Talks, Seminars, Events

Abstract
Stars are the building blocks of the Universe and have transformed the pristine Universe into the one we live in today. Massive stars in particular play a central role because they are cosmic powerhouses that give rise to copious amounts of ionising radiation, chemical elements and kinetic energy. They die as the most powerful explosions in the Universe and – if they lived in binary systems – their compact remnants are at the heart of gravitational wave astronomy. To understand the role of massive stars in the cosmos, a comprehensive
 picture of their lives and final fates is essential, but is as yet 
seriously incomplete. I will highlight some of the most pressing challenges in massive star research and then focus on two of them. First, I will discuss the stellar initial mass function of stars in the mass range 15-200 solar masses as observed in the local 30 Doradus starburst and implications for the maximum birth mass of stars and stellar feedback. Second, I will explain how stellar mergers may explain some of the enigmatic OBA stars that have strong, large-scale magnetic fields.

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In this colloquium, I present characterizations of molecular cloud properties with the THOR survey (The HI, OH and Radio Recombination Line (RRL) survey of the Milky Way). We analyze OH absorption at 18 cm within THOR and follow-up observations. We derive the abundance with respect to molecular hydrogen and the total number of hydrogen nuclei: 1) We find a decreasing OH abundance with increasing column density of molecular hydrogen. 2) Due to significant column densities of atomic hydrogen at low OH column density, the OH abundance with respect to the column density of hydrogen nuclei is approximately constant. 3) We detect OH components which are associated with gas that is not predominantly molecular or even CO-dark. We conclude that OH is a potential tracer for diffuse gas.
Regarding the impact of star clusters on molecular clouds, we detect signatures of feedback in RRL emission in the star forming region W49A. A comparison to the WARPFIELD models (one-dimensional models of feedback-driven shells) indicates that feedback is not yet strong enough to disperse its molecular cloud and that the shell is either in process of re-collapsing to initiate a new event of star formation or has already re-collapsed. This suggests that at least parts of the star formation in W49A is regulated by feedback.

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Planets form in dusty discs whose upper layers are bathed in energetic radiation, either deriving from the central star or from massive stars in the local environment. Heating of surface layers can drive powerful winds which can limit the lifetime of discs and their planet forming potential. I will describe some highlights of recent research into disc photoevaporation, emphasising in particular the evidence for substantial photoevaporative losses from environmental heating even in rather sparse birth environments. I will also show how recent studies in OB associations demonstrate that proximity to neighouring OB stars has a significant impact on disc lifetimes and can be used to test theories of disc photoevaporation.

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Dwarf spheroidal galaxies are dark matter dominated. Therefore, they are ideal to test any dark matter model. In this colloquium I will talk about my previous work on stellar substructures in dwarf spheroidal galaxies and how important they are to constrain dark matter models. I will continue to present our newly discovered stellar substructures in Sextans, Carina, Leo I and Leo II. Finally, I will present my current and future work.

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Dwarf spheroidal galaxies are dark matter dominated. Therefore, they are ideal to test any dark matter model. In this colloquium I will talk about my previous work on stellar substructures in dwarf spheroidal galaxies and how important they are to constrain dark matter models. I will continue to present our newly discovered stellar substructures in Sextans, Carina, Leo I and Leo II. Finally, I will present my current and future work.

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Many planets orbit within an AU of their stars, raising questions about their origins. Particularly puzzling are the planets found near the silicate sublimation front. We investigate conditions near the front in the protostellar disk around a young intermediate-mass star, using the first global 3D radiation non-ideal MHD simulations in this context.

The results show magnetorotational turbulence around the sublimation front at 0.5 AU. Beyond 0.8 AU is the dead zone, cooler than 1000 K and with turbulence orders of magnitude weaker. A local pressure maximum just inside the dead zone concentrates solid particles, allowing for efficient growth. Over many orbits, a vortex develops at the dead zone's inner edge, increasing the disk's thickness locally by around 10%.

We synthetically observe the results using Monte Carlo transfer calculations, finding the sublimation front is bright in the near-infrared. The models with vertical magnetic flux develop extended, magnetically-supported atmospheres that reprocess extra starlight, raising the near infrared flux 20%. The vortex throws a non-axisymmetric shadow on the outer disk.

Radiation-MHD models of the kind we demonstrate open a new window for investigating protoplanetary disks' central regions. They are ideally suited for exploring young planets' formation environment, interactions with the disk, and orbital migration, in order to understand the origins of the close-in exoplanets.

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The uncertainty of the measured Hubble constant (H0) has been reduced by more than a factor of 10 over the past three decades. Despite this significant improvement the discussion on H0 continues unabated. The value of the Hubble constant sets the size and age of the universe. Since H0 is based on an absolute measurement, it is regarded as the most difficult cosmological parameter to determine. In particular, a discrepancy between local determinations of H0 and the ones based on cosmological models has emerged. It has been speculated that if the tension is confirmed an additional cosmological parameter would have to be introduced. The critical measurements of the distance ladder and in particular the use of supernovae as distance indicators will be discussed within this context.

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Galactic globular cluster stars typically show light element abundance variations interpreted as signatures of multiple stellar populations. If multiple populations do indeed form exclusively in sufficiently massive clusters as our current knowledge suggests, they may be used to trace the globular cluster (GC) contribution to the field. Building on our earlier work, we analyzed approximately half a million stellar spectra from SDSS DR14 in order to identify halo field giants showing light element abundance anomalies traced by CN and CH. These chemical signatures are revealing candidates that were likely stripped from GCs. In fact, our search has more than quadrupled earlier samples, yielding the largest data set of candidate former globular cluster stars available so far. These data allow us to constrain the contribution of GCs to halo field stars. We find that the GC contribution is pronounced in the inner halo, but minor in the outer halo, which presumably formed mainly through the accretion of dwarf galaxies. We are now in the process of constraining the orbits and tracing the origin of our candidates using Gaia data.

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Stars are building blocks of our Universe. They determine its chemical evolution through nuclear synthesis, they host planets and they determine the evolution of galaxies. The characteristics of stars is predominantly determined by their masses. As such the stellar mass spectrum, also called the initial mass function (IMF), is truly a fundamental quantity to understand how our Universe works. A large number of studies have been performed to infer the IMF and it appears strikingly universal. That is to say, even when measured in rather different environments, the IMF presents no or modest variations. This is an intriguing fact as naive expectations would naturally relate the mass spectrum of stars to quantities such as the Jeans mass which depends significantly on the gas density and gas temperature. During the talk I will review some of the ideas that have been proposed to explain the IMF and discuss their success and failure. I will then present a large sets of simulations in which the initial conditions, the thermodynamics and the numerical resolution are all systematically varied. These simulations reveal that the initial conditions determine the power-law part of the IMF while the gas effective equation of state (EOS), which describe the isothermal to adiabatic transition, sets the peak of the stellar distribution. Analytical models are developed and compared with the simulation results. It is argued that the power-law part of the mass spectrum is due to an interplay between gravity and turbulence that determine the mass spectrum of gas reservoirs from which stars built their masses. The peak on the other hand, occurs at a mass which is 5-10 times the mass of the first Larson hydrostatic cores determined by the effective EOS. We propose that the very reason of the IMF weak variability is that the first hydrostatic core and immediate surrounding collapsing envelope are small scale processes which are nearly independent of the large scale environment characteristics. I will finish the talk by discussing remaining issues and suggests a possible "unifying picture".

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In the recent years, a big effort was made to develop deep photometric surveys aimed at studying galaxy structures down to the faintest levels of surface brightness. The VST Early-type GAlaxy Survey (VEGAS) has played a pivotal role in the field, by providing new insight on the faint regions of galaxies and on dwarf galaxies in all environments. About 30% of the VEGAS observing time was dedicated to the Fornax deep survey (FDS), the new multi-band deep survey of the Fornax cluster, covering the whole cluster out to the virial radius, with an area of 26 square degrees around the central galaxy NGC1399 and including the SW subgroup centred on the other bright member NGC 1316. With FDS we can map galaxy stellar halos down to μg ≃ 29−31 mag/arcsec2, detect new and faint (μg ≃ 28−30 mag/arcsec2) features in the intracluster space and trace the spatial distribution of candidate globular clusters inside ∼ 0.5 deg2 of the cluster core. Recently, the light and colour distribution of all the bright early-type galaxies inside the virial radius of FORNAX have been studied and a first comprehensive view of the galaxy structure and evolution as function of the cluster environment has been provided. In this talk I will review the main results obtained from VEGAS/FDS surveys.

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Over the course of the past few decades, it has become clear that the class of metal-poor stars known as carbon-enhanced metal-poor (CEMP) stars are powerful probes of a number of areas of interest to contemporary astrophysics. I review the multiple lines of evidence that demonstrate the association of CEMP-no stars (which do not exhibit neutron-capture element enhancements) with the nucleosynthesis products of the very first stars, their likely birth place in low-mass mini-halos, and (once accreted by the halo) their role as tracers of the outer-halo population of the Galaxy. The CEMP-s stars (which exhibit enhancements of the heavy s-process elements), by contrast, are likely to have been born in more massive mini-halos, and serve as tracers of the inner-halo population. The well-known increasing frequency of CEMP-no stars (and newly recognized relative constancy of CEMP-s stars) with declining metallicity, and the identification of the primary groups in the Yoon-Beers diagram of A(C) vs. [Fe/H], provide the means to explore these associations in more detail, and to constrain numerical models of the formation of the Milky Way.

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During this seminar I will describe my research program aiming at a better understanding of the physics of supernovae and of the origin of nuclei by increasing the quality and predictive power of numerical models as well as nucleosynthesis calculations. Supernovae play essential roles in the frameworks of many branches of astrophysics: star formation, galaxy dynamics, high-energy astrophysics, galactic chemical evolution, and cosmology. In spite of their ubiquitous presence in astrophysics, there are many uncertainties related to progenitor systems, treatment of the explosions, cross section determinations at such high temperatures, and comparisons with spectra. Most popular results in the field of nucleosynthesis during explosions are still mostly based on one-spatial dimension calculations. The pioneering and very innovative aspect today is the possibility of coupling nucleosynthesis to multidimensional simulations of different type of supernovae. I will show recent results and future perspectives in multi-dimensional calculations of thermonuclear as well as core-collapse supernovae, using tracer particle method for nucleosynthesis. I will illustrate detailed comparison of 1D and 3D supernova models with nucleosynthesis calculations and discussing the needs of multi-D (and where it is needed). Despite the huge investments in nuclear physics experiments, theoretical studies establishing priority lists of reactions to be measured and precision required for astrophysics are currently very limited. During this seminar I will also discuss a priority list for future experiments and improvements in predictions of key nuclear reactions for explosive nucleosynthesis. My expertise in Galactic Chemical Evolution modelling lead to the possibility to study a dependence of the SNe yields on metallicity and their contribution over the galactic age up to reproducing the Solar System composition. During my talk I will refer different times to result of chemical evolution studies with the need of a more clear understanding of the impact of supernovae at the earliest stages of the evolution of galaxies, and their contribution to the Solar System composition. The wealth of information from galactic surveys makes this the ideal time for a theorist to understand the formation and evolution of galaxies with a new generation of chemical evolution models. To this goal, at the end of my talk, I will describe a novel project to model chemo-dynamical evolution of the cosmos, based on a N-body SPH RAMSES code making use of the framework on a moving mesh, adjusting automatically spatial resolution but using a large number of isotopes.

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Globular clusters are the relics of extreme star formation in high-redshift galaxies. Their enormous potential as tracers of high-redshift galaxy formation is broadly recognised, but concrete applications of this link have remained out of reach. The key missing ingredient has been to construct an end-to-end model for star cluster formation and evolution in a cosmological context. I will review recent efforts towards formulating models for globular cluster formation and evolution during galaxy formation, showcasing the variety of techniques used, as well as their strengths and weaknesses. I will then present results from the E-MOSAICS project, in which we carry out fully self-consistent, cosmological zoom-in hydrodynamics simulations of the co-formation and evolution of globular clusters and their host galaxies. This work has led to two crucial insights. The first is that the formation of young massive clusters and old globular clusters can be described by a single modelling framework, showing that globular clusters are the relics of regular star formation in high-redshift environments. The second is that the high-pressure formation environment of globular clusters has shaped a wide range of their present-day properties, enabling their direct use as tracers of high-redshift galaxy growth. We demonstrate how globular cluster metallicities, masses, ages, kinematics, and spatial distributions provide a new and exciting window for reconstructing the host galaxy merger history, distinguishing between in-situ and ex-situ galaxy growth, and probing the conditions of cloud-scale star formation and feedback at high redshift. Specifically, I will demonstrate the power of unifying cluster formation and destruction processes across cosmic time by using the E-MOSAICS simulations to derive the formation and assembly history of the Milky Way, culminating in the reconstruction of its merger tree.

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The High Altitude Water Cherenkov (HAWC) observatory in Mexico (4100 m .a.s.l.) has just finished its first year of data taking and produced some interesting results. Because of its wide field of view it is currently a unique detector to study extended sources (>2 degree) at energies above a few TeV. One of the largest structures in the gamma-ray sky are the so-called Fermi bubbles, extending to the North and South from the milky way centre. At energies around 1TeV, and above, the HAWC observatory is in a unique position to make observations (or constrain the flux) of the bubbles. From the results of HAWC, modelling of the bubbles with a focus on the energy range above 500 GeV and improving the HAWC event reconstruction, both energy accuracy and flux sensitivity are expected to improve.

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Strong gravitational lenses can map an extended background source to several highly distorted and magnified images. Analysing the properties of those images yields important information about the distribution of the deflecting mass and the background source. Common approaches to reconstruct the source or the deflecting mass distribution model the global properties of the source and the lens. They obtain a consistent description of the entire configuration by refining the model until it matches the observation to a predefined precision. We develop a new approach to infer local properties of the gravitational lens and to reconstruct the source only using the properties of the multiple images without assuming a lens or a source model. In the talk, I will introduce the method and its applications in comparison to standard lens modelling methods. Since our leading principle to separate data-based information from model assumptions can also be applied to a broader range of research questions, I will conclude with an outlook how this ansatz can be transferred to other topics, based on my former experience searching for open star clusters in the HSOY catalogue.