Recent Talks

25 years ago, a seminal letter was published in Nature where it was concluded that the enigmatic linear polarization signal observed in the solar sodium D1 line implies that the quiet solar chromosphere is practically unmagnetized, in contradiction with other observational inferences and plasma physics arguments. This became known as the paradox of the solar sodium D1 polarization, which has puzzled theoretical physicists for many years, even leading some scientists to question the established quantum theory of radiation-matter interaction. In this talk I will briefly discuss the theoretical basis for this intriguing paradox and present its resolution in terms of a radiative transfer investigation published in Physical Review Letters. The observed linear polarization pattern across the sodium D2 and D1 lines can be reproduced to a remarkable degree in the presence of magnetic fields in the gauss range, if one accounts for the variations in the anisotropy of the solar radiation field over the small spectral interval spanned by the various hyperfine structure components of the sodium D2 and D1 lines. In addition, I will present the results of a series of radiative transfer investigations focused on the D lines of other alkali species, namely K I and Ba II, discussing their interest for probing the magnetism of the solar chromosphere.

Primera tanda de las charlas de instrumentación de los becarios de verano.

As a measure to fight global climate change, legislation is essentially terminating coal mining in Germany. In order to help with the necessary transformation of the economy, the federal government has set aside a total of 40 billion Euros for investment into new infrastructure in the affected regions. The state of Saxony decided to use a significant fraction of these funds to create two major research centers -- in essence to invest in brains, and not only in concrete and steel. As a result of a two-stage competition, the proposal of a group of astrophysicists has won a grant of 1.4 billion Euros to create, over a period of 15 years, the Deutsches Zentrum für Astrophysik (DZA). 
The talk will explain the structure of the institute, the current status, and the main focus on (1) fundamental research in Astrophysics, (2) technology development, and (3) big data & eScience. As for technology development, several examples will be presented that are intended to exploit the regional strength in semiconductor and photonics industry, e.g. CMOS sensors, photonic integrated circuits, photonic lanterns, etc.
In a second part of the talk, I will make a connection from the intended DZA engagement in novel detector technologies to the future WST: perhaps the next big project of ESO once the ELT has been finished. I will briefly touch upon the concept of the telescope, and report on the science case about resolved stellar populations that was developed in collaboration with IAC and others, and published recently as part of the WST White Paper (Mainieri+2024).

Elemental abundances of Sun-like stars are crucial for understanding the detailed properties of their planets. However, measuring elemental abundances in M stars is challenging due to their faintness and pervasive molecular features in optical spectra. To address this, elemental abundances of Sun-like stars have been proposed to constrain those of M stars by scaling [X/H] with measured [Fe/H] – a practice is yet to be well tested. Here we compile elemental abundances for 43 M dwarfs for 10 major rock-forming elements (Fe, C, O, Mg, Si, Al, Ca, Na, Ni, and Ti) from high-resolution near-infrared stellar surveys (APOGEE, CARMENES and Subaru/IRD). We perform bootstrap-based linear regressions on the M dwarfs to determine the trends of [X/H] vs. [Fe/H] and compare them with GK dwarfs (from GALAH + APOGEE). A 2-sample, multivariate Mahalanobis Distance test is applied to assess the significance of differences in [X/H]—[Fe/H] trends for individual elemental pairs between M and GK dwarfs. The null hypothesis of no significant difference in chemical trends between M and GK dwarfs is strongly rejected for all elements except Si, for which rejection is marginal, and Na and Ni, for which results are inconclusive. This suggests that assuming no difference may lead to biased results and inaccurate constraints on rocky planets around M dwarfs. Therefore, it is crucial for both the stellar and exoplanet communities to recognise these differences. To better understand these differences, we advocate for dedicated modelling techniques for M dwarf atmospheres and more homogeneous abundance analyses. Our statistically constrained trends of [X/H]—[Fe/H] for M dwarfs offer a new constraint on estimating M-dwarf elemental abundances given measured [Fe/H], aiding in detailed characterisation of M dwarf-hosted rocky worlds in the era of JWST, PLATO and ELT.

With the arrival of large galaxy surveys such as KiDS, DES or HSC and their ability to observe millions of galaxies, statistical errors have shrunk, making cosmology a precision science. Now, systematic effects are becoming the main source of uncertainty, so dealing with them will be one of the main challenges that next-generation surveys, such as Euclid, DESI or LSST. In this presentation, we will focus on two sources of systematic uncertainty: observational systematics and atmospheric conditions. For the former, we will explore the methods and lessons learnt from the DES-Y3 galaxy clustering analysis, the ongoing work for Y6 and the prospects for LSST-DESC. For the latter, we will introduce LSST’s Auxiliary Telescope (AuxTel), whose purpose is to measure atmospheric transparency and to derive color corrections based on spectroscopic observations.

The chemical evolution of the Universe is largely guided by the lives of O- and B-type stars. These stars are born with a convective core on the main-sequence and are heavily influenced by additional mixing occurring at both the convective core boundary and in the radiative envelope. Such mixing transports additional hydrogen fuel from the envelope to the convective core, allowing the stars to live longer and to enhance their final helium core mass at the end of the main-sequence evolution. Consequently, chemical mixing has a high impact on the stellar evolution of both intermediate- and high-mass stars yet remains one of the dominant uncertainties in their stellar structure and evolution theory, along with their interior angular momentum transport. Asteroseismology is a powerful tool for studying stellar interiors through the detection and interpretation of stellar oscillations. With this talk, I will demonstrate how we can use such stellar oscillations to probe the internal mixing and rotation, as well as discuss recent results for O- and B-type stars observed by photometric space telescopes such as Kepler and TESS.

Link de Zoom: https://rediris.zoom.us/j/94666719527?pwd=Tcikc3PagbtnsIeZJGIiSbYd9VUZ8I.1

Cosmic rays interact with astrophysical systems over a broad range of scales. They go hand-in-hand with violent, energetic astrophysical environments, and are an active agent able to regulate the evolution and physical conditions of galactic and circum-galactic ecosystems. Depending on their energy, cosmic rays can also escape from their galactic environments of origin, and propagate into larger-scale cosmological structures. In this talk, I will discuss the impacts of cosmic rays retained in galaxies. I will show that they can deposit energy and momentum, modify the circulation of baryons around galaxies, and have the potential to regulate long-term galaxy evolution. I will highlight some of the astrophysical consequences of contained hadronic and leptonic cosmic rays in and around galaxies, how their influence can be probed using signatures ranging from sub-mm to X-rays and gamma-rays, and the opportunities soon to open-up that will allow us to pin-down the multi-scale effects of cosmic rays in galaxies near and far. I will also discuss what happens to the cosmic rays that escape from galaxies, including their interactions with the magnetized large-scale structures of our Universe, and the fate of distant high-energy cosmic rays that do not reach us on Earth. 

Understanding the abundance patterns of metal-poor stars and the production of heavy elements through various nucleosynthesis processes offers crucial insights into the chemical evolution of the Milky Way. We investigate the origins of light, alpha, Fe-peak, and r-process elements in metal-poor halo stars using data from the R-Process Alliance observed by the Gran Telescopio Canarias (GTC). Our analysis of these faint stars reveals intriguing patterns. We utilize the abundances of carbon, Fe-peak elements and the alpha-elements to probe the contributions from different nucleosynthesis channels in the progenitor supernovae. Additionally, we identify globular cluster stars at very low metallicity, which adds to the growing evidence of a lower metallicity floor for GCs. We also reveal differences in the trends of the neutron-capture element abundances from the RPA data releases, which provide constraints on their nucleosynthesis sites and subsequent evolution. Complementing this, we use early data from the new dual-fibre high-resolution spectrograph GHOST at Gemini South to study globular clusters. We identify first and second-generation stars in the metal-poor globular cluster NGC 2298 through light element anti-correlation, obtaining precise abundances for over 45 elements, including 20 neutron-capture elements up to thorium. A larger dispersion in n-capture and Fe-peak elements is observed among first-generation stars, along with variations in the universal r-process pattern. Increases in Sr and Ba with Mg, significant trends in light, alpha, and Fe-peak abundances, and correlations between light and r-process abundances are noted. These findings enhance our understanding of the Milky Way's chemical evolution by integrating data on metal-poor stars and globular clusters, elucidating the nucleosynthetic pathways shaping our Galaxy's elemental composition.

Using only the speckle pattern of an unresolved source in the image-plane, it is possible to reconstruct the pupil-plane wavefront for use in an adaptive optics loop using Deep Neural Networks.  My thesis has involved exploring and refining this technique with applications from real time correction of the atmosphere for ELF telescopes, to sensing residual, non-common-path aberrations in coronagraphic instruments -- where dedicated WFS hardware becomes much more challenging.  I will present some background and results of both simulated as well as successful on-telescope testing.  Further, I may spend a bit of time on my recent exploration into using Deep Reinforcement Learning: its uses in bridging the sim-to-real gap, and for solving problems where exact solutions aren't available.

Seminarios Investigacion le está invitando a una reunión de Zoom programada.

Less than 30 years ago, we did not know whether planets exist outside our solar system. Fast forward to 2024, astronomers have discovered well over 5000 planets orbiting other stars similar to our Sun, including some that may have the right conditions to host life. As we learned that the formation of planets seems to go hand-in-hand with the birth of stars, we begin to wonder:

•     What happens to planetary systems when their host stars run out of fuel, and turn into Earth-sized white dwarfs?
•     Are those systems, if they exist, detectable?
•     What will happen to our solar system, and to the Earth?
•     And what are the possible implications for life?

I will discuss the late evolution of planetary systems, the observational fingerprints of planets and their debris orbiting white dwarfs, and how studying these exotic systems  improves our general understanding of the formation of planets.