Found 4 talks width keyword Wolf-Rayet

"Classical Wolf-Rayet (WR) stars" represent a class of hot, hydrogen-depleted stars wtih powerful stellar winds and are prominent progenitors of black holes. Next to their unparalleled radiative and mechanical energy feedback, they offer unique probes of massive-star evolution at the upper-mass end. To become a classical WR star, single stars require substantial mass-loss to strip their outer, hydrogen-rich layers, implying that only very massive stars could enter the WR phase. However, mass-transfer in binaries can further aid in the stripping of stars and form Wolf-Rayet stars, or more generally helium stars, at lower masses.  Due to the decrease of mass-loss with metallicity, it has been predicted that WR stars at low metallicity tend to form in binaries. However, this has so far not been supported by observations.

In my talk, I will give an overview on our current knowledge of the properties of Wolf-Rayet populations in the Galaxy and the Magellanic Clouds based on exhaustive spectral analyses. I will illustrate why binary formation does not necessarily dominate the evolution of WR stars at low metallicity, and highlight important discrepancies between theory and observations of WR stars. I will discuss the observed rarity of intermediate mass helium stars, and present recent reports of unique helium stars in the exotic binaries LB-1 and HR 6819.

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.

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.

 Chemical abundances derived using emission-line spectra in ionized gaseous nebulae are between the most useful properties that can be derived to understand the evolution of galaxies from the local Universe up to very high redshifts. Since nitrogen is one of the most abundant metals in the gas-phase of galaxies and its emission-lines can be measured many times instead of those emitted by oxygen, it is important to be aware of the implications of the variations in the nitrogen-to-oxygen ratio for the derivation of total metallicity and what are the advantages of using this abundance ratio to derive other evolutionary properties in different emission-line objects. We will also see the utility of some observational techniques, such integral field spectroscopy, to disentangle between different processes implied in the excess of observed nitrogen as derived from integrated observations.