Abstracts of the Thursday Programme

Possible origins of the first interstellar object 'Oumuamua

by Coryn Bailer-Jones, David Farnocchia, Karen Meech, et al.

The first detected interstellar object 'Oumuamua that passed within 0.25au of the Sun on 2017 September 9 was presumably ejected from a stellar system. We use its non-Keplerian trajectory together with the reconstructed Galactic orbits of 7 million stars from Gaia DR2 to identify stars that 'Oumuamua came close to in the past. Such an "encounter" could reveal the home system from which 'Oumuamua was ejected.

We find four stars that can be considered as plausible home stars. Ejection of 'Oumuamua by scattering from a giant planet in one of the systems is a plausible origin mechanism; but it requires a rather unlikely configuration to achieve the relative high velocities found (>= 10km/s). A binary star system is more likely to produce the observed velocities. Given that our sample of 7 million stars is dominated by nearby stars - and is only a small fraction of stars in the Galaxy - then the probability that one of our candidates is the home system depends on 'Oumuamua's age. If it is young - i.e. was ejected in the past few million years - then there is a reasonable chance that we have identified the home system. 'Oumuamua's low velocity with respect to the local standard of rest is consistent with youth.

If; on the other hand; 'Oumuamua is old (more than tens of Myr); then it could have traversed a significant fraction of the Galaxy before reaching the Sun; meaning that it will be essentially impossible to ever identify its origin. We conclude by discussing the prospects of discovering new interstellar objects.

Globular clusters in the Gaia era: precision astrometry for precision dynamics

by Paolo Bianchini, Anna Lisa Varri, Elena Pancino Alice Zocchi, et al.

The traditional picture of globular clusters (GCs) as simple stellar systems is being radically revolutionized by the synergistic efforts of theoretical advances and the advent of precision astrometry led by Gaia DR2. We now know that GCs present rich kinematic features; such as the presence of internal rotation; and complex stellar populations. Both these elements strongly puzzle our understanding of their formation in the early universe. A comprehensive assessment of the internal kinematics of GCs in a regime <1 km/s (typically <0.05 mas/yr) would offer the unique possibility to unveil the fossil records of their formation. To fully leverage on the power of precision astrometry a thorough investigations of the data is first required.

The challenge consists of disentangling complex physical dynamical processes from subtle data systematics at the sub-km/s regime. We show that the study of the mean radial proper motion profiles of GCs offers an ideal benchmark to assess the presence of subtle systematics in crowded fields. For this purpose; we investigate fundamental dynamical processes that could alter the shape of the mean radial proper motion profiles of old Galactic GCs; and show that any deviations present in the data can be ascribed to the presence of systematics.

An analysis of the closest 14 GCs shows that Gaia DR2 performs excellently in recovering the expected radial proper motion profiles; indicating that systematics are below the random measurement errors; reaching a precision of ~0.015 mas/yr for mean proper motion measurements. This result gives the first taste of the unparalleled power of Gaia DR2 to unlock our understanding of GCs formation and evolution; in preparation for the subsequent data releases.

Occultation investigations of small bodies in our solar system enabled by Gaia

by Marc Buie

Stellar occultations by solar system objects have always been a prized dataset rivaled only by in situ investigations. This observational tool has been limited by the precision of available star catalogs. With the advent of Gaia DR2; we finally have a catalog good enough to support far more in-depth small body investigations. Several projects I am involved with have used Gaia results.

This presentation will show come recent key results. 1) Four key stellar occultations of 2014MU69 used to support science planning and navigation for New Horizons. 2) Two occultations of one of the Lucy Mission targets. 3) Broader studies of D>100km Kuiper Belt objects for shape; size; and duplicity as pursued by the RECON project. All of these examples are useful case studies for the types of systematic investigations of small bodies that are now within our grasp. These investigations will require effort and care but these observations confirm that we are no longer limited by star catalog precision in the pursuit of occultations.

The stellar halo of our Galaxy; as revealed by Gaia

by Paola Di Matteo, Misha Haywood, Matthew Lehnert, et al.

The Gaia DR2 has permitted to understand that the major part of what was thought to be the in-situ Galactic halo of the Milky Way is in fact the remnant of one important merger that probably occurred more than 9 Gyr ago. These results have opened new interesting questions: what are the characteristics of this accretion event? Where is the in-situ Galactic halo? How is it connected to the Galactic thick disk ? We will present these results and discuss the new perspective about the old stellar populations of our Galaxy.

Invited talk: Progress made in asteroid research with Gaia DR2

by Josef Ďurech

The Gaia DR2 contains astrometry and photometry for about 14,000 asteroids. DR2 is for asteroids research the same as was DR1 for stellar astronomy - an appetizer before the main dish, which is the next data release. Measured by the amount of data and the number of asteroids for which the data are available, DR2 is only a fraction of what DR3 is expected to be. Nevertheless, DR2 was a milestone in asteroid research that has triggered a number of studies.

In my talk I will (i) describe the properties of asteroid data in DR2, (ii) summarize the progress in asteroid research that was enabled by DR2 asteroid photometry and astrometry, and (iii) describe in detail the process and results of asteroid shape and spin reconstruction from DR2 photometry and its combination with complementary data.

Occultation astrometry with DR2: Assessing the capabilities of robotic telescopes

by Joao Ferreira, Paolo Tanga, Pedro Machado, et al.

Stellar occultation events are a unique opportunity to obtain from the ground very accurate astrometry; close to the performance of Gaia. In fact; when an asteroid hides the light of a star; the uncertainty of its instantaneous position can be similar to that of the target star. By exploiting the accuracy of Gaia DR2 on both asteroids and stars; stellar occultation prediction and exploitation becomes an effective method to systematically collect asteroid astrometry.

At the same time; asteroid occultations can offer the possibility to confirm or discover double stars; in a range of small angular separations very complementary to the resolution accessible to Gaia itself. We will present statistics and simulations showing the improvement expected in the prediction of asteroid occultations thanks to Gaia astrometry.

By a bayesian approach; we determine in the parameter space (duration; flux drop; star brightness) the domain of detectable events from a single site. Our study prepares the exploitation of a 0.5-m robotic telescope at Plateau de Calern (southern France); for which we determine the range of asteroid size and star brightness that we expect to reach. This facility will start operations in a few months.

Invited talk: New insights into the formation of the Milky Way from Gaia

by Amina Helmi

I will describe recent analysis to pin down the assembly history of the Galaxy using stars from Gaia DR2, supplemented by existing ground based spectroscopic surveys. The focus of the talk will be on what we have learned so far about the merger history of the Galaxy by studying its stellar halo.

Multi-messenger study of the Milky Way with LISA; Gaia and LSST

by Valeriya Korol

Ultra-short period double white dwarf (DWD) binaries are unique multi-messenger tracers of the Milky Way. They can be detected in large numbers through electromagnetic radiation by Gaia and through gravitational waves by the upcoming LISA mission. I will show that the synergies between gravitational waves and optical observations of DWDs allow the study of the Milky Way baryonic structure. The success of this synergy is due to LISA's ability to localize binaries through virtually the whole Galactic plane; thus mapping its shape; while Gaia observations yield the motion of stars; tracing the underlying total enclosed mass.

Stellar 3D kinematics in dwarf spheroidal galaxies with Gaia+HST

by Davide Massari, Amina Helmi, Alessio Mucciarelli, et al.

Dynamical modelling of dark-matter dominated dwarf spheroidal satellites of the Milky Way has so far relied on line-of-sight velocity measurements only; and has thus suffered from degeneracies between the total mass of these systems and their stellar orbital distribution; expressed in terms of the anisotropy parameter. By combining data from Gaia and HST; we were able to measure for the first time ever the proper motions in one of these systems; the Sculptor dwarf spheroidal. Coupling them with available line of sight velocities we could thus measure the anisotropy and break such a limiting degeneracy.

With the more complete and precise Gaia DR2 dataset in hand; we have repeated the experiment on the Draco dwarf spheroidal; this time adding more spectroscopic data purposely acquired with the DEIMOS spectrograph at the Keck II telescope. The results of these two novel studies are presented; together with their implications in terms of the properties of the dark matter haloes of the two investigated galaxies; and their meaning for the challenges of cold dark matter cosmology.

Asteroid proper phase curves from Gaia DR2 photometry

by Karri Muinonen, Antti Penttilä, Julia Grön, et al.

The photometric phase curve of an asteroid describes the dependence of its disk-integrated brightness on the solar phase angle (the Sun-Object-Observer angle). We derive the parameters of the H; G1; G2 photometric phase function (e.g.; Penttilä et al. 2016; Planet. Space Sci. 123; 117) from the sparse data available from Gaia Data Release 2. An asteroid's lightcurve; i.e.; its observed brightness as a function of time; depends on the shape and spin state of the asteroid; as well as its surface scattering properties. It follows that these properties can be estimated from the observations; to an extent allowed by the given data.

We utilize Lommel-Seeliger ellipsoids (L-S ellipsoids) and low-resolution convex shapes in the statistical inversion of Gaia photometry. In the L-S scattering model for a surface element on the asteroid; we introduce a single-scattering phase function that enforces the H; G1; G2 disk-integrated photometric phase curve for a fictitious spherical asteroid (Muinonen et al. 2015; Planet. Space Sci. 118; 227). By utilizing the L-S model; we are able to derive H; G1; G2 parameters for asteroids observed by Gaia.

These parameters refer to the intrinsic surface properties of an asteroid and can be called proper H; G1; G2 parameters. Based on our preliminary study; the Gaia DR2 photometric data can allow us to constrain the proper H; G1; G2 parameters for asteroids with more than 15 observations; that is; for more than 2100 asteroids. In summary; Gaia DR2 facilitates photometric classification of a large number of asteroids and testifies to the impressive potential of the forecoming data releases.

Galactic Science with Gaia

by Ralph Schoenrich

While the Gaia data release has been a revolution for the field of Galactic astronomy; a proper scientific use requires removal of systematic biases. I will present our method for assessing distances in the Gaia DR2 dataset (which in contrast to astroseismic measurements uses and derives distances for all stars) and provide a quantification of the parallax bias in the Gaia data release; as well as a set of safe use rules for the data. After this; I will present new scientific results obtained with these data that very much have changed our picture of Galactic structure.

New detections of the Yarkovsky effect with Gaia DR2

by Federica Spoto, Paolo Tanga, Benoit Carry

The Yarkovsky effect (the result of the recoil force acting on rotating bodies as a consequence of their thermal emission) is the most important non-gravitational perturbation for asteroids. It produces a semi-major axis drift; which changes the orbit of small asteroids over millions of years. It is fundamental to understand the aging process of asteroid families and compute an impact risk for near-Earth asteroids experiencing close encounters with the Earth. It is also responsible of the transport mechanism from the asteroid belt to the inner Solar System and ultimately of the meteorite delivery to the Earth.

Direct detection of Yarkovsky is challenging because it can be measured only for small asteroids with very accurate orbits. The second Gaia data release (DR2; April 2018) contains about 2;000;000 observations for 14;099 Solar System Objects (SSOs). While available ground-based observations have a typical accuracy of the order of 300-400 mas; Gaia SSO observations reach an accuracy between 2 and 5 mas for the faintest asteroids (G~20.5) and at the sub-milliarcsecond level for brighter objects. However; Gaia DR2 observations cannot be used alone to detect this effect because of the limited duration covered by the observations.

To exploit Gaia observations together with other existing data; we have developed a new method to debias ground-based astrometry (see P. Tanga abstract) and a new weighting scheme taking into account the performances of each observer for each catalog used. We have chosen more than 70 asteroids from Gaia DR2 with a very accurate orbit and small enough to possibly detect the Yarkovsky effect. We show that the use of Gaia DR2 data is essential to detect the Yarkovsky effect for more than 10 asteroids for the first time and to improve the accuracy in the detections when the Yarkovsky effect had already been measured.

New life for old asteroid astrometry: correcting systematic errors by Gaia DR2.

by Paolo Tanga, Federica Spoto

Several scientific applications of the asteroid astrometry obtained by Gaia require to jointly use existing positions; mostly obtained by ground-based instruments. Despite their much lower accuracy; these archive data; spanning several decades; permit a large expansion of the time coverage; with a clear benefit on the accuracy of orbits and on the detection of subtle; secular dynamical effects. The exploitation of astrometric positions calibrated by using old catalogs (pre-DR2) and available at the Minor Planet Center; is a very challenging task. They are very heterogeneous; as they were obtained by different techniques (from traditional emulsion photography to CCD imaging and radar ranging) and a variety of instruments.

However; for a large fraction of them; information on the reference catalog used to obtain the archived position is provided. This is sufficient to try to apply corrections that can be derived by comparing local stellar astrometry in the original catalog; to a reference catalog such as DR2. We recall that without such corrections; orbital modelling is strongly limited in accuracy and precisions; and many observations affected by stronger systematics are even rejected as outliers. In the past a useful correction scheme (also known as "catalog debasing") was derived with respect to a subset of PPMXL stars used as a reference; on local sky patches defined by a healpix tessellation.

We present here a completely new approach; aiming to exploit the full accuracy of positions and proper mitions provided by Gaia DR2. Our method consists in computing zonal error not for single catalogs; but on single asteroid observations. This approach is much more flexible as the choice of the correction zone around the measured positions can be adjusted in size; shape; and magnitude depth; to better reproduce the instrumental constraints of each survey or instrument. Our first applications show that in fact; this debiasing approach works better and has more potential for further; future improvements. We present the results of the orbital fitting of several 1000s asteroids; all with astrometric observations published in DR2. We intend to release a large set of corrected asteroid data; for free access by interested researchers.

Dynamics of the Oort Cloud in the Gaia Era

by Santiago Torres, Maxwell Cai, Anthony Brown, et al.

Comets in the Oort cloud evolve under the influence of internal and external perturbations; such as giant planets; stellar passages; and the galactic tide. We aim to study the dynamical evolution of the comets in the Oort cloud; accounting for the perturbation of the galactic tidal field and passing stars. We base our study on three main approaches; analytic; observational and numerical. We first construct an analytical model of stellar encounters. We find that the cumulative effect of passing stars can strip the Oort cloud on a very short time scale(<500 Myr).

Using proper motions; parallaxes; and radial velocities from Gaia DR2 and combining them with the radial velocities from external surveys; we construct an astrometric catalogue of the 14;659 stars that are closer than 50pc to the Sun. We calculate the encounters that are closer than 2.5pc and that the Sun has had in the last 10Myr and will have in the near future 10Myr. Finally; we study the dynamical evolution of the comets in the Oort cloud under the effect of multiple stellar encounters from stars that pass within 2.5pc from the Sun and the galactic tide over ±10Myr.

For this; we use the Astrophysical Multipurpose Software Environment (AMUSE); and the GPU-accelerated direct N-body code ABIE. We find that the cumulative effect of relative distant stellar encounters together with the galactic tidal field; proves to be an efficient mechanism in the creation of interstellar comets. This raises the question about the formation; evolution and current status of the Oort cloud; as well as; the existence of an Interstellar Oort Cloud. A larger sample of stars constructed through less filtering or an incompleteness correction can only increase the comet loss.

Exploring Magellanic Interactions Through Bridge and Small Magellanic Cloud Dynamics

by Paul Zivick, Nitya Kallivayalil, Gurtina Besla, et al.

In recent years; our view of the Magellanic Clouds has shifted due to a rapidly expanding pool of data. Hubble Space Telescope-measured proper motions (PMs) of the Clouds revealed their first infall and the timescale on which they most recently interacted. With the advent of Gaia however; a new avenue of exploration has opened into the history of the Clouds.

Here we present measurements of the PMs in Magellanic stars from Gaia across the Magellanic Bridge compared against simulations of different interaction histories of the Clouds; revealing a strong preference for a recent and direct last interaction. We additionally present an analysis of the internal dynamics of the Small Magellanic Cloud (SMC). In particular; we show our search for rotation within the stellar populations of the SMC; hinted at by HI studies and stellar radial velocity measurements. In combination with simulations of interactions between the Clouds; we attempt to improve our physical understanding and intuition for the complex dynamic scenario that is the Magellanic Clouds.

Abstracts of the Thursday Pitch Session

How many young stars does Gaia see?

by Erika Varga-Verebelyi, Gábor Marton, Péter Ábrahám

Although Gaia is not sensitive to infrared it can see a lot of young stars around us. But how many? In Konkoly Observatory we are trying to figure it out. Our first step to answer this question was to collect all the available young stellar objects (YSOs) from the literature can see in optical bands, to see how many of them are in the second Gaia Data Release (DR2). Our second step was to combine DR2 data with WISE and Planck measurements and made an all-sky probabilistic catalogue of YSOs using machine learning techniques, such as Support Vector Machines, Random Forests, or Neural Networks. Our results will be on the poster.

Open Clusters Membership by Clusterix 2.0 for Gaia DR2

by Lola Balaguer-Nuñez, Mauro Lopez del Fresno, Enrique Solano, et al.

We present an advanced version of Clusterix (http://clusterix.cab.inta-csic.es), a tool for the determination of membership probabilities in stellar clusters from proper motion data adapted to the new wealth of Gaia data. Clusterix is a VO web-based, interactive application that allows the computation of membership probabilities from proper motions through a fully non-parametric method.

Clusterix 2.0 has been adapted to the exploitation of Gaia Data Release 2 and now features an improved user interface for a faster, easier and more accurate interactive definition of the cluster and field proper motion distributions. The system provides fast feedback between membership probability determinations and the distribution of the observables for the most probable members and field stars.

We present the first results of Clusterix for different cases to show the capabilities of the tool. An area of five degrees around NGC 2682 (M67), an old, well known cluster that extends further than in any previous study. A young cluster NGC 2516 with a striking radial structure extended up to four degrees. NGC 1750 & NGC1758, a pair of partly overlapping clusters found without applying any a priori knowledge. In the area of NGC 1817 we find a little known cluster, Juchert 23. And in an area with many neighbouring clusters we are able to disentangle the existence of two clusters where only one was known: Ruprecht 26 and the new, Clusterix 1.

Open cluster chemistry in the era of Gaia

by Angela Bragaglia

Stellar clusters; being groups of stars sharing the same age; distance; and initial chemical composition are ideal laboratories to test models of stellar and galactic formation and evolution (e.g. Freeman & Bland-Hawthorn 2002). Gaia observes almost the whole known population of Galactic OCs; providing crucial information on membership and high precision photometry (see e.g. Gaia Collaboration; Babusiaux et al. 2018; Cantat Gaudin et al. 2018); while the spectroscopic information (radial velocity; abundance) is more limited (Katz et al. 2018).

Ground based follow-up with higher resolution; larger spectral coverage; and reaching deeper than the Gaia RVS is then required. This has been/is/will be pursued by large surveys such as Gaia-ESO; GALAH; WEAVE; etc. A full characterisation -and an accurate age derivation- for open clusters requires to know also the metallicity and possibly the full set of detailed abundances.

Presently; less than 10% of OCs have been studied using high-resolution spectroscopy (e.g. Netopil et al. 2016; Magrini et al. 2017; Smiljanic; Donati; Bragaglia et al. 2018); so also "private" observational programs; such as our large program SPA at the TNG with HARPS-N+Giano and the OSTTA project at NOT+FIES; are important. I will present our projects (a collaboration of researchers of INAF Bologna; Catania; Firenze; Padova; Barcelona; Bordeaux) and the first results. We concentrate on the brighter stars of OCs in the solar vicinity; within about 2kpc; where Gaia precision is at its best and the membership determination is more reliable so that we can trust it also for clusters immersed in a strong contaminating field component. This permits to very efficiently observe only high-probability cluster members.

For these OCs we will provide a detailed and accurate chemical characterization (elements of all nucleosynthetic chains; probing different formation sites) on a par with the astrometric and photometric information provided by Gaia.

A new view of Cygnus OB2 from Gaia DR2

by Sara Rodríguez Berlanas, Nick Wright, Artemio Herrero, et al.

The structure of young star clusters and associations is fundamental to our understanding of their formation and dynamical evolution; as well as of their stellar content. In this contribution; we present the study of the 3-dimensional structure of Cygnus OB2 using Gaia DR2 astrometry. We find significant line-of-sight substructure within the association; which we quantify using a parameterised model that reproduces the observed parallax distribution.

We identify two different stellar groups superposed on the association and separated by several hundred parsecs: the main Cygnus OB2 group at  ~1760 pc; further away than recent estimates have envisaged; and a foreground group at ~1350 pc. This work represents an important example of how Gaia DR2 astrometry can be used to unveil the substructure of Galactic OB associations.

Membership of open clusters by combining Gaia-DR2 astrometry with Gaia-ESO survey data

by Fay E. Wilde, Robin D. Jeffries, Nicholas J. Wright, et al.

Open clusters are cosmic laboratories to study stellar evolution and galactic structure. Before an open cluster data set can be used it is necessary to establish membership of a cluster independent of any stellar property under investigation. This poster discusses such a method applied to a large set of clusters observed by the Gaia-ESO (GES) survey.

By combining Gaia-DR2 astrometry with homogeneously determined radial velocities and spectroscopic information from GES, we can provide estimates of membership probability, based on a flexible set of indicators, that are unbiased with respect to parameters of interest (e.g. lithium abundance or kinematics).

Runaway star candidates from the ONC with Gaia DR2

by Christina Schoettler, Jos de Bruijne, Eero Vaher

Simulations suggest that we should be able to find ejected, unbound stars of all masses around many young star-forming regions. Most observations of these fast stars, so-called runaway stars, are high-mass stars, i.e. O or B stars. Until now, observational limitations/bias have made it difficult to search for and find any lower-mass runaways, but this has changed with the advent of Gaia.

Using Gaia DR2, we have searched the vicinity of the Orion Nebula Cluster (ONC) and find a number of runaway stars and so-called walkaway stars (at lower velocity) that could have been ejected from the ONC during its previous dynamical evolution.