AB Aur Research Articles

Observational signatures of circumbinary discs II: Kinematic signatures in velocity residuals

Abstract Kinematic studies of protoplanetary discs are a valuable method for uncovering hidden companions. In the first paper of this series, we presented five morphological and kinematic criteria that aid in asserting the binary nature of a protoplanetary disc. In this work we study the kinematic signatures of circumbinary discs in the residuals of their velocity maps. We show that Doppler-flips, spiral arms, eccentric gas motion, fast flows inside of the cavity, and vortex-like kinematic signatures are commonly observed. Unlike in the planetary mass companion case, Doppler-flips in circumbinary discs are not necessarily centred on a companion, and can extend towards the cavity edge. We then compare the kinematic signatures in our simulations with observations and see similarities to the Doppler-flip signal in HD 100546 and the vortex-like kinematic signatures in HD 142527. Our analysis also reveals kinematic evidence for binarity in several protoplantary disks typically regarded as circumstellar rather than circumbinary, including AB Aurigae and HD 100546.

Sulfur monoxide (SO) as a shock tracer in protoplanetary disks: Case of AB Aurigae

Context. Sulfur monoxide (SO) is known to be a good shock tracer in molecular clouds and protostar environments, but its abundance is difficult to reproduce, even with state-of-the-art astrochemical models. Aims. We investigate the properties of the observed SO emission in the protoplanetary disk of AB Auriga, a Herbig Ae star of 2.4 M⊙ in mass, located at 156 pc. The AB Aur system is unique because it exhibits a dust trap and at least one young putative planet orbiting at about 30 au from the central star. Methods. We reduced ALMA archival data (projects 2019.1.00579.S, 2021.1.00690.S, and 2021.1.01216.S) and analyzed the three detected SO lines (SO 65 − 54, 67 − 56 and 56 − 45). We also used C17O and C18O 2–1 data to complement the interpretation of the SO data. Results. For the three SO lines, the maximum SO emission in the ring is not located in the dust trap. Moreover, the inner radius of the SO ring is significantly larger than the CO emission inner radius, ∼160 au versus ∼90 au. The SO emission traces gas located in part beyond the dust ring. This emission likely originates from shocks at the interface of the outer spirals, observed in CO and scattered light emission, as well as those in the molecular and dust ring. Also, SO is detected within the cavity, at a radius of ∼20 − 30 au and with a rotation velocity compatible with the protoplanet P1. We speculate that this SO emission originates from accretion shocks onto the circumplanetary disk of the putative protoplanet P1. Conclusions. These observations confirm that SO is a good tracer of shocks in protoplanetary disks and could serve as a powerful new tool for detecting embedded (proto)planets.

Direct Imaging Detection of the Protoplanet AB Aur b at Wavelengths Covering Paβ

Recently, Biddle et al. claimed a non-detection of the protoplanet AB Aurigae b in Keck/NIRC2 Paβ imaging. I reprocess these newly public data and compare them to data from the extreme AO platform (SCExAO/CHARIS) used to discover AB Aur b. AB Aur b is decisively imaged with SCExAO/CHARIS at wavelengths covering Paβ. The Biddle et al. non detection of AB Aur b results from a far poorer image quality that is non competitive with SCExAO/CHARIS. Their contrast limits and thus constraints on accretion are overestimated due to an inaccurate AB Aur b source model. Irrespective of image quality, single-band Paβ imaging is ill suited to conclusively identify accretion onto AB Aur b. Instead, high-resolution Hα spectroscopy may provide accretion signatures. Aside from PDS 70, AB Aurigae remains the system with the strongest evidence for having a directly imaged protoplanet.

Deep Paβ Imaging of the Candidate Accreting Protoplanet AB Aur b

Giant planets grow by accreting gas through circumplanetary disks, but little is known about the timescale and mechanisms involved in the planet-assembly process because few accreting protoplanets have been discovered. Recent visible and infrared imaging revealed a potential accreting protoplanet within the transition disk around the young intermediate-mass Herbig Ae star, AB Aurigae (AB Aur). Additional imaging in Hα probed for accretion and found agreement between the line-to-continuum flux ratio of the star and companion, raising the possibility that the emission source could be a compact disk feature seen in scattered starlight. We present new deep Keck/NIRC2 high-contrast imaging of AB Aur to characterize emission in Paβ, another accretion tracer less subject to extinction. Our narrow band observations reach a 5σ contrast of 9.6 mag at 0.″6, but we do not detect significant emission at the expected location of the companion, nor from other any other source in the system. Our upper limit on Paβ emission suggests that if AB Aur b is a protoplanet, it is not heavily accreting or accretion is stochastic and was weak during the observations.

AB Aur, a Rosetta stone for studies of planet formation

Context. Observational constraints on dust properties in protoplanetary disks are key to better understanding disk evolution, their dynamics, and the pathway to planet formation, but also surface chemistry, the main driver of chemical complexity. Aims. We continue our exploration of the protoplanetary disk around AB Aur by characterizing its dust properties at different millimeter wavelengths. Methods. We present new ALMA observations at 2.2 mm and VLA observations at 6.8 mm. Together with previous ALMA and NOEMA observations at 0.87 and 1.1 mm, these new observations are used to compute global spectral index profiles as well as spectral index maps to probe the dust properties throughout the disk. On the interpretation side, we present the results of a simple isothermal slab model to help constrain dust properties along the non-axisymmetric ring of continuum emission outside the millimeter cavity. We also present new results of dust radiative transfer calculations based on a disk-planet hydrodynamical simulation to explain how the azimuthal contrast ratio of the ring emission varies with millimeter wavelength. Results. The spectral energy distribution and the radial profiles of the spectral index indicate that the radiation from the compact source towards the center is not dominated by dust thermal emission, but most likely by free-free emission originating in the radio jet; it constitutes 93% of the emission at 6.8 mm, and 37% at 0.87 mm. The protoplanetary disk has a typical spectral index of 2.3, computed using the 0.87, 1.1, and 2.2 mm bands. We estimate a dust disk mass of 8 × 10−5 M⊙ which, assuming a mean gas-to-dust ratio of 40, gives a total disk mass of 3.2 × 10−3 M⊙. The azimuthal contrast ratio of the ring outside the millimeter cavity is smaller at 2.2 mm than at 1.1 mm, in agreement with previous findings. The VLA image shows several knots of 5σ emission all along the ring, which, with the help of our dust radiative transfer calculations, are consistent with the ring emission being nearly axisymmetric at that wavelength. The decrease in the azimuthal contrast ratio of the ring emission from 0.87 to 6.8 mm can be explained by a dust-losing decaying vortex at the outer edge of a planet gap.

Spectroastrometric Survey of Protoplanetary Disks with Inner Dust Cavities

We present high-resolution spectra and spectroastrometric (SA) measurements of fundamental rovibrational CO emission from nine nearby (≲300 pc) protoplanetary disks where large inner dust cavities have been observed. The emission-line profiles and SA signals are fit with a slab disk model that allows the eccentricity of the disk and intensity of the emission to vary as power laws. Six of the sources are well fit with our model, and three of these sources show asymmetric line profiles that can be fit by adopting a nonzero eccentricity. The three other sources have components in either their line profile or SA signal that are not captured by our disk model. Two of these sources (V892 Tau and CQ Tau) have multi-epoch observations that reveal significant variability. CQ Tau and AB Aur have CO line profiles with centrally peaked components that are similar to line profiles which have been interpreted as evidence of molecular gas arising from a wide-angle disk wind. Alternatively, emission from a circumplanetary disk could also account for this component. The interpretations of these results can be clarified in the future with additional epochs that will test the variability timescale of these SA signals. We discuss the utility of using high-resolution spectroscopy for probing the dynamics of gas in the disk and the scenarios that can give rise to profiles that are not fit with a simple disk model.

UV-optical Emission of AB Aur b Is Consistent with Scattered Stellar Light

The proposed protoplanet AB Aur b is a spatially concentrated emission source imaged in the millimeter-wavelength disk gap of the Herbig Ae/Be star AB Aur. Its near-infrared spectrum and absence of strong polarized light have been interpreted as evidence supporting the protoplanet interpretation. However, the complex scattered-light structures in the AB Aur disk pose challenges in resolving the emission source and interpreting the true nature of AB Aur b. We present new images of the AB Aur system obtained using the Hubble Space Telescope Wide Field Camera 3 in the ultraviolet (UV) and optical bands. AB Aur b and the known disk spirals are recovered in the F336W, F410M, and F645N bands. The spectral energy distribution of AB Aur b shows absorption in the Balmer jump, mimicking that of early-type stars. By comparing the colors of AB Aur b to those of the host star, the disk spirals, and predictions from scattered light and self-luminous models, we find that the emission from AB Aur b is inconsistent with planetary photospheric or accretion shock models. Instead, it is consistent with those measured in the circumstellar disks that trace scattered light. We conclude that the UV and visible emission from AB Aur b does not necessitate the presence of a protoplanet. We synthesize observational constraints on AB Aur b and discuss inconsistent interpretations among different data sets. Considering the significance of the AB Aur b discovery, we advocate for further observational evidence to verify its planetary nature.

Relation between metallicities and spectral energy distributions of Herbig Ae/Be stars

Context. Most studies devoted to Herbig Ae/Be stars (HAeBes) assume solar metallicity. However, the stellar metallicity, [M/H], is a fundamental parameter that can strongly differ depending on the source and may have important implications for planet formation. It has been proposed that the deficit of refractory elements observed in the surfaces of some HAeBes may be linked to the presence of cavities in their disks and is likely caused by Jovian planets that trap the metal-rich content. Aims. This work aims to provide a robust test on the previous proposal by analyzing the largest sample of HAeBes characterized by homogeneously derived [M/H] values and stellar and circumstellar properties. Methods. The spectra of 67 HAeBes, along with their well-known properties drawn from our previous work, have been collected from the ESO Science Archive Facility. Their [M/H] values were derived based on the comparison with Kurucz synthetic models. Statistical analyses were carried out with the aim to test the potential relation between [M/H] and the Meeus group I sources, with spectral energy distributions (SEDs) associated with the presence of cavities potentially carved by giant planets. We critically analyzed the eventual link between [M/H], the SED groups, and the presence of such planets. Results. Our statistical study robustly confirms that group I sources tend to have a lower [M/H] (typically ~ −0.10) than that of group II HAeBes (~ +0.14). A similar analysis involving SED-based transitional disks, with infrared excess only at wavelengths of ≥2.2 µm, does not reveal such a relation with [M/H]. This result indicates that not all processes capable of creating holes in the inner dust disks end up having an effect on the stellar abundances. The spatial distributions of group I and II sources are similar, at least within the available range of distances to the galactic centre and the galactic plane, for which the observed [M/H] differences are not driven by environmental effects. In addition, group I sources tend to have stronger (sub-) mm continuum emission presumably related to the presence of giant planets. Indeed, literature results indicate that disk substructures probably associated with the presence of giant planets are up to ten times more frequent in group I HAeBes than in group II. Finally, along with the metallicities derived for the whole sample, surface gravities and projected rotational velocities are additional outcomes reported in this work. Conclusions. We provide indirect evidence to suggest that giant planets are more frequent around group I/low [M/H] stars than around the rest of the HAeBes. However, a direct test of the previous hypothesis requires multiple detections of forming planets in their disks. Such detections have so far been limited to the candidate around the metal depleted ([M/H] = −0.35 ± −0.25) group I HAeBe star AB Aur, which is consistent with our findings.

Gap-opening Planets Make Dust Rings Wider

As one of the most commonly observed disk substructures, dust rings from high-resolution disk surveys appear to have different radial widths. Recent observations on PDS 70 and AB Aur reveal not only planets in the disk, but also the accompanying wide dust rings. We use three-dimensional dust-and-gas disk simulations to study whether gap-opening planets are responsible for the large ring width in disk observations. We find that gap-opening planets can widen rings of dust trapped at the pressure bump via planetary perturbations, even with the midplane dust-to-gas ratio approaching order unity and with the dust back-reaction accounted for. We show that the planet-related widening effect of dust rings can be quantified using diffusion-advection theory, and we provide a generalized criterion for an equilibrated dust ring width in three-dimensional disk models. We also suggest that the ring width can be estimated using the gas turbulent viscosity α turb, but with cautions about the Schmidt number being greater than order unity.

Probing Photon Statistics in Adaptive Optics Images with SCExAO/MEC* * Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

We present an experimental study of photon statistics for high-contrast imaging with the Microwave Kinetic Inductance Detector (MKID) Exoplanet Camera (MEC) located behind the Subaru Coronagraphic Extreme Adaptive Optics System at the Subaru Telescope. We show that MEC measures the expected distributions for both on-axis companion intensity and off-axis intensity, which manifests as quasi-static speckles in the image plane and currently limits high-contrast imaging performance. These statistics can be probed by any MEC observation due to the photon-counting capabilities of MKID detectors. Photon arrival time statistics can also be used to directly distinguish companions from speckles using a post-processing technique called Stochastic Speckle Discrimination (SSD). Here, we give an overview of the SSD technique and highlight the first demonstration of SSD on an extended source—the protoplanetary disk AB Aurigae. We then present simulations that provide an in-depth exploration as to the current limitations of an extension of the SSD technique called Photon-Counting SSD (PCSSD) to provide a path forward for transitioning PCSSD from simulations to on-sky results. We end with a discussion of how to further improve the efficacy of such arrival-time-based post-processing techniques applicable to both MKIDs, as well as other high-speed astronomical cameras.

AB Aur, a Rosetta stone for studies of planet formation

Context. The sulfur abundance is poorly known in most environments. Yet, deriving the sulfur abundance is key to understanding the evolution of the chemistry from molecular clouds to planetary atmospheres. We present observations of H2S 110–101 at 168.763 GHz toward the Herbig Ae star AB Aur. Aims. We aim to study the abundance of sulfuretted species toward AB Aur and to constrain how different species and phases contribute to the sulfur budget. Methods. We present new NOrthern Extended Millimeter Array (NOEMA) interferometric observations of the continuum and H2S 110–101 line at 168.763 GHz toward AB Aur. We derived radial and azimuthal profiles and used them to compare the geometrical distribution of different species in the disk. Assuming local thermodynamical equilibrium (LTE), we derived column density and abundance maps for H2S, and we further used Nautilus to produce a more detailed model of the chemical abundances at different heights over the mid-plane at a distance of r = 200 au. Results. We have resolved H2S emission in the AB Aur protoplanetary disk. The emission comes from a ring extending from 0.67″ (~109 au) to 1.69″ (~275 au). Assuming T = 30 K, nH = 109 cm−3, and an ortho-to-para ratio of three, we derived a column density of (2.3 ± 0.5) × 1013 cm−2. Under simple assumptions, we derived an abundance of (3.1 ± 0.8) × 10−10 with respect to H nuclei, which we compare with Nautilus models to deepen our understanding of the sulfur chemistry in protoplanetary disks. Chemical models indicate that H2S is an important sulfur carrier in the solid and gas phase. We also find an important transition at a height of ~12 au, where the sulfur budget moves from being dominated by ice species to being dominated by gas species. Conclusions. We confirm that present-day models still struggle to simultaneously reproduce the observed column densities of the different sulfuretted species, and the observed abundances are still orders of magnitude away from the cosmic sulfur abundance. Studying sulfuretted species in detail in the different phases of the interstellar medium is key to solving the issue.

Constrained Reference Star Differential Imaging: Enabling High-fidelity Imagery of Highly Structured Circumstellar Disks * * Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

High-contrast imaging presents us with the opportunity to study circumstellar disks and the planets still embedded within them, providing key insights into the formation and evolution of planetary systems. However, the postprocessing techniques that are often needed to suppress stellar halo light typically result in significant and variable loss of circumstellar light, even when using relatively conservative approaches like reference star differential imaging (RDI). We introduce “constrained reference star differential imaging” (constrained RDI), a new class of RDI point-spread-function (PSF) subtraction techniques for systems with circumstellar disks. Constrained RDI utilizes either high-resolution polarized-intensity (PI) images or disk models to severely limit or even eliminate the signal loss due to oversubtraction that is common to RDI. We demonstrate the ability of constrained RDI utilizing polarimetric data to yield an oversubtraction-free detection of the AB Aurigae protoplanetary disk in total intensity. PI-constrained RDI allows us to decisively recover the spectral signature of the confirmed, recently discovered protoplanet, AB Aurigae b. We further demonstrate that constrained RDI can be a powerful analysis tool for soon-to-be-acquired James Webb Space Telescope coronagraphic imaging of disks. In both cases, constrained RDI provides analysis-ready products that enable more detailed studies of disks and more robust verification of embedded exoplanets.

HST/WFC3 Hα Direct-imaging Detection of a Pointlike Source in the Disk Cavity of AB Aur

Accreting protoplanets enable the direct characterization of planet formation. As part of a high-contrast imaging search for accreting planets with the Hubble Space Telescope (HST) Wide Field Camera 3, we present Hα images of AB Aurigae (AB Aur), a Herbig Ae/Be star harboring a transition disk. The data were collected in two epochs of direct-imaging observations using the F656N narrowband filter. After subtracting the point-spread function of the primary star, we identify a pointlike source located at a position angle of 182.°5 ± 1.°4 and a separation of 600 ± 22 mas relative to the host star. The position is consistent with the recently identified protoplanet candidate AB Aur b. The source is visible in two individual epochs separated by ∼50 days, and the Hα intensities in the two epochs agree. The Hα flux density is F ν = 1.5 ± 0.4 mJy, 3.2 ± 0.9 times the optical continuum determined by published HST/STIS photometry. In comparison to PDS 70 b and c, the Hα excess emission is weak. The central star is accreting and the stellar Hα emission has a similar line-to-continuum ratio as seen in AB Aur b. We conclude that both planetary accretion and scattered stellar light are possible sources of the Hα emission, and the Hα detection alone does not validate AB Aur b as an accreting protoplanet. Disentangling the origin of the emission will be crucial for probing planet formation in the AB Aur disk.

Images of embedded Jovian planet formation at a wide separation around AB Aurigae

Direct images of protoplanets embedded in disks around infant stars provide the key to understanding the formation of gas giant planets like Jupiter. Using the Subaru Telescope and Hubble Space Telescope, we find evidence for a jovian protoplanet around AB Aurigae orbiting at a wide projected separation (93 au), likely responsible for multiple planet-induced features in the disk. Its emission is reproducible as reprocessed radiation from an embedded protoplanet. We also identify two structures located at 430-580 au that are candidate sites of planet formation. These data reveal planet formation in the embedded phase and a protoplanet discovery at wide, > 50 au separations characteristic of most imaged exoplanets. With at least one clump-like protoplanet and multiple spiral arms, the AB Aur system may also provide the evidence for a long-considered alternative to the canonical model for Jupiter's formation: disk (gravitational) instability.

Detection of Near-infrared Water Ice at the Surface of the (Pre)Transitional Disk of AB Aur: Informing Icy Grain Abundance, Composition, and Size

Abstract We present near-infrared Large Binocular Telescope LMIRCam imagery of the disk around the Herbig Ae/Be star AB Aurigae. A comparison of the surface brightness at K s (2.16 μm), H2O narrowband (3.08 μm), and L ′ (3.7 μm) allows us to probe the presence of icy grains in this (pre)transitional disk environment. By applying reference differential imaging point-spread function subtraction, we detect the disk at high signal-to-noise ratios in all three bands. We find strong morphological differences between the bands, including asymmetries consistent with the observed spiral arms within 100 au in L ′ . An apparent deficit of scattered light at 3.08 μm relative to the bracketing wavelengths (K s and L ′ ) is evocative of ice absorption at the disk surface layer. However, the Δ(K s − H2O) color is consistent with grains with little to no ice (0%–5% by mass). The Δ ( H 2 O − L ′ ) color, conversely, suggests grains with a much higher ice mass fraction (∼0.68), and the two colors cannot be reconciled under a single grain population model. Additionally, we find that the extremely red Δ ( K s − L ′ ) disk color cannot be reproduced under conventional scattered light modeling with any combination of grain parameters or reasonable local extinction values. We hypothesize that the scattering surfaces at the three wavelengths are not colocated, and that the optical depth effects in each wavelength result from probing the grain population at different disk surface depths. The morphological similarity between K s and H2O suggests that their scattering surfaces are near one another, lending credence to the Δ(K s − H2O) disk color constraint of <5% ice mass fraction for the outermost scattering disk layer.

Large Binocular Telescope Search for Companions and Substructures in the (Pre)transitional Disk of AB Aurigae

Abstract Multiwavelength high-resolution imaging of protoplanetary disks has revealed the presence of multiple, varied substructures in their dust and gas components, which might be signposts of young, forming planetary systems. AB Aurigae bears an emblematic (pre)transitional disk showing spiral structures observed in the inner cavity of the disk in both the submillimeter (Atacama Large Millimeter/submillimeter Array (ALMA); 1.3 mm, 12CO) and near-infrared (Spectro-polarimetric High-contrast Exoplanet Research; 1.5–2.5 μm) wavelengths, which have been claimed to arise from dynamical interactions with a massive companion. In this work, we present new deep K s (2.16 μm) and L′ (3.7 μm) band images of AB Aurigae obtained with the L/M-band Infrared Camera on the Large Binocular Telescope, aimed for the detection of both planetary companions and extended disk structures. No point source is recovered, in particular at the outer regions of the disk, where a putative candidate (ρ = 0.″681, PA = 7.°6) had been previously claimed. The nature of a second innermost planet candidate (ρ = 0.″16, PA = 203.°9) cannot be investigated by the new data. We are able to derive 5σ detection limits in both magnitude and mass for the system, going from 14 M Jup at 0.″3 (49 au) down to 3–4 M Jup at 0.″6 (98 au) and beyond, based on the ATMO 2020 evolutionary models. We detect the inner spiral structures (<0.″5) resolved in both CO and polarimetric H-band observations. We also recover the ring structure of the system at larger separation (0.″5–0.″7) showing a clear southeast/northwest asymmetry. This structure, observed for the first time at L′ band, remains interior to the dust cavity seen at ALMA, suggesting an efficient dust trapping mechanism at play in the disk.

AB Aur: A Rosetta stone for planet formation theories

AB Aur is a Herbig Ae star that hosts a well-known protoplanetary disk that depicts a plethora of features that can be attributed to the presence of a forming planet, such as a large inner cavity, a dust trap, and spiral arms. We present NOEMA observations of molecular species towards this transition disk around AB Aur, including CO, 13CO, C18O, HCN, HCO+, SO, H2CO, and H2S. These observations reveal complex chemistry with strong chemical differentiation and the presence of a cavity-crossing filament.

Molecules with ALMA at Planet-forming Scales (MAPS). XIX. Spiral Arms, a Tail, and Diffuse Structures Traced by CO around the GM Aur Disk

Abstract The concentric gaps and rings commonly observed in protoplanetary disks in millimeter continuum emission have lent the impression that planet formation generally proceeds within orderly, isolated systems. While deep observations of spatially resolved molecular emission have been comparatively limited, they are increasingly suggesting that some disks interact with their surroundings while planet formation is underway. We present an analysis of complex features identified around GM Aur in 12CO J = 2 − 1 images at a spatial resolution of ∼40 au. In addition to a Keplerian disk extending to a radius of ∼550 au, the CO emission traces flocculent spiral arms out to radii of ∼1200 au, a tail extending ∼1800 au southwest of GM Aur, and diffuse structures extending from the north side of the disk up to radii of ∼1900 au. The diffuse structures coincide with a “dust ribbon” previously identified in scattered light. The large-scale asymmetric gas features present a striking contrast with the mostly axisymmetric, multi-ringed millimeter continuum tracing the pebble disk. We hypothesize that GM Aur’s complex gas structures result from late infall of remnant envelope or cloud material onto the disk. The morphological similarities to the SU Aur and AB Aur systems, which are also located in the L1517 cloud, provide additional support to a scenario in which interactions with the environment are playing a role in regulating the distribution and transport of material in all three of these Class II disk systems. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

AB Aurigae: possible evidence of planet formation through the gravitational instability

ABSTRACT Recent observations of the protoplanetary disc surrounding AB Aurigae have revealed the possible presence of two giant planets in the process of forming. The young measured age of 1–4 Myr for this system allows us to place strict time constraints on the formation histories of the observed planets. Hence, we may be able to make a crucial distinction between formation through core accretion (CA) or the gravitational instability (GI), as CA formation time-scales are typically Myr whilst formation through GI will occur within the first ≈104–105 yr of disc evolution. We focus our analysis on the 4–13MJup planet observed at R ≈ 30 au. We find CA formation time-scales for such a massive planet typically exceed the system’s age. The planet’s high mass and wide orbit may instead be indicative of formation through GI. We use smoothed particle hydrodynamic simulations to determine the system’s critical disc mass for fragmentation, finding Md,crit = 0.3 M⊙. Viscous evolution models of the disc’s mass history indicate that it was likely massive enough to exceed Md,crit in the recent past; thus, it is possible that a young AB Aurigae disc may have fragmented to form multiple giant gaseous protoplanets. Calculations of the Jeans mass in an AB Aurigae-like disc find that fragments may initially form with masses 1.6–13.3MJup, consistent with the planets that have been observed. We therefore propose that the inferred planets in the disc surrounding AB Aurigae may be evidence of planet formation through GI.

Radiative Scale Height and Shadows in Protoplanetary Disks

Abstract Planets form in young circumstellar disks called protoplanetary disks. However, it is still difficult to catch planet formation in situ. Nevertheless, from recent ALMA/SPHERE data, encouraging evidence of the direct and indirect presence of embedded planets has been identified in disks around young stars: co-moving point sources, gravitational perturbations, rings, cavities, and emission dips or shadows cast on disks. The interpretation of these observations needs a robust physical framework to deduce the complex disk geometry. In particular, protoplanetary disk models usually assume the gas pressure scale height given by the ratio of the sound speed over the azimuthal velocity H/r = c s /v k. By doing so, radiative pressure fields are often ignored, which could lead to a misinterpretation of the real vertical structure of such disks. We follow the evolution of a gaseous disk with an embedded Jupiter-mass planet through hydrodynamical simulations, computing the disk scale height including radiative pressure, which was derived from a generalization of the stellar atmosphere theory. We focus on the vertical impact of the radiative pressure in the vicinity of circumplanetary disks, where temperatures can reach ≳1000 K for an accreting planet and radiative forces can overcome gravitational forces from the planet. The radiation pressure effects create a vertical, optically thick column of gas and dust at the protoplanet location, casting a shadow in scattered light. This mechanism could explain the peculiar illumination patterns observed in some disks around young stars such as HD 169142 where a moving shadow has been detected or the extremely high aspect ratio H/r ∼ 0.2 observed in systems like AB Aur and CT Cha.