Found 9 talks width keyword stellar atmospheres
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.
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.
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).
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.
(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
Understanding the atmospheric and evolutive properties of very low mass stars, brown dwarfs, and gas giant exoplanets have been important challenges for modelers around the world since the discovery of the first brown dwarfs in the Pleiades cluster (Rebolo et al. 1995) and in the field (Nakajima et al. 1995). The early studies of brown dwarfs have provided rich insights into atmospheric physics, with discoveries ranging from cloud formation (Tsuji et al. 1996), methane bands (Oppenheimer et al. 1995) and ammonia bands (Delorme et al. 2008), to the formation of wasi-molecular KI-H2 absorption (Allard et al. 2007), and to disequilibrium chemistry (Yelle & Griffith 2001). New classical 1D models yield spectral energy distribution (SED) that match relatively well despite these complexities. These models have for instance explained the spectral transition from M to L, T and now Y brown dwarf spectral types (Allard et al. 2013). However, in presence of surface inhomogeneities revealed recently for a nearby (2 pc) brown dwarf (Crossfield et al. 2014), the SED may well fit even exactly, but the model parameters could be far from exact, e.g. with the effective temperature by several hundred kelvins too cool in the case of dusty brown dwarfs and young gas giant exoplanets! I will review the progress achieved in reproducing the spectral properties of very low mass stars, brown dwarfs and gas giant exoplanets, and review progress in modeling more accurately their atmospheres using Radiation HydroDynamical (RHD) simulations.
We discuss the role and significance of molecules in the modern astrophysics. Molecular opacities govern the structure of model atmospheres of late-type stars and ultracool dwarfs. Some problems of computations of model atmosphere and synthetical spectra of cool stars are discussed. We present some successful attempts of the application of the molecular spectroscpy for the studies of late -type stars and ultracool dwarfs. Finally, some problems of fitting theoretical spectra to the observed SED are discussed.
Instituto de Astrofísica de Canarias, Spain
We have selected the Galactic HII region M43, a close-by apparently spherical nebula ionized by a single star (HD37061, B0.5V) to investigate several topics of recent interest in the field of HII regions and massive stars. We perform a combined, comprehensive study
of the nebula and its ionizing star by using as many observational constraints as possible. For this study we collected a set of high-quality observations, including the optical spectrum of HD3706, along with nebular optical imaging and long-slit spatially resolved spectroscopy. On the one hand, we have carried out a quantitative spectroscopic analysis of the ionizing star from which we have determined the stellar parameters of HD37061 and the total number of ionizing photons emitted by the star; on the other hand, we have done a
empirical analysis of the nebular images and spectroscopy from which we have find observational evidence of scattered light from the Huygens region (the brightest part of the Orion nebula) in the M43 region. We show the importance of an adequate correction of this scattered light in both the imagery and spectroscopic observations of M43 in accurately determining the total nebular Halpha luminosity, the nebular physical
conditions. and chemical abundances. We have computed total abundances for three of the analyzed elements (O, S, and N), directly from
observable ions (no ionization correction factors are needed). The comparison of these abundances with those derived from the spectrum of the Orion nebula indicates the importance of the atomic data and, specially in the case of M42, the considered ionization correction factors.
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