arXiv subjects

Thomas J. Haworth

Publications and source records attributed to Thomas J. Haworth.

At least 37 records · Page 2Linked to original sources

arXiv2025

XUE 10. The CO2-rich terrestrial planet-forming region of an externally irradiated Herbig disk

We investigate the James Webb Space Telescope (JWST) MIRI MRS gas molecular content of an externally irradiated Herbig disk, the F-type XUE 10 source, in the context of the eXtreme UV Environments (XUE) program. XUE 10 belongs to the massive star cluster NGC 6357 (1.69 kpc), where it is exposed to an external far-ultraviolet (FUV) radiation $\approx$ 10$^3$ times stronger than in the Solar neighborhood. We modeled the molecular features in the mid-infrared spectrum with Local Thermodynamic Equilibrium (LTE) 0D slab models. We derived basic parameters of the stellar host from a VLT FORS2 optical spectrum using PHOENIX stellar templates. We detect bright CO2 gas with the first simultaneous detection (> 5$σ$) of four isotopologues (12CO2, 13CO2, 16O12C18O, 16O12C17O) in a protoplanetary disk. We also detect faint CO emission (2$σ$) and the HI Pf$α$ line (8$σ$). We also place strict upper limits on the water content, finding a total column density $\lesssim$ 10$^{18}$ cm$^{-2}$. The CO2 species trace low gas temperatures (300-370 K) with a range of column densities of 7.4 $\times$ 10$^{17}$ cm$^{-2}$ (16O12C17O)-1.3 $\times$ 10$^{20}$ cm$^{-2}$ (12CO2) in an equivalent emitting radius of 1.15 au. The emission of 13CO2 is likely affected by line optical depth effects. 16O12C18O and 16O12C17O abundances may be isotopically anomalous compared to the 16O/18O and 16O/17O ratios measured in the interstellar medium and the Solar System. We propose that the mid-infrared spectrum of XUE 10 is explained by H2O removal either via advection or strong photo-dissociation by stellar UV irradiation, and enhanced local CO2 gas-phase production. Outer disk truncation supports the observed CO2-H2O dichotomy. A CO2 vapor enrichment in 18O and 17O can be explained by means of external UV irradiation and early on (10$^{4-5}$ yr) delivery of isotopically anomalous water ice to the inner disk.

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arXiv2025

Line ratio identification of external photoevaporation

External photoevaporation of protoplanetary discs, by massive O stars in stellar clusters, is thought to be a significant process in the evolution of a disc. It has been shown to result in significant mass loss and disc truncation, ultimately reducing the lifetime of the discs, and possibly affecting potential planet populations. It is a well-studied process in the Orion Nebula Cluster (ONC) where the cometary morphology of proplyds is spatially resolvable due to its proximity to Earth. However, we need to study external photoevaporation in additional stellar clusters to better understand its prevalence and significance more globally. Unfortunately, more massive stellar clusters where the majority of stars form are much farther away than the ONC. In these more distant clusters the proplyds are spatially unresolvable with current facilities, hence the cometary morphology is not a useful identification of external photoevaporation. Therefore, in order to identify and interpret external photoevaporation, the only observations we have are of spatially unresolved emission lines. To resolve this issue we have used the CLOUDY code to develop an approximate general model of the emission lines emanating from the hot ionized wind of a proplyd. We have used the model to determine which line ratios are most sensitive to the distance from an OB star, and found that the most sensitive line ratios vary by multiple orders of magnitude over an FUV field of between 10$^3$ G$_0$ to 10$^6$ G$_0$. By identifying spatial gradients of line ratios in stellar clusters, we can identify regions of ongoing external photoevaporation.

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arXiv2025

Solar C/O ratio in planet-forming gas at 1 au in a highly irradiated disk

The chemical composition of exoplanets is thought to be influenced by the composition of the disks in which they form. JWST observations have unveiled a variety of species in numerous nearby disks, showing significant variations in the C/O abundance ratio. However, little is known about the composition and C/O ratio of disks around young stars in clusters exposed to strong ultraviolet (UV) radiation from nearby massive stars, which are representative of the environments where most planetary systems form, including ours. We present JWST spectroscopy of d203-504, a young 0.7 $\rm M_{\odot}$ star in the Orion Nebula with a 30 au disk irradiated by nearby massive stars. These observations reveal spectroscopic signatures of CO, H$_2$O, CH$_3^+$, and PAHs. Water and CO are detected in absorption in the inner disk ($r\lesssim 1$ au), where the estimated gas-phase C/O ratio is 0.48, consistent with the Solar value and that of the Orion Nebula. In contrast, \ch{CH3+} and PAHs are found in the extended surface layers of the disk. These results suggest that gas in the inner disk is chemically shielded from UV radiation while the surface layers of the disk experience UV-induced chemistry, potentially depleting their carbon content.

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arXiv2025

The observable impact of runaway OB stars on protoplanetary discs

UV radiation from OB stars can drive ``external'' photoevaporative winds from discs in clusters, that have been shown to be important for disc evolution and planet formation. However, cluster dynamics can complicate the interpretation of this process. A significant fraction of OB stars are runaways, propagating at high velocity which might dominate over the wider cluster dynamics in setting the time variation of the UV field in part of the cluster. We explore the impact of a runaway OB star on discs and the observational impact that may have. We find that discs exposed to even short periods of strong irradiation are significantly truncated, and only rebound slightly following the ``flyby'' of the UV source. This is predicted to leave an observable imprint on a disc population, with those downstream of the OB star vector being more massive and extended than those upstream. Because external photoevaporation acts quickly, this imprint is less susceptible to being washed out by cluster dynamics for faster runaway OB stars. The Gaia proper motion vector of the B star 42 Ori in NGC 1977 is transverse to the low mass stellar population and so may make a good region to search for this signature in resolved disc observations.

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arXiv2025

Searching for star formation towards the Eos molecular cloud

The Eos cloud, recently discovered in the far ultraviolet via H$_2$ fluorescence, is one of the nearest known dark molecular clouds to the Sun, with a distance spanning from $\sim94-136$pc. However, with a mass ($\sim5.5\times10^3$M$_\odot$) just under $40$ per cent that of star forming clouds like Taurus and evidence for net molecular dissociation, its evolutionary and star forming status is uncertain. We use Gaia data to investigate whether there is evidence for a young stellar population that may have formed from the Eos cloud. Comparing isochrones and pre-main sequence evolutionary models there is no clear young stellar population in the region. While there are a small number of $<10$Myr stars, that population is statistically indistinguishable from those in similar search volumes at other Galactic latitudes. We also find no unusual spatial or kinematic clustering toward the Eos cloud over distances $70-150$pc. Overall we conclude that the Eos cloud has most likely not undergone any recent substantial star formation, and further study of the dynamics of the cloud is required to determine whether it will do so in the future.

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arXiv2025

The past, present and future of observations of externally irradiated disks

Recent years have seen a surge of interest in the community studying the effect of ultraviolet radiation environment, predominantly set by OB stars, on protoplanetary disc evolution and planet formation. This is important because a significant fraction of planetary systems, potentially including our own, formed in close proximity to OB stars. This is a rapidly developing field, with a broad range of observations across many regions recently obtained or recently scheduled. In this paper, stimulated by a series of workshops on the topic, we take stock of the current and upcoming observations. We discuss how the community can build on this recent success with future observations to make progress in answering the big questions of the field, with the broad goal of disentangling how external photoevaporation contributes to shaping the observed (exo)planet population. Both existing and future instruments offer numerous opportunities to make progress towards this goal.

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arXiv2025

A Nearby Dark Molecular Cloud in the Local Bubble Revealed via H$_2$ Fluorescence

A longstanding prediction in interstellar theory posits that significant quantities of molecular gas, crucial for star formation, may be undetected due to being ``dark" in commonly used molecular gas tracers, such as carbon monoxide. We report the discovery of Eos, the closest dark molecular cloud, located just 94 parsecs from the Sun. This cloud is the first molecular cloud ever to be identified using H$_2$ far ultra-violet (FUV) fluorescent line emission, which traces molecular gas at the boundary layers of star-forming and supernova remnant regions. The cloud edge is outlined along the high-latitude side of the North Polar Spur, a prominent x-ray/radio structure. Our distance estimate utilizes 3D dust maps, the absorption of the soft X-ray background, and hot gas tracers such as O\,{\sc vi}; these place the cloud at a distance consistent with the Local Bubble's surface. Using high-latitude CO maps we note a small amount (M$_{\rm{H}_2}\approx$20-40\,M$_\odot$) of CO-bright cold molecular gas, in contrast with the much larger estimate of the cloud's true molecular mass (M$_{\rm{H}_2}\approx3.4\times 10^3$\,M$_\odot$), indicating most of the cloud is CO-dark. Combining observational data with novel analytical models and simulations, we predict this cloud will photoevaporate in 5.7 million years, placing key constraints on the role of stellar feedback in shaping the closest star-forming regions to the Sun.

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arXiv2025

How the microphysical properties of external photoevaporation influence the global evolution of protoplanetary discs

External photoevaporation is one of the dominant mechanisms for mass loss from protoplanetary discs. However this mass loss is theoretically expected to depend upon the microphysical properties of protoplanetary discs, which are currently poorly constrained in observations. In this work we explore the impact of microphysics on the bulk evolution of discs. The polycyclic aromatic hydrocarbon (PAH) abundance, and the extent to which grain growth has occurred in the disc have profound effects on the strength of mass loss rates due to external photoevaporation, which in turn can have a significant impact on the disc evolution, impacting disc radii and accretion rates over time. The strongest sensitivity is to whether grain growth has occurred in the disc, which reduces the amount of dust entrained in the wind to shield the disc, thus increasing the rate at which gas is lost. Additionally, larger PAH abundances result in stronger heating and higher mass loss rates, but to a lesser extent than grain growth. We find that plausible variations in the PAH abundance and disc dust evolution can leave observable differences in disc populations. This work highlights the importance of obtaining observational constraints of the microphysical properties of protoplanetary discs. Future observations from JWST should soon be able to provide these constraints.

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arXiv2025

XUE. Thermochemical Modeling Suggests a Compact and Gas-Depleted Structure for a Distant, Irradiated Protoplanetary Disk

Unveiling the physical structure of protoplanetary disk is crucial for interpreting the diversity of the exoplanet population. Until recently, the census of the physical properties of protoplanetary disks probed by mid-infrared observations was limited to the solar neighborhood ($d \lesssim 250$ pc); however, nearby star-forming regions (SFRs) such as Taurus -- where no O-type stars reside -- are not representative of the environments where the majority of the planet formation occurs in the Galaxy. The James Webb Space Telescope (JWST) now enables observations of disks in distant high-mass SFRs, where strong external Far-Ultraviolet (FUV) radiation is expected to impact those disks. Nevertheless, a detailed characterization of externally irradiated disks is still lacking. We use the thermochemical code ProDiMo to model JWST/MIRI spectroscopy and archival visual/near-infrared photometry aiming to constrain the physical structure of the irradiated disk around the solar-mass star XUE 1 in NGC 6357 ($d \approx 1690$ pc). Our findings are: (1) Mid-infrared dust emission features are explained by amorphous and crystalline silicates with compositions similar to nearby disks. (2) The molecular features detected with MIRI originate within the first $\sim 1$ au, consistent with slab models' results. (3) Our model favors a disk truncated at $10$ au with a gas-to-dust ratio of unity in the outskirts. (4) Comparing models of the same disk structure under different irradiation levels, we find that strong external irradiation raises gas temperature tenfold and boosts water abundance beyond $10$ au by a factor of $100$. Our findings suggest the inner disk resists external irradiation, retaining the elements necessary for planet formation.

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arXiv2025

Thermal emission from bow shocks. III. Variable diffuse X-ray emission from stellar-wind bow shocks driven by dynamical instabilities

X-ray emission from wind-driven bow shocks is both difficult to measure and predict, but may give important insights into the energy budget of the hot phase of the ISM by quantifying mixing at the interface between hot and warm gas phases. We investigate the effect of magnetic fields and numerical resolution on predicted X-ray emission and other observable properties of bow shocks, to study convergence properties and assess robustness of predicted observables from simulations. A suite of 2D and 3D HD and MHD simulations of bow shocks were run and analysed to generate synthetic emission maps and light curves in X-ray and infrared emission. Resolving the Kelvin-Helmholtz (KH) instability at the wind-ISM contact discontinuity is crucial for obtaining converged results and for predicting X-ray emission and the properties of the hot shocked wind. When sufficient spatial resolution is used, we measure time variation of X-ray emission of at least an order of magnitude on a timescale comparable to the advection timescale of the wake downstream from the bow shock. Good correspondence is found between 2D and 3D simulations with comparable resolution, and 3D simulations can achieve the required resolution with reasonable computing resources. Development of the KH instability is inhibited for shear flows parallel to the ISM magnetic field, compared with what is seen in the perpendicular direction, resulting in synthetic IR emission maps of bow shocks that are smooth when seen from one perspective but show strong distortions from another. Measuring the X-ray morphology and luminosity in bow shocks may be useful for constraining mixing and energy-transfer rates between hot and warm gas phases of the ISM. Dynamical instabilities at the wind-ISM interface are a crucial ingredient in determining the properties of the hot-gas phase in stellar bow-shocks, in particular to capture its time dependence.

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arXiv2025

Discovery of Radio Recombination Lines from Proplyds in the Orion Nebula Cluster

We present new Atacama Large Millimeter/submillimeter Array observations that, for the first time, detect hydrogen and helium radio recombination lines from a protoplanetary disk. We imaged the Orion Nebula Cluster at 3.1 mm with a spectral setup that covered the $n=42 \rightarrow 41$ transitions of hydrogen (H41$α$) and helium (He41$α$). The unprecedented sensitivity of these observations enables us to search for radio recombination lines toward the positions of ${\sim}200$ protoplanetary disks. We detect H41$α$ from 17 disks, all of which are HST-identified `proplyds.' The detected H41$α$ emission is spatially coincident with the locations of proplyd ionization fronts, indicating that proplyd H41$α$ emission is produced by gas that has been photoevaporated off the disk and ionized by UV radiation from massive stars. We measure the fluxes and widths of the detected H41$α$ lines and find line fluxes of ${\sim}30-800$ mJy km s$^{-1}$ and line widths of ${\sim}30-90$ km s$^{-1}$. The derived line widths indicate that the broadening of proplyd H41$α$ emission is dominated by outflowing gas motions associated with external photoevaporation. The derived line fluxes, when compared with measurements of 3.1 mm free-free flux, imply that the ionization fronts of H41$α$-detected proplyds have electron temperatures of ${\sim}6,000-11,000$ K and electron densities of ${\sim}10^6-10^7$ cm$^{-3}$. Finally, we detect He41$α$ towards one H41$α$-detected source and find evidence that this system is helium-rich. Our study demonstrates that radio recombination lines are readily detectable in ionized photoevaporating disks, providing a new way to measure disk properties in clustered star-forming regions.

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arXiv2025

A possible trail of dust from a young, highly-extincted brown dwarf in the outskirts of the Trapezium Cluster

We present the JWST discovery of a highly-extincted ($A_V\sim52$) candidate brown dwarf ($\sim0.018$M$_\odot$) in the outskirts of the Trapezium Cluster that appears to be coincident with the end of a $\sim 1700\,$au long, remarkably uniformly wide, dark trail that broadens only slightly at the end opposite the point source. We examine whether a dusty trail associated with a highly-extincted brown dwarf could plausibly be detected with JWST and explore possible origins. We show that a dusty trail associated with the brown dwarf could be observable if dust within it is larger than that in the ambient molecular cloud. For example, if the ambient cloud has a standard $\sim0.25$$μ$m maximum grain size and the trail contains micron-sized grains, then the trail will have a scattering opacity over an order of magnitude larger compared to the surroundings in NIRCam short-wavelength filters. We use a simple model to show that a change in maximum grain size can reproduce the high $A_V$ and the multi-filter NIRCam contrast seen between the trail and its surroundings. We propose and explore two possible mechanisms that could be responsible for the trail: i) a weak FUV radiation-driven wind from the circum-brown dwarf disc due to the O stars in the region and ii) a Bondi-Hoyle-Lyttleton accretion wake. The former would be the most distant known case of the Trapezium stars' radiation driving winds from a disc, and the latter would be the first known example of ``late'' infall from the interstellar medium onto a low mass object in a high-mass star-forming region.

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arXiv2025

The Molecular Cloud Lifecycle I: Constraining H2 formation and dissociation rates with observations

Molecular clouds (MCs) are the birthplaces of new stars in galaxies. A key component of MCs are photodissociation regions (PDRs), where far-ultraviolet radiation plays a crucial role in determining the gas's physical and chemical state. Traditional PDR models assume chemical steady state (CSS), where the rates of H$_2$ formation and photodissociation are balanced. However, real MCs are dynamic and can be out of CSS. In this study, we demonstrate that combining H$_2$ emission lines observed in the far-ultraviolet or infrared with column density observations can be used to derive the rates of H$_2$ formation and photodissociation. We derive analytical formulae that relate these rates to observable quantities, which we validate using synthetic H$_2$ line emission maps derived from the SILCC-Zoom hydrodynamical simulation. Our method estimates integrated H$_2$ formation and dissociation rates with an accuracy $\approx 30$ % (on top of uncertainties in observed H$_2$ emission maps and column densities). Our simulations, valid for column densities $N \leq 2 \times 10^{22}$ cm$^{-2}$, cover a wide dynamic range in H$_2$ formation and photodissociation rates, showing significant deviations from CSS, with 74 % of the MC's mass deviating from CSS by a factor greater than 2. Our analytical formulae can effectively distinguish between regions in and out of CSS. When applied to actual H$_2$ line observations, our method can assess the chemical state of MCs, providing insights into their evolutionary stages and lifetimes. A NASA Small Explorer mission concept, Eos, will be proposed in 2025 and is specifically designed to conduct the types of observations outlined in this study.

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arXiv2025

Impact of photoevaporative winds in chemical models of externally irradiated protoplanetary disks

Most stars form in dense clusters within high-mass star-forming regions, where protoplanetary disks may be exposed to intense UV radiation from nearby massive stars. While previous studies have typically focused on isolated sources in low-mass regions, recent observational campaigns have started to probe the chemistry of irradiated disks in unprecedented detail. Interpreting this data requires complex chemical models, yet few studies have examined these disks' chemistry, and none have incorporated the photoevaporative wind launched by external UV fields into their physical structure. In this study, we post-process radiation hydrodynamics simulations of externally irradiated protoplanetary disks using the thermochemical code DALI, comparing models with and without the wind to assess its impact on disk chemistry. Results show that UV radiation is rapidly attenuated by the disk in both cases. However, thermal re-emission from the wind at longer wavelengths enhances disk heating, increasing the gas-phase abundances of some key volatiles. Synthetic line fluxes vary by orders of magnitude between wind and windless models, primarily due to emission from the wind itself rather than abundance variations within the disk. Our findings demonstrate that the photoevaporative wind significantly influences the physical and chemical structure, and observational characteristics, of externally irradiated disks. We conclude that incorporating the wind into chemical models is essential for accurately predicting chemical abundances, interpreting observations, and ultimately understanding planet formation in these common yet complex environments.

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arXiv2024

A tell-tale tracer for externally irradiated protoplanetary disks: comparing the [CI] 8727 A line and ALMA observations in proplyds

The evolution of protoplanetary disks in regions with massive OB stars is influenced by externally driven winds that deplete the outer parts of disks. These winds have previously been studied via forbidden oxygen emission lines, which also arise in isolated disks in low-mass star forming-regions (SFRs) with weak external UV fields in photoevaporative or magnetic (internal) disk winds. It is crucial to determine how to disentangle external winds from internal ones. Here, we report a proxy for unambiguously identifying externally driven winds with a forbidden line of neutral atomic carbon, [C i] 8727 A. We compare for the first time the spatial location of the emission in the [O i] 5577 A, [O i] 6300 A, and [C i] 8727 A lines traced by VLT/MUSE-NFM, with the ALMA Band 7 continuum disk emission in a sample of 12 proplyds in the Orion Nebula Cluster (ONC). We confirm that the [O i] 5577 A emission is co-spatial with the disk emission, whereas the [O i] 6300 A is emitted both on the disk surface and on the ionization front of the proplyds. We show for the first time that the [C i] 8727 A line is also co-spatial with the disk surface in proplyds, as seen in the MUSE and ALMA data comparison. To verify whether the [C i] 8727 A line is detected in regions where external photoevaporation is not expected, we examine VLT/X-Shooter spectra for young stars in low-mass SFRs. Although the [O i] lines are well detected in all these targets, there is <<10% detection rate in the case of the [C i] 8727 A line. This number increases substantially to a ~40% detection rate in sigma-Orionis, a region with intermediate UV radiation. The spatial location of the [C i] 8727 A line emission and the lack of its detection in isolated disks in low-mass SFRs strongly suggest that this line is a tell-tale tracer of externally driven photoevaporative winds, which agrees with recent excitation models.

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arXiv2024

A multi-ion non-equilibrium solver for ionised astrophysical plasmas with arbitrary elemental abundances

While many astrophysical plasmas can be modelled successfully assuming ionisation and thermal equilibrium, in some cases this is not appropriate and a non-equilibrium approach is required. In nebulae around evolved stars the local elemental abundances may also strongly vary in space and time. Here we present a non-equilibrium multi-ion module developed for the fluid-dynamics code \textsc{pion}, describing the physical processes included and demonstrating its capabilities with some test calculations. A non-equilbrium ionisation solver is developed that allows arbitrary elemental abundances for neutral and ionised (but not molecular) gas, for the elements H, He, C, N, O, Ne, Si, S and Fe. Collisional ionisation and recombination, photoionisation and charge-exchange reactions are included, and ion-by-ion non-equilibrium radiative cooling is calculated based on the instantaneous ion fractions of each element. Element and ion mass-fractions are advected using passive scalars, operator-split from the microphysical processes. The module is validated by comparing with equilibrium and non-equilibrium calculations in the literature. Effects of charge exchange on ion abundances in cooling plasmas are discussed. Application to modelling shocks and photoionised H~\textsc{ii} regions is demonstrated. The time-dependent expansion of a Wolf-Rayet nebula is studied, including photoionisation and collisional processes, and spectral-line luminosities calculated for non-equilibrium and equilibrium plasma states. The multi-ion module enables simulation of ionised plasmas with spatially varying elemental abundances using self-consistent ion abundances and thermal evolution. This allows prediction of spectral lines in UV, optical, IR and X-ray even in cases where the plasma is out of ionisation equilibrium.

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arXiv2024

Kaleidoscope of irradiated disks: MUSE observations of proplyds in the Orion Nebula Cluster. I. Sample presentation and ionization front sizes

In the Orion Nebula Cluster (ONC), protoplanetary disks exhibit ionized gas clouds in the form of a striking teardrop shape as massive stars irradiate the disk material. We present the first spatially and spectrally resolved observations of 12 proplyds, using Integral Field Spectroscopy observations performed with the MUSE instrument in Narrow Field Mode (NFM) on the VLT. We present the morphology of the proplyds in seven emission lines and measure the radius of the ionization front (I-front) of the targets in four tracers, covering transitions of different ionization states for the same element. We also derive stellar masses for the targets. The measurements follow a consistent trend of increasing I-front radius for a decreasing strength of the far-UV radiation as expected from photoevaporation models. By analyzing the ratios of the I-front radii as measured in the emission lines of Ha, [OI] 6300, [OII] 7330, and [OIII] 5007, we observe the ionization stratification, that is, the most ionized part of the flow being the furthest from the disk (and closest to the UV source). The ratios of I-front radii scale in the same way for all proplyds in our sample regardless of the incident radiation. We show that the stratification can help constrain the densities near the I-front by using a 1D photoionization model. We derive the upper limits of photoevaporative mass-loss rates by assuming ionization equilibrium, and estimate values decreasing towards lower impinging radiation. We do not find a correlation between Mloss and stellar mass. The highest mass-loss rate is for the proplyd 244-440. These values of Mloss, combined with estimates of the disk mass with ALMA, confirm previous estimates of the short lifetime of these proplyds. This work demonstrates the potential of this dataset and offers a new set of observables to be used to test current and future models of external photoevaporation.

astro-ph.SR↗

arXiv2024

A far-ultraviolet-driven photoevaporation flow observed in a protoplanetary disk

Most low-mass stars form in stellar clusters that also contain massive stars, which are sources of far-ultraviolet (FUV) radiation. Theoretical models predict that this FUV radiation produces photo-dissociation regions (PDRs) on the surfaces of protoplanetary disks around low-mass stars, impacting planet formation within the disks. We report JWST and Atacama Large Millimetere Array observations of a FUV-irradiated protoplanetary disk in the Orion Nebula. Emission lines are detected from the PDR; modelling their kinematics and excitation allows us to constrain the physical conditions within the gas. We quantify the mass-loss rate induced by the FUV irradiation, finding it is sufficient to remove gas from the disk in less than a million years. This is rapid enough to affect giant planet formation in the disk.

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