Found 132 talks archived in Stars

Thursday May 4, 2023
Dr. Paul Beck


Understanding stellar structure and evolution significantly impacts our understanding of the tight-knit evolution of galaxies and exoplanet systems. However, hidden behind the luminous layers of the stellar atmosphere, the deep interior of a star is eluding from direct measurements. The seismic study of waves propagating the deep interior provides the only way to measure the internal structure, dynamics, and mixing in any given star and compare it to theoretical models.

With the photometric data from space missions, such as the NASA Kepler telescope, a golden age has begun for seismology. In particular, the seismic studies of thousands of solar-like have led to numerous breakthroughs in our understanding of the stellar structure of red-giant stars. Complimentary information on stellar binarity, tidal forces, rotation, and lithium abundance provide additional constraints to characterize the advanced evolution of stars further and provide high-resolution insights into complex internal adjustments. Approaching a sample of ~1000 identified solar-like oscillators in binary systems, provided by the ESA Gaia and NASA TESS missions draws an exciting picture on the interaction of stellar and orbital evolution.
ID de reunión: 892 7515 0368

Código de acceso: 101169

Thursday April 20, 2023
Dr. Eva Laplace
HITS, Heidelberg


Gravitational-wave observations have revealed the population of stellar remnants from a new angle. Yet their stellar progenitors remain uncertain, in particular in the case of black holes. At least a fraction of these progenitors is believed to form in isolated binary systems. In this talk, I will discuss how binary mass transfer affects the late evolution and final fate of massive stars. The focus will be on stars that transfer their outer layers to a companion star and become binary-stripped. Binary-stripped stars develop systematically different core structures compared to single stars. I will discuss consequences for supernova progenitors, black hole formation, supernova nucleosynthesis, and gravitational-wave observations.

Thursday March 30, 2023
Dr. Noel Castro Segura
University of Southampton



Disc winds and jets are ubiquitous among accreting systems on all scales, from active galactic nuclei (AGN) down to young stellar objects. They represent a key mechanism through which these systems interact with their environment (“feedback") and may be responsible for triggering the mysterious state changes observed in X-ray binary stars (XRBs).

Transient low-mass X-ray binaries (LMXBs), harbouring a black hole or a neutron star, provide us with a natural laboratory for studying the connection(s) between accretion discs, jets and winds.  These systems undergo outbursts, during which they brighten dramatically across the whole electromagnetic spectrum. The outbursts typically last hundreds of days, recur on timescales of decades, and reflect a sudden increase in the accretion rate onto the compact object. Over the course of an outburst, LMXBs exhibit two distinct spectral states. These spectral states are thought to be a consequence of different accretion geometries close to the central object.

Remarkably, the two distinct accretion states also appear to produce two distinct types of outflows. Steady compact radio jets are only seen in the hard state, whereas evidence of disc winds originally came in the form of blue-shifted X-ray absorption lines associated with Fe ions detected only during the soft state. However, recent observations of disc winds in the far-UV, optical and NIR lines reveal a multiphase nature of these outflows that may be present across the entire outburst.  

I will discuss the current status of disc winds in LMXBs with special emphasis in the latest results from far-UV spectroscopy obtained with the Hubble Space Telescope.



Meeting ID: 868 2664 6040
Passcode: 610738



Tuesday January 24, 2023
Anish Amarsi
Uppsala University


Spectroscopic analyses of stellar chemical compositions are model-dependent, and shortcomings in the models often limit the accuracy of the final results.  For late-type stars like our Sun, two of the main problems in present-day methods are that they assume the stellar atmosphere is a) one-dimensional (1D) and hydrostatic, and b) satisfies local thermodynamic equilibrium (LTE).  We can relax these assumptions simultaneously by performing detailed 3D non-LTE radiative transfer post-processing of 3D radiative-hydrodynamic model stellar atmospheres.  I shall give a brief overview of this approach, and illustrate its impact on carbon, oxygen, and iron abundances in late-type stars.

Tuesday November 29, 2022
Christoph Schürmann
University of Bonn; Argelander-Institut für Astronomie


Most high mass X-ray binaries contain neutron stars as companions to an OB star, while high mass black hole binaries are very rare. We use rapid binary population synthesis to predict the number and properties of OB stars with compact companions, while varying uncertain physics assumptions. We find that synthetic populations which agree with the population properties of Be stars, Wolf-Rayet stars, and neutron stars forecast a large and so far undetected population of massive black hole binaries with orbital periods between a few days and 1000 days. To find or rule out this population is key for quantifying the contribution of isolated binaries to the merging massive black holes found through their gravitational wave emission.

Thursday October 13, 2022
Dr. Alexey Bobrick
Technion University, Israel


Exciting things may have happened sometimes to the stars we see in the sky today. For example, Betelgeuse, also known as Alpha-Ori, an M-type red supergiant, the 10th brightest sky in the sky (usually), may well have been a binary star in the past. Its rapid rotation, peculiarly large Galactic velocity, and unusual chemical abundances all point to it being kicked out from the birth environment and merging as a binary star. By comparing a Monte-Carlo stellar cluster population model with the observed populations of Galactic O- and B- type stars (progenitors of red supergiants), I will show that the story of Betelgeuse is not at all uncommon. In distant galaxies, closely related scenarios may give rise to peculiar core-collapse supernovae. I will conclude by briefly discussing how the diversity of such binary and triple stellar evolution histories reflects in the variety of the currently discovered core-collapse supernovae.

Tuesday September 20, 2022
Dr. Asif ud-Doula
Penn State University


Massive stars (at least eight times as massive as the Sun) possess strong stellar winds driven by radiation. With the advent of the so called MiMeS collaboration, an increasing number of these massive stars have been confirmed to have global magnetic fields. Such magnetic fields can have significant influence on the dynamics of these stellar winds which are strongly ionized. Such interaction of the wind and magnetic field can generate copious amount of X-rays, they can spin the star down, they can also help form large scale disk-like structures. In this presentation I will discuss the nature of such radiatively-driven winds and how they interact with magnetic fields.

Tuesday April 5, 2022
Prof. Brian Welsch
University de Wisconsin


On the Sun, the presence of magnetic flux at the photosphere is closely linked to (1) steady heating of the overlying atmosphere and (2) transient brightenings, the largest of which are flares.   I will discuss statistical properties of both phenomena, with an emphasis on aspects of each that might apply to other astrophysical objects, such as other stars or stellar remnants, and perhaps AGNs.  Regarding heating, power-law scalings have been found to relate magnetic flux with steady coronal emission in both soft X-ray (SXR) and EUV ranges.  A key observation is that the details of magnetic structure (field strengths and their spatial gradients, including measured electric currents) appear not to affect heating rates. Similar SXR scalings have been reported for G,K, and M dwarfs and classical T-Tauri stars.  Departures from such scalings, whether on the Sun, other stars, or other objects, might reveal important aspects of the heating mechanisms that drive steady emission, and should be sought.   Regarding flaring, again a power-law scaling between magnetic flux and flare SXR emission has been found, but with a different exponent.  Differences in these scalings suggest that steady heating fundamentally differs from flare heating, disfavoring the “nanoflare” hypothesis (i.e., that steady coronal heating arises from many weak, unresolved flares that are essentially scaled-down versions of larger flares).  Analogous differences in the scalings of steady vs. flaring luminosities with magnetic flux on other objects could constrain processes driving each type of emission.  Another key property of flares is that they extract energy from the magnetic field, which in the solar case leads to measurable changes in field strengths after flares – photospheric field strengths tend to increase, coronal fields tend to decrease.  It is possible that analogous changes could be observed on other stars or objects (via, e.g., Zeeman or synchrotron  methods). 

Thursday February 24, 2022
Dr. Diego Godoy-Rivera


Rotation plays an important role in the life of stars and offers a potential diagnostic to infer their ages and that of their planets. This idea is known as gyrochronology, and if properly calibrated, its applications to Galactic, stellar, and exoplanetary astrophysics would be far-reaching. Nevertheless, while potentially fruitful over a wide range of ages and masses, recent results have raised concerns regarding gyrochronology’s applicability. In this talk, I will present the opportunities that the Gaia astrometry has opened to address these issues. First, regarding rotation’s classical calibrators, I will illustrate the impact that removing the non-member contamination has on the rotational sequences of open clusters. Second, I will present a novel method that tests the state-of-the-art gyrochronology relations in under-explore domains using wide binary stars. Finally, I will discuss the prospects for expanding the existing rotational constraints in unprecedented regimes using data from the TESS mission.

Thursday January 27, 2022
Prof. Axel Brandenburg
Nordita (Sweden)


Following Cowling's anti-dynamo theorem of 1933, there was a long period during which the very existence of dynamos was unclear. Even with the emergence of three dimensional simulations in the late 1980s, people were careful to distinguish true dynamos from just some sort of amplification. Meanwhile, we know of many examples of true dynamos - not only from simulations, but also from several laboratory experiments. Nevertheless, there are still problems, fundamental ones and also very practical ones. After all, we are really not sure how the solar dynamo works. Today, global three-dimensional simulations seem to have an easier time to reproduce the behaviors of superactive stars, but not really the group of inactive stars, to which also the Sun belongs. The Sun itself may actually be special; it has so well defined cycles and it is at the brink of becoming very different. Theoretically, slightly slower rotators should have antisolar rotation, but it is possible that some of those stars never become that slow if stellar breaking ceases for some reason. Sun and starspots are very evident indicators of solar and stellar activity. Their formation is also not well understood. Polarimetry reveals their magnetic helicity, which can be detected even with the solar wind.

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