Multiplicity in Young Stars

26 Aug 2025, 08:30 → 29 Aug 2025, 15:00 Europe/Copenhagen

Niels Bohr Institute

Multiplicity in young stars Program.pdf

Tuesday 26 August

08:30 → 09:00 Registration and coffee

09:00 → 09:50 Introduction

09:00 Welcome

Speaker: Prof. Troels Haugbølle (Niels Bohr Institute)

09:10 Multiplicity - a historical review

Speaker: Hans Zinnecker (Astrophysical Institute Potsdam)

09:50 → 10:30 Formation pathways

09:50 A Comprehensive Characterization of Protostellar Multiplicity within 500 pc

The advent of efficient surveys at high resolution and sensitivity with ALMA (and the VLA) has enabled a comprehensive and robust characterization of protostellar multiplicity in the solar neighborhood (within 500 pc). We will present a characterization of multiplicity statistics for ~600 systems, comprising Orion, Serpens-Aquila, Perseus, Ophiuchus, and Taurus, representing the majority of low- to intermediate-mass protostars in the solar neighborhood. We have measured the separations of companion protostars from 1000s of au down to ~30 au. The distribution of separations typically exhibits two peaks, one at ~100 au and another at ~3000 au, and these peaks thought to result from disk and turbulent fragmentation, respectively. We compare the separation distributions between regions and to more-evolved samples, finding that protostars in the nearby star-forming regions are statistically indistinguishable from each other, but statistically inconsistent with more-evolved populations. Furthermore, there is tentative evidence for changes in multiplicity statistics as a function of evolution (i.e., protostellar class). Molecular line observations of close (< 500 au) multiples points to ~50% of close multiples forming within rotationally-supported structures, consistent with disk fragmentation, and outflows tend to be driven orthogonal to the projected plane of close companion sources. Finally, observations of close multiples at ALMA's highest available resolutions find correlated alignment of circumstellar disks around each protostar, further pointing to disk fragmentation for multiples with separations < 100 au.

Speaker: Dr John Tobin (NRAO)

10:30 → 11:00 Coffee break

11:00 → 12:20 Formation pathways

11:00 A multi-scale ALMA view of starless and protostellar dense cores in Aquila

Star formation is governed by many complex physical processes that intertwine at all scales, some of which include turbulent motions, mass inflow, thermal pressure and magnetic fields. These physical processes have direct impacts on the formation of many types of systems (e.g., single stars, multiple star systems, stellar clusters). Recent work studying a survey of all starless cores in Orion B North with ALMA (Fielder+2024) showed that protostellar cores and only a few percent of starless cores have high-density material (>10^7 cm^-3) on small scales (~100’s of au), with only a few complex fragmenting regions. We extend this view into fragmentation with a new archival ALMA main- and compact-array analysis of the 100 most gravitationally unstable cores in the Aquila region. We utilize data from many spatial scales to show how the material is arranged on small and larger scales. A few % of the starless cores are high-density (>10^7 cm^-3) on similarly small scales (~100’s of au), while around half of the starless cores have moderately high densities (>10^6 cm^-3) on larger spatial scales (~1000’s of au). We also find a significant number (~20%) of unstable cores that show highly complex fragmentation morphologies, which are strong candidates for higher-order multiple systems.

Speaker: Mr Samuel Fielder (University of Victoria)

11:20 Formation of high-mass multiple-star systems: early migration and stellar mergers

Massive stars, those several times the mass of the Sun, are typically found in multiple-star systems. A key unresolved question in astronomy is how close binary-star systems, those with separations ≲ 1 au, form. Because protostellar objects are generally too extended to initially fit into these tight configurations, migration is considered to play an essential in their formation. In this talk, we explore the formation and migration of massive stars at solar metallicity using a simulation that begins with a 6300 solar mass cloud. This simulation achieves spatial and mass resolutions of approximately 1 au and 0.01 solar masses, respectively, while also incorporating feedback processes. Our results indicate that stars more massive than 2 solar masses predominantly assemble in binary- or triple-star configurations, in agreement with observations. In most of these systems, the inner binary hardens by 1–3 orders of magnitude during the first 2 Myr, which represents the total duration of the simulation. Notably, disks are nearly ubiquitous during the hardening phase. Finally, stars more massive than 2 solar masses may undergo multiple stellar merger events during their evolution –up to four for the most massive systems. This suggests a tentative correlation between mass, multiplicity, and merger events in high-mass stars.

Speaker: Dr Alejandro Vigna Gomez (Max Planck Institute for Astrophysics)

11:40 Orbital statistics of multiple systems formed from small-N subclusters

We explore the multiplicity statistics (semi-major axis, period, eccentricity, degree of dynamical biasing, binary and tertiary mass ratios, mutual orbital inclination, and ejection velocity) of stars born in small-N subclusters using numerical N-body experiments. In these experiments, subclusters are evolved as if they are the fragmentation products of a single isolated prestellar core from which most of the natal gas has already been dispersed. Only two parameters are important: the number of stars in the subcluster, N, and the fraction of kinetic energy in ordered rotation, α_rot. We find that increasing N has the effect of systematically decreasing the binary semimajor axis and period, the degree of dynamical biasing, and the stellar ejection timescale, while increasing the number of high-eccentricity orbits and multiples formed. Changing N has very little effect on the semi-major axes or periods of higher-order orbits, or on binary or tertiary mass ratios. The main effect of α_rot is in moderating the distribution of mutual orbital inclination, with moderate α_rot producing a distribution of orbital inclinations for triple systems which is most consistent with observed values. Triples frequently form in high-inclination orbits without the assistance of von Zeipel-Lidov-Kozai cycles, and a significant proportion (21 ± 1%) of the cores produce more than one multiple.

Speaker: Ms Hannah Ambrose (Cardiff University)

12:00 Soft No More: Gas Shielding Protects Soft Binaries from Disruption in Gas-rich Environments

Binaries in dense environments are traditionally classified as soft or hard based on their binding energy relative to the kinetic energy of surrounding stars. Heggie's law suggests that stellar encounters tend to soften soft binaries and harden hard binaries, altering their separations. However, interactions with gas in such environments can significantly modify this behavior. This study investigates the impact of gas on binary softening and its consequences. We find that gas interactions can actually harden binaries, extending the soft–hard boundary to larger separations. This introduces a "shielding radius" within which binaries are likely to harden due to gas interactions, surpassing the traditional soft–hard limit. Consequently, a notable portion of binaries initially classified as "soft" may become "hard" when both gas and stars are considered. We propose a two-stage formation process for hard binaries: initial soft binary formation, either dynamically or through gas-assisted capture, followed by gas-induced hardening before eventual disruption. In environments with low gas density but high gas content, the shielding radius could exceed the typical hard–soft limit by 1 order of magnitude, leading to a significant fraction of originally soft binaries effectively becoming hard. Conversely, in high-gas-density environments, gas-induced hardening may dominate, potentially rendering the entire binary population hard. Gas hardening emerges as a crucial factor in shaping binary populations in gas-rich settings, such as clusters, star-forming regions, and possibly active galactic nucleus disks. This highlights the complex interplay between gas dynamics and stellar interactions in binary evolution within dense environments.

Speaker: Dr Mor Rozner (Cambridge & IAS)

12:20 → 13:40 Lunch break

13:40 → 14:40 Formation pathways

13:40 The interplay of multiplicity and infall

Traditionally, star formation has been viewed as a distinct phase where a collapsing gas and dust cloud forms a protostar and its disk. However, recent observations and simulations reveal that star formation is prolonged and dynamic, especially in binary and multiple star systems. Chaotic late infall continues long after the initial collapse, feeding planet-forming disks through filamentary streamers and interacting with the surrounding medium. In binary systems, these interactions are further influenced by companion stars. Using 3D non-ideal magnetohydrodynamics simulations, we show the significance of post-collapse accretion modes, including massive infall and Bondi-Hoyle-Lyttleton accretion, in both single and multiple star systems. Infall replenishes the disk’s mass for planet formation and induces disk misalignment, which may be observable. In binary systems, companion stars can enhance these effects, leading to more complex accretion patterns. Our results explain the correlation between stellar mass and disk size, and the intrinsic scatter in this relationship, particularly in multiple star systems. We also explore the implications of infall for the solar system's history and propose that Peter Pan disks—disks around stars older than 10 million years—may be younger than previously thought. This phenomenon could be more prevalent in multiple star systems. Overall, our results emphasize interpreting planet formation within the dynamic framework of star formation in both single and multiple star systems.

Speaker: Dr Michael Küffmeier (University of Copenhagen)

14:20 Low mass binaries are bound from birth

Observations show that most main sequence stars, unlike our Sun, are members of systems with two or more stars. Exactly how these systems form and when these stars become gravitationally bound together remains debated, since resolving the earliest stages of star formation is challenging. I will discuss how multiple star systems form and evolve in simulations of star cluster formation representative of typical Milky Way conditions that include all key physics and stellar feedback mechanisms. In particular, I will show ~70-80% of binaries form as bound systems, rather than from capture of initially unbound stars.

Speaker: Dr Aleksey Generozov (University of Texas at Austin)

14:40 → 15:10 Coffee break

15:10 → 16:30 Formation pathways

15:10 Exploring the Role of Star-forming Environment on the Formation of Stellar Multiples

Multiplicity is a common outcome of the star formation process. Previous stellar multiplicity studies in young, star-forming regions have identified a trend in wide (100 - 10,000 au) companion frequency with stellar density where higher frequencies are found in lower density regions, but it is unclear if this is primordial due to star formation physics or from dynamical interactions. Companions at separations < 100 au are likely to survive to reach the Galactic field and are important tracers of their birth environment. Using Gaia pre-selection, we performed large multiplicity surveys in Orion OB1a and OB1b (10 and 5 Myr, respectively) with Gemini speckle interferometry and Keck/NIRC2 imaging and aperture-masking interferometry down to 10 au. With the Hubble Space Telescope Advanced Camera for Surveys, we characterized the companion population in NGC 1333 (1-3 Myr) by fitting empirical point-spread function models to Hubble Space Telescope ACS data. We present our results which allow us to explore the origin of the Galactic field population and the role of environment on the formation of stellar multiples.

Speaker: Dr Matthew De Furio (University of Texas at Austin)

15:30 Poster Lightning Presentations

16:30 → 18:30 Poster session and welcome reception

Wednesday 27 August

09:00 → 10:20 Multiplicity in clustered environments

09:00 Invited review

Speaker: Dr Abigail Frost (ESO)

09:40 The Formation of Low-mass Multiple Systems in1 High-mass Cluster-forming Regions

Most stars form in multiple systems, profoundly influencing numerous astronomical phenomena intrinsically linked to multiplicity. However, our knowledge of the process by which multiple stellar systems form remains incomplete and is biased toward nearby molecular clouds forming only low-mass stars, which do not represent the typical stellar population of the Galaxy. Most stars form within dense cores in clusters alongside high-mass stars, as the Sun likely did. Here, we report deep ALMA dust continuum observations at 160 au spatial resolution, revealing an unprecedented 67 low-mass multiple systems embedded in 23 high-mass cluster-forming regions. The companion separation distribution exhibits a distinct peak at ~1000 au, in contrast to the 4000 au peak observed in nearby low-mass regions, likely resulting from core fragmentation. We demonstrate that the environment in which multiple systems form affects the characteristic initial fragmentation scale, with multiples formed in high-mass cluster-forming regions having separations more similar to those found in main-sequence stars. In addition, the multiplicity fraction remains constant as stellar density increases.

Speaker: Ms Qiuyi Luo (Shanghai Astronomical Observatory, CAS)

10:00 Probing Multiplicity in Young and Embedded Massive Galactic Star Clusters with NACO/VLT

The formation of stellar multiples is a frequent occurrence during the formation and evolution of massive stars. The characterisation of stellar multiplicity rates, separations, and mass ratios is a crucial input for star formation theories, and for investigating how the natal clusters influence the process. However, the multiplicity of young, massive stars remains poorly understood because of the challenges in observing them and the complexity of the dense cluster environments they are found in, especially during the first few million years of their formation. In this work, we focus on investigating close multiplicity (separations < 2000 AU) of young massive stars in embedded clusters at distances of 1.3 – 1.75 kpc and ages of 1 – 2 .75 Myr, aiming to fill the gap in our understanding of their multiplicity. We use 28′′ × 28′′ NACO/VLT near-infrared K-band images mapping multiple fields of four Galactic massive star clusters at Z ∼ Z⊙, called DBS 113, DBS 121, Hourglass, and RCW 108, to determine their multiplicity fractions (MF) and companion fractions (CF). We identify stars down to K ≈ 18 around cluster members previously identified through JHK colour-colour analysis using VVV data. We then assess the true companionship of stars around the cluster members by statistical means using chance alignment probabilities. This is achieved by estimating stellar density for each cluster using the Besancon stellar population synthesis model. We trace back the parenthood of the companions using a tree-based clustering algorithm called dendrogram. We see that both MF and CF increase with probed separations, while we do not notice a strong correlation with cluster age. This suggests that the cluster environment and its properties—such as the gas and dust environment, and initial mass function—might play a more pronounced role in influencing multiplicity at such early stages of evolution.

Speaker: Mr Chinmaya Nagar (University of Cologne)

10:20 → 10:50 Coffee Break

10:50 → 12:10 Multiplicity in clustered environments

10:50 Wide binaries in the Orion OB association

Some studies indicate that OB associations are not just structures that evolved from dispersing dense stellar clusters, but most likely were formed in a configuration similar to how they appear today - i.e. as an assembly of loose stellar groups. Early evolution of multiple stellar systems in such low density subgroups might be significantly different from their evolution in a dense stellar cluster environment. While wide binaries are dynamically destroyed in a dense stellar environment, in agreement with the observed low fractions of wide binaries in young stellar clusters, they could be numerous in OB associations. The fraction of wide binaries in OB associations is therefore an important diagnostics not only for binary star formation studies but also for testing the aforementioned finding. In this contribution I will present the results of our imaging survey for wide binaries in the Orion OB 1a/1b associations. With a total target sample size of ~1150 stars we are able to put our derived wide binary fraction on solid statistical grounds. We compare our results with the expected fractions of companions found in dense young stellar clusters and in T-associations, and analyse our findings in the context of binary star formation in OB-associations.

Speaker: Dr Monika Petr-Gotzens (European Southern Observatory)

11:10 The Multiplicity of Massive Young Stellar Objects

The question of the formation of massive stars, and massive binaries in particular is still very open. More than 90%, and perhaps all, high-mass main sequence stars are found in binary systems, while close massive binaries are responsible for some of the most energetic phenomena in the Universe. In order to understand massive stars and their evolution, it is therefore essential to find out how they formed in binary or multiple systems and how these primordial binaries evolve into the Main Sequence systems observed. This requires studying them at the earliest stages possible, however, data on young massive binary systems is very sparse. We are carrying out a project to determine the Multiplicity of Young MAssive Stars at all scales. The project uses methods probing different separations: with increasing separation, these range from spectroscopy, interferometry, proper motion studies to imaging. I will give an overview of the project and results obtained so far. Amongst others, these indicate that the binarity of massive stars is already large at a young stage, that mass ratios are close to one rather than randomly sampled from the Initial Mass Function, while there is evidence that binaries harden as a function of time. I will present recent modelling results indicating that the large multiplicity fractions of Massive Young Stars at all scales implies that massive stars predominately form in multiple systems rather than binary systems. This is supported by the observations when data probing the various separation scales are combined.

Speaker: Dr René Oudmaijer (Royal Observatory of Belgium)

11:30 From Wide to Close: how migration shapes massive binary systems

Massive stars, characterized by their extreme luminosities and crucial roles in stellar and galactic evolution, are often found in complex multiple systems. Understanding their primordial multiplicity provides key insights into their formation, interactions, and ultimate fates. One proposed mechanism to explain the relatively high rate (>30%) of massive short-period (< few months) binaries is the migration scenario, where close binaries form through companion hardening, either via disk-driven inward migration or orbital shrinkage caused by a tertiary, outer companion. To test this scenario it is essential to constrain the true end product of massive binary formation, which in turn requires observations of very young (<1 Myr) massive star-forming regions. This talk will first present interferometric observations (GRAVITY/VLTI) of six young massive stars in M17, detecting companions within 2–120 AU. The presence of numerous relatively massive companions at separations comparable to accretion disk sizes (10–1000 AU) provides strong evidence supporting the migration scenario. In the second part, we will discuss recent findings on a handful of main-sequence massive hierarchical triples, highlighting how the presence of a third companion, detected via interferometry (VLTI/PIONIER), can drive close binarity through Kozai-Lidov oscillations. These oscillations, governed by the relative inclination and eccentricity of the subsystems, induce periodic changes in orbital parameters, ultimately leading to orbital shrinking and circularization, forming short-period binaries. Additionally, we will explore realistic mass and separation distributions for such systems and discuss their long-term stability criterion. By comparing typical timescales, we show that both channels are not mutually exclusive and may work together to explain the observed relatively high fraction of few-Myr-old short-period massive binaries. This talk will explore two key aspects of the migration scenario, showcasing how optical long-baseline interferometry constrains multiplicity and orbital properties in young massive stars. In conclusion, we will briefly discuss ongoing observational efforts to expand our statistical sample and foresee potential collaborations to extend this study to other star-forming regions or young clusters.

Speaker: Dr Emma Bordier (I. Physics Institute - University of Cologne)

11:50 Multiplicity properties of massive stars through high-contrast imaging

Detailed observations of the multiplicity properties of massive stars probing the full mass ratio and separation range are crucial for constraining massive star and binary formation models. However, the low-mass end of the companion mass function around massive stars remains largely unexplored due to the contrast limitations of previous spectroscopic and interferometric studies. Recently, the Carina High-contrast Imaging Project of massive Stars (CHIPS) showed that high-contrast imaging (VLT/SPHERE) enables us to explore the brown dwarf mass regime around massive stars at separations between 0”.15 and 6” (~400-15000 AU). These observations provide key insights into whether low-mass (sub)stellar companions can form and survive in the harsh UV radiation fields of massive stars. In this talk, I will present the bias-corrected multiplicity fractions obtained from recent high-contrast imaging surveys of massive stars in various environments (Carina region, Sco OB1, M17). Additionally, I will discuss follow-up observations of multiple low-mass stellar and substellar companions in Sco OB1 and M17.

Speaker: Ms Tinne Pauwels (KU Leuven)

12:10 → 12:20 Conference Photo

12:20 → 13:40 Lunch

13:40 → 14:40 Planet formation and stability in multiple systems

13:40 Growing binary/multiple star systems as sources of free floating planets and FU Ori outbursts

We report 3D SPH and 2D fixed grid simulations of massive protoplanetary discs fed by external mass deposition. We find that systems in which catastrophic disc fragmentation is triggered exhibit very complex dynamics of multiple fragments interacting with each other and with the disc. We find these systems to be natural birth places of binary/multiple stellar systems AND of free floating planets (FFPs). The former grow by rapid gas accretion and/or mergers with other fragments; the latter are less massive fragments that are usually ejected from the disc by the most massive secondary object. We show that this scenario for the origin of FFPs can be observationally distinguished from the scenarios advanced in the Core Accretion field through the age, mass function and the velocity dispersion of FFPs. Additionally, the oligarchic growth of protostars in these discs may be a promising framework for FU Ori outbursts in the scenarios where they arise due to super-Jupiter mass fragments being accreted by the growing protostars (e.g., Vorobyov & Base 2015; Nayakshin, Owen & Elbakyan 2023).

Speaker: Prof. Sergei Nayakshin (University of Leicester)

14:00 The ODISEA Project: Planet Formation in the Ophiuchus Molecular Cloud

Ophiuchus is one of the closest and youngest star-forming regions to our planet, making it a perfect laboratory to study star and planet formation. The Ophiuchus DIsk Survey Employing ALMA (ODISEA) originally targeted ~300 YSOs at different stages of evolution (from SED Class I to III) in Band-6 and at modest resolution. After several follow up programs, there are now observations on multiple frequencies, sensitivities and angular resolutions. That data allows us to explore the “big picture” of disk evolution and planet formation at different scales, from outflows and envelopes, to planet-induced substructures in protoplanetary disk. In this talk, I will present a summary of the main results from the ODISEA project, including on environment around very low mass stars and proto brown dwarfs, the large gaseous structures interacting with the ISM, the statistics for disk masses and sizes around single and multiple systems, and the evolution of substructures within massive protoplanetary disks. Furthermore I will also present ongoing projects on the statistics of molecular line structures along the whole ODISEA survey, the multi frequency analysis of 100 Ophiuchus sources, the detection of substructure in new Band-8 data, and the very recent modeling of planet-induced substructures driving the observed diversity of disk morphology (see companion figure). In conclusion, this is going to be a “brief” summary of the main past, present and future results on disk evolution and planet formation from the ODISEA survey.

Speaker: Dr Camilo González-Ruilova (Universidad de Santiago de Chile)

14:20 Polar circumbinary discs and planets around binary star systems

Most stars form within dense stellar clusters and reside in binary systems, where circumbinary discs of gas and dust commonly envelop the binary pair. Observations reveal frequent misalignments between these discs and the binary orbital plane. Over time, such misaligned circumbinary discs dynamically evolve toward either coplanar or polar (perpendicular to the binary plane) configurations. Here, I present 3D hydrodynamical simulations incorporating multi-fluid physics (gas and dust) alongside linear theory to investigate the evolution of highly tilted circumbinary discs. A key finding is the formation of localized dust concentration regions—termed "dust traffic jams"—resulting from differential nodal precession between the gas and dust components within the tilted disc. These regions create favorable conditions for the potential formation of misaligned or polar circumbinary planets. This process persists across varied disc and binary parameters, underscoring its robustness. Additionally, I review candidate systems with tentative evidence for the first polar circumbinary planets. These results advance our understanding of protoplanetary disc evolution in binary environments, offering critical insights into planet formation pathways and the diversity of exoplanetary systems.

Speaker: Dr Jeremy Smallwood (University of Oklahoma)

14:40 → 15:10 Coffee break

15:10 → 16:30 Planet formation and stability in multiple systems

15:10 Invited review

Speaker: Prof. Kaitlin Kratter (University of Arizona)

15:50 Disk evolution and planet formation in the Triple HD104237 System

Exoplanets are rare in close (a$<$10~au) binaries. Consequently, close-on binary pairs are believed to halt planet formation by depleting planet-building material from the disk. We observed the young, gas-rich, multiple system HD~104237 at 0.1\arcsec resolution with the Atacama Large Millimeter/submillimeter Array (ALMA) to study the effects of binarity on the disk's evolution. The new ALMA observations resolve the gas and dust disks for the first time. We find that the dust disk has a radius of $\sim$7-8~au, consistent with previous upper limits from the literature, and an inclination of 19.5$\pm$5.9$^{\circ}$. The $^{12}$CO(2--1) data is used to constrain the gas disk geometry. The gas disk inclination is 22.6$\pm$0.6$^{\circ}$ and from the moment 0 image we estimate a radius of $\sim$26~au, also consistent with previous estimates of the gas disk geometry. The dust and gas disks are therefore compact and coplanar with the binary orbit. The outer disk radius aligns with semi-analytical predictions of truncation by the external binary companion, HD~104237-B. The disk has a mass similar to those of of much larger protoplanetary disks, suggesting that the presence of the close-in and the external companion does not deplete the disk of planet-forming material, but does halt planet formation beyond 10-20~au. We also report the serendipitous detection of protoplanetary disk emission around another member of $\epsilon$ Cha association, HD~104237-E, known to have excess dust emission. The dust and gas disks around this object fell within the observation's primary beam. We report the 1.3~mm and $^{12}$CO(2--1) integrated fluxes around this source.

Speaker: Dr Antonio Hales (NRAO)

16:10 The critical role of cooling in shaping circumbinary disk cavities: hydrodynamics and observational comparisons

Multiwavelength and time-variable observations of circumbinary disks and their inner cavities have been made possible thanks to recent advancements in interferometry with ALMA and the VLTI instruments. These disks show large, eccentric cavities whose properties depend on the disk's aspect ratio, viscosity, binary eccentricity and mass ratio. Recent studies show that radiative cooling significantly affects the shape and size of gaps/cavities created by planets. We study the role of different thermodynamical models—locally isothermal, radiatively cooled, and parameterized β-cooling—on the cavity properties of circumbinary disks using 2D hydrodynamical simulations for varying binary eccentricities, and over very long timescales. While radiative and locally isothermal models yield comparable cavity shapes, the inner disk structure and precession rates differ. With β-cooled models, the shape and size of the cavity changes dramatically towards values of β = 1. We propose a parameterized β model that captures the shorter cooling timescales within the cavity, closely reproducing radiative model results, emphasizing the importance of accurate thermodynamic treatment. Additionally, new GRAVITY and PIONIER reductions for HD 45677, combined with Brγ imaging, reveal orbital motion of a dusty feature and high-velocity deviations indicative of a potential secondary companion. Our hydrodynamical models and mock observations constrain the cavity temperature and place an upper limit on the binary mass ratio, offering insights into the system's dynamics.

Speaker: Ms Prakruti Sudarshan (MPIA Heidelberg)

Thursday 28 August

09:00 → 10:20 Discs in multiple systems

09:00 Chopping and changing: How multiplicity shapes disc evolution

The last 10 years have brought spectacular images of discs around young stars. One of the main things we have learnt is that planet formation takes place already during the 'violent' phase of star formation, where interactions with other stars are important. This can help shed light on old mysteries in disc evolution, such as how accretion bursts and dust melting events may be triggered by stellar flybys. I will argue (controversially) that much of disc evolution might actually be controlled by tidal interactions, both from the inside and outside. If true, this would be a radical departure from a picture of steady viscous disc evolution over 10s of millions of years. I will also discuss how infall can dramatically change disc evolution, and why infall and multiplicity are correlated.

Speaker: Prof. Daniel Price (Monash University)

09:40 The Dynamics of Disks in Multi-Star Systems

The process of star formation frequently produces multi-star systems, with nearly half of Sun-like stars formed as part of a pair. Protoplanetary disks in these systems experience additional gravitational interactions and are subject to environments unlike those around single stars, influencing their evolution and subsequent planet formation. We investigate a potential source of variability in misaligned circumbinary disks due to changing illumination from the central stars. We also study numerical simulations of disks around triple star systems, and find that misaligment angles between the disk and stellar components are frequently nonzero due to orbital precession of the stellar orbits, in rough agreement with the observed population of circumtriple disks.

Speaker: Dr Ian Rabago (Universita Degli Studi di Milano)

10:00 On the lower limit of circumbinary disk fragmentation

In recent years, many wide orbit circumbinary giant planets have been discovered. These may have formed by gravitational fragmentation of circumbinary disks. The aim of this work is to investigate the lower limit of circumbinary disc fragmentation and to compare it to the lower limit of circumstellar disc fragmentation. Using the smoothed particle hydrodynamics code SEREN with the Lombardi method of radiative transfer, we run a series of simulations varying the binary separation, mass ratio and eccentricity to see their effect on disk fragmentation and planet formation. For the circumstellar disk, we find a lowest disk-to-star mass ratio that allows for fragmentation to be ~0.3 which is in agreement with past studies. However, for the circumbinary disk case fragmentation is possible down to a mass ratio of 0.14 - 0.17. We conclude that disk fragmentation is easier around circumbinary disks, facilitating the formation of giant planets.

Speaker: Mr Matt Teasdale (University of Central Lancashire)

10:20 → 10:50 Coffee break

10:50 → 12:10 Discs in multiple systems

10:50 Where do grains grow in binary systems ? A 3D hydrodynamical approach

Most of young stars are part of multiple stellar systems, where star-disc interactions shape protoplanetary disc and impact the planet formation processes. In particular, tidal perturbations from companion stars imprint high dust velocity dispersion, hindering dust particle growth. However the formation of substructures, like spiral arms or horseshoes, leads to the creation of regions of high density that enhance dust growth. The balance of these competing effects remains unclear. We performed 3D hydrodynamical simulations of binary systems accounting for the dust growth and fragmentation. Binary stars were initialized with varying eccentricity, with either circumstellar or circumbinary discs. Our analysis focused on dust spatial and size evolution, linking substructures to grain growth and assessing the potential for streaming instability. Preliminary results suggest that substructures can mitigate high collisional velocity and allow for dust aggregation. Solids within circumstellar discs face growth complications due to companion star perturbations, while circumbinary discs circumbinary discs can concentrate dust at the cavity edge, promoting growth. Conditions favorable to planet formation are present but occur over smaller regions than in single-star systems. This work contributes to a better understanding of the early stages of planetesimal formation in the complex environments of multiple stellar systems.

Speaker: Mr Antoine Alaguero (CNRS / IPAG)

11:10 Sculpting the inner edge - themodynamic effects on circumbinay discs

Binaries carve unique inner cavities into their young surrounding protoplanetary disc. These cavities can be as large and eccentric like GG Tau A. However, with recent observation around tighter binaries like CsCha reveal circular, smaller cavities when compared to the separation of the binary stars. Since similar dynamics carves these cavities, what are we missing when to understand the difference in the shape in these systems? On relevant difference is the heating and cooling in the disc. As viscous heating loses its effect and emission cooling becomes more efficient further out where discs are thinner, the thermodynamic state of the disc changes with distance. In our work, we perform perform 2D hydrodynamic simulations including these thermodynamic effects within the disc. The local cooling time can effect how efficient wave propagate within the disc and, thereby, effect the shape of the circumbinary disc depending on their absolute size. Through those simulations we can explain the observed difference between the inner disc structure self-consistently.

Speaker: Dr Anna Penzlin (LMU - Munich)

12:10 → 13:20 Lunch

13:20 → 14:40 Accretion and variability in multiple systems

13:20 Accretion variability and multiplicity

In this talk, I will summarize our current knowledge about accretion in multiple systems. I will talk about the structure of the circumstellar environment depending on the separation of the components and its potential effects on the disc-to-star accretion rate. I will discuss two special cases: one is that of the pulsed accretors. These are close spectroscopic SB2 binaries on eccentric orbit, where the accretion rate onto the central binary from the circumbinary disc is modulated at the orbital period, having a peak at periastron. The other special case is protostars with periodic variability typically seen at far-infrared or millimetre wavelengths. While the central object in these deeply embedded systems is hidden, they are believed to be binaries as well. I will show results from our optical-infrared accretion variability study of several T Tauri systems, both single stars and binaries. Finally, I will discuss the effect of variable accretion on the disc properties and, consequently, its potential influence on the planet-forming material.

Speaker: Dr Ágnes Kóspál (Konkoly Observatory)

14:00 Simulated Analogues: connecting observations with simulations using Deep Learning

Recent advances in observations and theory show increasing evidence that star formation is a multi-scale problem. Global simulations that properly account for the connection from the large-scale gas flow to the accreting protostar can be used to understand protostellar systems in such a context. Given the turbulent nature of the initial conditions, it is challenging to compare models directly to specific observations. Both because a perfect match between unknown initial and boundary conditions are hard to establish, and also because observations are two dimensional projections, at one point in time. To overcome this challenge, we have used Deep Convolutional Neural Networks (DCNN) to make unbiased matching between a specific observation and large catalogues of synthetic observations using image similarity. A set of simulated analogues, with metadata such as age, mass, disk size, and other parameters, matching visually the observations may then be used as an aid in understanding and interpreting the physical and chemical structure in the observations. Using global models and statistical matching is of particular relevance for multiple protostellar systems. Only global models can provide the mass flux from the large environment, the correct anchoring of magnetic fields, and the torques and tidal forces that generate the resulting binary system. Statistical matching help us tremendously in understating the complicated kinematic structure of multiple inflows, outflows, and spiral bridges present in a protostellar binary. We will present ab initio MHD zoom-in models of three binary protostellar systems forming due to core-collapse in a large 4 pc box where we use deep adaptive meshes refinement to reach down to 0.8 to 3.2 AU resolution while following the binary stars for up to 150 kyr. This unprecedented data set allow us to better investigate the formation process of binaries in a realistic environment, provide us with a large catalogue of synthetic observations, and allow us to perform a down-selection from large datasets of synthetic images to a handful of matching candidates using Deep Convolutional Neural Networks (DCNN). The networks are used to infer several system parameters for the observed system IRAS-2A.

Speaker: Mr Vito Tuhtan (Niels Bohr Institute)

14:20 The NGC 6334-43 field with ALMA: protostellar multiplicity and a young streamer-fed protobinary

Massive stars (> 8 Msun) are likely found in binaries or higher order multiples throughout their lives; however, due to observational challenges, the relative importance of the formation mechanisms giving rise to this multiplicity are not well constrained. The youngest multiple systems, whose system parameters best constrain theoretical models, are the most deeply embedded, and in the high-mass regime only a few examples of young multiples -that have not yet developed ultracompact/hypercompact HII regions or bright IR emission- have been identified. I will present high angular resolution (~0.27”, 357 AU) ALMA observations taken as part of the The Complex Chemistry in hot Cores with ALMA (CoCCoA) survey that offer sufficient sensitivity and angular resolution to undertake a case study of multiplicity in the massive star forming environment NGC 6334-43. We identify nine protostellar and prestellar sources that reside within a single filamentary structure. A simple energy analysis using our derived system parameters is consistent with a quadruple system formed via filament fragmentation and a triple system that contains two hot core components and a young protobinary system both formed via core fragmentation. The 1530 AU protobinary is of particular interest: its components have a gas mass of 0.34 and 0.53 Msun, show very low continuum brightness temperatures (< 2 K) and display weak and sparse line emission, indicating a lack of significant central heating, which suggests a young evolutionary state. However, the nature of the system is confirmed as protostellar due to the detection of a single outflow lobe in CO. Further, the system displays a prominent spiral-arm-like feature traced by continuum and H13CO+ emission, which we characterise as a 3400 AU streamer. Two independent analyses find a substantial mass flow rate along this streamer of between 1 x 10^-5 to 8 x 10 ^-5 Msun yr^-1.

Speaker: Mr David Taylor (University of St Andrews)

14:40 → 15:10 Coffee break

15:10 → 15:50 Accretion and variability in multiple systems

15:10 Multiplicity and Inner Disk Reshaping in the Extreme Accretion Event of FUor V960 Mon

V960 Mon is a recent FUor that underwent an accretion outburst in 2014. These extreme accretion events are transient stages in the stellar evolution, each detection is a rare gem for our understanding of how stars gain their mass and interact with their environment. Particularly, V960 has been observed shortly after its outburst with VLT/SPHERE and ALMA. Recently, these observations revealed interaction with companion stars on scales >1’000au, raising the question of how the outburst is related to the large-scale environment. Given the diversity of detected features and events, V960 presents an exceptional laboratory for the study of protoplanetary environments under extreme conditions. The accretion process itself that triggers the FUor event occurs on short time scales, consistent with an origin from close to the primary of the system. An inner disk has been proposed and modeled for the purpose of understanding the effects of accretion outburst on its morphology. To test this innermost region directly, V960 has been observed in 2015 during the outburst peak and post-outburst between 2016 and 2017 by VLTI/AMBER. In this occasion, I present an analysis of these data to detect and characterize the inner disk structure which presents the most-important link between the disrupted environment and the outbursting YSO. I will focus, specifically on its size and temperature, how it interrelates with the outbursting event and then discuss the impact of V960 Mon's multiplicity on the evolution of the inner disk and the sudden trigger of the accretion flow.

Speaker: Mr Daniel Daza Valdebenito (Niels Bohr Institute)

15:30 Accretion Variability and Binarity in Young Stars: Insights from DQ Tau and Transition Disks

Accreting binary systems interact with their circumstellar and circumbinary environments, leading to variability across multiple wavelengths and timescales. Understanding these variations is essential for constraining accretion physics and disk structures. With its nearly pole-on orientation, DQ Tau allows a detailed investigation of accretion variability across different wavelengths. DQ Tau is a highly eccentric (e = 0.6), equal-mass binary that exhibits sharp accretion events at periastron passage. The short-period nature of the system (15.7 days) enables orbital motion to clear the central region around the binary, leading to the formation of up to three accretion disks. The circumbinary accretion streams are strongly influenced by the system's orbital parameters. Using VLT/UVES and VLT/X-Shooter, we have applied the broadening function (BF) technique to calculate radial velocities (RVs) across multiple epochs. The BF analysis confirms the presence of two RV signatures corresponding to the equal-mass components, as previously reported in the literature. Over multiple epochs, we observe variations in the BF peaks of both components, including changes in height (flux ratio), which depend on their orbital positions. Additionally, these variations provide insights into the veiling effect on both components. In this talk, I will demonstrate the results of the spectral disentangling carried on DQ Tau, deriving the accretion rates of both stars. I will also show the influence of the pulsed accretion on the measured RVs across 10 orbits provided by a recent simultaneous JWST, LCO, and XShooter campaign and that of UVES. The study of DQ Tau has influenced our understanding of young binary systems. More specifically, how much further can we push the study of binarity in variable accretors? In the second part of the talk, I will highlight disks with big dust-free cavities shown by ALMA continuum images, and often where the inner disk is casting shadows on the outer disk, proposing that they are carved by a sub/stellar companion. I will tackle the observed RV variations with ESPRESSO/VLT for a sample of 12 transition disks (e.g., RY Lup ), with 2 clear RV signatures, across 3 years and the possible binary configurations that might explain the cavities seen by ALMA. Finally, I will point out how to disentangle the variations caused by stellar activity.

Speaker: Ms Hala Alqubelat (European Southern Observatory)

16:00 → 21:00 Conference Excursion and Dinner

We will leave with bus from the conference venue at 16.00 sharp, and arrive at the Kronborg castle (https://www.kronborg.dk/en) around 17.00. We then have 1 hour to see the castle, and afterwards we will walk to the nearby "Værftets madmarked" for the conference dinner (https://www.vaerftetsmadmarked.dk). It is easy to get back to Copenhagen on your own using the trains that run every 20 minutes from the nearby station.

Friday 29 August

09:00 → 10:20 Interactions with the environment

09:00 Invited review

Speaker: Dr Maria Teresa Valdivia Mena (European Southern Observatory)

09:40 Unveiling Central ortho-H2D+ Depletion at Sub-kau Scales in Prestellar Core G205.46−14.56 M3

Prestellar cores represent the initial conditions of star formation. The Orion B prestellar core G205.46 M3 has been reported to exhibit two substructures, B1 and B2, which have been proposed as the stellar embryos of a future protobinary system. At this stage, heavy molecules such as CO are significantly depleted in these cold, dense environments, limiting our ability to probe core centers. In contrast, deuterated molecular ions, particularly oH2D+, emerge as key tracers due to enhanced deuterium fractionation at low temperatures. We present the ALMA oH2D+ and 820um continuum maps at ~300au resolution, showing oH2D+ depletion in the prestellar core G205.46 M3. We identify a significant oH2D+ depletion zone of ~600au in diameter toward B1. Chemo-dynamical modeling reproduces the observed deuteration profiles with a core age of approximately 0.42 Myr, comparable to the free-fall time. This suggessts that the substructures formed via turbulent fragmentation through rapid contraction rather than through slow, quasi-static contraction. Our observations also reveal that the gas between B1 and B2 exhibits nearly thermal velocity dispersion, which is consistent with a turbulent scenario in which turbulence dissipates in no more than a few free-fall times. Our results highlight the critical role of deuterated ions in probing both the chemical evolution and dynamical state of dense cores.

Speaker: Dr Sheng-Jun Lin (ASIAA)

10:00 Tangled infall signatures in the L1448N region in the Perseus molecular cloud

The formation of stars has been subject to extensive studies in the past decades from molecular cloud to protoplanetary disk scales. It is still not fully understood how the surrounding material in a protostellar system, that often shows asymmetric structures with complex kinematic properties, is delivered onto the disk and how this may shape the properties of the protostar and disk. We present 1 mm NOEMA observations of the PRODIGE large program and analyze the kinematic properties of molecular lines in the L1448N region located in the Perseus molecular cloud tracing scales from 6000 au down to the disk scales at 300 au. This data set is complemented by sensitive IRAM 30m maps of the ANTIHEROES large program expanding the spatial scales up to several 10000s au. This region harbors three Class 0/I protostellar sytems with distinct properties. IRS3A is a single protostar and has a ring-like disk, IRS3B is one of the largest known disks with spiral arms harbouring a triple system, and within the binary IRS3C each of the protostars have a compact disk. The high spectral resolution data of molecular lines reveal extended molecular gas with complex kinematic properties. The clustering algorithm DBSCAN is used to disentangle velocity components into the underlying physical structure. We discover an extended gas bridge (≈3000 au) surrounding both the IRS3A and IRS3B systems. The velocity gradients along the gas bridge are on the order of 100 km/s/pc and point towards the IRS3A system. We find that the observed velocity profile is consistent with analytical streamline models of gravitational infall towards IRS3A. Towards IRS3C we find multiple infalling structures detected each in distinct molecular tracers that may feed each of the disks within the binary. The PRODIGE and ANTIHEROES molecular gas observations reveal that these systems are still embedded within a common large-scale mass reservoir with a complex spatial morphology as well as velocity profiles tracing not only disk/envelope rotation and outflows, but also large-scale infall.

Speaker: Dr Caroline Gieser (Max Planck Institute for Astronomy)

10:20 → 10:50 Coffee break

10:50 → 12:10 Interactions with the environment

10:50 Streamers and Multiplicity: Is there a connection?

The process of star and disk formation is known to involve interactions with the surrounding environment. Recent observations have revealed evidence of several young, embedded objects showing elongated, asymmetric infall. We refer to these features as streamers when they appear unconnected to the parental dense core. Such streamers can deliver substantial mass and angular momentum, potentially disrupting the disk formation process. However, a systematic analysis across a broader sample of objects has not yet been carried out comparing singles and multiples. Here, we present results from the PRODIGE NOEMA large program, which surveyed 32 young stellar objects (YSOs) in the Perseus cloud. We report the overall detection rate of streamers in the full sample and specifically compare the occurrence between single and multiple systems. We investigate whether the presence of streamers correlates with the degree of multiplicity. These results represent the first systematic study of this kind.

Speaker: Dr Jaime E Pineda (Max-Planck-Institut für extraterrestrische Physik)

11:10 Walkaway Stars: Evidence for Stellar Mergers

Speaker: Dr Bo Reipurth (University of Hawaii)

11:30 Dust Dynamics in Protoplanetary Discs after Stellar Flybys

An open problem in planet formation theory is the growth of dust grains from centimetre to metre sizes. The streaming instability has been proposed as a mechanism to overcome barriers in dust grain growth; however, it is only triggered in regions of sufficiently high dust to gas ratio. We therefore require protoplanetary discs to contain substructures where dust grains can collide and planetesimal formation can be triggered. In this talk I will discuss the substructures induced in discs perturbed by stellar flybys, i.e. interactions with an unbound stellar-mass companion. I will present the results of 3D smoothed particle hydrodynamics simulations of discs after a range of flyby encounters, and study the dust dynamics in flyby-induced substructures. I will demonstrate that dust particles remain in the dust overdensities induced by the flyby for long periods of time and that their dust to gas ratios reach values sufficient to trigger the streaming instability and planetesimal formation.

Speaker: Ms Vasundhara Prasad (Institute of Astronomy, University of Cambridge)

11:50 On the use of Gaia astrometry to indentify companions in Young Stars

I will show how to use Gaia astrometry to indentify binaries and companions in Young Stars. I will exemplify the power of this technique with a few science cases, from newly identified protoplanets within protoplanetary disks to a reassesment of the multiplicity of the populations of circumbinary and transition disks. I will conclude looking forward to the upcoming Gaia DR4 and the revolution it will bring for multiplicity analyses, once all individual epochs are published.

Speaker: Dr Miguel Vioque (European Southern Observatory)

12:10 → 13:40 Lunch

13:40 → 15:00 Summary

13:40 Conference Summary

Speaker: Prof. Dominique Segura-Cox (University of Rochester)

14:40 Closing remarks

Speaker: Prof. Troels Haugbølle (Niels Bohr Institute)