Events, Seminars, Talks

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The recent space age of uninterrupted high-precision photometry finally offered us a way to look deep inside stars. This deep probing is possible thanks to the detection of numerous stellar oscillations. The observed properties of such oscillations allow us to derive how stars age, how they rotate, and how they mix their gas throughout their life. In this Colloquium, we start with the basic ingredients of asteroseismology for the non-expert. We discuss the diversity of nonradial oscillations of stars, introducing pressure, gravity, magnetic, and tidal waves. We then emphasize some of the breakthroughs from space asteroseismology, focusing mostly on results from the 4-year Kepler light curves. Examples involve high-precision sizing, weighing, and ageing of stars throughout our Milky Way. We review the current status of the internal rotation of stars and offer a sneak-preview on internal mixing inside stars with a convective core as new observational input to calibrate stellar evolution theory. We end with a future outlook for this glorious research field of astrophysics, touching upon how to achieve probing of internal magnetism and tidal asteroseismology, thanks to the ongoing NASA TESS and future ESA PLATO space missions.

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Stellar feedback connects the smallest, densest scales of the galaxy to the largest, most diffuse components of the galaxy halo. The detailed, fractal structure of the interstellar medium (ISM) and the evolution of stars sets the local coupling efficiency of energy from stellar winds, radiation and supernovae. This feedback can then drive outflows from most galaxies, removing mass and metals to the diffuse, hot circumgalactic medium. This low-density halo can act as a reservoir for material, slowly allowing it to re-accrete and fuel ongoing star formation. This in turn influences the formation of new molecular clouds within the ISM, connecting the process of star formation to all scales of the galaxy's gas content. In this talk, I will present recent results that have helped to clarify the details as to how the details of these processes can impact the overall life of galaxies, and how we can use new observational and statistical methods to constrain these details.

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CARMENES is a high-resolution optical and near-infrared spectrograph(s) mounted at the 3.6m telescope in Calar Alto, Spain. The CARMENES exoplanet survey began in 2016, targeting over 350 nearby M-dwarf stars. Since then, CARMENES has proven to be one of the most effective Radial Velocity (RV) exoplanet finders currently in operation. I will briefly present the CARMENES-GTO Doppler survey, its scientific goals, and I will focus on some of the exciting CARMENES-GTO exoplanet discoveries. I will show that CARMENES is fully capable of discovering complex multiple-planet systems around late M-dwarf stars, which cannot be studied by other instruments. I will reveal the powerful synergy between CARMENES and the Transiting Exoplanet Survey Satellite (TESS), leading to a dozen exoplanet discoveries. One of the most exciting CARMENES discoveries, confirmed by TESS, is a nearby transiting rocky exoplanet that will be a prime target for atmospheric investigation and characterization with the JWST. The precise orbital determination based on CARMENES precision RVs is vital for probing the planetary formation mechanisms and evolution scenarios around low-mass stars and in general.

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The diverse architectures of discovered exoplanetary systems have provoked an equally diverse range of explanations of the processes governing their formation and evolution. However, the star formation environment is a factor that is frequently overlooked in studies that aim to synthesise the observed planet properties. I review the growing evidence suggesting that environmental feedback plays a significant role during and after planet formation. Two mechanisms that may be particularly influential are external photoevaporation due to irradiation of the protoplanetary disc by neighbouring OB stars, and subsequently star-star encounters that can generate instability in the mature planetary system. I discuss observational constraints and examples of these mechanisms caught in the act, and how they might alter a system from that which forms ?in isolation'. Finally, I contextualise the Solar System in terms of possible environmental sculpting.

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Modern wide-field surveys will provide us with alert streams generating millions of alerts every night and by that providing a comprehensive picture of the variable sky. Traditionally, microlensing events have been found in the most crowded place in the Milky Way. Thanks to the footprint, microlensing events in the disk will play an increasingly important role. The opportunities opening up by these surveys will lead to more discoveries and the characterization of extrasolar planets (bound and unbound), black holes and insights into the binarity of stars and brown dwarfs. The most audacious surveys will certainly provide the best alert stream but some of the aforementioned science cases still require automated follow-up observations. The "fire-hose of alerts" demands cloud-based tools to trigger heterogeneous follow-up observations with a homogeneous software package - the TOM (Target and Observation Manager) Toolkit. We will show early results from the most recent observing programs running at Las Cumbres Observatory (LCO) global network using a TOM and how that can be applied to the surveys of the future.

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Direct N-body simulations are the only simulation method capable of properly resolving dense star clusters from the scales between close interacting stars in a binary all the way up to their interaction with distant halo stars. Nevertheless, they are frequently complemented with approximate Monte-Carlo methods, because these are much quicker computationally. Recently, there have been a number of code advances concerning the stellar evolution. This is crucial, as the evolution of individual stars has a huge effect on the overall dynamical evolution of a star cluster and thus amongst others the formation of gravitational wave emitting sources that may be detectable with LIGO and VIRGO. In this talk, I will highlight the state of stellar evolution routines in the direct N-body code NBODY6++GPU and the Henon-type Monte-Carlo code MOCCA by comparing the results of small star cluster simulations performed with both. The results of these are important groundwork to ensure that the upcoming million-body simulations of globular and nuclear star clusters are astrophysically up-to-date.

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The source of about half of the heaviest elements in the Universe has been a mystery for a long time. Although the general picture of element formation is well understood, many questions about the astrophysical details remain to be answered. Here I focus on recent advances in our understanding of the origin of the heaviest and rarest elements in the Universe.

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On September 2nd 2020, the LIGO-Virgo collaboration announced the detection of GW190521, a gravitational wave source associated with the merger of two black holes (BHs), 66 and 85 Mo masses, which left behind an intermediate-mass black hole (IMBH) with a mass of 142 Mo. This binary merger is peculiar because its primary mass falls in the so-called upper mass-gap, a region of stellar masses where modern stellar evolution predicts the absence of remnants, and the final remnant represents the first specimen of a "light" IMBH. In this seminar, I will describe a novel channel suitable to explain the properties of GW190521, namely a sequence of three mergers among stellar mass black holes. We discovered serendipitously such a process in a high-resolution N-body model of a dense star cluster. We combine these simulations with an analysis based on numerical relativity fitting formulae and on observed properties of globular, young, and nuclear clusters, to show that if GW190521 originated via such a mechanism its observation gives us insights on the distribution of stellar BH natal spins and on the environment that harboured such a system, most likely a dense and young star cluster.

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Supermassive black holes of million solar mass and above are commonly hosted by massive galaxies, but are also present in local dwarf galaxies. Black holes are a fundamental component of galaxies and galaxy evolution, but their origin is still far from being understood. Large-scale cosmological simulations are crucial to understand BH growth and their interplay with their host galaxies. We recently compared the black hole population of six of these large-scale cosmological simulations (Illustris, TNG100, TNG300, Horizon-AGN, EAGLE, and SIMBA) and I will review how the simulation sub-grid models affect the build-up of the BH population and their correlations with galaxies properties. The next two decades will be dedicated to the exploration of the high-redshift Universe with upcoming space missions such as LynX, Athena, JWST, WFIRST, and LISA. I will present how we can use cosmological simulations to prepare these missions and maximize their scientific return.

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