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Events on Thursday, February 2nd, 2023

R. G. Herb Condensed Matter Seminar
Emergence of Quantum Order
Time: 10:00 am - 6:00 pm
Place: 5310 Chamberlin
Speaker: Elio Konig , Max Planck (Stuttgart)
Abstract: The patterns of self-organization in complex quantum systems are traditionally understood using the concept of spontaneous symmetry breaking. In contrast, quantum order in the absence of symmetry breaking, in particular highly entangled topologically ordered states, describe fundamentally distinct phases of matter which are of great interest both in present-day condensed matter research and in quantum information science. In this talk, I will spend some time reviewing the concepts of topological and non-symmetry broken quantum order. I will then focus on the interplay of such exotic states with gapless fermions, i.e., metals. I will discuss their importance as solid-state realizations of lattice gauge theories, mechanisms in which a Fermi sea stabilizes miniature variants of topological order, and experimental setups in which metallic contacts are used to probe quantum order. I will conclude with an outlook summarizing the multiple synergies of quantum information theory and quantum materials science.
Host: Victor Brar
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Astronomy Colloquium
The discovery and properties of binary-stripped helium stars
Time: 3:30 pm - 4:30 pm
Place: 4421 Sterling Hall
Speaker: Ylva Götberg, Carnegie Observatories
Abstract: Massive stars stripped of their H-rich envelopes through mass transfer or common envelope ejection are thought to be the main progenitors of H-poor supernovae, to emit large amounts of hard ionizing radiation, and to constitute two necessary steps in the binary evolution pathways towards compact objects merging in gravitational wave events. Despite their importance, these stripped helium stars have remained elusive. With new UV photometry combined with optical magnitudes, and follow-up optical spectroscopy, we identified a first sample of dozens of such stripped star systems in the Magellanic Clouds. We obtain estimates for their stellar parameters by fitting their optical spectra to a newly computed grid of helium star atmosphere models. Aligned with theoretical expectations, we find that stripped stars are hot (Teff~50-100 kK), compact (log g ~ 5), He-rich (Y_surf ~0.6-1), and H-poor (X_surf ~0-0.4). Furthermore, by matching the spectroscopic fits with the photometrical data, we find small radii (~1 Rsun), a range of luminosities (L ~ 1,000-100,000 Lsun), and masses that are sufficient to lead to core-collapse (~2-8 Msun). There are strong indications that the stellar winds are surprisingly weak, suggesting that binary-stripped helium stars are the main responsible for both IIb and Ib supernovae. Apart from providing an observational anchor for both binary evolution models and simulations of common envelope ejection, this sample of stars proves that the full mass range of helium stars exists, forming a bridge between subdwarfs and Wolf-Rayet stars.
Host: Ke Zhang
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Astronomy Colloquium
The Discovery and Properties of Binary-Stripped Helium Stars
Time: 3:30 pm - 4:30 pm
Place: 4421 Sterling Hall
Speaker: Dr. Ylva Götberg , Carnegie Observatories
Abstract: Massive stars stripped of their H-rich envelopes through mass transfer or common envelope ejection are thought to be the main progenitors of H-poor supernovae, to emit large amounts of hard ionizing radiation, and constitute two necessary steps in the binary evolution pathways towards compact objects merging in gravitational wave events. Despite their importance, these stripped helium stars have remained elusive. With new UV photometry combined with optical magnitudes, and follow-up optical spectroscopy, we identified the first sample of dozens of such stripped star systems in the Magellanic Clouds. We obtain estimates for their stellar parameters by fitting their optical spectra to a newly computed grid of helium star atmosphere models. Aligned with theoretical expectations, we find that stripped stars are hot (Teff~50-100 kK), compact (log g ~ 5), He-rich (Y_surf ~0.6-1), and H-poor (X_surf ~0-0.4). Furthermore, by matching the spectroscopic fits with the photometrical data, we find small radii (~1 Rsun), a range of luminosities (L ~ 1,000-100,000 Lsun), and masses that are sufficient to lead to core-collapse (~2-8 Msun). There are strong indications that the stellar winds are surprisingly weak, suggesting that binary-stripped helium stars are the main ones responsible for both IIb and Ib supernovae. Apart from providing an observational anchor for both binary evolution models and simulations of common envelope ejection, this sample of stars proves that the full mass range of helium stars exists, forming a bridge between subdwarfs and Wolf-Rayet stars.
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