Abstracts of the Wednesday Programme


Hypervelocity discoveries with Gaia DR2

by Douglas Boubert, Denis Erkal, Ken Shen

After only two weeks; Gaia DR2 had revolutionised the study of fast stars by enabling the discovery of white dwarfs moving at 1000s of km/s; the elimination of hundreds of candidate hypervelocity stars; and the proof of an 8 solar mass star having been ejected from the LMC at 870 km/s. I will present these three results and discuss the implications for the occurrence of both double-degenerate Type Ia supernova and intermediate mass black holes in star clusters.

I will go on to summarise how the field has advanced in the subsequent months; giving particular focus to my own work on the mystery of the fastest star in the Gaia DR2 RVS sample and the extragalactic origin of the main sequence hypervelocity stars in the halo.


Invited Talk: Using stellar clusters to trace the properties of the Milky Way in the Gaia era

by Tristan Cantat-Gaudin

The second Gaia data release (Gaia DR2) has allowed immediate and significant progress in our ability to identify and study stellar clusters.

In this talk I will review some of the main results of the first months of scientific exploitation of the Gaia DR2 data, including the discovery of new clusters and the improved picture of the Galactic structure enabled by Gaia.

Gaia DR2 astrometry also allows us, for the first time, to characterise the 3D position and motion of individual stars in a large number of objects. Its unprecedented precision has opened a new era for studies of internal mechanisms at work in old clusters and of the complex spatial and kinematic structure of the Vela-Puppis association and other young stellar aggregates.


How complete is the Open Cluster census?

by Alfred Castro-Ginard, Carme Jordi, Xavier Luri

In the time of massive surveys such as Gaia; machine learning (ML) has shown to be a powerful tool to explore the catalogue searching for hitherto unknown stellar structures. In particular; the combination of both unsupervised and supervised learning algorithms allows to significantly improve the detection of open clusters (OCs). We use a combination of ML techniques to systematically mine the Gaia archive; looking for overdensities in the five dimensional astrometric space (positions; parallax and proper motions).

On the one hand; the DBSCAN algorithm identifies overdensities in this space that can correspond to physical clusters. On the other hand; the clusters found are then separated in physical clusters (OCs) and random statistical clusters using an artificial neural network (ANN) to recognise the isochrone pattern of the true OCs in the color magnitude diagram (CMD) of the member stars. With this methodology we were able to detect 23 previously unknown OCs in the TGAS dataset; and later confirm them using Gaia DR2 data.

Using Gaia DR2; we explore a region near the galactic anticentre (120 <= l <= 205 and -10 <= b <= 10); with the goal of fine tuning the method to adapt to different density regions; where we detect a significant amount of new OCs compared to the previously catalogued in this region. The planned application of the method to an all sky search will substantially increase the existing; and continuously growing; OCs catalogues.


Unravelling the structure and kinematics of the Taurus star-forming region with Gaia-DR2 and VLBI astrometry

by Phillip Galli, Laurent Loinard, Hervé Bouy, et al.

The second data release of the Gaia space mission (Gaia-DR2) delivered; for the first time; trigonometric parallaxes and proper motions for a large number of young stellar objects (YSO) that could not be measured by other techniques. In recent years significant progress has also been made to deliver state-of-the-art astrometry using multi-epoch Very Long Baseline Interferometry (VLBI) observations from the ground in the radio domain. In particular; the Gould's Belt Distances Survey (GOBELINS) uses the Very Long Baseline Array (VLBA) operated by the National Radio Astronomy Observatory (NRAO) to measure trigonometric parallaxes and proper motions of YSOs in nearby star-forming regions with the same level of precision of the Gaia satellite.

We present recent results of the GOBELINS project towards the Taurus star-forming region and compare them with Gaia-DR2 astrometry. In a subsequent analysis; we combine Gaia-DR2 and VLBI data to investigate the structure and kinematics of the various molecular clouds in the Taurus complex with the best astrometry available to date. We apply a hierarchical clustering algorithm to the sample of known YSOs in Taurus and our results reveal the existence of multiple populations of YSOs with different properties that provide important clues to the star formation episode in this region.


Orion A with Gaia DR2: how 3D changes everything

by Josefa Grossschedl, Joao Alves, Stefan Meingast, et al.

The giant molecular cloud Orion A has been extensively studied in the past; being the closest massive star-forming region to Earth (~400pc). It contains the famous Orion Nebula Cluster (ONC) towards the North; and the low-mass star-forming regions L1641 and L1647 toward the South; and it is the cluster and massive star formation benchmark; always assumed to be the straight 40 pc filament one sees from radio and submillimeter surveys. While performing an analysis of the gas mass distribution; and star formation rate and efficiency across the cloud; we found that the gas is roughly distributed uniformly across the cloud; while; oddly; the ONC region produced about a factor of ten more stars compared to the rest of the cloud.

To try to understand this counter-intuitive relation between dense gas and star formation rate (SFR); we used Gaia DR2 data to reconstruct the 3D shape of the cloud; using the distances of young stellar objects as a proxy for cloud distances. We find that the ONC region indeed lies at about 400 pc; as expected; while the low-mass star-forming parts are; surprisingly; inclined about 70 deg from the plane of the sky; reaching about 500 pc towards the Southern end of the cloud. Orion A is then an approximately 90 pc long filament (nearly twice as long as previously assumed); comet-shaped; with its "Head" (the ONC region) "bent" likely due to external feedback; responsible for the observed increased SFR.

This new view of Orion A is an example of how Gaia is opening a new "3D window" on the topology of the dense; star-forming ISM; a critical missing ingredient in our understanding of star formation.


Dust Distribution in the Milky Way Disk

by Clément Hottier, Carine Babusiaux, Frédéric Arenou

We have developed the FEDReD algorithm (Field Extinction-Distance Relation Deconvolver) to study the distribution of stars and dust in the galactic disk; by using the Gaia data release 2 combined with infrared photometric surveys (2MASS; UKIDSS; VVV). We first built an empirical HR diagram corrected from selection effects with local GDR2 stars crossmatched with 2MASS. This empirical diagram is then used as a reference to analyse our data set.

We analyse the galactic disk field by field. For each star in the field; we use our reference to get an individual 2D probability distribution (distance / extinction). We then proceed to a Bayesian deconvolution; taking into account the surveys selection functions; to get the join distribution probability of the field of view. By applying FEDReD to several fields along the galactic plane; we are able to draw a 3 dimensional extinction map of the Milky Way disk.

We will present our first extinction map obtained by this algorithm with 2MASS and Gaia DR2; and the galactic structures that appear in it.


Gaia-2MASS 3D maps of Galactic interstellar dust within 3 kpc

by Rosine Lallement, Jean-Luc Vergely, Carine Babusiaux, et al.

3D maps of the interstellar dust provide complementary information to Gaia stellar measurements and are a tool for a wide range of uses. In a first step towards maps; Gaia DR2 photometric data were combined with 2MASS measurements to derive extinction towards stars that possess accurate photometry and relative uncertainties on DR2 parallaxes smaller than 20%. In a second step; we applied to these individual extinctions a new hierarchical inversion algorithm adapted to large datasets and to a non homogeneous target distribution. Each step associates a regularized Bayesian inversion along the radial directions and azimuthal merging by means of subsequent full-3D inversion. Each inverted distribution serves as a prior for the subsequent step and the spatial resolution is progressively increased.

We present the resulting 3D distribution of the dust in a 6 x 6 x 0.8 kpc3 volume around the Sun. Its main features are found to be elongated along different directions that vary from below to above the mid-plane: the outer part of Carina-Sagittarius; mainly located above the mid-plane; the Local arm/Cygnus Rift around and above the mid-plane and the fragmented Perseus arm are oriented close to the direction of circular motion. The more than 2 kpc long spur that extends between the Local Arm and Carina-Sagittarius; the compact near side of Carina-Sagittarius and finally the Cygnus Rift below the Plane are all three oriented along a 40-55 degrees Galactic longitude direction.

Dust density in vertical planes reveal in some regions a wavy pattern and show that the solar neighborhood within 500 pc remains atypical by its extent above and below the Plane. We show map validations; comparisons with the locations of molecular clouds; HII regions; O stars and masers. The link between the dust concentration and these tracers is markedly different from one region to the other.


Searching for the fastest stars in the Galaxy with Gaia DR2

by Tommaso Marchetti, Elena M. Rossi, Anthony G.A. Brown

The second data release (DR2) of the ESA satellite Gaia gives us the first opportunity to search for high velocity stars in the largest stellar catalogue ever produced. Categories of great interest are runaway stars; high velocity halo stars; and hypervelocity stars (HVSs). In this talk I will present our results on the high velocity tail of the velocity distribution; using the subset of ~7 million stars with a radial velocity measurement from Gaia DR2. This study reveals the presence of a population of ~20 unbound stars; half of them with a possible extra-Galactic origin.

I will also present ongoing results on the data mining routine we specifically developed to search for the fastest stars; HVSs; in the full (~1.3 billion sources) Gaia DR2 catalogue. Our published estimates show that hundreds to thousands of HVSs are expected to be contained in the full Gaia catalogue; but their identification is not trivial because of the missing radial velocity information; which needs to be obtained with a spectroscopic follow-up. I will conclude presenting HVSs as a powerful probe to map the dark halo of the Milky Way: a population of ~200 Gaia HVSs could be used to nail down the Galactic halo potential parameters with sub-percent precision.


Extended stellar systems in the solar neighborhood - The Hyades tidal tails and beyond

by Stefan Meingast, Joao Alves, Verena Fürnkranz

Already the first few years of the Gaia mission have brought a wealth of new insights into the velocity distribution of the stellar population in the solar neighborhood. While the complex ridge-like pattern in the large-scale velocity distribution seems to be caused by non-axisymmetric components of the Galaxy; we can find stellar clusters and groups imprinted as small-scale fluctuations on those larger ridges in velocity space. One prominent representative of these groups is the nearest open cluster to the Sun; the Hyades.

I will show how Gaia enabled the remarkable confirmation of the long-predicted tidal tails of the Hyades open star cluster. In particular; I will highlight the striking similarity to the predicted shape of the tidal tails; as well as the expected velocity dispersion profile of the cluster. Looking beyond the Hyades; I will highlight the richness of the solar neighborhood in kinematically cold stellar associations and clusters. The structure and spatial arrangement of these groups outlines a highly dynamic evolution in the global gravitational potential of the Galaxy.

Most notably; these results favour a picture where star clusters; that are bound in the Galactic disk; are; given time; transformed into thin; dynamically-cold streams of stars; their shapes reminiscent of tidally disrupted globular clusters.


The Gaia Archive. Present and evolution for future Data Releases

by Alcione Mora, Juan Gonzalez-Nuñez, Jorgo Bakker, et al.

The data produced by the ESA Gaia mission, and analysed by the DPAC consortium, are provided to the astronomical community via the Gaia Archive located at ESAC, and its partner and affiliated data centres.

The bulk of Data Release 2 is a big catalogue (gaia_source), distributed using the Virtual Observatory TAP protocol, complemented with some ad-hoc extensions. The main extra features include a private space for each registered user (persistent uploads), and the possibility to share these resources among other researchers for a collaborative data analysis. An Astroquery module provides Python programmatic acces. These capabilities make the Archive a key component in the pre-release validation, when the data are ingested but only accessible to the dedicated DPAC team.

Additional resources are also available at the Archive. Notably, detailed information on a sample of variable stars, together with their associated light curves, and epoch data for a selection of known asteroids. A set of external catalogues from major surveys and their pre-computed cross-matches against Gaia positions are also provided for convenience.  A number of technical improvements have been introduced; like the use of arrays, which is a novelty for TAP based archives. As opposed to the first data release, the light curves are not stored as a plain table but in a separate infrastructure, based on the Virtual Observatory DataLink protocol, and specifically designed to support the massive downloads expected for Gaia Data Release 3 and later; e.g. when average and epoch spectra (billions); astrophysical parameter distribution functions (billions) and epoch photometry and astrometry (trillions of points) will be progressively available. 

Additional features have also been incorporated, such as the possibility to rigorously propagate Gaia astrometry and their uncertainties to any other epoch for precise cross-catalogue comparison  Finally, a summary of the system behaviour and users' feedback after the release will be presented.  


3D map of the dust distribution towards the Orion-Eridanus superbubble with Gaia DR2

by Sara Rezaei Kh., Coryn Bailer-Jones , Juan D. Soler, et al.

We present a study of the 3D distribution of dust towards the Orion-Eridanus superbubble; a nearby expanding structure that spans more than 1600 square degrees in the sky. We use Gaia photometry and parallax together with photometry from 2MASS and WISE to get distances and extinctions for stars towards this region. Then by considering neighbouring correlations using the Gaussian Processes; we derive the probability distribution of dust density at any arbitrary point towards the Orion-Eridanus region. Using the resulting maps; we examine the relation of the inferred 3D structure with the gas dynamics and the magnetic field morphology observed toward this prototypical object.


Open clusters from Gaia DR2

by Antonella Vallenari, Ricardo Carrera, Diego Bossini, et al.

Gaia DR2 opened a new window on open Galactic clusters; including high quality astrometry and photometry. This has allowed a re-determination of the open cluster parameters and a better view on the relation between clusters and field stars. Here we focus on the age determination and on the cluster dissolution processes.


The Optical to Mid-Infrared Extinction Law Based on the APOGEE, Gaia DR2, Pan-STARRS1, SDSS, APASS, 2MASS and WISE Surveys

by Shu Wang, XiaoDian Chen

A precise interstellar extinction law is critically important to properly interpret observations. In principle; both the color excess ratio (CER) and the relative extinction value are usually regarded as indicators of the extinction law. With the information of observations and knowledge of intrinsic color indices; the color excess and CER can be easily derived.

However; to derive an estimate of the wavelength-dependent interstellar extinction law A_lambda/A_lambda1 is more challenging. It requires an independent determination of extinction value or the distance information to the target source. In this work; we combine spectroscopic data; astrometric data; and photometric data to derive both accurate CERs and relative extinction in optical--mid-infrared (IR) bands. A group of red clump (RC) stars are selected as extinction tracers by stellar parameters from APOGEE survey.

The multi-bands photometric data are collected from projects including Gaia; APASS; SDSS; Pan-STARRS1; 2MASS; and WISE surveys. The CERs are determined by the color--excess method. After detailed simulations of the statistical and systematic errors; we conclude that our measurements of CERs have only ~1--2% uncertainties. With accurate parallaxes from Gaia DR2; the relative extinction A_GBP/A_GRP=1.700+-0.014 is derived by the color--extinction method. Finally; the optical--mid-IR multibands extinction values A_lambda/A_GRP have been determined in total twenty-one bands.

As this work has no bias toward any specific environment; our extinction law represents the average extinction law with Rv=3.11+-0.24. In optical bands; our extinction law can be well fitted by Rv = 3.1 curve. However; in near-IR bands; it can be perfectly fitted by the power law with index alpha=2.10+-0.02. The extinction coefficients at Gaia bands are determined as well. Based on our extinction law; we have estimated the extinction and effective wavelengths of Gaia bands; and predicted the relative extinction in the near-IR bandpasses of the HST and the JWST.


3D mapping of young stars in the solar neighbourhood with Gaia DR2

by Eleonora Zari, Hossein Hashemi, Anthony G. A. Brown

We use Gaia DR2 to explore the three dimensional arrangement of young stars in the solar neighbourhood and we provide a new; original view of the spatial configuration of the star-forming regions within 500 pc of the Sun. To do so; we construct three dimensional (3D) density maps for early-type stars (upper-main sequence; UMS) and pre-main sequence (PMS) sources. Both density maps show three prominent structures; Scorpius-Centaurus; Orion; and Vela. The PMS map also shows a plethora of lower-mass star-forming regions; such as Taurus; Perseus; Cepheus; Cassiopeia; and Lacerta; which are less visible in the UMS map due to the lack of large numbers of bright; early-type stars.

We estimate ages for the PMS sample and we study the distribution of PMS stars as a function of their age. We find that younger stars cluster in dense; compact clumps; and are surrounded by older sources; whose distribution is instead more diffuse. Finally; we conclude that the 3D density maps show no evidence for the existence of the ring-like structure which is usually referred to as the Gould Belt.


The substructure and halo population of the Double Cluster h and chi Persei

by Jing Zhong, Li Chen, M.B.N. Kouwenhoven

In order to study the stellar population and possible substructures in the outskirts of Double Cluster h and chi Persei; we investigate using the Gaia DR2 data a sky area of about 7.5 degrees in radius around the Double Cluster cores. We identify member stars using various criteria; including their kinematics (viz; proper motion); individual parallaxes; as well as photometric properties.

A total of 2168 member stars in the parameter space were identified as members. Based on the spatial distribution of the member stars; we find an extended halo structure of h and chi Persei; about 6-8 times larger than their core radii. We report the discovery of filamentary substructures extending to about 200~pc away from the Double Cluster. The tangential velocities of these distant substructures suggest that they are more likely to be the remnants of primordial structures; instead of a tidally disrupted stream from the cluster cores.

Moreover; the internal kinematic analysis indicates that halo stars seems to be experiencing a dynamic stretching in the RA direction; while the impact of the core components is relatively negligible. This work also suggests that the physical scale and internal motions of young massive star clusters may be more complex than previously thought.