arXiv subjects

Simon Daley-Yates

Publications and source records attributed to Simon Daley-Yates.

12 recordsLinked to original sources

arXiv2026

Dust cloud lifetimes of Scallop-shell stars

We investigate the survival of dust trapped in magnetically confined cool gas clouds (or {\it prominences}) around rapidly rotating M-dwarfs exhibiting the ``scallop-shell'' light-curve morphology. Using a two-dimensional magnetohydrodynamic simulation, we extend previous coronal prominence models to include a passive tracer field to allow for a single injection of collisionally charged dust grains. The tracer evolution reveals how recurrent centrifugal breakouts--the slingshot process--remove dust and gas from the prominence while chromospheric evaporation replenishes gas from below. For our simulated star, which has $R_{\ast} = 0.6 R_{\odot}$, $M_{\ast} = 0.3 M_{\odot}$, and $P_{\ast} = 0.32$ days, the resulting dust content decays exponentially with a minimum half-life of approximately 6 stellar rotations, representing a lower limit set by our assumption of fully coupled dust and gas dynamics. Synthetic velocity-phase diagnostics show a single, phase-locked feature that fades steadily, reproducing the behaviour of dips seen in TESS and K2 light curves. Comparison with observed river plots suggests a natural classification: (i) persistent, non-decaying features formed by quiescent prominences below co-rotation; (ii) gradually fading features produced by slingshot prominences near co-rotation; and (iii) abrupt disappearances linked to magnetic reconnection and flare-driven ejections. These results demonstrate that dust-bearing prominences--undergoing repeated slingshots--can persist for tens of rotations, linking the observed longevity of the scallop-shell photometric features with the dynamic cycle of prominence slingshot ejections.

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arXiv2025

WASP-12, shrouded in mystery or just cold gas?

Observations of the planet-hosting star WASP-12 show a distinctive depression in the \ion{Mg}{ii} and \ion{Ca}{ii} resonance lines. This has been interpreted as a marker of atmospheric loss from the close-in hot Jupiter WASP-12b and the resulting formation of a gas torus around the star. In this paper we quantify the \ion{Mg}{ii} absorption from this torus, compared to that provided by the stellar wind, the stellar astrosphere and the ISM. To do this we piece together the full density profile of \ion{Mg}{ii} from WASP-12 to an observer on Earth using a combination of hydrodynamical simulations and observations. We find that the bulk of the gas along the line of sight is contained within a dense torus close to WASP-12. However, the temperatures in this torus are sufficient to promote Mg into a doubly (\ion{Mg}{iii}) or higher ionized state. As a result, the singly ionized fraction (\ion{Mg}{ii}) is low. We find that most of the \ion{Mg}{ii} is not in the torus but in the ISM. Despite this, the total column density of \ion{Mg}{ii} is two orders of magnitude lower than required to explain observations of the system. To resolve this discrepancy, we note that the torus gas is at a temperature where it will cool efficiently. We speculate that the onset of the cooling instability will cause the torus to fragment, forming cold clumps with a higher fraction of \ion{Mg}{ii}, capable of explaining the observed absorption.

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arXiv2025

Filament Mass Losses Forced by Magnetic Reconnection in the Solar Corona

Recent observations of the solar atmosphere in cool extreme ultraviolet (EUV) lines have reported the prevalence of coronal rain falling from coronal cloud filaments that are associated with the magnetic dips of coronal X-point structures. These filaments mysteriously appear as clouds of mass in the corona that subsequently shrink and disappear due to mass losses that drain as coronal rain along arced field lines. Using a two and a half dimensional, magnetohydrodynamic model, we investigated evaporation-condensation as the formation mechanism of the subset of coronal cloud filaments that form above coronal X-points. Our simulation included the effects of field-aligned thermal conduction and optically thin radiation and used the state-of-the-art Transition Region Adaptive Conduction (TRAC) method to model the formation, maintenance, and mass loss of a filament above a coronal X-point. This paper presents a physical model that demonstrates magnetic reconnection as a filament loss mechanism, producing hybrid filament/coronal rain via mass losses through the X-point. A detailed analysis of how the mass of the filament forces the field to reconnect is also presented, revealing three phases that characterize the evolution of the reconnecting current sheet and associated mass losses. We conclude that the formation of certain coronal cloud filaments and subsequent mass losses via coronal rain can be explained by the evaporation-condensation model combined with filament mass losses forced by magnetic reconnection. We also report that rebound shocks generated by the impact of coronal rain condensations on the chromosphere together with retractive upflows can cause upward propagating condensations to form through a dynamic thermal runaway process.

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arXiv2024

Simulating stellar coronal rain and slingshot prominences

We have numerically demonstrated that simulated cool star coronae naturally form condensations. If the star rotates slowly, with a co-rotation radius greater than the Alfv\'{e}n radius (i.e. $R_{\mathrm{K}} > R_{\mathrm{A}}$), these condensations will form below the co-rotation radius $R_{\mathrm{K}}$ and simply fall back to the stellar surface as coronal rain. If, however, the star is more rapidly rotating, ($R_{\mathrm{K}} < R_{\mathrm{A}}$), not only rain will form but also ``slingshot prominences''. In this case, condensations collect into a large mass reservoir around the co-rotation radius, from which periodic centrifugal ejections occur. In this case, some $51\%$ of the coronal mass is cold gas, either in rain or prominences. We find that 21\% of the mass lost by our simulated fast rotating star is cold gas. Studies of stellar mass-loss from the hot wind do not consider this component of the wind and therefore systematically underestimate mass-loss rates of these stars. Centrifugal ejections happen periodically, between every 7.5 - 17.5 hours with masses clustering around $10^{16}$ g, These results agree well with observational statistics. Contrasting the fast and slow rotating magnetospheres, we find that there are two distinct types of solutions, high lying and low lying loops. Low lying loops only produce coronal rain whereas high lying loops produce both rain and slingshots.

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arXiv2023

Heating and cooling in stellar coronae: coronal rain on a young Sun

Recent observations of rapidly-rotating cool dwarfs have revealed H$\alpha$ line asymmetries indicative of clumps of cool, dense plasma in the stars' coronae. These clumps may be either long-lived (persisting for more than one stellar rotation) or dynamic. The fastest dynamic features show velocities greater than the escape speed, suggesting that they may be centrifugally ejected from the star, contributing to the stellar angular momentum loss. Many however show lower velocities, similar to coronal rain observed on the Sun. We present 2.5D magnetohydrodynamic simulations of the formation and dynamics of these condensations in a rapidly rotating ($P_{\rm rot}~=~ 1 \ \mathrm{day}$) young Sun. Formation is triggered by excess surface heating. This pushes the system out of thermal equilibrium and triggers a thermal instability. The resulting condensations fall back towards the surface. They exhibit quasi-periodic behaviour, with periods longer than typical periods for solar coronal rain. We find line-of-sight velocities for these clumps in the range $50 \ \mathrm{km} \ \mathrm{s}^{-1}$ (blue shifted) to $250 \ \mathrm{km} \ \mathrm{s}^{-1}$ (red shifted). These are typical of those inferred from stellar H$\alpha$ line asymmetries, but the inferred clump masses of $3.6\times 10^{14}\ \mathrm{g}$ are significantly smaller. We find that a maximum of $\simeq~3\%$ of the coronal mass is cool clumps. We conclude that coronal rain may be common in solar like stars, but may appear on much larger scales in rapid rotators.

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arXiv2023

3D MHD models of the centrifugal magnetosphere from a massive star with an oblique dipole field

We present results from new self-consistent 3D MHD simulations of the magnetospheres from massive stars with a dipole magnetic axis that has a non-zero obliquity angle ($\beta$) to the star's rotation axis. As an initial direct application, we compare the global structure of co-rotating disks for nearly aligned ($\beta=5^o$) versus half-oblique ($\beta=45^o$) models, both with moderately rapid rotation ($\sim$ 0.5 critical). We find that accumulation surfaces broadly resemble the forms predicted by the analytic Rigidly Rotating Magnetosphere (RRM) model, but the mass buildup to near the critical level for centrifugal breakout against magnetic confinement distorts the field from the imposed initial dipole. This leads to an associated warping of the accumulation surface toward the rotational equator, with the highest density concentrated in {\em wings} centered on the intersection between the magnetic and rotational equators. These MHD models can be used to synthesize rotational modulation of photometric absorption and H$\alpha$ emission for a direct comparison with observations.

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arXiv2021

Idealised 3D simulations of diabatically-forced Ledoux convection. Application to the atmosphere of hot rocky exoplanets

We investigate the impact on convective numerical simulations of thermo-compositional diabatic processes. We focus our study on simulations with a stabilizing temperature gradient and a destabilizing mean-molecular weight gradient. We aim to establish the possibility for a reduced temperature-gradient in such setups. A suite of 3D simulations were conducted using a numerical hydrodynamic code. We used as a simplified test case, a sample region of the secondary atmosphere of a hot rocky exoplanet within which the chemical transition CO+O $\leftrightarrow$ CO$_{2}$ could occur. Newtonian cooling and a chemical source term was used to maintain a negative mean molecular weight gradient. Our results demonstrate that this setup can reduce the temperature gradient, a result which does not converge away with resolution or over time. We also show that the presence of the reduced temperature gradient is a function of the forcing timescales. The above transition leads to a bifurcation of the temperature profile when the chemical forcing is fast, reminiscent of the bifurcation seen in the boiling crisis for steam/liquid convection. With the reduced temperature gradient in these idealized setups, there exists the possibility for an analogy of the reddening (currently observed in the spectra of brown dwarfs) in the spectra of rocky exoplanet atmospheres. Detailed 1D modelling is needed, in order to characterize the equilibrium thermal and compositional gradients, the timescales, and the impact of a realistic equation of state, in order to assess if the regime identified here will develop in realistic situations. This possibility cannot, however, be excluded a priori. This prediction is new for terrestrial atmospheres and represents strong motivation for the use of diabatic models when analysing atmospheric spectra of rocky exoplanets that will be observed with e.g. the James Webb Space Telescope.

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arXiv2019

3D MHD simulations and synthetic radio emission from an oblique rotating magnetic massive star

We have performed 3D isothermal MHD simulation of a magnetic rotating massive star with a non-zero dipole obliquity and predicted the radio/sub-mm observable lightcurves and continuum spectra for a frequency range compatible with ALMA. From these results we also compare the model input mass-loss to that calculated from the synthetic thermal emission. Spherical and cylindrical symmetry is broken due to the obliquity of the stellar magnetic dipole resulting in an inclination and phase dependence of both the spectral flux and inferred mass-loss rate, providing testable predictions of variability for oblique rotator. Both quantities vary by factors between 2 and 3 over a full rotational period of the star, demonstrating that the role of rotation as critical in understanding the emission. This illustrates the divergence from a symmetric wind, resulting in a two armed spiral structure indicative of a oblique magnetic rotator. We show that a constant spectral index, $α$, model agrees well with our numerical prediction for a spherical wind for $ν~<~10^{3} \ \mathrm{GHz}$, however it is unable to capture the behavior of emission at $ν~>~10^{3} \ \mathrm{GHz}$. As such we caution the use of such constant $α$ models for predicting emission from non-spherical winds such as those which form around magnetic massive stars.

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arXiv2018

Hot Jupiter accretion: 3D MHD simulations of star-planet wind interaction

We present 3D MHD simulations of the wind-wind interactions between a solar type star and a short period hot Jupiter exoplanet. This is the first such simulation in which the stellar surface evolution is studied in detail. In our simulations, a planetary outflow, based on models of FUV evaporation of the exoplanets upper atmosphere, results in the build-up of circumstellar and circumplanetary material which accretes onto the stellar surface in a form of coronal rain, in which the rain is HJ wind material falling onto the stellar surface. We have conducted a suite of mixed geometry high resolution simulations which characterise the behaviour of interacting stellar and planetary wind material for a representative HJ hosting system. Our results show that magnetic topology plays a central role in forming accretion streams between the star and HJ and that the nature of the accretion is variable both in location and in rate, with the final accretion point occurring at $ϕ~=~227^{\circ}$ ahead of the sub-planetary point and $θ~=~53^{\circ}$ below the orbital plain. The size of the accretion spot itself has been found to vary with a period of $67 \ \mathrm{ks}$. Within the accretion spot, there is a small decrease in temperature accompanied by an increase in density compared with ambient surface conditions. We also demonstrate that magnetic fields cannot be ignored as accretion is highly dependent upon the magnetic topology of both the HJ and the host. We characterise this behaviour as Star Planet Wind Interaction (SPWI)

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arXiv2018

Inhibition of the electron cyclotron maser instability in the dense magnetosphere of a hot Jupiter

Hot Jupiter (HJ) type exoplanets are expected to produce strong radio emission in the MHz range via the Electron Cyclotron Maser Instability (ECMI). To date, no repeatable detections have been made. To explain the absence of observational results, we conduct 3D adaptive mess refinement (AMR) magnetohydrodynamic (MHD) simulations of the magnetic interactions between a solar type star and HJ using the publicly available code PLUTO. The results are used to calculate the efficiency of the ECMI at producing detectable radio emission from the planets magnetosphere. We also calculate the frequency of the ECMI emission, providing an upper and lower bounds, placing it at the limits of detectability due to Earth's ionospheric cutoff of $\sim 10 \ \mathrm{MHz}$. The incident kinetic and magnetic power available to the ECMI is also determined and a flux of $0.069 \ \mathrm{mJy}$ for an observer at $10 \ \mathrm{pc}$ is calculated. The magnetosphere is also characterized and an analysis of the bow shock which forms upstream of the planet is conducted. This shock corresponds to the thin shell model for a colliding wind system. A result consistent with a colliding wind system. The simulation results show that the ECMI process is completely inhibited by the planets expanding atmosphere, due to absorption of UV radiation form the host star. The density, velocity, temperature and magnetic field of the planetary wind are found to result in a magnetosphere where the plasma frequency is raised above that due to the ECMI process making the planet undetectable at radio MHz frequencies.

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arXiv2017

Winds of Massive Magnetic Stars: Interacting Fields and Flow

We present results of 3D numerical simulations of magnetically confined, radiatively driven stellar winds of massive stars, conducted using the astrophysical MHD code Pluto, with a focus on understanding the rotational variability of radio and sub-mm emission. Radiative driving is implemented according to the Castor, Abbott and Klein theory of radiatively driven winds. Many magnetic massive stars posses a magnetic axis which is inclined with respect to the rotational axis. This misalignment leads to a complex wind structure as magnetic confinement, centrifugal acceleration and radiative driving act to channel the circumstellar plasma into a warped disk whose observable properties should be apparent in multiple wavelengths. This structure is analysed to calculate free-free thermal radio emission and determine the characteristic intensity maps and radio light curves.

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arXiv2017

Interacting Fields and Flows: Magnetic Hot Jupiters

We present Magnetohydrodynamic (MHD) simulations of the magnetic interactions between a solar type star and short period hot Jupiter exoplanets, using the publicly available MHD code PLUTO. It has been predicted that emission due to magnetic interactions such as the electron cyclotron maser instability (ECMI) will be observable. In our simulations, a planetary outflow, due to UV evaporation of the exoplanets atmosphere, results in the build-up of circumplanetary material. We predict the ECMI emission and determine that the emission is prevented from escaping from the system. This is due to the evaporated material leading to a high plasma frequency in the vicinity of the planet, which inhibits the ECMI process.

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