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 arxiv.org/abs/2306.03744. 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.)

XXXI Canary Islands Winter School: Computational Fluid Dynamics in Astrophysics (Photos)

Two weeks of interesting lectures, intensive hands-on exercises, discussions and meeting new people... A couple of photos by Claudio Dalla Vecchia that perfectly capture the spirit of the school. 

Deep-learning about horizontal velocities at the solar surface

The velocity fields are of great importance for understanding dynamics and structure of the solar atmosphere. The line of sight velocities are coded in the wavelength shifts of the spectral lines, thanks to the Doppler effect, and relatively easy to measure. On the other hand, the orthogonal ("horizontal") components of the velocity vector are impossible to measure directly.

The most popular method for estimating the horizontal velocities is so-called local correlation tracking (LCT, November & Simon, 1988). It is based on comparing successive images of the solar surface in the continuum light and transforming their differences into information about the horizontal fields. However, the LCT algorithm suffers from several limitations.

In a paper by Andres Asensio Ramos and Iker S. Requerey (with a small contribution from my side) accepted by A&A and published on Arxiv some weeks ago (2017arXiv170305128A) this problem is tackled by the deep-learning approach. A deep fully convolutional neural network is trained on synthetic observations from 3D MHD simulations of the solar photosphere and then applied to the real observation with the IMaX instrument on board the SUNRISE balloon (Martinez Pillet et al, 2011; Solanki, 2010). The method is validated using simulation snapshots of the quiet sun produced with the MANCHA code that I have been developing in the last couple of years.

Atlantis over Tenerife

Japanese astronaut Sochi Noguchi took this iconic picture of the Space Shuttle Atlantis over the Atlantic Ocean and the island of Tenerife. The picure was taken from the International Space Station on Sunday, May 16 2010 at 10:28 am EDT (1428 GMT) while the Atlantis was getting ready for docking. At that time the ISS was about 350 km above the ground. This was the flight before the last one for the shuttle. In the highest resolution, the white towers of the solar telescopes at the Observatorio del Teide (IAC) are visible between the clouds. To help your eyes, I took a snapshot from the Google Maps: look for the distinctive dark patches in the NASA picture, the towers are tiny white dots just above them.

Credit: NASA/Sochi Noguchi (click for hi-res)

Credit: Google Maps (click to go to the interactive map)

Talk at IAC solar seminar

At the IAC solar seminar I gave a talk on fast horizontal flows in solar granulation and their spectroscopic signatures. The pdf file with the presentation is here. Most of that work was done in collaboration with my Utrecht colleagues Catherine Fischer, Alexander Vogler and Christoph Keller and it has already been published in A&A (Vitas et al, 2011). Thanks those who attended the talk for a nice and lively discussion!

Figure shows profiles of Fe I 6301.5 A computed at the location of a large supersonic event that produced a shock front. They are computed at $\mu = 0.4$ (a position away from the center of the solar disk) and for 8 azimuthal directions. It's like we've been moving around the shock. The profiles are computed at high resolution, but degraded in both spatial and spectral domain to mimic the capabilities of the SP/SOT instrument at Hinode spacecraft. It is interesting to note how the asymmetry of the line changes with azimuth. When we see the flow is moving toward us (0 degrees), the blue wing of the profile is extended and distorted. When it is moving away from us (180 degrees), the red wing is affected. That behavior is due to the well-known Doppler effect. The figure tells us that the high velocity is not sufficient for the supersonic flows to be identified through line deformation. The velocity had to be roughly aligned with the line of sight of the observer.

New coordinates, new challenges

This spring I changed my job. After a nice and inspiring year that I spent doing the Earth's observation at SRON Netherlands Institute for Space Research, I am back to the solar physics. My new affiliation is Instituto de Astrofísica de Canarias (IAC) in La Laguna (Tenerife, Spain). Here I will be working on an exciting and challenging project: "Solar Partially Ionized Atmosphere (ERC-2011-StG 277829-SPIA)", financed by the European Research Council in the frame of the FP7 Specific Program IDEAS. The PI of the project is Dr. Elena Khomenko, while the other team members are Dr. Angel de Vicente, Dr. Manuel Luna Bennasar, Dr. Antonio Díaz Medina and Prof. Dr. Manuel Collados Vera. More details about the project are available here.

Address: Instituto de Astrofísica de Canarias (IAC) C/ Vía Láctea, s/n E38205 - La Laguna (Tenerife), España Phone: (+34) 922 60 5746 Email: n.vitas@iac.es