Found 120 talks archived in Stars

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.

Youtube: https://youtu.be/yJHMQFmLsGE

(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
   lines.

"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

Wide hot subdwarf B (sdB) binaries with main-sequence companions are outcomes of stable mass transfer from evolved red giants. The orbits of these binaries show a strong correlation between their orbital periods and mass ratios. The origins of this correlation have, so far, been lacking a conclusive explanation.
We have performed a small but statistically significant binary population synthesis study with the binary stellar evolution code MESA. We have used a standard model for binary mass loss and a standard Galactic metallicity history.  We have achieved an excellent match to the observed period - mass ratio correlation without explicitly fine-tuning any parameters. Furthermore, our models produce a good match to the observed period - metallicity correlation.
We demonstrate, for the first time, how the metallicity history of the Milky Way is imprinted in the properties of the observed post-mass transfer binaries. We show that Galactic chemical evolution is an important factor in binary population studies of interacting systems containing at least one evolved low-mass (Mi < 1.6 Msol) component. Finally, we provide an observationally supported model of mass transfer from low-mass red giants onto main-sequence stars.

Zoom link: https://rediris.zoom.us/j/98017007654