Wednesday 7 June 2023

Opacity for realistic 3D MHD simulations of cool stellar atmospheres

The first paper of Andrea Perdomo Garcia is just submitted for publication in Astronomy & Astrophysics, and out on The paper is all about computing the opacities for realistic modelling of cool stellar atmospheres. It is divided in three unities. First (Section 3) it describes the computation of detailed monochromatic opacity including millions of atomic and molecular spectral lines and millions of wavelength points. For this the code SYNSPEC (Hubeny and Lanz, 2011, 2017a, b) is used. Then (Section 4) the monochromatic opacities are used to construct opacity distribution function which reduces the number of wavelength points from millions to thousands. The results are compared in detail with ones produced by Kurucz. Some striking similarities and some warning differences are found. Finally (Section 5), the opacity distribution function to construct opacity bins. This method, originally proposed by Nordlund (1982) is the key ingredient for realistically simulating stellar atmospheres in 3D as it reduced the problem further, from thousands of wavelength points to only a few. However, the method depends on a choice of some free parameters. In our paper the possible choices are carefully analyzed and some interesting conclusions are offered. 

In Sect.3 there are two figures (Figs.2 and 3) that I find very useful and illustrative. The monochromatic opacity (Fig.2) and the radiative heating rate (Fig.3) are shown as 2D functions of wavelength (X-axis) and height in the atmosphere (Y-axis) for four different cool stars (all with solar metalicity). Optical depths in the continuum and continuum+lines are overplotted.

(Andrea is the final year PhD student at Instituto de Astrofisica de Canarias and Univeridad de La Laguna, supervised by Manolo Collados Vera and myself. Stay tuned, more cool stuff is coming out from her research this year.)

Saturday 25 March 2023

Charles Hermite (1822 - 1901)

Sunday morning in Paris was opportunity to walk to the Montparnasse cemetery and pay respect to some of my personal heroes buried there. While the graves of Beckett, Cortázar and Poincaré attract quite many attention and visitors, it is less known that the great French mathematician Charles Hermite is buried there as well. Not only that he was a mentor to Henri Poincaré, Henri Padé, Thomas Stieltjes and Mihajlo Petrovic Alas, but his work on interpolation and function approximation is at the very core of the modern numerical methods used in computational fluid dynamics and radiative transfer (even when this is not so obvious or properly acknowledged). His work from 1878 ("Sur la formule d'interpolation de Lagrange") should be read by anyone interested in function approximation. The name on his grave stone are barely readable nowadays.