Abstracts of the poster sessions


Close encounters of the stellar kind

by Coryn Bailer-Jones, Jan Rybizki, Rene Andrae, et al.

Passing stars may play an important role in the evolution of our solar system. They can perturb the Oort cloud; thereby injecting comets into the inner solar system which may then collide with the Earth or other bodies. We have searched for close stellar encounters to the Sun among all 7.2 million stars in Gaia DR2 that have six-dimensional phase space data. We characterize encounters by integrating their orbits through a Galactic potential and propagating the correlated uncertainties via a Monte Carlo resampling.

We find several stars with good quality data which have passed - or which will pass - within 1pc of the Sun. The possible influence of these stars on the Oort cloud is investigated. By modelling the Gaia selection function; we can use these encounters to estimate the total encounter rate over the past/future few million years. We examine the current limitations of this work; and look at prospects for extending it with future Gaia and non-Gaia data.


Albireo, a pretty pair: stable stellar marriage or celestial one-night stand?

by Ulrich Bastian

In a short paper published in November 2018 (arXiv astro-ph 1811.01665); we used Gaia DR2 to discuss the physical configuration of one of the most beautiful and prominent multiple stars in the sky: Albireo; beta Cygni. Taking all available information at face value (including the new aspects from Gaia DR2); astrophysically paradoxical conclusions are drawn. The paper proposes specific follow-up observations. By the time of the 53rd ESLAB symposium we expect to have clarified the true mass of Albireo Aa (bloated dwarf or normal giant?) and the nature of Albireo AB: physical or optical pair?


Data mining Gaia DR2: the quest for Pre-Main Sequence Stars (and their discs)

by Héctor Cánovas, Carles Cantero, Lammers Uwe, et al.

As the birth-sites of planets; protoplanetary discs have become the object of intense study during the last years. Constraining their typical lifetimes; masses and/or sizes is crucial to understand the process of planet formation. These objects are a natural by-product of early stellar evolution; and therefore a high fraction of Pre-Main Sequence Stars (PMS) are surrounded by a protoplanetary disc. The current census of PMS (and discs) has been limited due to instrumental restrictions (i.e. sensitivity); but this situation is going to change thanks to projects like Gaia.

In this talk I will present the results of applying machine learning; density-based clustering algorithms to the Gaia DR2 dataset with the aim of identifying new members of the Ophiuchus star forming region. I will briefly describe our methodology and present the ~280 potential new PMS that we find.


Synergies between Gaia Data Release 2 and OCCASO survey in the study of open clusters

by Juan Carbajo-Hijarrubia, Laia Casamiquela, Ricardo Carrera, et al.

Galactic Open Clusters (OCs) are crucial to investigate the formation and evolution of the Galactic disc. The Open Clusters Chemical Abundance from Spanish Observatory survey (OCCASO) aims to provide high precision radial velocity (typical uncertainties are between 100 to 200 m/s) and to determine abundances for more than 20 chemical species in around 30 Northern OCs (Casamiquela et al; 2016 MNRAS 458; 3150). We use high resolution (R > 65;000) high signal-to-noise (∼70) spectra of at least 6 Red Clump stars; for each cluster.

The advent of Gaia Data Release 2 (DR2; Gaia Collaboration 2018; A&A 616; A1) has been a breakthrough in our knowledge of the OCs allowing the identification of previously unknown clusters (Castro-Ginard et al; 2018; A&A 618; A59) and the redetermination of the membership for all known clusters (Cantat-Gaudin et al; 2018; A&A 618; A93). Gaia DR2 mean parallaxes and proper motions and OCCASO radial velocities have been combined to obtain 3D spatial velocities and peculiar velocities with respect to the Regional Standard of Rest. With this information we have redetermined the orbits for our previously observed clusters and calculated orbits for recently discovered ones.

The computation of the orbits uses gravitational potential that includes the Galactic bar and spiral arms resembling those of the Milky Way (Pichardo et al 2003; 2004; ApJ 582; 230). Galactic trends (with radius; distance from the plane and age) of Fe peak and alpha elements have been studied for the 21 OCs currently observed in the OCCASO survey and compared with chemodynamical models of the Milky Way; with special emphasis on clusters in the inner disk (like M11).


Chemically characterising nearby evolved open clusters

by Laia Casamiquela, Caroline Soubiran, Yoann Tarricq, et al.

Open Clusters (OCs) are excellent tracers of the formation and evolution of the Galaxy; as well as an ideal laboratory to test theories of stellar evolution. In particular; nearby OCs spanning a wide age range are commonly used as benchmark objects for galactic and stellar studies owing to the highest precision that is achievable. We have designed a project to perform an in-depth study of ~30 nearby (closer than 500 pc) and old (age>300Myr) OCs. After Gaia DR2; very accurate memberships; distances; motions and ages are available for these OCs; allowing us to revise their physical properties. In particular we are determining their chemical composition in a homogeneous way using a strictly line-by-line differential method with high-resolution; high signal-to-noise spectra; either retrieved from public archives or from our own observations.

We will present the first results of this project where we mesure the internal dispersions of abundances within the studied OCs and compare their chemical signatures. We will show how abundance ratios correlate with age and kinematics.


The inversion of merged Hipparcos and Gaia DR2 photometric data for asteroids

by Alberto Cellino, Daniel Hestroffer, Xiaoping Lu, et al.

Sparse photometric data can be used to determine the spin properties (rotation period; orientation of the spin axis) and infer information about the shapes of asteroids. The algorithm adopted for the inversion of Gaia photometric data assumes; for the sake of simplicity and to minimize CPU execution time; that the objects have triaxial ellipsoid shapes. In the past; this algorithm was tested against photometric data obtained by the Hipparcos satellite. Only limited numbers of Hipparcos measurements were available for a small sample of bright asteroids; and the data were affected by large uncertainties. Nevertheless; it was possible to obtain correct determinations of the spin period for several asteroids present in the Hipparcos photometric catalogue.

Now; after the second Gaia data release; which includes photometric data for a large sample of objects; it is possible to merge new Gaia data with older Hipparcos measurements; in order to test the performances of the photometric inversion algorithm using a larger data set. In so doing; we use an improved version of the inversion algorithm; which includes now the treatment of a Lommel-Seeliger scattering relation; specially developed for the case of triaxial ellipsoid shapes. In addition to the simple triaxial ellipsoid shape model; in some cases we also make tests using a more refined shape model; based on the so-called “cellinoid” shapes. The results of our analysis are encouraging.

With respect to the old inversion of Hipparcos data carried out in the past; we are able now to improve the number of successful determinations of asteroid spin periods and poles. This is a relevant; indirect validation of Gaia DR2 photometry of asteroids; taking into account that the asteroids of our sample include some of the brightest and largest asteroids; which are certainly not ideal Gaia targets.


Benford's Law in Gaia DR2

by Jos de Bruijne, Jurjen de Jong

Benford's Law states that the frequency distribution of significant digits of particular data sets is non-uniform; with a strong preference for small numbers. As an example; Benford's Law states that 1 appears as the leading significant digit 30.1% of the time while 9 occurs as significant digit for only 4.6% of the data points. Earlier work has demonstrated that the ~100;000 Hipparcos parallaxes; converted to distances by simple inversion; follow Benford's Law. We present an investigation into the intriguing question whether also the 1.3 billion Gaia DR2 parallaxes; and the associated Bayesian-inferred distances; follow Benford's Law.


Mapping M Dwarf Metallicities

by Victoria DiTomasso, Sarah Schmidt

We use distances derived from Gaia parallaxes cross-matched with SDSS; WISE and 2MASS photometry to assemble a sample of approximately 2 million M dwarfs; defined as stars with SDSS colors redder than r-i = 0.3 and i-z = 0.2. Previous work has found relations between SDSS r-z and WISE W1-W2 colors and the effective temperature and metallicities of cool stars (3550-4200 K). Using our sample and these previously derived relationships; we measure these stars' metallicities and effective temperatures. We calculate the metallicity distribution gradient as a function of galactic height for M dwarfs within 1000 pc of the Sun; which ultimately lends insight to the kinematic and chemical evolution of the Galaxy.


Systematic astrometric biases in stellar catalogues as compared to the Gaia DR2

by Karolina Dziadura, Dagmara Oszkiewicz, Agnieszka Kryszczyñska

We search for systematic astrometric biases in the stellar catalogues as compared to the Gaia DR2 catalogue for asteroid astrometry. We take into account star positions in the Tycho-2; UCAC2; USNO B1.0; USNO A2.0 and USNO A1.0 catalogues. Chesley et al. (2010) showed that biases in stellar catalogues result in systematic errors (even up to 1.5 arcsecond) in asteroid astrometry. We made maps of spatially resolved RA and DEC offsets with respect to GAIA DR2 positions for above mentioned star catalogues with the goal of increasing accuracy of existing asteroid astrometry similarly to Chesley et al. (2010). Those maps will be used to remove systematic biases in asteroid astrometry; depending on asteroid position on the sky. The corrections will be applied to asteroid astrometry in the Minor Planet Center database. Those corrections are important for improving asteroids orbits; detection of Yarkowski effect and better prediction of asteroids collisions.


Statistical detection of the tidal streams of the globular clusters using Gaia data

by Carles Garcia Palau

The distribution of mass in the various components of the Milky Way and its gravitational potential are still unknown. With the current data; one of the most unknown components is the dark-matter halo. The shape of this component can be constrained using stellar streams. Streams form when a satellite galaxy like a globular cluster gets tidally disrupted by the Milky Way gravitational field; mainly when the satellite crosses the plane disk or passes near the inner galaxy region. The mass loss due to the tidal stripping trace a track close to the satellite orbit extending from the front and the back of the satellite position. Some of this streams have been discovered and studied mainly Pal-5 and GD-1; but now we have access to parallaxes; proper motions and radial velocities for a billion stars due to Gaia satellite. This improves significantly our data of the observed positions and velocities of halo stars and will allow us to discover more stellar streams.

The stellar streams are proved to be very sensitive to the shape of the dark-matter halo and the overall shape of the Milky Way potential. For this reason a statistical method has been developed in order to detect low density streams; inclusive individual stars far from its source satellite. These stars are selected as a candidates to be a member of a stream computing a probability attending on its position in phase-space and other properties such as metallic abundances among others. Each identified stream will provide a new constrain for the shape of the Milky Way potential; in particular its three-dimensional characteristics like the minor-to-major axis ratio (a/b). The three dimensional shape of Milky Way's dark-matter halo has been studied using stellar streams finding ratios a/b between 0.9 - 0.95 but the uncertainties do not allow to get a strong conclusion.

Another methods have been used; one of them uses the observed positions and velocities of halo stars and a model based on the Jeans equations to infer the triaxilaity of the gravitational potential. Despite the efforts; these measurements give a wide range of values for the minor-to-major axis ratio; from strongly oblate (a/b ~ 0.4) to prolate potentials. On the other hand; numerical simulations of the formation of dark-matter halos suggest that halos follow a triaxial Navarro-Frenk-White profile. In absence of baryons halos tend to be strongly triaxial but the presence of a baryonic disk causes the halos to become more spherical (a/b ~ 0.7 to 0.8). These uneven results are expected to be improved using the Gaia data and the statistical method developed to identify stellar streams.


GAIA DR2 Observations of Galactic post-AGB stars

by Pedro Garcia-Lario

Gaia Data Release 2 (DR2) data provides a unique opportunity to review the distances; luminosities; as well as the overall photometric and variability properties of Galactic Post-AGB stars. In this contribution we review the information contained in the Gaia DR2 archive for all post-AGB stars contained in the Torun Catalogue of Post-AGB stars (https://www.ncac.torun.pl/postagb); the largest sample of stars in this short evolutionary phase so far compiled and derive some preliminary conclusions.


Long-term monitoring of unclassified Gaia transients at the Terskol Observatory

by Vira Godunova, Andrii Simon, Inna Izviekova, et al.

We report on optical observations of stellar objects; which were detected within the framework of the Gaia mission. The program on long-term monitoring of Gaia transients has started at the Terskol Observatory in 2016. The telescopes Zeiss-2000 and Zeiss-600 provide good enough opportunities for follow-up studies of these objects. Special attention is given to unclassified transients discovered by ESA Gaia; DPAC and the Photometric Science Alerts Team (gsaweb.ast.cam.ac.uk/alerts). Photometric observations through BVRI filters have been used to perform a quick analysis of the imaging data in order to reveal short- or long-term variability in the brightness of sources; as well as changes of their color indices. Appropriate software developed has been applied for processing and analysis of datasets gathered in 2016-2018 for more than 40 objects; a lot of them have been continuously observed over the years. In this paper; we present recent results obtained for transients Gaia17agr; Gaia17cuh; Gaia18aen; and Gaia18cct.


The PLZ-relation in classical Cepheids: A cleaner sample

by Martin Groenewegen

In Groenewegen (2018 A&A 619; A8) I investigated the period-luminosity-metallicity relation of classical cepheids in the V-; K- and Wesenheit WVK bands based on Gaia DR2 data. The results depended critically on the adopted parallax zeropoint offset in the GDR2 data. Another uncertainty is the actual classification of the object as a classical cepheid. For a fraction in the sample different classifications exist in the literature. To investigate this further; the spectral energy distributions of the 450 stars in the sample were constructed (using as much as possible magnitudes averaged over the light curve) and fitted with model atmospheres; to derive effective temperatures and luminosities (for a given parallax or distance). For some stars a strong mid-infrared excess is found; typical of that seen in RV Tau stars or the new class of dusty post-RGB stars (often of the [peculiar] W Vir class); suggesting that they are Type-II Cepheids. The effect on the PLZ-relation is presented when these non-classical cepheids are removed from the sample.


Variability analysis of transient Gaia16bnz

by Inna Izviekova, Andrii Simon, Vira Godunova

We present the results of long-term BVRI photometry of the blue source Gaia16bnz; which has been performed at the Terskol Observatory since 2016. Observations were obtained using the 60-cm Cassegrain telescope (Zeiss-600) and a SBIG STL-1001 CCD with a field of view of 10.9x10.9 arcmin. A set of reference stars in the field of view was selected and continuously used to reduce the photometric measurements. As a result; we detected low-amplitude variability in V and R bands on the time scale of tens of minutes. Amplitude of variations changed from night to night; and the highest one of 0.17m was detected on 2018 December 2; while in most of the previous nights the amplitude did not exceed 0.06m. Furthermore; observations allowed us to determine the object's BVRI colors; which also can be used to classify Gaia16bnz.


Gaia isosurface visualization

by Kevin Jardine

Isosurfaces are a convenient way to visualize large scale three dimensional data.The isosurface threshold value can be star density; average extinction; brightness temperature or some other appropriate measure. 3D software can be used to render the resulting isosurface meshes. Multiple enclosed isosurfaces can be visualized in many ways; including translucency; dissolve animations; stacking or fly throughs. This poster will give several examples of isosurface visualization techniques as applied to Gaia data.


Hirearchical star formation in young stellar associations

by Janez Kos

In recent years; observations and theoretical models of star formation suggested that even systems with 10^4 and 10^5 solar masses show hierarchical star formation. If parts of such star-forming regions were formed at different times; their chemical homogeneity can be compromised. Gaia DR2 parameters are good enough that intra-cluster dynamics can be observed; so the kinematical history and interactions of young stellar associations can be revealed. I am combining Gaia data with radial velocities and chemical abundances from the GALAH survey. I will present a chemo-dynamical study of the Orion complex and Scorpius-Centaurus association and discuss implications for chemical homogeneity and chemical tagging.


Improvements in stellar ages in the era of Gaia

by Alexey Mints, Saskia Hekker

We present estimates of the distances; ages and masses for nearly 5;5 million stars from public spectroscopic surveys. To derive these estimates we use Gaia DR2 parallax values; spectroscopic parameters as well as infra-red photometry. These estimates were obtained with the third version of the Unified tool to estimate Distances; Ages; and Masses (UniDAM); which introduced the use of optical photometry and (where available) asteroseismic data.


A comprehensive study of IC4665: membership; empirical isochrones; spatial distribution; luminosity and mass functions

by Núria Miret-Roig, Hervé Bouy, Javier Olivares, et al.

The study of star formation is extremely challenging due to the lack of complete and clean samples of young; nearby; clusters and star forming regions. The recent Gaia DR2 catalog has started to change this picture providing the precisions required to obtain reliable lists of members. We aim to provide a comprehensive membership analysis of the 30 Myr open cluster IC4665 which allows to study its properties (empirical isochrones; spatial distribution; luminosity and mass functions). We use the extremely accurate Gaia DR2 catalog together with the deep; ground based COSMIC DANCe catalog to look for members. We use modern statistical techniques to compute Bayesian membership probabilities.

We obtain a final list of ~900 members of IC4665 with high membership probabilities; ~500 of which are new members. This represents a significant increase in the quantity and quality (low-contamination) with respect to previous studies. Our members include objects of sub-stellar masses which we will spectroscopically follow-up in order to search for possible companions. The relatively young age of IC4665 provides a key anchor to study the planetary formation process.


Ruprecht 147: A comprehensive study of the oldest open cluster in the solar vicinity.

by Javier Olivares, Hervé Bouy, Luis Manuel Sarro, et al.

Using a combined Gaia+DANCe astro-photometric data set together with an improved membership selection method; we obtain the most complete (in photometry) and extended (in the sky) census of Ruprecht 147 cluster members. With the latter we derive the cluster luminosity; mass and spatial distributions. The COSMIC-DANCe project compiles public multi-archive astrometry and photometry of nearby open clusters. We complement these data with our own observation to derive precise proper motions and photometry of millions of sources in the region of the target cluster. In the case of Ruprecht 147; the DANCe data set encompasses the central 8x6 deg; thus providing proper motions and Pan-STARRS+2MASS photometry reaching up to 23 mag in i band.

We complement this catalogue with the exquisite Gaia data (replacing DANCe proper motions by those reported by Gaia when available) obtained in a 6 deg radius area around the cluster centre. We analyse Gaia+DANCe data set with a comprehensive membership selection methodology that incorporates the properties of both the cluster (i.e. single; binaries and WD populations) and the data (full uncertainties). We find 260 candidate members; of which 30% are new. These candidates define a cluster photometric sequence that perfectly matches the theoretical isochrones.

Amongst our candidate members we found 16 white dwarfs and several spectroscopic and eclipsing binaries. The luminosity and mass distribution of the cluster clearly show a lack of low-mass stars; effects of mass segregation; and the Wielen dip. By fitting diverse profiles (e.g. King; EFF) we quantify the cluster luminosity segregation and elongation towards the galactic plane. The detailed properties of this old cluster will help us to understand the origin and evolution of open clusters.


Clustering properties of Herbig Ae/Be stars

by Alice Perez-Blanco, Rene Oudmaijer, Ricardo Perez-Martinez, et al.

It is a well-established result that many stars do not form in isolation; young stars are usually found to be members of clusters. But little is known or understood about the origin of the clusters. In particular; evidence that pre-main sequence stars of intermediate (2-10Msun) and higher masses are found in clusters has been found in several studies at optical and infrared wavelengths. Additionally; there has been an increased interest in the study of intermediate-mass stars in the past ten years. Here we study Herbig Ae/Be stars which are optically visible pre-main sequence stars of intermediate-mass. They represent the most massive objects to experience an optically visible pre-main sequence phase; bridging the gap between low- and high-mass stars.

Studies in the nineties into the occurrence of young stellar clusters around Herbig Ae/Be stars conclude; based on near-infrared imaging data; that there is a difference in clustering properties between low and high mass stars. Here we are investigating the presence of clusters around previously known and newly discovered intermediate-mass pre-main sequence Herbig Ae/Be stars with the detailed astrometric data offered by Gaia. This will enable us to determine the position of the Herbig Ae/Be stars and the possible lower mass companions in the HR diagram and allow us to detect and confirm the presence of the clusters around them.

We outline the results obtained using Gaia DR2 with the clustering detection algorithm (CEREAL) we developed for the enquiry and analysis of the clusters and clustering properties of the Herbig Ae/Be stars.


Relics of the formation of the Galactic halo from Gaia and APOGEE

by Paola Re Fiorentin, Mario G. Lattanzi, Alessandro Spagna

We explore the vicinity of the Milky Way through the use of high quality astrometric and spectroscopic data from the most recent releases by Gaia and APOGEE. We adopt the method described by Re Fiorentin et al. (2015) to select halo stars and search for kinematic substructures in the high velocity tail; as possible debris of past accretion events. Finally; we analyse the chemical properties of the kinematically selected moving groups in order to investigate the accretion history of the stellar halo.


Analysis of CV candidates and blue hostless transients from Gaia alerts data

by Andrii Simon, Reshetnyk Volodymyr, Vira Godunova, et al.

We have carried out detailed analysis of transient events, which were discovered by Gaia and published by ESA Gaia, DPAC and the Photometric Science Alerts Team (http://gsaweb.ast.cam.ac.uk/alerts). Among them, we detected many objects with the similar features in their lightcurves presented at Gaia alerts webpage. Special attention was given to CV candidates and blue hostless transients. Using the available data, we could identify some different groups of objects. We performed successful follow-up observations for one of such objects, namely Gaia17cuh, based on our prediction of its activity and revealed the subsequent outburst.

Now we develop a data processing tool which will allow one to predict further outburst activity for about one hundred objects in the Northern hemisphere including determination of their amplitude and periodicity. This new so-called alerting subsystem should help for a faster classification of Gaia transients.


The thick disk rotation-metallicity correlation; comparison with Galactic cosmological simulations

by Alessandro Spagna, Anna Curir, Marco Giammaria, et al.

Although the existence of a thick disc in the Milky Way was revealed 35 years ago and its spatial; kinematic; and chemical properties are today better defined; its origin is still matter of debate. Proposed scenarios include the heating of a pre-existing thin disc through a minor merger; accretion of dwarf galaxies stars from disrupted satellites; or stars formed in situ from gas-rich mergers at high redshift.

Recently; by means of a new chemo-kinematic catalogue based on Gaia DR2 and APOGEE DR14; Re Fiorentin; Lattanzi & Spagna (2018) showed evidence that the thick disk rotation-metallicity correlation is persistently positive from R=5 kpc to 13 kpc and that such correlation increases as a function of the Galactic radius. Here; we analyze the results derived from fully cosmological simulations of Milky Way-like disk galaxies and discuss the evolution of the Galactic radial metallicity gradient in a consistent inside-out formation scenario.


The shape of open clusters and its evolution with age

by Yoann Tarricq, Laia Casamiquela-Floriach, Caroline Soubiran

Thanks to Gaia DR2; new determinations of membership; distance; motion and age are available for galactic open clusters; allowing us to revise their physical properties. For a number of clusters we have fitted a 3D ellipsoid on the spatial distribution of the members and determined its axis ratios and orientation. We have investigated how these parameters are correlated with the age; mass; kinematics and environment of the clusters.


Identification of escaped alpha Persei members from traceback computations of Gaia DR2 sources

by Eero Vaher

Identification of escaped members of nearby open clusters could tell us a lot about both the Milky Way gravitational potential and the intrinsic expansion that young clusters are believed to undergo as a result of gas expulsion. Detecting structure in the tidal streams of open clusters might also inform us about the interactions between open clusters; giant molecular clouds or other massive objects. Identification of the members of nearby tidal streams requires an all-sky astrometric survey and is finally enabled by Gaia.


New catalogue of intermediate mass Pre-Main Sequence objects in Gaia DR2 using machine learning

by Miguel Vioque, Rene Oudmaijer, Deborah Baines, et al.

The intermediate mass Herbig Ae/Be stars are young stars approaching the Main Sequence and are key to understanding the differences in formation mechanisms between magnetic low mass stars and the non-magnetic high mass stars. However; the study of the general properties of these objects is arduous as only around 270 of them are known; with many presenting a doubtful nature.

Gaia Data Release 2 has improved and greatly increased the number of sources with an astrometric solution available. In combination with other catalogues; it constitutes a splendid big data breeding ground for applying Machine Learning techniques and algorithms. We present our plan to discover new Herbig Ae/Be stars by first creating a robust training set from this very reduced set of known objects. Several features were chosen for identifying new objects of the class based on our current knowledge of this group; which normally spotlights in infrared excesses; photometric variabilities and Halpha emission lines. This feature selection was complemented with Principal Component Analysis.

The training set and the final set of features were used to train a Neural Network; which we later used for looking for new Herbig Ae/Be stars and Pre-Main Sequence objects in general among Gaia DR2 sources. Evaluation on test set concludes that we reach a precision over 90% and a recall over 70%; this meaning that we retrieve more than 7000 new Pre-Main Sequence objects spread all over the galactic plane and; as a side effect; over 1000 classical Be stars; with which they share many characteristics. This in turn will allow us to study the Pre-Main Sequence evolution as a function of mass; age and location in the galaxy to an unprecedented precision.


Abundances of neutron capture elements in dwarf stars of the Solar neighborhood located towards the north celestial pole

by Carlos Viscasillas Vázquez, Vilius Bagdonas, Graźina Tautvašiene, et al.

Chemical compositions of stars provide us with unique information about their nucleosynthesis processes and the chemical enrichment of the Galaxy. Elements heavier than iron cannot be produced by nuclear fusion and require capturing neutrons by lighter elements. However; the conditions and environment needed for such processes are still relatively uncertain. While it is well established that asymptotic giant branch stars play an important role in the production of s-process chemical elements; recent studies show that their contribution was underestimated. As dwarf stars have a very long; relatively stable lives and do not show any substantial stellar evolution; their surface abundances are very helpful for studying the environment and time in which they were formed.

Our aim was to determine abundance patterns of neutron-capture elements and probe the evolution of the Milky Way stellar disc. For this purpose we used spectra obtained with a Vilnius University Echelle Spectrograph and 1.65 m telescope for a sample of almost 300 F and G spectral type bright (V < 8 mag) dwarf stars (logg > 3.5) in two nearby fields towards the north celestial pole direction; with metallicities between -0.7 < [Fe/H] < +0.5. The method used for the analysis of chemical abundances is based on the spectral synthesis using the code TurboSpectrum combined with the MARCS stellar atmospheres models. We provide the chemical composition and time evolution for Sr II; Y II; Zr I; Zr II; Ba II; La II; Ce II; Nd II; Pr II; Sm II; and Eu II at different mean Galactocentric distances.

For majority of the stars investigated in this work; it is the first time that abundances of neutron-capture elements were determined. The investigated stars were used to trace how the s-process was influencing the production of n-capture elements during the evolution of the Galactic thin disc.


Spatial structure of the Galactic ISM obtained with extensive analysis of several dozen DIBs

by Rok Vogrinçiç, Janez Kos

Diffuse interstellar bands (DIBs) can trace different conditions of the interstellar medium along the line-of-sight between us and the observed star. We use spectra from the GALAH survey to measure profiles of around a dozen DIBs towards 100;000s stars - hot and cool alike. Our study includes mostly lines-of-sight in the thin (70%) and thick (20%) disks at distances up to 5 kpc. Combined with Gaia's excellent distances we can observe interstellar gas structures at a ~10 kpc scale by measuring correlations between nearby lines-of-sight.