A non ambiguous definition of stellar radial velocity (RV) is non trivial. Early comparison between astrometric and spectroscopic RVs revealed only a qualitative agreement and hint to potential quantitative differences between them. We compare for the first time in detail spectroscopic and astrometric RVs for stars of the Hyades open cluster.

Astrometric RVs are available for the Hyades’ stars, based on Hipparcos and on the first Gaia data release. We obtained HARPS spectra for a large sample of Hyades stars, and we homogeneously analysed them, either by applying a zero point correction deduced from solar observations and equal for all stars, or by correcting each star for the convective shifts in the stellar atmosphere computed theoretically from 3D atmospheric models. After cleaning the sample from binaries, RV variables, and outliers, 71 stars remained.

The distribution of the RV difference (spectroscopic – astrometric) is skewed and possibly double-peaked, and it depends on the star right ascension. This is consistent with the Hyades cluster rotating at 43±10 m s−1pc−1.We find that the spectroscopic radial velocities published in literature have a mean offset of 689 m s−1.We detect in the two single cluster giants clearly Gravitational Redshift in perfect agreement with predictions.The mean difference between spectroscopic and astrometric RV is of −33 m s−1 and the median is of −16 m s−1 when considering the Gaia-based RVs (corrected for cluster rotation) and the theoretical spectroscopic RV corrections, and of −85 m s−1 and +48 m s−1 when considering the Hipparcos and the empirically determined zero correction.  Thus it is possible to reach accurate RV measurements.

We finally discuss the phenomena that can influence the astrometri and spectroscopic RVs, such as cluster expansion, stellar activity, general relativity, Galactic potential; clusters within the reach of current telescopes are expected to show differences of several hundreds m s−1, depending on their position in the Galaxy, but will not have precise enough astrometric RV.

Cataclysmic variables (CVs) are interacting binaries composed of a white dwarf undergoing stable mass transfer from (usually) a low-mass main sequence star and they are expected to exist in non-negligible numbers in globular clusters (GCs) that are natural laboratories for testing theories of stellar dynamics and evolution. In the first part of this talk, I will describe two methods for investigating CVs in GCs, which are direct N-body integrations and Monte Carlo approach. I will then explain why the Monte Carlo approach is the most convenient for investigating exotic objects in GCs. In the second part, I will present the main aspects related to our understanding regarding GC CV populations based on the analysis of MOCCA (Hypki & Giersz 2013) numerical simulations performed by Belloni et al. (2016,2017a,2017b) and Hong et al. (2017). Emphasis will be given to the main differences in present-day CV properties due to dynamics in comparison with CVs in non-crowded environments. Finally, I will discuss how good are the agreements with respect to recent searches for CV candidates in four specific GCs, namely NGC 6397 (Cohn et al. 2010), Omega Cen (Cool et al. 2013), NGC 6752 (Lugger et al. 2017), and 47 Tuc (Rivera-Sandoval et al., sub. to MNRAS).

At the heart of interpreting starlight, infrared and radio continuum (all typically dominated by massive stars) in terms of star-formation rate (SFR) and stellar mass in individual galaxies or averaged over cosmological volumes, the stellar initial mass function (IMF) is often described as a nearly-universal function in the Galaxy and nearby galaxies. Classical determinations of the IMFin local galaxies are traditionally made at ultraviolet, optical and near-infrared wavelengths that cannot be adopted for dust-obscured galaxies, and even more so in distant starbursts selected at submillimetre (rest-framefar-infrared) wavelengths, exactly the type of galaxies for which galaxy evolution models often predict an IMF biased towards massive stars.  We bring along the state-of-the-art chemical evolution models with well-calibrated stellar yields, time-delay effects, and various star formation history, combine with abundances of carbon and oxygen isotopologues measured in molecular gas,opening up an unexpected new gate of exploring the IMFs in dust-rich galaxies,especially for those in the early Universe. We will also  discuss the physical origins for the top-heavy IMF in starburst systems, presenting our new ALMA results in a few strongly lensed dusty starbursts at high redshift, and compare them with different types of local galaxies.

Ultra-diffuse galaxies (UDGs) are a class of low surface brightness (LSB) galaxies with stellar masses typical of dwarfs (M*~10^7-10^8) and large sizes (Rh>1.5kpc) for their luminosity. UDGs are ubiquitous and abundant in nearby galaxy clusters, but their formation mechanism is still unclear. I will review the recent proposals and show that a scenario in which UDGs represent the tail into the LSB regime of the abundant dwarf galaxy populationreproduces their abundance and size-distribution in clusters. I will also expand on an effort to assess the richness of their GC systems. This has shown that most Coma LSB galaxies have GC abundances that are in line with what expected for their stellar mass. A small fraction of systems, however, displays ‘over-abundant’ GC systems. Interestingly, not all of the latter are extended, implying that the physical mechanisms responsible for the large sizes of the UDGs and for the enhanced GC abundances of some cluster dwarfs are not identical.

We will share our experience of teaching at the West African International Summer School for Young Astronomers, held in Ghana this summer and Nigeria in 2015. We will summarize the obstacles faced by West African physics students in pursuing a career in science, and explore how we as professional astronomers could contribute to improving the situation.

We will informally discuss the recently announced kilonova event associated with the gravitational wave GW 170817, with particular emphasis on the ePESSTO collaboration paper.

Lithium, fragile and scarce, sensitive and primitive, is one of the most complicated elements in astrophysics. Its abundance in stars, in both the main sequence phase and the giant branch phase, has plagued our current understanding of cosmology, stellar evolution, and metal sources of the interstellar medium. I will discuss problems and puzzles that Li introduces in astrophysics, including cosmological Li problem, Li-rich giants problem, Li problem in the Sun, nucleosynthesis of Li production, and Interstellar Li content, then focus on the Galactic Li evolution.

It is generally accepted that the distribution in colour and magnitude (temperature and luminosity) of the Pre MainSequence (PMS) objects in the Orion Nebula Cluster (ONC) shows an intrinsic spread. At least two are the interpretations of such observed spread:1) a genuine spread of the ages of few Myr, inconsistent with that of a coeval stellar population and in agreement with a star formation activity lasting between 1.5 and 3.5 Myr; 2) the spread in temperature and luminosity is a consequence of the accretionhistory taking place at the early stages of the formation of the stars. Recent results obtainedwith multi-band optical photometry with OmegaCAM@VST indicate the presenceof 3 discrete populations of PMS in the ONC. I will discuss how discreteness might prompt a revised look at the formation mode and early evolution of stars in the ONC.

Due to the limitations of current astronomical instrumentation and data reduction techniques, the Ultra-Low Surface Brightness (ULSB) universe,  which lies over two orders of magnitude below that of the sky background , is one of the last niche that remains to be explored in observational parameter space. So far, the first pioneering observations using small telescopes and telephoto lens have revealed  a wealth of stellar tidal streams and shells, diffuse stellar systems and a possible hitherto unknown type of galaxies (ultra-diffuse galaxies) whose properties could be  different from those at brighter levels. In this talk, I'll discuss this topic and our deep exploration of the interaction of the Magellanic Clouds using telephoto lens.

We would like to discuss with you the time allocation process at ESO – how does (should) it support the scientific goals of the community? We will briefly review the outcomes of the Time Allocation Working Group, look at what other observatories are doing (HST, Gemini, …) and present fun facts about request statistics and OPC gradings.

First of all, I shall give an overview on the research status about stellar magnetic activity and exoplanetary systems at Yunnan Observatories. Then, some more detailed results derived during recent years will be introduced. For stellar activity, I shall talk about the study on chromospheric and photospheric activities by using high-resolution spectroscopy, magnetic field by using Zeeman Doppler imaging method. For exoplanetary systems, I shall introduce the study on wide field transit survey project, TTV follow-up observations. Finally, the prospects in the near future will be given.

We present a tomographic method allowing to recover the velocity field at different optical depths in a stellar atmosphere. It is based on the computation of the contribution function to identify the depth of formation of spectral lines in order to construct numerical spectral masks probing different optical depths.
These masks are cross-correlated with observed spectra to extract information about the average shape of lines forming at a given optical depth and to derive the velocity field projected on the line of sight. We apply this method to time-series of spectra of the red supergiant star mu Cep and derive velocities
in different atmospheric layers. The main results will be discussed during the talk.

In order to study kinematics of galaxies, one uses spectroscopic observations, which provide us with the line-of-sight velocity component only. What about the two other velocity components? Can we get a handle on them for external galaxies, where proper motions are too small to detect? I will show that if a galaxy contains a feature with a mirror symmetry, like a bar, then from the observed line-of-sight velocity one can recover two velocity components in its disc, assuming that the vertical velocity is zero. What can we learn from having at hand these two velocity components? I will be glad to hear your ideas, but I will also present what I've got thus far: the estimate of dark matter contribution in central parts of galaxies, and the evaluation of pattern speed of the bar.

Symbiotic stars (SySt) are long-period interacting binaries composed of a hot compact star --generally a white dwarf-- an evolved giant star, and a tangled network of gas and dust nebulae. Inspired by the fact that SySt are a promising  channel to supernovae Ia, it is clearly of prime relevance to determine their  population. We discovered a number of SySt in the Local Group galaxies, while using imaging techniques to search for planetary nebulae. Most of them showing a particular  emission line that is almost exclusive of symbiotic systems, the O VI Raman scattered line at 6825A. In this opportunity I will discuss the first results of our pilot project of an innovative technique to search for SySt, which --if proved to work--, will be far more efficient them those commonly used.