Vorträge, Seminare, Ereignisse

Abstract
On October 19, 2017 the Pan-STARRS1 telescope discovered a rapidly moving object. Additional astrometry obtained with pre-discovery observations on October 18 through data obtained with the Canada-France-Hawaii-Telescope on October 22 showed that the object had the highest hyperbolic eccentricity ever detected, confirming that this object clearly originated from outside the solar system. 1I/2017 U1 passed perihelion on September 9, 2017 and had made its Earth close approach at 63 lunar radii on October 14. The official name of ‘Oumuamua, meaning visitor from the distant past, was approved by the IAU on November 6. Beginning on October 22 there was an intense effort to secure observing resources to characterize the object. Because it was receding rapidly from the Earth and Sun, within a week of discovery the brightness had dropped by a factor of 10 and in less than a month it had dropped by a factor of 100. Thus, there was a period of just over a week where the target could be relatively easily characterized. Deep images of ‘Oumuamua showed no hint of cometary activity, with limits on the amount of dust that could be present at less than 7-8 orders of magnitude that of a typical comet at similar distances. Light curve observations showed that the object was rotating with an instantaneous rotation period of 7.34 hours, and a light curve range of 2.5 magnitudes, implying an extremely elongated axis ratio perhaps as large as 10:1, but certainly larger than 5:1. Spitzer observations suggest an average diameter somewhere between 98-440 km depending on model-dependent surface thermal properties. As more time series data were obtained, it was evident that ‘Oumuamua was in an excited spin state with the long axis precessing around the total angular momentum vector with an average period of 8.67±0.34 hr. The timescale for damping an excited spin in a body this size is very long, so the spin state may reflect the violent process of ejection of ‘Oumuamua from its host planetary system. The color of ‘Oumuamua was found to be quite red with a spectral slope of 23%±3% per 100 nm, consistent with comet surfaces, the dark side of Iapetus, and other minerals. Precision astrometric measurements obtained from the Hubble Space Telescope and the ground allowed us to do a detailed study of the orbit. Analysis of 207 astrometric positions showed that the orbit cannot be fit by a purely gravity-only trajectory, but are well matched (at the 30-sigma level) by the addition of a radial acceleration. We explored several explanations for the non-gravitational motion, and found that cometary outgassing is the most physically plausible, but requires that ‘Oumuamua has a somewhat different nature from solar system comets. Many attempts were made to trace ‘Oumuamua back to it’s home system, the most detailed after the release of the Gaia 2 catalog, but no convincing candidates have been found. Whether this will ever be feasible depends on how long ago it was ejected. ‘Oumuamua has challenged many of our assumptions about what small bodies from another solar system would look like, and has triggered an avalanche of papers, some highly speculative. In this talk I’ll share the story of the discovery of ‘Oumuamua and discuss what we know about our first known interstellar visitor – including new information from papers in press.

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In 2017 the LIGO/Virgo gravitational wave observatories detected the first binary neutron star merger event (GW170817), a discovery followed by the most ambitious electromagnetic (EM) follow-up campaign ever conducted. Within 2 seconds of the merger, a weak burst of gamma-rays was discovered by the Fermi and INTEGRAL satellites. Within 11 hours, a bright but rapidly-fading thermal optical counterpart was discovered in the galaxy NGC 4993 at a distance of only 130 Million light years. The properties of the optical transient match remarkably well predictions for “kilonova” emission powered by the radioactive decay of heavy nuclei synthesized in the expanding merger ejecta by rapid neutron capture nucleosynthesis (r-process). The rapid spectral evolution of the kilonova emission to near-infrared wavelengths demonstrates that a portion of the ejecta contains heavy lanthanide nuclei. Two weeks after the merger, rising non-thermal X-ray and radio emission were detected from the position of the optical transient, consistent with delayed synchrotron afterglow radiation from an initially off-axis relativistic jet. I will describe efforts to create a unified scenario for the range of EM counterparts from GW170817 and their implications for the astrophysical origin of the r-process and the properties of neutron stars (particularly their uncertain radii and maximum mass, which are determined by the equation of state of dense nuclear matter). Time permitting, I will preview the upcoming era of multi-messenger astronomy, in the current O3 run and once Advanced LIGO/Virgo reach design sensitivity and a neutron star merger is detected every few weeks.

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I will review current observational and modelling results on neutron stars with strong magnetic fields (aka magnetars): in particular their outburst activity, their predicted evolution, birth and possible connection with GRBs and SLSNe. Furthermore, I will present new discoveries that strengthen somehow the relation between magnetars and other neutron star classes, and that argue on the ubiquitous presence of strong field non-dipolar component in possibly any young neutron star.

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The early stages of circumstellar disk evolution are likely influenced by the relatively large gas mass, driving gravitational perturbations that result in turbulence and disk fragmentation. Fragmentation of the disk produces dense gaseous objects with masses on the order of a few Jupiter masses an up, and preferentially occurs in the cool distant regions of the disk. This provides a possible formation channel for directly imaged planets which are difficult to account for otherwise. Unfortunately, despite filling in this niche of planet formation, several factors make gravitational instabilites a minor factor in the overall planet formation paradigm. I will discuss the viability of gravitational instabilities as a planet formation theory, including recent developments in both observation and theory. These developments perhaps point away from the formation of massive gaseous planets and towards the early concentration of solid material in the vein of traditional core accretion models, but within the first million years of the lifetime of the disk.

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The giant molecular cloud (GMC) lifetime provides an upper bound on the local time-scale for star-formation, linking cloud-scale and sub cloud-scale physics to galaxy-scale trends in the star formation rate. Conversely, the galactic environment plays an important role in setting the cloud lifetime, leading to a complex interplay of physical mechanisms over a range of scales in the interstellar medium (ISM), from galactic dynamics to small-scale turbulence and feedback. Previous theories of GMC lifetimes have made predictions based on just one mechanism of cloud evolution, relevant only in a fraction of Galactic and extragalactic star-forming environments. That approach is inconsistent with recent observations, which show that a diverse range of entities are observationally-identifiable as clouds, and reveal environmentally-driven correlations between their gravitational boundedness and the galaxy-scale star formation rate. I present an analytic theory for GMC lifetimes, dependent on the large-scale dynamical environment of the ISM, including its local gravitational stability, cloud-cloud collisions, epicyclic perturbations, galactic shear, and interaction with galactic bars and spiral arms. Our analytic predictions depend on just five observable properties, accessible through measurements of the rotation curve, surface density and velocity dispersion of the host galaxy, and are applicable over a wide range of redshifts. In this contribution, I will present predicted cloud lifetimes and properties across a range of galactic dynamical environments. I will compare these results to hydrodynamic simulations performed using the moving-mesh code Arepo, where the influence of dynamics is combined with sub-cloud physics such as supernova feedback, HII-region feedback, and ISM chemistry. These theoretical and numerical results are consistent with pioneering observational results currently obtained with ALMA. Together, this combination of analytic, numerical and observational results show that the galactic dynamic environment plays a crucial role in determining GMC lifecycles and thus the star formation rates of their host galaxies.

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ith about 4000 extrasolar planets orbiting about 3000 different host stars known today, we are now clearly in the realm of detailed statistical studies of planet and host star properties. Furthermore, rarer systems are found, which sometimes can constrain planet formation and evolution theories single-handedly. Here I will review the main results of two Doppler surveys: the Lick G and K giant survey, focusing on evolved and relatively massive host stars, as well as the Carmenes survey which targets M dwarfs, the least massive host stars. I will discuss both, ensemble properties as well as individual systems, and will summarize what we have learned so far.

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Stars are formed by the gravitational collapse of dense, gaseous and dusty cores within magnetized molecular clouds. Understanding the complexity of the numerous physical processes involved in the very early stages of star formation requires detailed thermodynamical modeling in terms of radiation transport and phase transitions. I will discuss the outcome of our spherically symmetric radiation hydrodynamic simulations with which we investigate the collapse of molecular cloud cores including the stages of first and second hydrostatic core formation. We investigate the properties of Larson’s first and second cores and expand these collapse studies for the first time to span a wide range of initial cloud masses from 0.5 Msun to 100 Msun. I will highlight the strong dependence of a variety of first core properties on the initial cloud mass. Furthermore, based on our new 2D radiation hydrodynamic simulations, I will discuss the impact of different cloud properties on the formation of early disks.

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Theories of planet formation predict the existence of a population of free-floating planets that are not gravitationally tethered to any host star. Gravitational microlensing provides a unique tool for studying these objects. The first results of Sumi et al. (2011) claimed that Jupiter-mass free-floating planets are as common as main-sequence stars. However, these results disagree with censuses of substellar objects in young clusters and star-forming regions and with predictions of planet formation theories. I will present new results of the analysis of a ten times larger sample of microlensing events discovered by the OGLE-IV survey during the years 2010-2018, which shed new light on the population of free-floating planets. I will also discuss prospects for detecting free-floating planets with the future missions, like Euclid and WFIRST.

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In the course of their accretion phase, massive (proto)stars impact their natal environment in a variety of feedback effects such as thermal heating, MHD-driven protostellar jets and outflows, radiation forces, and photoionization / HII regions. Here, I present our most recent simulation results in terms of the relative strength of the feedback components and the size of the reservoir from which the forming stars gain their masses. For the first time, these simulations include all of the feedback effects mentioned above which allows us to shed light on the physical reason for the upper mass limit of present-day stars. Furthermore, we predict the fragmentation of massive circumstellar accretion disks as a viable road to the formation of spectroscopic massive binaries and the recently observed strong accretion bursts in high-mass star forming regions.

To advertise our latest code development, I will also overview the most recent results obtained in a variety of other astrophysical research fields from the formation of embedded Super-Earth planets' first atmospheres (Cimerman et al. 2017, MNRAS) to the formation of the progenitors of the first supermassive black holes in the early universe (Hirano et al. 2017, Science).

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