Planet formation in multiple systems

Dust structures and 'traffic jams' in misaligned circumbinary discs

Hossam Aly (Centre de Recherche Astrophysique de Lyon (CRAL))

Binary systems exert a gravitational torque on misaligned discs, which induces radially deferential precession. While gas communicates the resulting warp via pressure and viscosity, weakly coupled dust particles try to precess as test particles. I will present results from recent numerical simulations exploring the dust structures caused by different precession profiles of the gas and dust components

A misaligned circumbinary planet model for HD143006 

Giulia Ballabio (Queen Mary University of London) 

In the last 10 years or so there have been huge developments with telescopes like ALMA and it is becoming evident that protoplanetary discs are characterised by substructures. Understanding the origin of these substructures is one of the few challenges to shed light on the possible mechanisms involved in planet formation. Among the myriad of observed substructures, asymmetries are definitely the most intriguing and, in particular, the ones originated by misaligned or broken discs. Misalignments within protoplanetary discs are now commonly observed, and features such as shadows in scattered light images are clear signatures of departure from a co-planar geometry. A recent and interesting case is the disc around HD143006. Observations at both optical and millimeter wavelengths show multiple substructures within this disc and are proving to be puzzling to interpret. We present a promising scenario able to explain most of the observed features and possibly making HD143006 a very unique system.

Chemistry and Kinematics of L1551 IRS 5

Fernando Cruz Saenz de Miera (Konkoly Observatory)

FU Orionis-type stars (FUors) are young stellar objects experiencing brief periods of an enhanced mass accretion rate. The episodic nature of the accretion outbursts has been proposed as a solution for the "luminosity problem" which states that protostars are less luminous that theoretically expected. These outbursts have significant effects on their surroundings and on their disks. In the case of their surroundings, these events help clear the material from their envelopes and, by driving powerful jets and outflows, it has been suggested that these outbursts can cause a change in the local stellar formation rate. Concerning the effects on the disks, the sudden dramatic increase in temperatures produces changes in the chemistry of the disk and desorbs previously frozen molecules, enabling their detection. In this work we present our ~20 au resolution observations of L1551 IRS 5, a well known protobinary in which one of the protostars is experiencing a FUor-like accretion outburst, and the results of our kinematic and chemical analysis. In our low spectral resolution observations, we detected several complex organic molecules and other interesting molecules, including extended emission from H2S, and we show how eruptive young stars can be used to study the chemical composition of protoplanetary disks.

Insight into the circumbinary disc HD142527

Himanshi Garg (Monash University)

I will be presenting results from a recently submitted paper on HD142527, for which we used ALMA observations of 12CO, 13CO and C18O J=2-1 transitions, along with the 1.3mm continuum emission, at an angular resolution of ~0.3". The primary results are: large scale spiral arm structures observed in intensity, velocity and velocity dispersion for the 12CO and 13CO gas tracers. A newly detected 12CO spiral is seen to originate from the dust horseshoe and potentially connects up a previously detected spiral, thus spanning more than 360 degrees; the spirals are observed to be rotating at super-Keplerian and/or vertically ascending, whilst the inter-spiral gas is rotating at sub-Keplerian velocities; a spatial offset between the 12CO and 13CO spirals is observed, to which we hypothesise that the spirals are surfing on the vertical temperature gradient; we reconstruct column density maps of the outer disc and compute a Stokes number of ~1 within the dust horseshoe, where column density is seen to peak, thus confirming radial and azimuthal trapping within the horseshoe; an eccentricity between 0.3 to 0.45 is measured for the central cavity.

Spirals, shadows and precession in HD 100453

Jean-François Gonzalez (Centre de Recherche Astrophysique de Lyon, France)

The protoplanetary disc of HD 100453 exhibits a curious combination of spirals, shadows and a relative misalignment between the observed outer disc and inferred inner disc. This disc is accompanied by a secondary star on a bound orbit exterior to the outer disc and recent observations have suggested there may be an additional low-mass companion residing within the disc inner cavity. We study the system by running SPH simulations of the gas and dust disc and computing synthetic observations from their results. We first consider seven orbital configurations for the outer companion, taken from astrometric fits, and find that the best agreement with observations is obtained for an eccentric orbit, inclined by ~60° relative to the outer disc plane. This large misalignment between the outer disc and orbit planes is compatible with the tidal evolution of a circumprimary disc in an eccentric, unequal-mass binary star. In a second set of simulations adopting this orbit, we investigate the properties of the inner companion and the origin of the misalignment between the inner and outer disc. We show that the disc structure and kinematics are consistent with a <~ 5 M_J planet located at 15-20 au and find that the disc evolution is governed by differential precession and to a lesser extent, the Kozai-Lidov effect. In our proposed model the misalignment observed between the outer and inner disc arises naturally as a result of the misaligned outer companion driving the outer disc to precess more rapidly than the inner disc.

A young triple star system with misaligned disk/orbit planes shaped by disk tearing

Stefan Kraus (University of Exeter)

GW Ori is a T Tauri-star triple system with an inner pair (242 day period) and an outer companion (4217 day period). We monitored the orbital motion of the system over the full 11 years period and derived a full RV+astrometry orbit solution, which reveals that the orbital plane of the inner and outer companion are misaligned with respect to each other. Furthermore, we image the circumtriple disc using ALMA, SPHERE, and GPI and discover three rings in thermal light and an asymmetric structure with radial shadows in scattered light. The inner-most ring is eccentric (e=0.3; 43 au radius) and strongly misaligned both with respect to the orbital planes and with respect to the outer disc. Based on the measured triple star orbits and disc properties, we conducted smoothed particle hydrodynamic simulations which show that the system is susceptible to the disc tearing effect, where the gravitational torque of the misaligned companion tears the disc apart into distinct rings that precess independently around the central objects. The ring might offer suitable conditions for planet formation, providing a mechanism for forming wide-separation planets on highly oblique orbits. Modeling the scattered light signatures and the shape of the shadows cast by the misaligned ring allows us derive the shape and 3-dimensional orientation of the disc surface, revealing that the disc is strongly warped and breaks at a radius of about 50 au.

Searching for Hidden Companions With T Tauri Jets

Aisling Murphy (Maynooth University)

Young stars exhibit prominent outflow features demonstrating a dynamic history of mass loss and interaction with the protoplanetary disk. MUSE is a VLT instrument providing optical IFS observations over an exceptionally large field of view and spectral range, which offers a valuable tool for examining the structure and origin of these jets. Using MUSE we have detected wiggling in the microjet from the Classical T Tauri star Th28, which we examine as a potential signature of a substellar companion.

Spirals, shadows and precession in HD100453 - Part 2

Rebecca Nealon (University of Leicester/Warwick)

The protoplanetary disc HD 100453 shows evidence of spirals arms, shadows, a misaligned inner disc, a massive inner cavity and an outer bound companion. In Part I of this work (presented by Jean-François Gonzalez) we constrain the orbit of the outer companion, establishing that it is on an orbit that is misaligned by more than 60 degrees to the main disc. Using numerical simulations and mock observations, here we explain the origin of the massive cavity, the misaligned inner disc and it's accompanying shadows. We establish that the configuration of HD 100453 is uniquely driven by the outer, bound companion.

HL tau's neighboring star is born from a dwarf disk

Mayra Osorio (Instituto de Astrofísica de Andalucía ,CSIC)

From the ALMA Science Verification data focused on the famous protostar HL tau, we found that its neighboring star, XZ Tau B, is surrounded by a disk of only 3au. The high quality image obtained from this data reveals a small cavity in the disk, so we believe it may be a miniature transition disk. Subsequent observations obtained with ALMA, this time focused on XZ Tau, confirm the size and flux density of the disk at 1.3 mm. However, the new observations do not allow us to confirm whether the disk is a transition disk, since the UV coverage was poorer than the case of the Science Verification data obtained for HL Tau. We think that if it is confirmed that XZ Tau B has a miniature transition disk, it would provide a novel window to the study of planet formation because its evolution could be faster than that of the large disks and because these small disks could be the precursors of compact exoplanetary systems.

Circummultiple/circumstellar Structures and Outflow near its Launching Point: the IRAS 16293-2422 Source A Case

Yoko Oya (The University of Tokyo)

Recently, high-resolution observations have delineated various substructures, such as rings, gaps, and spirals, in protoplanetary disks. The role of these substructures is of great interest in the planetary formation study, and at the same time, the origin of them is an important target in the disk formation study. In addition, formation of disk/envelope systems is closely related with the outflow launching via the angular momentum of the gas. We have been tackling with the early phase of the disk formation process by performing ALMA observations toward young low-mass protostellar sources. In this talk, we report our new observational results for the Class 0 close-multiple source IRAS 16293-2422 Source A. We detected the substructure in the 1-mm and 3-mm dust continuum emission, tracing the protostars A1 and A2 involved in Source A. As well, we revealed the circummultiple structure surrounding Source A and the circumstellar disk associated with the protostar A1. Their kinematic structures traced by the C17O and H2CS lines are reproduced by simple models for disk/envelope systems. We evaluated the physical parameters of models, and in particular, the specific angular momenta of these structures.
We also detected an outflow structure near its launching point in the SO line. IRAS 16293-2422 Source A has a quadruple outflow structure, and we observed the northwest-southeast one. The outflow is found to accompany a rotating motion, whose direction is consistent with that of the rotation in the disk/envelope system. Comparing the specific angular momenta of the outflow and the disk/envelope structures, we find that the outflow can extract the angular momentum of the gas in the circumstellar disk. These results provide us with a novel information on the formation of disk/envelope systems in this complex multiple source.

Binary-induced spiral arms inside the disc cavity of AB~Aurigae

Pedro Poblete (Friedrich-Schiller-Universität Jena)

In this work we demonstrate that the inner spiral structure observed in AB~Aurigae can be created by a binary star orbiting inside the dust cavity. We find that a companion with a mass-ratio of 0.25, semi-major axis of 40 au, eccentricity of 0.5, and inclination of 90° produces gaseous spirals closely matching the ones observed in 12CO (2-1) line emission. Based on dust dynamics in circumbinary discs (Poblete+2019) we constrain the inclination of the binary with respect to the circumbinary disc to range between 60° and 90°. We predict that the stellar companion is located roughly 0.18 arcsec from the central star towards the east-southeast, above the plane of the disc. Should this companion be detected in the near future, our model indicates that it should be moving away from the primary star at a rate of 6 mas/yr on the plane of the sky. Since our companion is inclined, we also predict that the spiral structure will appear to change with time, and not simply co-rotate with the companion.

Constraining the Circumbinary Disk Tilt in the KH 15D system

Michael Poon (University of Toronto)

KH 15D is a system which consists of a young, eccentric, eclipsing binary, and a circumbinary disk which obscures the binary as the disk precesses. We develop a self-consistent model that provides a reasonable fit to the photometric variability that was observed in the KH 15D system over the past more than 60 years. Our model suggests that the circumbinary disk has an inner edge at ~1 au, an outer edge at  ~ a few au, and that the disk is misaligned relative to the stellar binary by ∼3-15 degrees, with the inner edge more inclined than the outer edge. The difference between the inclinations (warp) and longitude of ascending nodes (twist) at the inner and outer edges of the disk are of order ∼10 degrees and ∼15 degrees, respectively. We also provide constraints on other properties of the disk, such as the precession period and surface density profile. Our work demonstrates the power of photometric data in constraining the physical properties of planet-forming circumbinary disks.

ALMA Observations of the Circumbinary Disk in the Protostellar Binary System L1551 IRS 5: Resolving the Gas Motion inside the Hill Radii

Shigehisa Takakuwa (Kagoshima University, Japan)

We will report ALMA results of the protostellar binary system L1551 IRS 5 in the 0.9-mm dust-continuum and C18O (3-2) emission at an angular resolution of ~0.1 arcsec. The 0.9-mm dust-continuum image reveals disks associated with the individual binary protostars, circumstellar disks (hereafter CSDs), as well as a disk surrounding the binary, circumbinary disk (CBD). The CBD exhibits a two spiral-arm feature, and each spiral arm connects to the CSD of each binary star. The C18O (3-2) emission shows rotating and expanding motions in these spiral arms. Our numerical simulations reproduce the spiral feature and the expanding gas motion, which are caused by the gravitational torques from the binary. Our ALMA data also show transition from the CBD rotation to the individual CSD rotations around the inferred L2 and L3 Lagrangian points. Furthermore, between the binary stars a gas component with a linear velocity gradient passing through the L1 stagnation point is also found. The identified gas component and the velocity structure can be interpreted as a bridging gas stream that connects the two CSDs through the L1 point. These observed gas motions inside the Hill radii are also reproduced with our numerical simulations. In our presentation we will review these observed features along with our numerical simulations of the binary system.