Found 10 talks width keyword stellar activity
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.
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.
The field of Galactic archaeology has been very active in recent years, with a major influx of data from the Gaia satellite and large spectroscopic surveys. The major science questions in the field include Galactic structure and dynamics, the accretion history of the Milky Way, chemical tagging, and age-abundance relations. I will give an overview of GALAH as a large spectroscopic survey, and describe how it is complementary to other ongoing and future survey projects. I will also discuss recent science highlights from the GALAH team and compelling questions for future work.
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.
Planetary systems have been found systematically orbiting main sequence stars and red giants. But the detection of planets per se during the white dwarf phase has been more elusive with only 3 systems. We have, however, ample indirect evidence of the existence of planetary debris around these systems in the form of material acreted onto the white dwarf, disks and even planetesimals. In this talk, I will review how we can put the pieces together: how we can reconcile what we see in white dwarfs with what we can infer regarding the evolution of planetary systems from the main sequence phase.
(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
Stellar magnetic activity generates astrophysical noise on the collected data in the quest for what might be called Earth 2.0. This noise poses obstacles and difficulties in the detection and accurately determining small-sized exoplanets properties. Characterising the relation between stellar photometric variability and radial velocity jitter can help us to define optimal observational strategies, and also to better model and mitigate the activity noise. Moreover, stellar activity will remain as one of the biggest challenges in detecting and assessing the exoplanetary atmosphere’s signal, even in the era of upcoming missions. I will present the current view of the intricate relationship between exoplanets and activity, discuss some of the latest developments, and show some of our first results.
AbstractThe RV method is responsible for discovering the majority of planets that orbit stars other than our Sun. However, one problem with this technique is that stellar jitter can cause RV variations that mimic or mask out a planet signature. There have been several instances in the past when stars have shown periodic RV variations which are firstly attributed to a planet and later found to be due to stellar spots, e.g. BD+20 1790 (Figueira, P et al. 2010) and CJ674 (Turnball et al. 204). So far the method of choice to overcome these problems is to avoid observing stars which show levels of high activity. However, this does not solve the problem: it merely avoids it. We have therefore been developing a code which separates out stellar jitter from the RVs to enable active planets to be looked at for planets. I will talk about our technique as well as show some exciting preliminary results.
AbstractIn the first (optical) part, we present our recent results on mass and luminosity function of Galactic open clusters, a new statistical study based on the ASCC-2.5 catalogue of bright stars, complete to about 1 kpc around the Sun. This includes a new determination of the fraction of field stars born in open clusters. It also briefly addresses the issue whether all massive stars are exclusively born in clusters. In the second (infrared) part, we discuss the prospects of a 42m European ELT to "see" the origin of massive stars in dense embedded protoclusters, by penetrating dense proto- cluster clouds up to 200 mag of visual extinction at 2-5 microns. High-angular resolution AO imaging as well as 3D integral field spectroscopy are required to study the stellar density, binary content, and dynamical properties of these highly obscured, massive, compact star clusters.
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