arXiv subjects

Richard Alexander

Publications and source records attributed to Richard Alexander.

At least 19 recordsLinked to original sources

arXiv2026

JWST/MIRI Reveals the Evolution from Molecular to Atomic Disk Winds

The evolution and dispersal of protoplanetary disks--governed by accretion, magnetically launched jets and winds, and photoevaporative winds--fundamentally shape planetary systems. Determining how these mass-loss processes co-evolve is crucial for constraining planet formation pathways. We analyze archival JWST/MIRI/IFU data of 72 inclined (i>40deg) mostly ClassII disks to identify and characterize spatially resolved jets and winds, focusing on [NeII] and H2 lines. Extended emission in H2 S(1), S(3), S(5), S(7) and/or [NeII] is detected toward 66 disks, revealing diverse morphologies. We develop a framework to identify conical H2 winds and high-velocity [NeII] jets perpendicular to the disk, detecting them toward 46 and 40 disks, respectively. All sources with [NeII] jets exhibit a corresponding wind traced in either H2 (85%) or [OI], establishing a connection between jets and winds. The detection fractions of [NeII]-jets and H2-winds correlate positively with mass accretion rate, with no dependence on disk inclination or stellar mass. Conversely, marginally resolved low-velocity [NeII] winds are found preferentially toward lower accretors. Among sources with H2 winds, detection of hotter winds traced by S(7) and S(5) declines more rapidly with decreasing accretion rate than the colder S(1) component. Comparison with high-resolution [OI]6300\text{\AA} spectroscopy reveals [OI] LVC and extended H2 wind detections preferentially toward moderate-to-high accretors (>~10^{-8.5}~Msun/yr), whereas lower accretors exhibit only [OI] and [NeII] winds. Together, these results indicate that atomic jets and atomic+molecular winds, consistent with an MHD disk-wind origin, dominate during early, actively accreting disk phases, while at lower accretion rates, jets weaken and winds become predominantly atomic.

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arXiv2026

Revisiting the picture of circumbinary disc truncation

Circumbinary discs are observed to develop central cavities carved by the gravitational influence of the binary. Analytical estimates of cavity sizes predict truncation at $\sim 2 \textrm{--} 3$ binary separations, depending on the binary properties. However, numerical studies show only qualitative agreement with these predictions: cavity sizes often evolve on long timescales and can exceed substantially the analytically predicted values. In this work, we revise this paradigm, suggesting that tidal truncation in circumbinary discs responds to additional dynamical parameters that have so far been neglected. We analyse a suite of 80 numerical simulations of circumbinary discs to re-examine the physical mechanism responsible for cavity truncation and to provide a prescription for the cavity size independent of the state of evolution of the system. We find that truncation depends not only on the binary parameters $a_{\rm bin}$, $e_{\rm bin}$, and mass ratio $q$, but also on the instantaneous cavity eccentricity $e_{\rm cav}$ and the relative apsidal orientation $\varpi_{\rm bin}-\varpi_{\rm cav}$. These quantities jointly determine the pericentre of the innermost stable disc orbit $R_{\rm p}$, in a way that shares some similarities with orbital stability in the restricted three body problem. Hydrodynamical effects introduce secondary corrections, with the disc scale height $H$ and viscosity $\alpha$ mildly shifting the cavity edge relative to the purely gravitational prediction. We introduce a semi-analytical prescription that captures these dependences for $R_{\rm p}$ and cavity semi-major axis $a_{\rm cav}$. We conclude that cavity truncation for binaries with mass ratios $q>0.05$ is a process where the instantaneous orbital properties of the disc ($e_{\rm cav}$, $\varpi_{\rm cav}$) play a fundamental role and should be taken into account to accurately evaluate the truncation efficiency.

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arXiv2025

Turbulence and dust fragility in protoplanetary discs

Dust growth from micron- to planet-size in protoplanetary discs involves multiple physical processes, including dust collisions, the streaming instability, and pebble accretion. Disc turbulence and dust fragility matter at almost every stage. Previous studies typically vary one of them while fixing the other, failing to provide a complete picture. Here, we use analytical models and numerical dust evolution models DustPy to study the combinations of gas turbulence and dust fragility that can reproduce multi-wavelength ALMA observables. We find that only appropriate combinations -- fragile dust (fragmentation velocity $v_\mathrm{frag}$= 1-2 m/s) in discs with viscous $\alpha=10^{-4}$ or resilient dust ($v_\mathrm{frag}$= 6-10 m/s) in discs with viscous $\alpha=10^{-3}$ -- can reproduce observations. Our result is robust to two widely used opacities (DSHARP and Ricci opacities). Regardless of the strength of disc turbulence, reproducing observations requires observed dust rings to be optically thick at $\lambda=1.3$ and $3$ mm. As only small dust can be lifted above the midplane to reach the emitting layers, SED analysis probably yields lower limits on the maximum grain sizes. We highlight the challenge of creating detectable dust rings at large radii when incorporating bouncing in models, and the need for earlier formation of dust rings at smaller radii to reproduce the decreasing ring brightness with radius observed across ALMA wavelengths.

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arXiv2025

Leading & Trailing Spiral Arms in a Nearly Broken Protoplanetary Disc

We perform three-dimensional smoothed particle hydrodynamics simulations to investigate the formation of spiral arms in misaligned circumbinary discs. In a nearly broken disc the misaligned inner and outer discs interact at two nodes, launching leading spiral arms that do not rotate with the disc. These spirals vanish when the disc is fully broken or aligned. Our results show that the formation of leading spirals is driven by the relative misalignment of the inner and outer disc, and does not depend on the disc physics. With live radiative transfer, the shadows cast by the misaligned inner disc are also able to launch trailing spiral arms that only appear at high misalignments when the discs are disconnected. When the disc is strongly misaligned, leading and trailing spiral arms can both appear and interact with each other. At lower misalignments, the impact of shadows is negligible and leading spiral arms are seen instead. The presence of both leading and trailing spiral arms implies that the rotation of the disc cannot be assumed based on the orientation of the spiral arms alone. Unlike spirals formed by gravitational instability, the spirals in this work can also form in low-mass, gravitationally stable discs.

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arXiv2025

The ALMA Survey of Gas Evolution of PROtoplanetary Disks (AGE-PRO): VII. Testing accretion mechanisms from disk population synthesis

The architecture of planetary systems depends on the evolution of the disks in which they form. In this work, we develop a population synthesis approach to interpret the AGE-PRO measurements of disk gas mass and size considering two scenarios: turbulence-driven evolution with photoevaporative winds and MHD disk-wind-driven evolution. A systematic method is proposed to constrain the distribution of disk parameters from the disk fractions, accretion rates, disk gas masses, and CO gas sizes. We find that turbulence-driven accretion with initially compact disks ($R_0 \simeq 5-20~$au), low mass-loss rates, and relatively long viscous timescales ($t_{\nu,0} \simeq 0.4-3~$Myr or $\alpha_{SS} \simeq 2-4 \times 10^{-4}$) can reproduce the disk fraction and gas sizes. However, the distribution of apparent disk lifetime defined as the $M_D/\dot{M}_*$ ratio is severely overestimated by turbulence-driven models. On the other hand, MHD wind-driven accretion can reproduce the bulk properties of the disk populations from Ophiuchus to Upper Sco assuming compact disks with an initial magnetization of about $\beta \simeq 10^5$ ($\alpha_{DW} \simeq 0.5-1 \times 10^{-3}$) and a magnetic field that declines with time. More studies are needed to confirm the low masses found by AGE-PRO, notably for compact disks that question turbulence-driven accretion. The constrained synthetic disk populations can now be used for realistic planet population models to interpret the properties of planetary systems on a statistical basis.

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arXiv2025

Capture and Escape of Planetary Mean-motion Resonances in Turbulent Discs

Mean-motion resonances (MMRs) form through convergent disc migration of planet pairs, which may be disrupted by dynamical instabilities after protoplanetary disc (PPD) dispersal. This scenario is supported by recent analysis of TESS data showing that neighboring planet pairs in younger planetary systems are closer to resonance. To study stability of MMRs during migration, we perform hydrodynamical simulations of migrating planet pairs in PPDs, comparing the effect of laminar viscosity and realistic turbulence. We find stable 3:2 resonance capture for terrestrial planet pairs migrating in a moderately massive PPD, insensitive to a range of laminar viscosity (alpha = 0.001 to 0.1). However, realistic turbulence enhances overstability by sustaining higher equilibrium eccentricities and a positive growth rate in libration amplitude, ultimately leading to resonance escape. The equilibrium eccentricity growth rates decrease as planets migrate into tighter and more stable 4:3 and 5:4 MMRs. Our results suggest that active disc turbulence broadens the parameter space for overstability, causing planet pairs to end up in closer-in orbital separations. Libration within MMR typically lead to deviation from exact period ratio |Delta| \sim 0.5%, which alone is insufficient to produce the typical dispersion of |Delta| \sim 1 to 3% in TESS data, suggesting that post migration dynamical processes are needed to further amplify the offset.

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arXiv2025

Disc-planet misalignment from an unstable triple system: IRAS04125

The IRAS01425+2902 wide binary system was recently reported to have both a young planet and a puzzling geometric arrangement, where the planet and binary both orbit edge-on, but misaligned by 60 deg to the circumprimary disc. This is the youngest transiting planet yet to be detected but its misalignment to the disc is difficult to explain. In this paper we explore the dissolution of an unstable triple system as a potential mechanism to produce this system. We simulate the effects of an ejection interaction in models using a highly inclined, retrograde flyby centred on the primary star of IRAS01425. The escaping star of ~0.35 solar masses inclines both the disc and binary orbits such that they have a relative misalignment of greater than 60 deg, as inferred from observations. The planet orbit also becomes inclined relative to the disc, and our interpretation predicts that the binary should have a highly eccentric orbit (e > 0.5 from our simulations). We additionally demonstrate that despite the high relative misalignment of the disc it is unlikely to be vulnerable to von Zeipel-Kozai-Lidov oscillations.

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arXiv2025

Evidence of a Disk-wind Origin for Fluorescent H$_2$ in Classical T Tauri Stars

We use FUV spectra of 36 T Tauri stars, predominately from $\textit{Hubble Space Telescope}$'s ULLYSES program, to examine the kinematic properties of fluorescent H$_2$ emission lines for evidence of disk outflows. Leveraging improvements to the $\textit{HST}$-COS wavelength solution, we co-add isolated lines within four fluorescent progressions ([$\textit{v'}$,$\textit{J'}$] = [1,4], [1,7], [0,2], and [3,16]) to improve signal-to-noise, and we fit each co-added line profile with one or two Gaussian components. Of the high S/N line profiles (S/N $\geq$ 12 at the peak of the profile), over half are best fit with a combination of a broad and narrow Gaussian component. For profiles of the [1,4] and [1,7] progressions, we find a systematic blue-shift of a few km s$^{-1}$ between the broad and narrow centroid velocities and stellar radial velocities. For the [0,2] progression, we find centroid velocities consistently blueshifted with respect to stellar radial velocities on the order of -5 km s$^{-1}$ for the single and narrow components, and -10 km s$^{-1}$ for the broad components. Overall, the blueshifts observed in our sample suggest that the molecular gas traces an outflow from a disk wind in some sources, and not solely disk gas in Keplerian rotation. The low-velocity systematic blue-shifts, and emitting radii as inferred from line FWHMs, observed in our sample are similar to those observed with optical [O I] surveys of T Tauri stars. We estimate H$_2$ mass-loss rates of 10$^{-9}$ to 10$^{-11}$ $M_{\odot}$ yr$^{-1}$, but incomplete knowledge of wind parameters limits comparisons to global models.

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arXiv2025

Compact protoplanetary discs can be produced by dead zones

Radially compact protoplanetary discs (<=50 au) are ubiquitous in nearby star-forming regions. Multiple mechanisms have been invoked to interpret various compact discs. In this paper, we propose that fragmentation of fragile dust grains in moderate turbulence, as expected beyond the dead zone, provides an effective alternative mechanism to form compact discs which are consistent with current observations. We run 1-D dust transport and collision models with DustPy and generate synthetic observations, and find that discs formed by this mechanism have sizes determined by the extent of their dead zones. Accounting for dust porosity, and considering less fragile dust, do not change disc sizes significantly. The smooth dust morphology can be altered only when pressure bumps are present in the dead zone. However, when present at small radii (<=10 au), pressure bumps cannot effectively trap dust. Dust in these bumps fragments and replenishes the inner discs, effectively hiding dust traps in the optically thick inner disc from observations. We note a striking resemblance in the radial intensity profile between our synthetic observations and some recent high-resolution observations of compact discs. We discuss how such observations can inform our understanding of the underlying disc physics.

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arXiv2024

JWST captures a sudden stellar outburst and inner disk wall destruction

We present JWST/MIRI observations of T~Cha, a highly variable ($\Delta V \sim$3-5\,mag) accreting Sun-like star surrounded by a disk with a large ($\sim 15$\,au) dust gap. We find that the JWST mid-infrared spectrum is signiticantly different from the {\it Spitzer} spectrum obtained 17 years before, where the emission at short wavelengths ($5-10 \mu m$) has decreased by $\sim 2/3$ while at longer wavelengths ($15-25 \mu m$) it increased by up to a factor of $\sim 3$. This 'seesaw' behavior is contemporary with a fairly constant higher optical emission captured by the All Sky Automated Survey. By analyzing and modelling both SEDs, we propose that JWST caught the star during an outburst that destructed the asymmetric inner disk wall responsible for the high optical variability and lower $15-25$\,micron\ emission during the {\it Spitzer} time. The dust mass lost during this outburst is estimated to be comparable ($\sim 1/5$) to the upper limit of the total micron-sized dust mass in the inner disk of T~Cha now. Monitoring this system during possible future outbursts and more observations of its quiescent state will reveal if the inner disk can be replenished or will continue to be depleted and vanish.

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arXiv2024

Short-Lived Gravitational Instability in Isolated Irradiated Discs

Irradiation from the central star controls the temperature structure in protoplanetary discs. Yet simulations of gravitational instability typically use models of stellar irradiation with varying complexity, or ignore it altogether, assuming heat generated by spiral shocks is balanced by cooling, leading to a self-regulated state. In this paper, we perform simulations of irradiated, gravitationally unstable protoplanetary discs using 3D hydrodynamics coupled with live Monte-Carlo radiative transfer. We find that the resulting temperature profile is approximately constant in time, since the thermal effects of the star dominate. Hence, the disc cannot regulate gravitational instabilities by adjusting the temperatures in the disc. In a 0.1 Solar mass disc, the disc instead adjusts by angular momentum transport induced by the spiral arms, leading to steadily decreasing surface density, and hence quenching of the instability. Thus, strong spiral arms caused by self-gravity would not persist for longer than ten thousand years in the absence of fresh infall, although weak spiral structures remain present over longer timescales. Using synthetic images at 1.3mm, we find that spirals formed in irradiated discs are challenging to detect. In higher mass discs, we find that fragmentation is likely because the dominant stellar irradiation overwhelms the stabilising influence of PdV work and shock heating in the spiral arms.

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arXiv2024

A question of personalities: evolution of viscous and wind-driven protoplanetary discs in the presence of dead zones

Whether the angular momentum of protoplanetary discs is redistributed by viscosity or extracted by magnetised winds is a long-standing question. Demographic indicators, such as gas disc sizes and stellar accretion rates, have been proposed as ways of distinguishing between these two mechanisms. In this paper, we implement one-dimensional gas simulations to study the evolution of "hybrid" protoplanetary discs simultaneously driven by viscosity and magnetised winds, with dead zones present. We explore how the variations of disc properties, including initial disc sizes, dead zone sizes and angular momentum transport efficiency, affect stellar accretion rates, disc surface density profiles, disc sizes, disc lifetimes, and cumulative mass loss by different processes. Our models show that the expansion of the gas disc size can be sustained when the majority of angular momentum is removed by the magnetised wind for individual protoplanetary discs. However, when we can only observe discs via demographic screenshots, the variation of disc sizes with time is possibly diminished by the disc "personalities", by which we mean the variations of initial disc properties among different discs. Our "hybrid" models re-assess association of the two demographic indicators with mechanisms responsible for angular momentum transport and suggest additional diagnostics are required to assist the differentiation.

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arXiv2024

The PLATO Mission

PLATO (PLAnetary Transits and Oscillations of stars) is ESA's M3 mission designed to detect and characterise extrasolar planets and perform asteroseismic monitoring of a large number of stars. PLATO will detect small planets (down to <2 R_(Earth)) around bright stars (<11 mag), including terrestrial planets in the habitable zone of solar-like stars. With the complement of radial velocity observations from the ground, planets will be characterised for their radius, mass, and age with high accuracy (5 %, 10 %, 10 % for an Earth-Sun combination respectively). PLATO will provide us with a large-scale catalogue of well-characterised small planets up to intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and to better understand planet evolution. It will make possible comparative exoplanetology to place our Solar System planets in a broader context. In parallel, PLATO will study (host) stars using asteroseismology, allowing us to determine the stellar properties with high accuracy, substantially enhancing our knowledge of stellar structure and evolution. The payload instrument consists of 26 cameras with 12cm aperture each. For at least four years, the mission will perform high-precision photometric measurements. Here we review the science objectives, present PLATO's target samples and fields, provide an overview of expected core science performance as well as a description of the instrument and the mission profile at the beginning of the serial production of the flight cameras. PLATO is scheduled for a launch date end 2026. This overview therefore provides a summary of the mission to the community in preparation of the upcoming operational phases.

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arXiv2024

X-ray and UV radiation in the planet-forming T-Tauri system PDS 70. Signs of accretion and coronal activity

Planet formation takes place in protoplanetary discs around young T-Tauri stars. PDS 70 is one of the first confirmed examples of a system where the planets are currently forming in gaps in the disc, and can be directly imaged. One of the main early influences on planet formation is the lifetime of the protoplanetary disk, which is limited by the intense stellar X-ray and UV radiation. Stellar coronal activity and accretion of material onto the star are both potential sources of XUV radiation. Previous \textit{Swift} observations detected UV emission, which were consistent with a low rate of accretion. We present follow up observations with the XMM-Newton observatory, which observed PDS 70 simultaneously in X-ray and UV in order to determine intensity of XUV radiation in the system, and identify if the source is coronal, accretion, or both. We detect a strong source in both X-ray and UV, with an average X-ray 0.2-12 keV luminosity of $1.37\times10^{30}\ \mathrm{erg\ s}^{-1}$, and a possible flare which increased the luminosity to $2.8\times10^{30}\ \mathrm{erg\ s}^{-1}$. The UV flux density is in excess of what would be expected from chromospheric emission, and supports the interpretation that PDS 70 has continuing weak accretion less than $\sim10^{-10}\ \mathrm{M_{\odot}\ yr^{-1}}$. The implications of the detected X-ray and UV radiation are that the disc is likely to be in the final stages of dispersal, and will be completely evaporated in the next million years, bringing an end to the primary planet formation process.

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arXiv2024

Modeling JWST MIRI-MRS Observations of T Cha: Mid-IR Noble Gas Emission Tracing a Dense Disk Wind

[Ne II] 12.81 $\mu\mathrm{m}$ emission is a well-used tracer of protoplanetary disk winds due to its blueshifted line profile. MIRI-MRS recently observed T Cha, detecting this line along with lines of [Ne III], [Ar II] and [Ar III], with the [Ne II] and [Ne III] lines found to be extended while the [Ar II] was not. In this complementary work, we use these lines to address long-debated questions about protoplanetary disk winds regarding their mass-loss rate, the origin of their ionization, and the role of magnetically-driven winds as opposed to photoevaporation. To this end, we perform photoionization radiative transfer on simple hydrodynamic wind models to map the line emission. We compare the integrated model luminosities to those observed with MIRI-MRS to identify which models most closely reproduce the data and produce synthetic images from these to understand what information is captured by measurements of the line extents. Along with the low degree of ionization implied by the line ratios, the relative compactness of [Ar II] compared to [Ne II] is particularly constraining. This requires Ne II production by hard X-rays and Ar II production by soft X-rays (and/or EUV) in an extended ($\gtrsim 10$ au) wind that is shielded from soft X-rays - necessitating a dense wind with material launched on scales down to ~1 au. Such conditions could be produced by photoevaporation, whereas an extended MHD wind producing equal shielding would likely underpredict the line fluxes. However, a tenuous inner MHD wind may still contribute to shielding the extended wind. This picture is consistent with constraints from spectrally-resolved line profiles.

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arXiv2024

JWST MIRI/MRS Observations of T Cha: Discovery of a Spatially Resolved Disk Wind

Understanding when and how circumstellar disks disperse is crucial to constrain planet formation and migration. Thermal winds powered by high-energy stellar photons have long been theorized to drive disk dispersal. However, evidence for these winds is currently based only on small (~3-6 km/s) blue-shifts in [Ne II] 12.81 um lines, which does not exclude MHD winds. We report JWST MIRI MRS spectro-imaging of T Cha, a disk with a large dust gap (~30 au in radius) and blue-shifted [Ne II] emission. We detect four forbidden noble gas lines, [Ar II], [Ar III], [Ne II], and [Ne III], of which [Ar III] is the first detection in any protoplanetary disk. We use line flux ratios to constrain the energy of the ionizing photons and find that Argon is ionized by EUV whereas Neon is most likely ionized by X-rays. After performing continuum and Point Spread Function (PSF) subtraction on the IFU cube, we discover a spatial extension in the [Ne II] emission off the disk continuum emission. This is the first spatially resolved [Ne II] disk wind emission. The mostly ionic spectrum of T Cha, in combination with the extended [Ne II] emission, points to an evolved stage for any inner MHD wind and is consistent with the existence of an outer thermal wind ionized and driven by high-energy stellar photons. This work acts as a pathfinder for future observations aiming at investigating disk dispersal using JWST.

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arXiv2024

The Role of Drag and Gravity on Dust Concentration in a Gravitationally Unstable Disc

We carry out three dimensional smoothed particle hydrodynamics simulations to study the role of gravitational and drag forces on the concentration of large dust grains (St > 1) in the spiral arms of gravitationally unstable protoplanetary discs, and the resulting implications for planet formation. We find that both drag and gravity play an important role in the evolution of large dust grains. If we include both, grains that would otherwise be partially decoupled will become well coupled and trace the spirals. For the dust grains most influenced by drag (with Stokes numbers near unity), the dust disc quickly becomes gravitationally unstable and rapidly forms clumps with masses between 0.15 - 6 Earth masses. A large fraction of clumps are below the threshold where runaway gas accretion can occur. However, if dust self-gravity is neglected, the dust is unable to form clumps, despite still becoming trapped in the gas spirals. When large dust grains are unable to feel either gas gravity or drag, the dust is unable to trace the gas spirals. Hence, full physics is needed to properly simulate dust in gravitationally unstable discs. Dust trapping of large grains in spiral arms of discs stable to gas fragmentation could explain planet formation in very young discs by a population of planetesimals formed due to the combined roles of drag and gravity in the earliest stages of a disc's evolution. Furthermore, it highlights that gravitationally unstable discs are not just important for forming gas giants quickly, it can also rapidly form Earth mass bodies.

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arXiv2023

Lyman-alpha Scattering Models Trace Accretion and Outflow Kinematics in T Tauri Systems

T Tauri stars produce broad Lyman-alpha emission lines that contribute $\sim$88% of the total UV flux incident on the inner circumstellar disks. Lyman-alpha photons are generated at the accretion shocks and in the protostellar chromospheres and must travel through accretion flows, winds and jets, the protoplanetary disks, and the interstellar medium before reaching the observer. This trajectory produces asymmetric, double-peaked features that carry kinematic and opacity signatures of the disk environments. To understand the link between the evolution of Lyman-alpha emission lines and the disks themselves, we model HST-COS spectra from targets included in Data Release 3 of the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program. We find that resonant scattering in a simple spherical expanding shell is able to reproduce the high velocity emission line wings, providing estimates of the average velocities within the bulk intervening H I. The model velocities are significantly correlated with the K band veiling, indicating a turnover from Lyman-alpha profiles absorbed by outflowing winds to emission lines suppressed by accretion flows as the hot inner disk is depleted. Just 30% of targets in our sample have profiles with red-shifted absorption from accretion flows, many of which have resolved dust gaps. At this stage, Lyman-alpha photons may no longer intersect with disk winds along the path to the observer. Our results point to a significant evolution of Lyman-alpha irradiation within the gas disks over time, which may lead to chemical differences that are observable with ALMA and JWST.

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