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.