Found 125 talks archived in Stars

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). 

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.

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.

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.

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.