temperature and the equilibrium tidal friction model of Hut 1981, where the additional orbital energy loss from tidal friction is accounted for by increases in the primary surface temperature, and hence increasing radius. Applying this heating model to the current sample of binaries with ZTF, we predict temperature increases from the present day of up to $\sim$40\% before the onset of mass transfer. We find that helium white dwarfs are generically hot and large at the onset of mass transfer, even for the oldest DWDBs whose components can cool to be degenerate by the present day. In the population of Galactic DWDBs, we find that the onset of mass transfer should occur at orbital periods as long as 1000s (17 minutes), or binary gravitational wave frequency of 2 mHz. This is over three times longer than periods expected for degenerate WD (5 minutes). Since mass transferring DWDBs are progenitors for a variety of transients and stellar populations e.g. RCrB stars, AM CVn binaries, so-called Type .Ia supernova, the finite temperature of donor white dwarfs should be taken into account.

astro-ph.SR↗

arXiv2022

Differential Rotation in a 3D Simulation of Oxygen Shell Burning

We study differential rotation in late-stage shell convection in a 3D hydrodynamic simulation of a rapidly rotating $16M_\odot$ helium star with a particular focus on the convective oxygen shell. We find that the oxygen shell develops a quasi-stationary pattern of differential rotation that is neither described by uniform angular velocity as assumed in current stellar evolution models of supernova progenitors, nor by uniform specific angular momentum. Instead, the oxygen shell develops a positive angular velocity gradient with faster rotation at the equator than at the pole by tens of percent. We show that the angular momentum transport inside the convection zone is not adequately captured by a diffusive mixing-length flux proportional to the angular velocity or angular momentum gradient. Zonal flow averages reveal stable large-scale meridional flow and an entropy deficit near the equator that mirrors the patterns in the angular velocity. The structure of the flow is reminiscent of simulations of stellar surface convection zones and the differential rotation of the Sun, suggesting that similar effects are involved; future simulations will need to address in more detail how the interplay of buoyancy, inertial forces, and turbulent stresses shapes differential rotation during late-stage convection in massive stars. If convective regions develop positive angular velocity gradients, angular momentum could be shuffled out of the core region more efficiently, potentially making the formation of millisecond magnetars more difficult. Our findings have implications for neutron star birth spin periods and supernova explosion scenarios that involve rapid core rotation.

astro-ph.SR↗

arXiv2022

Probing Formation of Double Neutron Star Binaries around 1mHz with LISA

We propose a novel method to examine whether Galactic double neutron star binaries are formed in the LISA band. In our method, we assign an effective time fraction $τ$ to each double neutron star binary detected by LISA. This fraction is given as a function of the observed orbital period and eccentricity and should be uniformly distributed in the absence of in-band binary formation. Applying statistical techniques such as the Kolmogorov-Smirnov test to the actual list of $τ$, we can inspect the signature of the in-band binary formation. We discuss the prospects of this method, paying close attention to the available sample number of Galactic double neutron star binaries around 1mHz.

astro-ph.HE↗

arXiv2017

Gravitational Waves from Orphan Memory

Gravitational-wave memory manifests as a permanent distortion of an idealized gravitational-wave detector and arises generically from energetic astrophysical events. For example, binary black hole mergers are expected to emit memory bursts a little more than an order of magnitude smaller in strain than the oscillatory parent waves. We introduce the concept of "orphan memory": gravitational-wave memory for which there is no detectable parent signal. In particular, high-frequency gravitational-wave bursts ($\gtrsim$ kHz) produce orphan memory in the LIGO/Virgo band. We show that Advanced LIGO measurements can place stringent limits on the existence of high-frequency gravitational waves, effectively increasing the LIGO bandwidth by orders of magnitude. We investigate the prospects for and implications of future searches for orphan memory.

astro-ph.IM↗