Posters can be prepared with various widths, but up to 1.2m (47″) high. Thumbtacks, blue stick tack, and double sided stickers will be provided. Posters will remain on display at the venue throughout the meeting.
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Federico Abbate (Milano-Bicocca): Ionized Gas in 47 Tuc – A Detailed Study with Millisecond Pulsars
Globular clusters are known to be very poor in gas despite predictions telling us otherwise. The presence of ionized gas can be investigated through its dispersive effects on the radiation of the millisecond pulsars inside the clusters. This effect led to the first detection of any kind of gas in a globular cluster in the form of ionized gas in 47 Tucanae. With new timing results of these pulsars over longer periods of time we can improve the precision of this measurement and test different distribution models. We first use the parameters measured through timing to measure the dynamical properties of the cluster and the line of sight position of the pulsars. Then we test for gas distribution models. We detect ionized gas distributed with a constant density of n = (0.22 ± 0.05) cm−3. Models predicting a decreasing density or following the stellar distribution are highly disfavoured. Thanks to the quality of the data we are also able to test for the presence of an intermediate mass black hole in the center of the cluster and we derive an upper limit for the mass at ∼ 4000 M_sun.
Javier Alonso-García (Antofagasta): VVV and VVV-X Surveys – Unveiling the Innermost Galaxy
We find the densest concentrations of field stars in our Galaxy when we look towards its central regions. But our knowledge of the innermost Milky Way is seriously hampered by the dust and gas present at low latitude lines of sight. The VVV and the VVV-X surveys can beat these difficulties by observing the Galactic Bulge and inner Disk at near-infrared wavelengths were extinction is highly diminished. Using the 4m VISTA telescope located in Cerro Paranal, the VVV and VVV-X surveys provide wide-field high-resolution images of the highly reddened inner Galaxy in the Z, Y, J, H and Ks near infrared filters. In my talk I will present a new photometric catalog that we have built with nearly one billion sources from the inner Galactic regions surveyed by the VVV. Using the color-magnitude diagrams we build, we are able to disentangle and provide a detailed description of the inner Galactic stellar populations. I will also report on the proper characterization of the extinction law towards the inner Milky Way that we have obtained. Finally, I will describe VVV-X, the extended VVV survey, that we are currently running over a much bigger area in the inner Galaxy, and talk about possible synergies with Gaia and other Galactic surveys.
Jeff J. Andrews (Crete/FORTH): Double Neutron Star Formation: Supernova Dynamics and Orbital Decay due to Gravitational Wave Radiation
The formation and evolution of double neutron stars (DNS) has traditionally been studied using binary population synthesis in which one randomly generates a set of stellar binaries from initial conditions and evolves them forward using our best understanding of the physics involved. Here, I take an alternative approach by focusing only on the second supernova (SN) forming the DNS and the subsequent orbital decay and merger due to gravitational wave radiation. Since we have decoupled the effects of uncertainties in prior binary evolution, our results are quite general. Using analytic and numerical methods, we explore how different kick velocity distributions, pre-SN orbital separations, and progenitor He-star masses affect the derived delay time distributions, systemic velocities, and distances traveled before the DNS mergers. We find that DNSs typically have a systemic velocity of order the pre-SN orbital velocity, but often much less depending on the kick velocity. Furthermore, we provide realistic travel distances for systems that escape the gravitational potential of their host galaxies. Only relatively tight systems can leave host galaxies with escape velocities above a ~100 km/s. However, those that do escape may travel as far as ~Mpc before merging, possibly explaining the so-called host-less short gamma ray bursts that are found far from any galaxy.
Francisco Aros (MPIA/ESO): Axisymmetric Dynamical Models for Stellar Systems: an Application to Local Dwarf Spheroidal Galaxies
In the search to understand the nature of dark matter, the dynamical analysis of dark matter dominated galaxies is expected to be of special interest. In this sense, the local dwarf spheroidal galaxies are the ideal objects for this kind of studies. Up to now, most approaches have assumed spherical symmetry and explored dynamical models based on the Jeans equations. However, we know that the stellar component of these galaxies are not spherical, and thus their mass determination may have been biased. Dynamical models based on the Jeans Equations have been successful on determining global properties of these systems under the assumption of spherical symmetry; yet we do not know if these would also apply to axisymmetric models. Here we present a thorough analysis on the constraints for the intrinsic flattening of the dark matter halo and its degeneracy with the velocity anisotropy.
Alessandro Ballone (INAF Padova): Weighing the IMBH candidate CO-0.40-0.22* in the Galactic Centre
The high velocity gradient observed in the compact cloud CO-0.40-0.22, at a projected distance of 60 pc from the centre of the Milky Way, has led its discoverers to identify the closeby mm continuum emitter, CO-0.40-0.22*, with an intermediate mass black hole (IMBH) candidate. We describe the interaction between CO-0.40-0.22 and the IMBH, by means of a simple analytical model and of hydrodynamical simulations. Through such calculation, we obtain a lower limit to the mass of CO-0.40-0.22* of few 10^4 × M_sun. This result tends to exclude the formation of such massive black hole in the proximity of the Galactic Centre. On the other hand, CO-0.40-0.22* might have been brought to such distances in cosmological timescales, if it was born in a dark matter halo or globular cluster around the Milky Way.
Matteo Bonetti (Insubria): Gravitational Waves from Massive Black Hole Triplets
Massive black-hole binaries (MBHBs) are thought to be the main source of gravitational waves (GWs) in the low-frequency domain surveyed by ongoing and forthcoming Pulsar Timing Array (PTA) campaigns and future space-borne missions (LISA). However, MBHBs in realistic astrophysical environments may not always reach separations small enough to allow significant GW emission. This “final-parsec problem” can be eased by the appearance of a third massive black hole (MBH) whose action can force, under certain conditions, the former MBHB on a very eccentric orbit, hence allowing intense GW emission eventually leading to coalescence. A detailed assessment of the process requires a general relativistic treatment and the inclusion of environmental effects. In order to tackle the problem, we developed a three-body Post-Newtonian (PN) code framed in a spherical galactic potential, including GR corrections up to 2.5PN order as well as the effects of orbital hardening and dynamical friction.
I will present the results of an extensive set of simulations aimed at characterize the dynamics of such triple systems in a cosmological context. Finally, I will analyze the implications on the emitted GW signal.
Phil Breen (Edinburgh): Light Element Variations in Globular Clusters via Nucleosynthesis in Black Hole Accretion Discs
Ancient globular clusters contain multiple stellar populations identified by variations in light elements (e.g., C, N, O, Na). Although many scenarios have been suggested to explain this phenomenon, all are faced with challenges when compared with all the observational evidence. In this Letter, we propose a new scenario in which light element variations are determined by nucleosynthesis in accretion discs around black holes. Since the black holes form after a few Myrs, the cluster is expected to still be embedded in a gas rich environment. By using a simplified accretion model which assumes virial temperatures, we show that the correct light element anti-correlations could be produced in accretion flows around stellar-mass black holes. Assuming a Kroupa IMF, each black hole would only have to process 300Msun of material in order to produce multiple populations; over a period of 3Myr this corresponds to 10^-4 Msun/yr, which is within the range of values typically assumed for the formation of massive stars.
Katelyn Breivik (CIERA): Revealing Black Holes in Binaries with Gaia
We estimate the population of black holes with luminous stellar companions (BH-LCs) in the Milky Way (MW) observable by Gaia. We evolve a realistic distribution of BH-LC progenitors from zero-age to the current epoch taking into account relevant physics, including binary stellar evolution, BH-formation physics, and star formation rate, in order to estimate the BH-LC population in the MW today. We predict that Gaia will discover between 3800 and 12,000 BH-LCs by the end of its 5 year mission, depending on BH natal kick strength and observability constraints. We find that the overall yield, and distributions of eccentricities and masses of observed BH-LCs, can provide important constraints on the strength of BH natal kicks. Gaia-detected BH-LCs are expected to have very different orbital properties compared to those detectable via radio, X-ray, or gravitational-wave observations.
Lapo Casetti (Firenze): Cold Collapse and Violent Relaxation in a Toy Model of a Self-gravitating System
When starting from non-virialized initial conditions (e.g., “cold” ones), a self-gravitating system undergoes collective oscillations (referred to as virial oscillations) that damp out in a relatively short timescale not depending on the system’s size; after the damping of such oscillations the system is found in a virialized steady state that is typically very far from a thermal one, even in those cases where a thermal equilibrium state does exist for a finite-mass system, as in two dimensions [1]. This fast relaxation process, dubbed “violent relaxation” by the late Donald Lynden-Bell who first proposed a statistical-mechanical theory to tackle such a phenomenon [2], is completely non-collisional, at variance with the slow relaxation driven by two-body interactions that sets in on much longer timescales, growing with the system’s size and diverging in the limit of an infinite system. Hence, the states in which we now observe large systems such as elliptical galaxies are essentially the end states of a violent relaxation, being the two-body relaxation time much longer than the age of the Universe. In systems like dynamically old globular clusters collisional processes may well have driven the system out of the end states of violent relaxation, but still these states are relevant as initial conditions of the collisional relaxation process. Fifty years after Lynden-Bell’s seminal paper we do not have a satisfactory theory of violent relaxation yet, despite many advances [3]. What we have learnt is that violent relaxation is not a peculiarity of self-gravitating systems: it is a universal property of any long-range-interacting system. During violent relaxation the distribution function obeys the non-collisional Boltzmann (or Vlasov) equation, that, being invariant under time reversal, does not “naturally” describe a relaxation process. Indeed, the dynamics never stops, but it is moved to smaller and smaller scales in phase space as time goes on. This explains why observables that do not depend on fine-scale details appear as relaxed after a short time. We argue that an effective description of the violent relaxation process is possible, after a suitable coarse-graining procedure. We derive a general approximation scheme to explicitly describe the collisionless relaxation process [4]. The direct application of such procedure to a self-gravitating system is at present a difficult task, but the above mentioned universality of the violent relaxation process opens the possibility of using toy models to understand at least its most general aspects. We thus apply our method to the cold collapse of a simple toy model of a self-gravitating system, where explicit calculations are possible, obtaining results in very good agreement with numerical simulations [4].
[1] P. Di Cintio, S. Gupta, and L. Casetti, MNRAS 475, 1137 (2018)
[2] D. Lynden-Bell, MNRAS 136, 101 (1967)
[3] See e.g. Y. Levin et al., Phys. Rep. 535, 1 (2014)
[4] G. Giachetti and L. Casetti, in preparation
Jörg Dabringhausen (Prague): Dynamically Stable Models for the Galactic Center
A popular method to model galaxies in general and the center of the Milky Way in particular is Schwarzschild’s method. For this method, a grid of sample orbits of stars in an external potential is calculated, and a model for the stellar system is obtained through attributing specific weights to the orbits in a superposition of them. The models created with Schwarzschild’s method can fit many observed properties of the modeled stellar system (profiles of the luminosty density and velocity moments) with high precision, but they may suffer from the fact that the real stellar system is not a system of test particles in an external potential, but a self-gravitating system. Systems that are stationary as Schwarzschild models may therefore exhibit a strong time evolution if they are translated into models that explicitly account for the fact that it is the stars themselves that generate the potential in which they move. I will discuss these issues using a Schwarzschild model for the the Milky Way as an example, and how to turn this model into a stationary self-gravtiting model that inherits the good properties of the underlying Schwarzschild model.
Nazanin Davari (Rome): Close Approach of Stars and their Planets with Sgr A* Black hole
We present some preliminary results of our progressing work about the close interaction of binary stars and their planetary systems with the Super Massive black hole (SMBH) at the center of our galaxy. As first theorized by Hills (1988), the disruption of a binary star system by the massive black hole can lead to the capture of one star around Sgr A* and the ejection of its companion as a hypervelocity star (HVS). In this work, we consider that stars may have planetary systems and investigate the orbital properties of the binary star system and the fate of their planetary systems after close interaction with the SMBH at the Galactic Centre.
Pierfrancesco Di Cintio (CNR-IFAC & INFN – Firenze): N-body Chaos and Discreteness Effects and the Continuum Limit in Dense Stellar Systems
We revise the role of N-body chaos and the validity of the continuum limit in self gravitating systems using both orbit integration in frozen N-body potential and direct self-consistent N-body simulations. We find a strong dependence on single orbit energy in the approach to the continuum (Vlasov-Poisson) limit for self gravitating system. We discuss the implications with respect to the validity of the continuum limit itself as well as the robustness of models based on stochastic equations modelling discreteness effects (e.g. dynamical collisions or density inhomogeneities). In addition, we show some preliminary results on the dynamics of massive objects in dense environments.
Juan Farias (Chalmers): The Dynamics of Star Cluster Formation
We study the early dynamical evolution of star forming clusters, particularly focused on the effects of the formation timescale. The timescale for star cluster formation may be long compared to the dynamical time of the system, so that during this phase there can be significant dynamical evolution. This is the process we set out to model, here via direct N-body simulations, including realistic primordial binary fractions. Specifically, we explore how the dynamical evolution of forming clusters depends on the cluster formation timescale, as well as other assumptions that are generally needed for star formation subgrid models, such as degree of primordial mass segregation and binarity. We find that star clusters born slowly are able to virialize before exhausting/ejecting their natal gas and are more stable against expansion, independent of their initial density. Also, we are able to obtain a considerable amount of runaway stars even in low density environments, where as they do not form if star formation is fast. We present the first results of such modeling and discuss the future improvements that may include substructured formation and more realistic star cluster formation assumptions.
Eulalia Gallego Cano (IAA-CSIC): The Stellar Cusp Around the Milky Way’s Central Black Hole
Theoretical stellar dynamics firmly predicts the formation of a stellar cusp in a dense cluster around a massive black hole after a relaxation time. Contrary to expectations, this cusp has not yet been found at the center of the Milky Way. Instead, a core-like structure, indicating a relative lack of red giants, has been found in the surroundings of Sagittarius A*. We have taken on this problem again with improved data analysis techniques to push the limit of completeness due to crowding about one magnitude deeper. We show that K ~ 18 mag stars, which can be expected to be old and therefore dynamically relaxed, show a single power-law density distribution similar to the expected cusp. We conclude that there is good evidence for a stellar cusp at the center of the Milky Way. As a corollary, the lack of red giants may have been caused by collisions, which remove the envelope of the giants and render them invisible.
Doug Geisler (Conception): Chemistry, Kinematics and Ages of Bulge Globular Clusters
I will discuss a variety of current projects I am working on related to bulge globular clusters, including VVV, HST and GeMS photometry and Ca triplet and high resolution near IR spectroscopy to help improve our current very limited knowledge of this key Galactic globular cluster system.
Nicola Giacobbo (Padova): A Critical Look at Progenitors of Merging Black Hole Binaries
The recent detection of gravitational waves has proven the existence of massive stellar black hole binaries (BHBs), but the formation channels of BHBs are still an open question. One of the most powerful tools to investigate the origin of BHBs are the population-synthesis codes. In this talk, I describe my new code MOBSE, which is an updated version of the widely used binary population synthesis code, BSE (Hurley et al. 2002). In MOBSE, I have included the most recent models of star evolution, wind mass-loss and core-collapse supernovae, which are the key ingredients to determine the fate of massive stars. Based on the results of MOBSE, I show that only massive metal-poor stars (Z 0.002) can be the progenitors of gravitational-wave events like GW150914. Finally, I show that most of the binary systems leading to the formation of BHBs pass through the common envelope phase.
Arkadiusz Hypki (Adam Mickiewicz): BEANS — Interactive, Distributed Data Analysis of Huge Data Sets
BEANS is a web-based software for interactive distributed data analysis with a clear interface for querying, filtering, aggregating, and plotting data from an arbitrary number of data sets and tables. During the talk I would like to present a number of interesting, easy to understand but yet very powerful examples of data analysis made in the BEANS software. The example data sets are based on the data from MOCCA simulations which is one of the most powerful codes to simulate real-size star clusters. The example queries will be made against the data coming from over 2000 MOCCA simulations, where every simulation takes on average 20 GBs and contains around 10 different files. The BEANS software will be shown how one can manage the data sets which are large and contain also large number of files.
Jisu Kang (Seoul): Globular Clusters in NGC 4993 Hosting GW170817/GRB 170817A
NGC 4993 is a merger remnant early-type galaxy hosting a binary neutron star merger GW170817/GRB 170817A. Since this is the first case emitting both gravitational waves and electromagnetic waves, it provides us a unique chance to study binary neutron star mergers and compare the cosmic ladder distance with the distance based on the gravitational wave siren method. In this study we search for globular clusters in this galaxy, and derive a new distance using the globular cluster luminosity function (GCLF) method. We select the globular cluster candidates from the HST/ACS F606W archival images, using the structural parameters of detected sources. These candidates are centrally concentrated around NGC 4993 at the galactocentric distance r 50″ (~10 kpc), showing that most of them are the members of this galaxy. We derive a background- and extinction-corrected turnover magnitude of the Gaussian GCLF, F606W(max)_0 = 25.36 ± 0.08 (V(max)_0 = 25.52 ± 0.11). Adopting the calibration of the turnover magnitudes for globular clusters, we derive a GCLF distance to NGC 4993, d = 41.65 ± 3.00 Mpc ((m – M)_0 = 33.10 ± 0.16). This value is consistent with the previous distance estimates, and it can be used to constrain the inclination angle of the binary system in the models of gravitational wave analysis.
Tomoya Kinugawa (Tokyo): Low Metallicity Black Hole Binary Calculation for GAIA
Using the population synthesis method, we calculate the black hole binary distribution, considering the metallicity evolution. Black holes which are formed in the metal poor environment tend to be more massive than black holes formed in the solar metal environment. The metal-poor low-mass stars can arrive at present day and might pair with a black hole. We might be able to check the black hole mass distribution by GAIA.
Myung Gyoon Lee (Seoul): Star Clusters in Merger-remnant Galaxies
Massive galaxies grow via diverse merging processes, transforming finally to early-type galaxies. Merger-remnant galaxies are lenticular or elliptical galaxies which show intriguing substructures of merging process. Star clusters are an excellent tool to trace a merging history of these merger-remnant galaxies. They can be also the origin of interesting sources including neutron star binaries or progenitors of peculiar supernovae. We present a study of star clusters in some interesting merger-remnant galaxies.
Young-Wook Lee (Yonsei): Assembling the Milky Way Bulge from GCs
The two red clumps (RCs) observed in the H-R diagram of the Milky Way bulge is widely accepted as evidence for an X-shaped structure originated from the bar instability. We suggest, however, a drastically different interpretation based on the He-enhanced multiple stellar population phenomenon as is observed in globular clusters (GCs). Our synthetic H-R diagrams naturally predict super-He-rich stars to be placed on the bright RC in the metal-rich bulge population, while chemical evolution models reproduce the required strong metallicity dependence of He enhancement (dY/dZ ~ 6) for the second generation stars. We also report our observational discovery that the stars in the two RCs show a significant difference in CN-band strength, in stark contrast to the X-shaped bulge scenario. The difference in CN abundance and the population ratio between the two RCs are comparable to those observed in GCs between the first- and later generation stars. Since CN-strong stars trace a population with enhanced N, Na, and He abundances originated in GCs, this is direct evidence that the double RC is due to the multiple population phenomenon, and that a significant population of stars in the Milky Way bulge were provided by disrupted proto-GCs. Our result also calls for the major revision on the 3D structure of the Milky Way bulge as the current view is based on an erroneous interpretation of the double RC phenomenon.
Kirill Lezhnin (Princeton): Joint Evolution of Binary Supermassive BHs and Galactic Nuclei
Binary supermassive black holes (bSMBHs) are expected to be formed naturally during the galaxy merger process. After the dynamical friction phase, when the two SMBHs become gravitationally bound to each other, and a brief stage of initial rapid hardening, the orbit gradually continues to shrink due to the three-body interactions with stars that enter the loss cone of the bSMBH. Using the stellar-dynamical Monte-Carlo code RAGA, we explore the co-evolution of bSMBH and the nuclear star cluster in this slow stage, for various combinations of parameters (geometry of the star cluster, primary/secondary SMBH mass, initial eccentricity, inclusion of stellar captures/tidal disruptions). We compare the rates of stellar captures in bSMBH-hosting galactic nuclei to those of galaxies with a single SMBH. The difference between the two cases is moderate in triaxial stellar systems; however, the rates may be substantially higher in binary-hosting axisymmetric systems than in equivalent single-SMBH systems. We find that the hardening rate is not influenced by star captures, nor does it depend on eccentricity; however, it is higher when the difference between the black hole masses is greater. We confirm that the eccentricity of the binary tends to grow, which may significantly shorten the coalescence time due to earlier onset of gravitational-wave emission. We also explore the properties of the orbits entering the loss cone, and demonstrate that it remains nearly full throughout the evolution in the triaxial case, but significantly depleted in the axisymmetric case. Finally, we study the distribution of ejected hypervelocity stars and the corresponding massdeficits in the central parts of the galaxies hosting a binary.
Walid Majid (JPL, Caltech): Neutron Stars in Compact Orbits Around Sgr A*
The black hole in the center of the Milky Way, Sgr A∗ is an ideal candidate for testing general relativity in the strong-field regime. The tracking of young stars at very close orbits has already led to a direct measurement of its mass. The discovery and subsequent timing of a single radio pulsar in orbit around Sgr A∗ will also allow for a direct measurement of the black hole’s spin and quadrupole moment. The presence of a cluster of young, massive stars in orbit around Sgr A∗ makes it highly likely that a large population of such pulsars exist. Moreover, the detection of excess gamma-rays in this region of the galaxy by the Fermi LAT instrument provides further hints that a population of millisecond pulsars maybe the culprit in the production of anomalous gamma-rays. The difficulty, however, in detecting these pulsars lies in the fact that interstellar scattering heavily disperses and broadens the emission from each pulsar. This effect is known to be severe at standard pulsar search frequencies. We have begun a dedicated survey for millisecond pulsars in the Galactic Center region of the Galaxy, specifically designed to overcome the deleterious effects of scatter broadening by using a state-of-the-art pulsar machine on the largest radio telescope in the Southern Hemisphere. I will describe the details of the survey and will provide early results from our recent observations.
Yohai Meiron (ELTE): Resonant Relaxation in Globular Clusters
Resonant relaxation is mostly discussed in the context of the Galactic Center. Other spherically symmetric stellar systems, such as globular clusters, exhibit a restricted form of this effect where enhanced relaxation rate only occurs in the directions of the angular momentum vectors, but not in their magnitudes; this is called vector resonant relaxation (VRR). To explore this effect, we performed a large set of direct N-body simulations, with up to 512k particles and ~500 dynamical times. We show that the temporal behavior of the angular momentum vectors cannot be explained by 2-body relaxation alone (contrasting our simulations with Spitzer-style Monte Carlo simulations, that by design only exhibit 2-body relaxation). The fact that VRR operates in globular clusters may open way to use powerful tools in statistical physics for their description. In particular, since the distribution of orbital planes relaxes much more rapidly than the distribution of the magnitude of angular momentum and the radial action, the relaxation process reaches an internal statistical equilibrium in the corresponding part of phase space while the whole cluster is generally out of equilibrium, in a state of quenched disorder. In this case, statistical mechanics may be utilized to understand the long-term behavior of the system.
Francisco Nogueras-Lara (IAA/CSIC): GALACTICNUCLEUS: A High Angular Resolution Survey of the Galactic Center
The GALACTICNUCLEUS survey is a high angular resolution, 0.2”, $JHK_s$ survey of the central few thousand square parsecs of the Galactic centre (GC) that reaches a few magnitudes deeper than any existing, seeing-limited surveys (Nogueras-Lara et al, 2018). The centre of the Milky Way is of fundamental interest as it constitutes a unique laboratory to study the stellar nuclei and their role in the context of galaxy evolution. Our main goal is to obtain a far more global view of the GC’s stellar population, structure and history, overcoming the extreme crowding and extinction that characterize this very dense and extreme environment.
Kwangmin Oh (Chungnam): High Energy Observation in Globular Clusters
I will show and present our recent investigation of globular clusters in X-ray observations.
Simon Portegies Zwart (Leiden): Planetary Systems in Star Clusters
Simulations of star clusters with primordial planetary systems.
Steven Rieder (RIKEN): FDPS with AMUSE: High-performance Simulations with an Easy Python Interface
Developing a particle simulation code for one-core machines is quite a different thing than developing for supercomputers. With FDPS, we present a method that dramatically simplifies this, providing the libraries to create high-performance parallel N-body and SPH codes. We have now created an interface to the FDPS framework from AMUSE, making it possible to access FDPS codes from the commonly known AMUSE Python interface, allowing users to scale up their single-core simulation to a supercomputer-ready one with only minimal changes to their code. I will present examples for using N-body and SPH simulations developed with FDPS from AMUSE.
Riccardo Schiavi (Rome): The Future Milky Way and Andromeda Galaxy Merger
According to our current knowledge about physical and dynamical properties of the Milky Way-M31 system, it seems likely that these two galaxies will collide and eventually merge in a time very sensitive to initial conditions. Using the HiGPUs code (Capuzzo-Dolcetta et al., 2013), we have performed several numerical simulations to study the combined dynamics of the system, trying to define a relation between the present-day relative velocities of the two galaxies and the timescale of the merger. At the same time, we have followed the dynamics of the two massive black holes sitting in the galactic centers, to check (within the limit of the space and time resolution of our simulation) their relative motion upon the completion of the galaxies merger process.
Naoki Seto (Kyoto): Probabilistic Eccentricity Bifurcation for Stars Around Shrinking Massive Black Hole Binaries
Based on the secular theory, we discuss the orbital evolution of stars in a nuclear star cluster to which a secondary massive black hole is infalling with vanishing eccentricity. We find that the eccentricities of the stars could show sharp transitions, depending strongly on their initial conditions. By examining the phase-space structure of an associated Hamiltonian, we show that these characteristic behaviors are partly due to a probabilistic bifurcation at a separatrix crossing, resulting from the retrograde apsidal precession by the cluster potential. We also show that separatrix crossings are closely related to realization of a large eccentricity and could be important for astrophysical phenomena such as tidal disruption events or gravitational wave emissions.
Hiromichi Tagawa (Eotvos): Compact Object Mergers Driven by Gas Fallback
Several gravitational wave detections have shown evidence for compact object mergers. However, the astrophysical origin of merging binaries is not well understood. An interesting problem is that existing astrophysical models for neutron star mergers typically predict a lower merger rate than the observed rate. In such situation, we have proposed a new channel for mergers of compact object binaries, which may enhances the merger rate. We examine the binary evolution following gas expansion due to a weak failed supernova explosion, neutrino mass loss, core disturbance, or envelope instability. In such situations the binary is possibly hardened by ambient gas. We investigate the evolution of the binary system after a shock has propagated by performing N-body/smoothed particle hydrodynamics simulations. We find that significant binary hardening occurs when the gas mass bound to the binary exceeds that of the compact objects. This mechanism represents a new possibility for the pathway to mergers for gravitational wave events.
Santiago Torres (Leiden): Dynamics of the Oort Cloud in the Gaia Era
Comets in the Oort cloud evolve under the influence of internal and external perturbations from giant planets to stellar passages, the Galactic tides, and the interstellar medium. Using the positions, parallaxes and proper motions from Gaia DR1 and combining them with the radial velocities from the RAVE-DR5, Geneva-Copenhagen and Pulkovo catalogues, we calculated the closest encounters the Sun has had with other stars in the recent past and will have in the near future; and their influence on the dynamics of the minor bodies of the Solar System. We will present our results together with a comparison with the data from the upcoming Gaia DR2.
Alessandro Trani (Tokyo): Forming Circumnuclear Disks and Rings in Galactic Nuclei: a Competition Between Supermassive Black Hole and Nuclear Star Cluster
We investigate the formation of circumnuclear gas structures from the tidal disruption of molecular clouds in galactic nuclei, by means of smoothed particle hydrodynamics simulations. We model galactic nuclei as composed of a supermassive black hole (SMBH) and a nuclear star cluster (NSC) and consider different mass ratios between the two components. We find that the relative masses of the SMBH and the NSC have a deep impact on the morphology of the circumnuclear gas. Extended disks form only inside the sphere of influence of the SMBH. In contrast, compact rings naturally form outside the SMBH’s sphere of influence, where the gravity is dominated by the NSC. This result is in agreement with the properties of the Milky Way’s circumnuclear ring, which orbits outside the SMBH sphere of influence. Our results imply that the morphology of circumnuclear gas can be used as a probe for SMBH presence: the inner radius of circumnuclear rings represents an upper limit to the SMBH sphere of influence.
Eugene Vasiliev (Cambridge IoA): Internal dynamics of the Large Magellanic Cloud from Gaia DR2
We use the proper motions (PM) of half a million red giant stars in the Large Magellanic Cloud measured by Gaia to construct a 2d kinematic map of mean PM and its dispersion across the galaxy, out to 7 Kpc from its centre. We then explore a range of dynamical models and measure the rotation curve, mean azimuthal velocity, velocity dispersion profiles, and the orientation of the galaxy. We find that the circular velocity reaches ~100 km/s at 5 Kpc, and that the velocity dispersion ranges from ~40-50 km/s in the galaxy centre to ~20 km/s at 7 Kpc.
Newlin C. Weatherford (CIERA): Prediction of Stellar-Mass Black Hole Populations in Globular Clusters
Recent discoveries of black hole (BH) candidates in Galactic and extragalactic globular clusters (GCs) have ignited interest in understanding how BHs dynamically evolve in a GC and the number of BHs (NBH) that may still be retained by today’s GCs. Numerical models show that even if stellar-mass BHs are retained in today’s GCs, they are typically in configurations that are not directly detectable. We show that a suitably defined measure of mass segregation (Δ) between, e.g., giants and low-mass main-sequence stars, can be an effective probe to indirectly estimate NBH in a GC aided by calibrations from numerical models. Using numerical models including all relevant physics we first show that NBH is strongly anti-correlated with Δ between giant stars and low-mass main-sequence stars. We apply the distributions of Δ vs NBH obtained from models to three Milky Way GCs to predict the NBH retained by them at present. We calculate Δ using the publicly available ACS survey data for 47 Tuc, M 10, and M 22, all with identified stellar-mass BH candidates. Using these measured Δ and distributions of Δ vs NBH from models as calibration we predict distributions for NBH expected to be retained in these GCs. For 47 Tuc, M 10, and M 22 our predicted distributions peak at NBH ≈ 8, 15, and 40, whereas, within the 2σ confidence level, NBH can be up to ∼100, 50, and 200, respectively.
Andrew Winter (Cambridge): Constraining the Dynamical History of Cygnus OB2 using Protoplanetary Disc Fractions
In dense stellar environments, protoplanetary disc (PPD) evolution is dependent on the feedback of neighbouring stars. Two such mechanisms which can influence the evolution of a PPD are external photoevaporation, a result of irradiation of a disc by massive stars, and tidal truncation, dependent on star-disc encounters. We quantify the effect of these mechanisms in a given stellar environment and then apply these results to models of real clusters. This can further be applied to massive clusters, in order to constrain dynamical history by considering the disc populations as a function of the local environment. We present models of the dynamically complex association, Cygnus OB2, and show how the fraction of surviving PPDs can indicate likely scenarios for the early stages of cluster evolution. Based on this history we can also make statements about the present state of substructure and the future of the stellar components in the association.
Masaki Yamaguchi (Konan): Detecting Stellar-mass Black Holes in Binaries by Gaia
We estimate the number of stellar-mass black hole in binaries by the Gaia astrometry and discuss what we will find by using a number of these black hole. The astrometric measurements for stars with an unseen companion enables us to find binary elements, which is related to the mass of the companion. If this companion is a black hole, we may be able to identify it by using the measured mass. We find that 200-1000 black holes will be detected with Gaia (the uncertainty is due to several models of binary evolution). By using these black holes, we could constrain the relation between the black hole mass and its ZAMS mass.
Peter Zeidler (STScI): A Galactic Young Massive Star Cluster in Motion – The Bimodal Stellar Velocity Structure of Westerlund 2
We have studied Westerlund 2 (Wd2), a near-by Galactic young massive star cluster, and its surrounding HII region using high-resolution optical and near-infrared photometry obtained with the Hubble Space Telescope in combination with optical integral field spectroscopy obtained with VLT/MUSE. We analyzed a 64 × 64 arcsec* region, north of the Main Cluster, which we call the Northern Bubble” (NB), a circular cavity carved into the remaining gas of the cluster region. The NB hosts “the Sock”, a Pillar like structure, including multiple OB stars. We extracted 20 stellar spectra from the MUSE data and fitted template spectra and stellar absorption lines to determine their spectral types and to estimate their radial velocities (RVs) to an accuracy of ~2 km⁄s. We revealed that the cluster member stars have a bimodal velocity distribution with a median velocity difference of ~25 km⁄s, which is in agreement with studies supporting cloud-cloud collision as a formation scenario for Wd2. Most of the more luminous stars are part of the Gaia DR2 data, which we will use to improve the RV calibration and cluster membership selection. This is crucial for the extension of our analysis to the whole Wd2 cluster, especially to the newly obtained deep exposures, reaching pre-main-sequence stars down to 1 − 2 M⨀. Combining Gaia, MUSE, and 3 more years of approved HST observations, we will determine 3D motions (RVs and proper motions), reveal the cluster’s binary fraction, and study the feedback effects on the gas cloud to obtain a full 3D picture Wd2’s past and future evolution.
