I give an overview of our spectroscopic work on the old open cluster M67 and what it may tell us about the origin of the Sun, the existence of terrestrial planets around solar twins and effects that change the surface composition of stars. I will argue that much remains to be learned from studies of stars in different environments (globular clusters, open clusters, associations).

Approximately 10 per cent of massive OBA main-sequence (MS) and pre-MS stars harbour strong, large-scale magnetic fields. At the same time, there is a dearth of magnetic stars in close binaries. A process generating strong magnetic fields only in some stars must be responsible and several channels for the formation of magnetic massive stars have been proposed. In this talk, I will present recent results on the origin and evolution of such strong surface magnetic fields. Regarding the origin, mergers of MS and pre-MS stars have been proposed to form magnetic stars and I will highlight a method to probe this hypothesis observationally. Applying this new method to two magnetic massive stars, we find that they are indeed consistent with being MS merger products. Utilising a large sample of magnetic and non-magnetic OB stars, I will show that there is a dearth of evolved magnetic stars that suggests that magnetic fields disappear over time. I will argue that this is most likely caused by decaying magnetic fields.

 In this talk I will present the our work on an exotic group  
of evolved objects: post-AGB and post-RGB stars and the excellent  
constraints they provide for single and binary star evolution and  
nucleosynthesis. These objects have also revealed new evolutionary  
channels and AGB nucleosynthesis which is vital for understanding the  
complex chemical evolution of our Galaxy as well as external galaxies.

Stars originate by the gravitational collapse of a turbulent molecular cloud of a diffuse medium, and
are often observed to form clusters. Stellar clusters therefore play an important role in our
understanding of star formation and of the dynamical processes at play. However, investigating the
cluster formation is difficult because the density of the molecular cloud undergoes a change of
many orders of magnitude. Hierarchical-step approaches to decompose the problem into different
stages are therefore required, as well as reliable assumptions on the initial conditions in the clouds.
In this talk I will report for the first time the use of the full potential of NASA Kepler
asteroseismic observations coupled with 3D numerical simulations, to put strong constraints on the
early formation stages of old open clusters. Thanks to a Bayesian peak bagging analysis of about 50
red giant members of NGC 6791 and NGC 6819, the two most populated open clusters observed
in the nominal Kepler mission, I derive a complete set of detailed oscillation mode properties for
each star, with thousands of oscillation modes characterized. I therefore show how these
asteroseismic properties lead to a discovery about the rotation history of stellar clusters. Finally,
the observational findings will be compared with hydrodynamical simulations for stellar cluster
formation to constrain the physical processes of turbulence, rotation, and magnetic fields that are
in action during the collapse of the progenitor cloud into a proto-cluster.

Current planet formation theories are bound to comply with the observational constraint that protoplanetary disks have lifetime of ~3 Myr. This timescale is mostly based on spectroscopic studies of objects accreting matter from a circumstellar disk around pre-main sequence stars (PMS) located in low-density, nearby (d<1-2kpc) star forming regions. These objects do not reflect the conditions in place in the massive starburst clusters where most of star formation occurs in the universe. Using a new robust method to indentify PMS objects through their photometric excess in the Halpha band, we have studied with the HST and ground based facilities the PMS population several starburst clusters, namely NGC3603 in the Milky Way and several clusters in the Carina Nebula,  30 Doradus and the surrounding regions in the Large Magellanic Cloud and NGC 346 and NGC 602 in the Small Magellanic Cloud. We found a wide spread of ages (0.5 to 20 Myr) for PMS stars, clearly showing that accretion from circumstellar disks is still going on well past 10 Myr. This finding challenges our present understanding of protoplanetary disk evolution, and can imply a new scenario for the planet formation mechanism and of star clusters formation in general. Based on these results we were recently granted 175hr with OmegaCAM at the VST to carry out a deep optical wide field survey of nearby (<3kpc) star forming regions. These observations will provide physical parameters (including mass accretion rates) for over 10000 PMS stars and will establish whether the long timescales of circumstellar discs are common.

The initial mass function describes the distribution of masses for a population of stars and substellar objects when they are born. It defines the evolution of a population of stars and provides constrains on the star formation theory. The determination of the initial mass function in the substellar regime is still an open question in Astrophysics. Brown dwarfs do not have enough mass to sustain hydrogen fusion. As a consequence, mass and age are degenerate for these objects. An older high mass object may be indistinguishable from a younger low mass object. In my PhD thesis, through the characterization of brown dwarfs using several observational methods, I work towards solving the general problem of constraining the substellar initial mass function.

In my first project, I calculated trigonometric parallaxes of a sample of six cool brown dwarfs. I determined the luminosity for our objects and I found that one of them might be a brown dwarf binary. In my second project, I confirmed the youth of seven brown dwarfs (ages between 1 and 150 Myr) using spectroscopic data.In the last project of this PhD thesis, I aimed to refine the brown dwarf binary fraction using spectroscopic data in the optical and in the near infrared for 22 brown dwarfs. I found six new brown dwarf binary candidates, two of them were previously known.

The determination of distances, ages and the refinement of the brown dwarf binary fraction in this PhD thesis contribute to the determination of the initial mass function. In the next years, the Gaia satellite, the James Webb Space Telescope and the E-ELT will provide new data, allowing the discovery of new brown dwarf binaries, the constraining of atmospheric and evolutionary models, and the refinement of the initial mass function.

It is often assumed that when stars reach their Eddington limit, strong outflows are initiated, and that this happens only for extreme stellar
masses. I will show that in realistic models of stars up to 500 Msun, the Eddington limit is not reached at the stellar surface. Instead, I will argue that the Eddington limit is exceeded inside the stellar envelope, in hydrogen-rich stars above about 1 ... 30 Msun, and in Wolf-Rayet stars above 7 Msun, with drastic effects for their structure and stability. I will discuss the observational evidence for this, and outline evolutionary consequences.

All the elements from carbon to uranium present in the Solar System were produced by hundreds to thousands of stars belonging to different stellar generations that evolved and died during the presolar evolution of the Galaxy. Using the abundances of radioactive nuclei inferred from meteoritic analysis we can date the last of these stellar additions. We have found that the last contribution of elements such as carbon and slow neutron-capture elements to the Solar System from an asymptotic giant branch star occurred 15-30 Myr before the formation of the Sun. This provides us with an upper limit of the time when the precursor material of the Solar System became isolated from the bulk of the galactic material. Interestingly, it compares well to the lifetime of high-mass molecular clouds suggesting that the Sun was born in a very large family of stars.

In this talk we will present our most recent numerical and observational results on the formation, evolution, and X-ray emission from hot bubbles in nebulae around evolved stars. Our studies include hot bubbles around massive and low-mass stars, e.g., Wolf-Rayet nebulae and planetary nebulae. Our results show that the diffuse X-ray emission from these hot bubbles is a dynamic process that involves mixing of nebular material into the hot bubble due to hydrodynamical instabilities, photoevaporation, thermal conduction, and dust cooling. The formation of these hot bubbles is governed by the evolution of the stellar wind parameters, and its properties can be used to study stellar evolution.

The origins of neutron(n)-capture elements (atomic number Z > 30) have historically been discerned from the interpretation of stellar spectra. However, in the last decade nebular spectroscopy has been demonstrated to be a potentially powerful new tool to study the nucleosynthesis of n-capture elements. In this talk, I will discuss exciting new advances made in this field with near-infrared and optical observations of planetary nebulae, and atomic data investigations that enable the analysis of spectroscopic data.