25 research outputs found
Analysis of stellar spectra with 3D and NLTE models
Models of radiation transport in stellar atmospheres are the hinge of modern
astrophysics. Our knowledge of stars, stellar populations, and galaxies is only
as good as the theoretical models, which are used for the interpretation of
their observed spectra, photometric magnitudes, and spectral energy
distributions. I describe recent advances in the field of stellar atmosphere
modelling for late-type stars. Various aspects of radiation transport with 1D
hydrostatic, LTE, NLTE, and 3D radiative-hydrodynamical models are briefly
reviewed.Comment: 21 pages, accepted for publication as a chapter in "Determination of
Atmospheric Parameters of B, A, F and G Type Stars", Springer (2014), eds. E.
Niemczura, B. Smalley, W. Pyc
Acoustic-gravity wave propagation characteristics in 3D radiation hydrodynamic simulations of the solar atmosphere
There has been tremendous progress in the degree of realism of
three-dimensional radiation magneto-hydrodynamic simulations of the solar
atmosphere in the past decades. Four of the most frequently used numerical
codes are Bifrost, CO5BOLD, MANCHA3D, and MURaM. Here we test and compare the
wave propagation characteristics in model runs from these four codes by
measuring the dispersion relation of acoustic-gravity waves at various heights.
We find considerable differences between the various models. The height
dependence of wave power, in particular of high-frequency waves, varies by up
to two orders of magnitude between the models, and the phase difference spectra
of several models show unexpected features, including phase
jumps.Comment: 19 pages, 15 figure
High-resolution wave dynamics in the lower solar atmosphere
The magnetic and convective nature of the Sun's photosphere provides a unique
platform from which generated waves can be modelled, observed, and interpreted
across a wide breadth of spatial and temporal scales. As oscillations are
generated in-situ or emerge through the photospheric layers, the interplay
between the rapidly evolving densities, temperatures, and magnetic field
strengths provides dynamic evolution of the embedded wave modes as they
propagate into the tenuous solar chromosphere. A focused science team was
assembled to discuss the current challenges faced in wave studies in the lower
solar atmosphere, including those related to spectropolarimetry and radiative
transfer in the optically thick regions. Following the Theo Murphy
international scientific meeting held at Chicheley Hall during February 2020,
the scientific team worked collaboratively to produce 15 independent
publications for the current Special Issue, which are introduced here.
Implications from the current research efforts are discussed in terms of
upcoming next-generation observing and high performance computing facilities.Comment: 16 pages, 4 figures, Introduction to the "High-resolution wave
dynamics in the lower solar atmosphere" special issue of the Philosophical
Transactions A: https://walsa.team/u/rst
Solar and Stellar Photospheric Abundances
The determination of photospheric abundances in late-type stars from
spectroscopic observations is a well-established field, built on solid
theoretical foundations. Improving those foundations to refine the accuracy of
the inferred abundances has proven challenging, but progress has been made. In
parallel, developments on instrumentation, chiefly regarding multi-object
spectroscopy, have been spectacular, and a number of projects are collecting
large numbers of observations for stars across the Milky Way and nearby
galaxies, promising important advances in our understanding of galaxy formation
and evolution. After providing a brief description of the basic physics and
input data involved in the analysis of stellar spectra, a review is made of the
analysis steps, and the available tools to cope with large observational
efforts. The paper closes with a quick overview of relevant ongoing and planned
spectroscopic surveys, and highlights of recent research on photospheric
abundances.Comment: Invited review to appear in Living Reviews in Solar Physics. 39
pages, 7 figure