Found 121 talks archived in Stars
Galactic globular clusters have always been at the crossroad of several investigations in both Stellar and Galactic Astrophysics. For long time, they have been considered the prototypes of Simple Stellar Populations, and hence used for testing and calibrating stellar evolutionary models as well as population synthesis tools. Nowadays, after the discovery of the presence of multiple stellar populations in almost all Galactic GCs, we know that this assumption is no longer valid. The process(es) of formation and early evolution of these star clusters is (are) very far to be understood, and any scenario so far envisaged is severely challenged by the pletora of empirical evidence collected till now. In the same time, thanks to the availability of an impressive observational framework - collected by combining kinematic measurements from Gaia mission, with data provided by large spectroscopic and photometric surveys -, GCs are playing a crucial role for our understanding of the assembly history of the Milky Way. We will review our present knowledge about these important stellar systems, discussing the several, open issues related to their formation/evolution, and discuss how we can use them in our effort to depict the Milky Way assembly history.
The new generation of spectrometers designed for extreme precision radial velocities enable correspondingly precise stellar spectroscopy. It is now fruitful to theoretically explore what the information content would be if stellar spectra could be studied with spectral resolutions of a million or more, and to deduce what signatures remain at lower resolutions. Hydrodynamic models of stellar photospheres predict how line profiles shapes, asymmetries, and convective wavelength shifts vary from disk center to limb. Corresponding high-resolution spectroscopy across spatially resolved stellar disks is now practical using differential observations during exoplanet transits, thus enabling the testing of such models. A most demanding task is to understand and to model spectral microvariability toward the radial-velocity detection of also low-mass planets in Earth-like orbits around solar-type stars. Observations of the Sun-as-a-star with extreme precision spectrometers now permit searches for spectral-line modulations on the level of a part in a thousand or less, feasible to test against hydrodynamic models of various solar features.
Massive stars are generally fast rotators, however, with significant dispersion. We discuss the hypothesis that all OB stars are all born with very similar spins, with slower and faster rotators being produced by close binary evolution. We review supporting evidence from recent observations of young and rich star clusters, from OB star surveys, and from dense grids of detailed binary evolution models. We connect the OB star spins with the likelihood of evolved/compact binary companions, and with the variety of the explosive end states of massive stars.
(This seminar is organized by the IAU G5 commission on stellar and planetary atmospheres)
Task-based computing is a method where computational problems are split
into a large number of semi-independent tasks (cf.
2018MNRAS.477..624N). The method is a general one, with application not
limited to traditional grid-based simulations; it can be applied with
advantages also to particle-based and hybrid simulations, which involve
both particles and fields. The main advantages emerge when doing
simulations of very complex and / or multi-scale systems, where the
cost of updating is very unevenly distributed in space, with perhaps
large volumes with very low update cost and small but important regions
with large update costs.
Possible applications in the context of stellar atmospheres include
modelling that covers large scales, such as whole active regions on the
Sun or even the entire Sun, while at the same time allows resolving
small-scale details in the photosphere, chromosphere, and corona. In
the context of planetary atmospheres, models of pebble-accreting hot
primordial atmospheres that cover all scales, from the surfaces of
Mars- and Earth-size embryos to the scale heights of the surrounding
protoplanetary disks, have already been computed (2018MNRAS.479.5136P,
2019MNRAS.482L.107P), and one can envision a number of applications
where the task-based computing advantage is leveraged, for example to
selectively do the detailed chemistry necessary to treat atmospheres
saturated with evaporated solids, or to do complex cloud chemistry
combined with 3-D radiative transfer.
In the talk I will give a quick overview of the principles behind
task-based computing, and then use both already published and still
on-going work to illustrate how this may be used in practice. I will
finish by discussing how these methods could be applied with great
advantage to problems such as non-equilibrium ionization, non-LTE
radiative transfer, and partial redistribution diagnostics of spectral
"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.
The majority of massive stars are born in close binary systems with orbital periods of a few days. At some point during their core-hydrogen burning phase, both members of these close binaries inevitably overflow their Roche lobes simultaneously and get bound by a common equipotential surface. The characteristics of this `contact phase’ will determine the fate of the binary system: whether the stars will merge on the main sequence or evolve further towards becoming potential gravitational-wave progenitors. Although data is available for several of these massive contact binaries in the Magellanic Clouds and the Milky Way, there has not been a dedicated study of these systems so far. In this talk, I will present the first set of detailed binary models covering a wide range of initial masses (20-80 Msun) and initial periods (0.6-2 days), focusing especially on the properties of the contact phase. We find that our models can approximately reproduce the period-mass ratio trend of the observed binaries although for the higher masses of our grid, our model predictions do not match with what is observed. We also find that those binary models which are in contact over nuclear timescales evolve towards equal masses before ultimately merging on the main sequence. This first study of massive contact binaries has allowed us to gain insights into the physics of massive contact systems and also provide reasonable predictions for the final fate of close massive binary stars.
Globular clusters (GCs) are fascinating objects nearly as old as the Universe that provide insight on a large variety of astrophysical and cosmological processes. However, their formation and their early and long-term evolution are far from being understood. In particular, the classical paradigm describing GCs as large systems of coeval stars formed out of chemically homogeneous material has been definitively swept away by recent high-precision spectroscopic and deep photometric observations. These data have provided undisputed evidence that GCs host multiple stellar populations, with very peculiar chemical properties. In this talk, I will review the properties of these multiple populations, before presenting the different scenarios that have been proposed to describe their formation. I will focus on the (many) current theoretical issues and open questions.
Massive stars are often found to be in pairs. This configuration is both a blessing and a curse. From it, we can estimate their exact properties such as their masses but the interactions that result during their life considerably affect the way that the stars evolve.
Here, we provide an overview of progresses made through a number of medium and large surveys. These results provide new insights on the observed and intrinsic multiplicity properties of massive stars through a large range of masses and at different metallicities. Furthermore, to understand how the stars evolve when they are in pair and what are the effects of these interactions on the stellar properties, we undertook a large study of more than 60 massive binaries at Galactic and LMC metallicities using spectral disentangling, atmosphere modelling and light curve fitting to determine their stellar parameters, and surface abundances. This unique dataset is the largest sample of binaries composed of at least one O-type star to be studied in such a homogeneous way. It allows us to give strong observational constraints to test theoretical binary evolutionary tracks, to probe rotational and tidal mixings and mass transfer episodes.
The formation and evolution of planets in general is closely linked to the life of their host star. What happens to the planetary systems at the end stages of the life cycle of their star has been one of the questions that have received attention from a theoretical point of view but has had a lack of real life examples to study. Among more than 4000 known exoplanets to date only a few of these objects have been found orbiting around pulsars, but so far we have found nothing that resembles what our own solar system will be like long after the Sun leaves the main sequence.
In this talk we will discuss the recent announcement by A. Vanderburg et al. of a giant planet candidate detected by the transit method orbiting around a white dwarf. The candidate was discovered using data from the space-based NASA mission TESS and confirmed using GTC, Spitzer, and other ground-based facilities. We will talk about the role that GTC played in this discovery, the peculiarity of this candidate system, and the possibility of detecting atmospheres in rocky planets orbiting around white dwarfs.
Zoom link: https://rediris.zoom.us/j/95796802777
Youtube link: https://youtu.be/TX5KfTeJNAM
- IAU G5 -- The GALAH survey: science goals and highlights to dateSarah MartellTuesday January 25, 2022 - 10:30 GMT (Online)
- Dynamos, the drivers of solar and stellar activityProf. Axel BrandenburgThursday January 27, 2022 - 10:30 GMT (Online)