Irradiated Disk Research Articles

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

ABSTRACT Most stars form in dense clusters within high-mass star-forming regions, where protoplanetary discs may be exposed to intense UV radiation from nearby massive stars. While previous studies have typically focussed on isolated sources in low-mass regions, recent observational campaigns have started to probe the chemistry of irradiated discs in unprecedented detail. Interpreting this data requires complex chemical models, yet few studies have examined these discs’ 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 discs using the thermochemical code dali, comparing models with and without the wind to assess its impact on disc chemistry. Results show that UV radiation is rapidly attenuated by the disc in both cases. However, thermal re-emission from the wind at longer wavelengths enhances disc 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 disc. Our findings demonstrate that the photoevaporative wind significantly influences the physical and chemical structure, and observational characteristics, of externally irradiated discs. 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.

A tell-tale tracer for externally irradiated protoplanetary disks: Comparing the [C I] 8727 Å 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 these disks. The 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 Å. We compare for the first time the spatial location of the emission in the [O I] 5577 Å, [O I] 6300 Å, and [C I] 8727 Å 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 Å emission is co-spatial with the disk emission, whereas that of [O I] 6300 Å 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 Å line is also co-spatial with the disk surface in proplyds, as seen in the MUSE and ALMA data comparison. The peak emission is compatible with the stellar location in all cases, apart from one target with high relative inclination with respect to the ionizing radiation, where the peak emission is located at the disk edge in the direction of the ionizing radiation. To verify whether the [C I] 8727 Å line is detected in regions where external photoevaporation is not expected, we examined VLT/X-Shooter spectra for young stars in low-mass SFRs. Although the [O I] 5577 Å and 6300 Å lines are well detected in all these targets, the total detection rate is ≪10% in the case of the [C I] 8727 Å line. This number increases substantially to a ∼40% detection rate in σ-Orionis, a region with higher UV radiation than in low-mass SFRs, but lower than in the ONC. The spatial location of the [C I] 8727 Å 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.

Characterizing high and low accretion states in VY Scl CVs using ZTF and TESS data

ABSTRACT VY Scl binaries are a sub-class of cataclysmic variable (CV) which show extended low states, but do not show outbursts which are seen in other classes of CV. To better determine how often these systems spend in low states and to resolve the state transitions we have analysed Zwicky Transient Facility (ZTF) data on eight systems and Transiting Exoplanet Survey Satellite (TESS) data on six systems. Half of the sample spent most of the time in a high state; three show a broad range and one spends roughly half the time transitioning between high and low states. Using the ZTF data, we explore the colour variation as a function of brightness. In KR Aur, we identify a series of repeating outburst events whose brightness appears to increase over time. Using TESS data, we searched for periods other than the orbital. In LN UMa, we find evidence for a peak whose period varies between 3 and 6 d. We outline the current models which aim to explain the observed properties of VY Scl systems which includes disc irradiation and a white dwarf having a significant magnetic field.

The remarkable microquasar S26: A super-Eddington PeVatron

Context. S26 is an extragalactic microquasar with the most powerful jets ever discovered. They have a kinetic luminosity of Lj ∼ 5 × 1040 erg s−1. This implies that the accretion power to the black hole should be super-Eddington, of the order of Lacc ∼ Lj. However, the observed X-ray flux of this system indicates an apparent very sub-Eddington accretion luminosity of LX ≈ 1037 erg s−1. Aims. We aim to characterize the nature of S26, explain the system emission, and study the feasibility of super-Eddington microquasars as potential PeVatron sources. Methods. We first analyze multi-epoch X-ray observations of S26 obtained with XMM-Newton and model the super-Eddington disk and its wind. We then develop a jet model and study the particle acceleration and radiative processes that occur in shocks generated near the base of the jet and in its terminal region. Results. We find that the discrepancy between the jet and the apparent disk luminosities in S26 is caused by the complete absorption of the disk radiation by the wind ejected from the super-Eddington disk. The nonthermal X-rays are produced near the base of the jet, and the thermal X-rays are emitted in the terminal regions. The radio emission observed with the Australia Telescope Compact Array can be explained as synchrotron radiation produced at the reverse shock in the lobes. We also find that S26 can accelerate protons to PeV energies in both the inner jet and the lobes. The ultra-high energy protons accelerated in the lobes of S26 are injected into the interstellar medium with a total power of ∼1036 erg s−1. Conclusions. We conclude that S26 is a super-Eddington microquasar with a dense disk-driven wind that obscures the X-ray emission from the inner disk, and that the supercritical nature of the system allows the acceleration of cosmic rays to PeV energies.

Retrieval of Thermally Resolved Water Vapor Distributions in Disks Observed with JWST-MIRI

The mid-infrared water vapor emission spectrum provides a novel way to characterize the delivery of icy pebbles toward the innermost (<5 au) regions of planet-forming disks. Recently, JWST MIRI-MRS showed that compact disks exhibit an excess of low-energy water vapor emission relative to extended multigapped disks, suggesting that icy pebble drift is more efficient in the former. We carry out detailed emission-line modeling to retrieve the excitation conditions of rotational water vapor emission in a sample of four compact and three extended disks within the JWST Disk Infrared Spectral Chemistry Survey. We present two-temperature H2O slab model retrievals and, for the first time, constrain the spatial distribution of water vapor by fitting parametric radial temperature and column density profiles. Such models statistically outperform the two-temperature slab fits. We find a correlation between the observable hot water vapor mass and stellar mass accretion rate, as well as an anticorrelation between cold water vapor mass and submillimeter dust disk radius, confirming previously reported water line flux trends. We find that the mid-IR spectrum traces H2O with temperatures down to 180–300 K, but the coldest 150–170 K gas remains undetected. Furthermore the H2O temperature profiles are generally steeper and cooler than the expected “superheated” dust temperature in passive irradiated disks. The column density profiles are used to estimate icy pebble mass fluxes, which suggest that compact and extended disks may produce markedly distinct inner-disk exoplanet populations if local feeding mechanisms dominate their assembly.

Spectral characteristics of a rotating solar prominence in multiple wavelengths

We present synthetic spectra corresponding to a 2.5D magnetohydrodynamic simulation of a rotating prominence in the k Ly alpha , and Ly beta lines. The prominence rotation resulted from angular momentum conservation within a flux rope where asymmetric heating imposed a net rotation prior to the thermal-instability-driven condensation phase. The spectra were created using a library built on the framework called which provides boundary conditions for incorporating the limb-darkened irradiation of the solar disk on isolated structures such as prominences. Our spectra show distinctive rotational signatures for the k Ly alpha , and Ly beta lines, even in the presence of complex, turbulent solar atmospheric conditions. However, these signals are barely detectable for the and spectral lines. Most notably, we find only a very faint rotational signal in the line, thus reigniting the discussion on the existence of sustained rotation in prominences.

Unraveling the Hybrid Origins of the X-Ray Nonthermal Emission from IGR J17091–3624

We present a comprehensive study based on multiwavelength observations from the NuSTAR, NICER, Swift, Fermi, NEOWISE, and ATCA telescopes during the 2022 outburst of the black-hole X-ray binary IGR J17091–3624. Our investigation concentrates on the heartbeat-like variability in the X-ray emission, with the aim of using it as a tool to unravel the origin of the nonthermal emission during the heartbeat state. Through X-ray timing and spectral analysis, we observe that the heartbeat-like variability correlates with changes in the disk temperature, supporting the disk radiation pressure instability scenario. Moreover, in addition to a Comptonization component, our time-averaged and phase-resolved spectroscopy reveal the presence of a power-law component that varies independently from the disk component. Combined with the radio–X-ray spectral energy distribution fitting, our results suggest that the power-law component could originate from synchrotron self-Compton radiation in the jet, which requires a strong magnetic field of about B = (0.3–3.5) × 106 G. Additionally, assuming that IGR J17091-3624 and GRS 1915 + 105 share the same radio–X-ray correlation coefficient during both the hard and the heartbeat states, we obtain a distance of 13.7 ± 2.3 kpc for IGR J17091–3624.

Short-lived gravitational instability in isolated irradiated discs

ABSTRACT 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 $0.1M_\odot$ 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 time-scales. Using synthetic images at 1.3 mm, 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 stabilizing influence of $P\mathrm{d}V$ work and shock heating in the spiral arms.

PDRs4All

PDRs4All

JWST study of the DG Tau B disk-wind candidate

Context. The origin of outflows and their impact on protoplanetary disk evolution and planet-formation processes are still crucial open questions. DG Tau B is a Class I protostar associated with a structured disk and a rotating conical CO outflow and was recently identified by ALMA as one of the best CO disk wind candidate. It is therefore a perfect target for studying these questions. Aims. We aim to map and study any outflow component intermediate between the axial jet and the CO outflow in order to constrain the origin (irradiated or shocked disk wind or swept-up material) of the redshifted molecular outflows in DG Tau B. Methods. We analyzed observations obtained with James Webb Space Telescope NIRSpec-IFU and NIRCam, supplemented with IFU data from the SINFONI/VLT instrument. We investigated the morphology, kinematics, and excitation conditions of the ro-vibrational H2 emission lines and their relation with the atomic jet and CO outflow. We focus our analysis on the redshifted outflow lobe. Results. We observe a global layered structure of the redshifted outflows in DG Tau B, with the atomic jet inside the H2 cavity, which in turn is nested inside the CO conical outflow. We also find temperature, velocity, and collimation to be increasing towards the axis of the flow. The redshifted H2 emission traces a narrow conical cavity (semi-opening angle of 9.4°) that is wider than the axial jet but nested just inside the CO outflow. Both the jet and the H2 cavity originate from the innermost regions of the disk (r0 &lt; 6 au). The redshifted H2 cavity flows with a constant vertical velocity of Vz = 22.5 ± 0.8 km s−1, twice faster than the conical CO flow. The excitation conditions imply a hot H2 gas (Tex ≃ 2200 K) with an average mass flux of Ṁ(H2) = 3 × 10−11 M⊙ yr−1, which is significantly lower than the jet and CO values. Conclusions. The global layered H2-CO structure in temperature, velocity, and collimation in the DG Tau B redshifted lobe is consistent with a magneto-hydrodynamic disk wind scenario. The hot H2 could trace the inner, dense photodissociation layer in the wind. An H2 launching region at disk radii of 0.2−0.4 au combined with a large ejection efficiency (ξ ≃ 1) would account for the mass flux and kinematics. Alternatively, the near-IR ro-vibrational H2 could be emitted in the interaction layer driven by successive jet bow shocks into an outer disk wind or envelope. Further constraints on both scenarios will be obtained from the analysis of MIRI observations.

High-energy Neutrinos from Outflows Powered by the Kicked Remnants of Binary Black Hole Mergers in Active Galactic Nucleus Accretion Disks

Merging of stellar-mass binary black holes (BBHs) could take place within the accretion disks of active galactic nuclei (AGN). The resulting BH remnant is likely to accrete the disk gas at a super-Eddington rate, launching a fast, quasi-spherical outflow (wind). Particles will be accelerated by shocks driven by the wind, subsequently interacting with the shocked disk gas or radiation field through hadronic processes and resulting in the production of high-energy neutrinos and potential electromagnetic (EM) emission. This study delves into the intricate evolution of the shock driven by a merged BH wind within an AGN disk. Subsequently, we calculated the production of neutrinos and the expected detection numbers for a single event, along with their contributions to the overall diffuse neutrino background. Our analysis, which considers various scenarios, reveals considerable neutrino production and possible detection by IceCube for nearby events. The contribution of merged BH winds on the diffuse neutrino background is minor due to the low event rate density, but it can be improved to some extent for some optimistic parameters. We also propose that there could be two neutrino/EM bursts, one originating from the premerger BBH wind and the other from the merged BH wind, with the latter typically having a delay to the gravitational wave (GW) event of around tens of days. When combined with the anticipated GWs emitted during the BBH merger, such a system emerges as a promising candidate for joint observations involving neutrinos, GWs, and EM signals.

Chemistry in Externally FUV-irradiated Disks in the Outskirts of the Orion Nebula Cluster

Most stars are born in stellar clusters, and their protoplanetary disks, which are the birthplaces of planets, can, therefore, be affected by the radiation of nearby massive stars. However, little is known about the chemistry of externally irradiated disks, including whether or not their properties are similar to the so-far better-studied isolated disks. Motivated by this question, we present ALMA Band 6 observations of two irradiated Class II protoplanetary disks in the outskirts of the Orion Nebula Cluster to explore the chemical composition of disks exposed to (external) far-ultraviolet (FUV) radiation fields: the 216-0939 disk and the binary system 253-1536A/B, which are exposed to radiation fields of 102–103 times the average interstellar radiation field. We detect lines from CO isotopologues, HCN, H2CO, and C2H toward both protoplanetary disks. Based on the observed disk-integrated line fluxes and flux ratios, we do not find significant differences between isolated and irradiated disks. The observed differences seem to be more closely related to the different stellar masses than to the external radiation field. This suggests that these disks are far enough away from the massive Trapezium stars, that their chemistry is no longer affected by external FUV radiation. Additional observations toward lower-mass disks and disks closer to the massive Trapezium stars are required to elucidate the level of external radiation required to make an impact on the chemistry of planet formation in different kinds of disks.

Kaleidoscope of irradiated disks: MUSE observations of proplyds in the Orion Nebula Cluster

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 such objects, known as proplyds, using integral field spectroscopy observations performed with the Multi-Unit Spectroscopic Explorer (MUSE) instrument in Narrow Field Mode (NFM) on the Very Large Telescope (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 Hα, [O I] 6300 A, [O II] 7330 A, and [O III] 5007 A, 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 ionization 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 (Ṁloss) by assuming ionization equilibrium, and estimate values in the range 1.07–94.5 × 10−7 M⊙ yr−1, with Ṁloss values decreasing towards lower impinging radiation. We do not find a correlation between the mass-loss rate and stellar mass. The highest mass-loss rate is for the giant proplyd 244–440. These values of Ṁloss, combined with recent estimates of the disk mass with ALMA, confirm previous estimates of the short lifetime of these proplyds. This work demonstrates the potential of this MUSE dataset and offers a new set of observables to be used to test current and future models of external photoevaporation.

Buoyancy torques prevent low-mass planets from stalling in low-turbulence radiative discs

ABSTRACT Low-mass planets migrating inwards in laminar protoplanetary discs (PPDs) experience a dynamical corotation torque (DCT), which is expected to slow down migration to a stall. However, baroclinic effects can reduce or even reverse this effect, leading to rapid inward migration. In the radiatively inefficient inner disc, one such mechanism is the buoyancy response of the disc to an embedded planet. Recent work has suggested that radiative cooling can quench this response, but for parameters that are not necessarily representative of the inner regions of PPDs. We perform global 3D inviscid radiation hydrodynamics simulations of planet–disc interaction to investigate the effect of radiative cooling on the buoyancy-driven torque in a more realistic disc model. We find that the buoyancy response exerts a negative DCT – albeit partially damped due to radiative cooling – resulting in sustained, rapid inward migration. Models that adopt a local cooling prescription significantly overestimate the impact of the buoyancy response, highlighting the importance of a realistic treatment of radiation transport that includes radiative diffusion. Our results suggest that low-mass planets should migrate inwards faster than has been previously expected in radiative discs, with implications for the formation and orbital distribution of super-Earths and sub-Neptunes at intermediate distances from their host stars, unless additional physical processes that can slow down migration are considered.

Pulsed Iron Line Emission from the First Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124

We report the phase-resolved spectral results of the first Galactic pulsating ultraluminous X-ray source (PULX) Swift J0243.6+6124, modeling its 2017–2018 outburst peak using data collected by the Hard X-ray Modulation Telescope (Insight-HXMT). The broad energy coverage of Insight-HXMT allows us to obtain a more accurate spectral continuum to reduce the coupling of broad iron line profiles with other components. We use three different continuum spectrum models but obtain similar iron line results. For the first time, we detect the pulse characteristics of the broad iron line in a PULX. The variation in the width and intensity of this iron line with σ ∼ 1.2–1.5 keV has a phase offset of about 0.25 from the pulse phase. We suggest that the uneven irradiation of the thick inner disk by the accretion column produces the modulated variation of the broad iron line. In addition, the nonpulsed narrow line is suggested to come from the outer disk region.

The bright black hole X-ray binary 4U 1543-47 during 2021 outburst. A clear state transition from super-Eddington to sub-Eddington accretion revealed by Insight-HXMT

ABSTRACT We present a detailed analysis of the observations with the Hard X-ray Modulation Telescope of the black hole X-ray transient 4U 1543-47 during its outburst in 2021. We find a clear state transition during the outburst decay of the source. Using previous measurements of the black hole mass and distance to the source, the source luminosity during this transition is close to the Eddington limit. The light curves before and after the transition can be fitted by two exponential functions with short (∼16 d) and long (∼130 d) decay time-scales, respectively. We detect strong reflection features in all observations that can be described with either the relxillns or reflionx_bb reflection models, both of which have a black-body incident spectrum. In the super-Eddington state, we observe a Comptonized component characterized by a low electron temperature of approximately 2.0 keV. We suggest that this component appears exclusively within the inner radiation-pressure-dominated region of the supercritical disc as a part of the intrinsic spectrum of the accretion disc itself. This feature vanishes as the source transitions into the sub-Eddington state. The emissivity index of the accretion disc in the reflection component is significantly different before and after the transition, ∼3.0–5.0 and ∼7.0–9.0 in the super- and sub-Eddington states, respectively. Based on the reflection geometry of returning disc radiation, the geometrically thicker the accretion disc, the smaller the emissivity index. Therefore, we propose that the transition is primarily driven by the change of the accretion flow from a supercritical to a thin disc configuration.

Unveiling the origin of the optical and UV emission during the 2017 giant outburst of the Galactic ULX pulsar Swift J0243.6+6124

Context. From late September 2017 to January 2018, the Be X-ray binary (BeXB) Swift J0243.6+6124 underwent a giant outburst that was unprecedently bright. The reported X-ray luminosities were so high that the system was classified as an ultraluminous X-ray source (ULX). It was also the first BeXB pulsar to show radio jet emission. The source was not only bright in X-rays and radio, but also in the optical and ultraviolet (UV) wavelenghts as well. Aims. In this work, we aim to understand the origin of the observed optical/UV fluxes simultaneous to the X-ray emission. Methods. We studied the optical/UV light curves in comparison with the X-ray fluxes along the outburst. We considered the main mechanisms that can explain the optical/UV emission in X-ray binaries. Due to the tight correlation observed between the optical/UV and X-ray light curves, reprocessing of X-rays seems to be the most plausible explanation. We calculated the timescales of the light curve decays and studied the correlation indexes between the optical and X-ray emission. Finally, we built a physical model that considers the X-ray heating of the surface of the donor star, irradiation of the accretion disk, and emission from a viscously heated accretion disk, so that we could reproduce the observed optical/UV spectral energy distributions (SEDs) along the outburst. In our model, we considered the Be circumstellar disk to be co-planar to the orbit and then we neglected its irradiation in the current model. As an input for the model, we used as incident X-ray luminosities those calculated from the bolometric X-ray fluxes obtained from the spectral fit of the Swift/XRT and BAT observations. Results. The timescales of the exponential decay of the outburst are between two and four times longer for the UV and optical light curves than for the X-ray light curve. The correlation index between the optical/UV and X-ray fluxes varies between optical/UV filters and when different X-ray bands are considered and is larger for the rise than for the decay phase of the outburst for the fluxes at redder wavelengths. The modelling of the SED shows that X-ray heating of the companion star surface is the main mechanism contributing to the UV emission and contributes significantly to the optical emission during the whole outburst. The X-ray irradiation of the accretion disk is necessary to reproduce the optical observed fluxes from MJD 58047 to 58120 and contributes significantly to the UV fluxes close to the peak of the outburst. As a first attempt, the fits yield an increasing value of the outer radius of the accretion disk along the outburst. An alternative interpretation points to variations in the geometry of the inner flow and the fraction of reprocessed X-ray emission during the outburst. On the other hand, variations in the geometry of the Be circumstellar disk could also play a role, but they have not been considered in the current model. Conclusions. Reprocessing of X-rays via the X-ray heating of the Be star surface and as irradiation of the accretion disk is the main mechanism that can reproduce the observed optical/UV emission during the 2017−2018 giant outburst of Swift J0243.6+6124.

Evolution of accretion disk structure of the black hole X-ray binary MAXI J1820+070 during the rebrightening phase

Abstract To understand the evolution of global accretion disk structure in the “rebrightening” phase of MAXI J1820+070, we perform a comprehensive analysis of its near infrared/optical/UV to X-ray spectral energy distribution (SED) utilizing data obtained by OISTER, Las Cumbres Observatory (LCO), Swift, NICER, and NuSTAR in 2019. Optical spectra observed with Seimei telescope in 2019 and 2020 are also analyzed. On the basis of the optical and X-ray light curves and their flux ratios, we divide the whole phase into three periods, Periods I (flux rise), II (decay), and III (dim). In the first two periods, the source stayed in the low/hard state (LHS), where the X-ray (0.3–30 keV) and optical/UV SED can both be fitted with power-law models. We interpret that the X-ray emission arises from hot corona via Comptonization, whereas the optical/UV flux is dominated by synchrotron radiation from the jets, with a partial contribution from the irradiated disk. The optical/UV power-law component smoothly connects to a simultaneous radio flux, supporting its jet origin. Balmer line profiles in the optical spectra indicate that the inner radius of an irradiated disk slightly decreased from ∼2 × 105rg (Period I) to ∼1 × 105rg (Period II), where rg is the gravitational radius, implying a change of the hot corona geometry. In Period III, the SED can be reproduced by an advection-dominated accretion flow and jet emission. However, the double-peaked Hα emission line indicates that a cool disk remained at large radii.

Vertical shear instability in two-moment radiation-hydrodynamical simulations of irradiated protoplanetary disks

Context. The vertical shear instability (VSI) is a hydrodynamical instability predicted to produce turbulence in magnetically inactive regions of protoplanetary disks. The regions in which this instability can occur and the physical phenomena leading to its saturation are a current matter of research. Aims. We explore the secondary instabilities triggered by the nonlinear evolution of the VSI and their role in its saturation. We also expand on previous investigations on stability regions by considering temperature stratifications enforced by stellar irradiation and radiative cooling, and including the effects of dust-gas collisions and molecular line emission. Methods. We modeled the gas-dust mixture in a circumstellar disk around a T Tauri star by means of high-resolution axisymmetric radiation-hydrodynamical simulations including stellar irradiation with frequency-dependent opacities, considering different degrees of depletion of small dust grains. Results. The flow pattern produced by the interplay of the axisymmetric VSI modes and the baroclinic torque forms bands of nearly uniform specific angular momentum. In the high-shear regions in between these bands, the Kelvin–Helmholtz instability (KHI) is triggered. A third instability mechanism, consisting of an amplification of eddies by baroclinic torques, forms meridional vortices with Mach numbers up to ∼0.4. Our stability analysis suggests that protoplanetary disks can be VSI-unstable in surface layers up to tens of au for reasonably high gas emissivities. Conclusions. The significant transfer of kinetic energy to small-scale eddies produced by the KHI and possibly even the baroclinic acceleration of eddies limit the maximum energy of the VSI modes, likely leading to the saturation of the VSI. Depending on the gas molecular composition, the VSI can operate at surface layers even in regions where the midplane is stable. This picture is consistent with current observations of disks showing thin midplane millimeter-sized dust layers while appearing vertically extended in optical and near-infrared wavelengths.

Vertical shear instability in two-moment radiation-hydrodynamical simulations of irradiated protoplanetary disks

Context. Hydrodynamical instabilities are likely the main source of turbulence in weakly ionized regions of protoplanetary disks. Among these, the vertical shear instability (VSI) stands out as a rather robust mechanism due to its few requirements to operate, namely a baroclinic stratification, which is enforced by the balance of stellar heating and radiative cooling, and short thermal relaxation timescales. Aims. Our goal is to characterize the transport of angular momentum and the turbulent heating produced by the nonlinear evolution of the VSI in axisymmetric models of disks around T Tauri stars, considering varying degrees of depletion of small dust grains resulting from dust coagulation. We also explore the local applicability of both local and global VSI-stability criteria. Methods. We modeled the gas-dust mixture in our disk models by means of high-resolution axisymmetric radiation-hydrodynamical simulations including stellar irradiation with frequency-dependent opacities. This is the first study of this instability to rely on two-moment radiative transfer methods. Not only are these able to handle transport in both the optically thin and thick limits, but also they can be integrated via implicit-explicit methods, thus avoiding the employment of expensive global matrix solvers. This is done at the cost of artificially reducing the speed of light, which, as we verified for this work, does not introduce unphysical phenomena. Results. Given sufficient depletion of small grains (with a dust-to-gas mass ratio of 10% of our nominal value of 10−3 for &lt; 0.25 μm grains), the VSI can operate in surface disk layers while being inactive close to the midplane, resulting in a suppression of the VSI body modes. The VSI reduces the initial vertical shear in bands of approximately uniform specific angular momentum, whose formation is likely favored by the enforced axisymmetry. Similarities with Reynolds stresses and angular momentum distributions in 3D simulations suggest that the VSI-induced angular momentum mixing in the radial direction may be predominantly axisymmetric. The stability regions in our models are well explained by local stability criteria, while the employment of global criteria is still justifiable up to a few scale heights above the midplane, at least as long as VSI modes are radially optically thin. Turbulent heating produces only marginal temperature increases of at most 0.1% and 0.01% in the nominal and dust-depleted models, respectively, peaking at a few (approximately three) scale heights above the midplane. We conclude that it is unlikely that the VSI can, in general, lead to any significant temperature increase since that would either require it to efficiently operate in largely optically thick disk regions or to produce larger levels of turbulence than predicted by models of passive irradiated disks.