Substellar Astrophysics meeting in Tordesillas, Spain

 

Substellar science has emerged in the last few decades as new branch of Astrophysics that connects Stars, Exoplanets and the Solar System. The advent of new surveys such as Euclid and Rubin LSST is poised to increase the numbers of known substellar objects by more than an order of magnitude, while the James Webb Telescope is providing new details about their properties. Rapid advances in observational capabilities have spurred the development of a new generation of theoretical models, now reaching unprecedented levels of accuracy and physical completeness.

This meeting will bring together researchers interested in Substellar science with special emphasis on exploiting the wealth of data provided by Euclid and complementing it with other surveys and follow-up observations.

The conference is supported by the European Research Council Advanced grant nicknamed Substellar.

A total eclipse of the Sun will take place during the conference (August 12, 2026).

The proceedings of the conference will be formally published as part of the conference series of Astronomische Nachrichten.

 More information at the conference website. 

Program highlights

Key themes include:

• Detection methods (Searching methods, astrometry, photometry, spectroscopy)
• Confirmation of UCD candidates
• Atmospheric properties of UCD's
• UCD in connection with Milky Way
• Multiplicity, planetary systems, disks
• Substellar luminosity and mass functions
• Connection with exoplanets
• Synergies of Euclid and other surveys
• Big data, machine learning for substellar science
• Prospects for Exolife in Substellar Worlds
• Theory of UCD's (atmospheric and evolutionary models, microphysics)

Scientific Organizing Committee

• Eduardo Martin (chair)
• Maruša Žerjal (co-chair)
• Nikola Vitas (co-chair)
• Patricia Cruz
• Pin-Gao Gu
• Nuria Huélamo
• Nicolas Lodieu
• Koraljka Mužić
• Ngoc Phan
• Annie Robin
• Johannes Sahlmann
• Kun Wang

Invited speakers

• Khalid Barkaoui, Instituto de Astrofísica de Canarias, Tenerife, Spain
• David Barrado, Centro de Astrobiología, Madrid, Spain
• Beth Biller, Royal Observatory, Edinburgh, UK
• Clemence Fontanive, Royal Observatory, Edinburgh, UK
• Kevin Luhman, Pennsylvania State University, USA
• Elena Manjavacas, Space Telescope Science Institute, USA
• Javier Olivares, UNED, Madrid, Spain
• Antonio Pérez Garrido, Universidad Politécnica de Cartagena, Spain
• Rafael Rebolo, Instituto de Astrofísica de Canarias, Tenerife, Spain
• Céline Reylé, Observatoire de Besançon, France
• Richard Smart, Ossevatorio Astrofisico di Torino, INAF, Italy
• Enrique Solano, Centro de Astrobiología, Madrid, Spain
• Xianyu Tan, Shanghai Jiao Tong University, China
• Ramarao Tata, Ohio University, USA
• Maria Rosa Zapatero Osorio, Centro de Astrobiología, Madrid, Spain
• Jun-Yan Zhang, Western University, Ontario, Canada

A photometric search for ultracool dwarfs in the Euclid Deep Fields

 

A new paper from Euclid Q1, is just out on arxiv. I’m particularly happy to be part of!

In this work, led by Maruša Žerjal (my collaborator at the IAC and the SUBSTELLAR project), we explored what Euclid can already tell us about the population of ultracool dwarfs. Using a remarkably simple photometric selection based on the very red Euclid ($I_\mathrm{E}-Y_\mathrm{E}$) colour, we identified 5306 new ultracool-dwarf candidates in the three Euclid Deep Fields, ranging from late-M to late-T dwarfs. Around 1200 are L and T dwarfs, and 546 objects are spectroscopically confirmed.

Euclid was designed primarily for cosmology, but its combination of deep optical and near-infrared photometry and spectroscopy makes it an extraordinary machine for finding and characterising the faintest members of the stellar and substellar populations.

And Q1 is only a tiny taste of what is coming. We find roughly 100 UCDs per square degree, which suggests that the final Euclid Wide Survey could contain at least 1.4 million ucd candidates, including around 300 000 L dwarfs and thousands of T dwarfs.

These estimates come from deliberately strict selection criteria that favour purity over completeness, so they are essentially a lower limit. Millions of ultracool dwarfs waiting in the Euclid data, exciting times ahead!

Three-dimensional radiative MHD simulations of near-surface convection in main sequence cool stars

 Andrea Perdomo García has recently completed her PhD thesis, Three-dimensional radiative MHD simulations of near-surface convection in main sequence cool stars, supervised by Manuel Collados and myself.

Andrea’s thesis explored the atmospheres of cool main-sequence stars through realistic three-dimensional radiative MHD simulations with the MANCHA code. A major part of her work was devoted to one of the central challenges of such simulations: treating radiative transfer accurately while keeping it computationally feasible. She developed and tested opacity-binning strategies for stars ranging from F to M spectral types, investigated the increasingly important role of molecular opacity toward cooler stars, and studied how different opacity treatments affect atmospheric structure and radiative energy exchange.

She then applied these methods to 3D simulations of G2V, K0V, and M2V stars, exploring their convection and photospheric structure and extending the calculations to magnetic simulations in which fields generated through the Biermann battery were amplified by small-scale dynamo action.

It has been a true pleasure to work with Andrea over the years and to follow her development as a researcher. I wish her all the best in her future career and in her new position at the Max Planck Institute in Heidelberg. I am sure there are many interesting problems, simulations, and probably quite a few opacity tables still ahead!

Moving to the dark side!

 

I have some big news to share. From March 1, I’ll be starting a new chapter and moving to the dark side of astrophysics: the substellar world of ultracool dwarfs!

This seems like a natural moment to make the move. With the Euclid ESA mission now opening an enormous new window on the faint and cool populations of the Milky Way, we are entering an era in which the number of known ultracool and substellar objects will increase substantially. Turning this wealth of new observations into physical understanding will require careful work on the modelling side.

In some ways, though, I am not moving very far at all. My main interests will remain what they have been for years: developing numerical agoriths for radiative transfer, equations of state, opacities, and numerical modelling of atmospheres. The difference is that I will now be applying them much further down the temperature scale. At the temperatures of ultracool dwarfs, molecules dominate the opacity, chemistry becomes increasingly tricky, condensates and clouds appear, and the coupling between chemistry, radiation and atmospheric structure gets the central stage in modelling problem. 

I’ll be joining the SUBSTELLAR project led by Eduardo Martín, one of the co-discoverers of Teide 1, the first confirmed brown dwarf, announced in 1995. The work is supported by the European Research Council through the ERC Advanced Grant SUBSTELLAR, devoted to pushing the frontier of substellar science with the Euclid mission.

A new wavelength regime, a new class of objects, and plenty of new physics — but, fortunately, still lots of opacities, radiative transfer and numerical challenges. I hope that this move will also allow me to bring some of my experience in stellar astrophysics across the boundary between these fields. I’m looking forward to seeing where it leads.

May the (gravity and Lorentz) Force be with me!

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.)

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.

Robert Jelle Rutten (1942 - 2022)

So long, my teacher and friend. You will be hugely missed.