Recent Talks

Divulgation Lecture to Celebrate the International Year of Light and 150th Anniversary of Maxwell´s great paper on Electromagnetism of 1865.

Evidence is mounting for the presence of complex low surface brightness structures in the outer regions of galaxies. While the most spectacular examples are provided by systems hosting coherent debris streams, the most common examples may be extremely diffuse stellar envelopes. Wide-field imagers on large telescopes are allowing us to quantitatively explore the resolved stellar populations in these components within and well beyond the Local Group. I will highlight some recent  results from our work and discuss the insight these outer structures provide on understanding massive  galaxy assembly.  I will also discuss how we are using deep HST studies of M31's outer regions to probe its evolutionary history in unprecedented detail.

( * Please bring your laptop or your smartphone with you * )

Astronomers are not only skilled computer users, but also experienced Internet users. Their work could benefit from latest improvement in the world of the web. In particular, new client-side, server-side, and database technologies can be used to set up:

1) extremely performing data mining applications;

2) multi-thread CPU/GPU simulation or reduction software;

3) efficient ways to communicate results or to exchange data;

4) modern web-based tools for astronomic research and outreach.

After a quick introduction and a basic history of the evolution of web technologies during these last 15 years, we will present interactive examples of the above-mentioned technologies, focusing on client-side innovations.

Progress in our understanding of the Solar corona and its underlying heating and acceleration processes, depends critically on our ability to measure fundamental plasma parameters, such as magnetic field, density, temperature and composition. In this talk, I will introduce my main research topics which concern measurement of fundamental plasma parameters in the Solar corona and the development of new plasma diagnostic tools, in order to provide constraints for the various proposed physical mechanisms.

Coronal observations are always integrated along a line of sight (LOS). Because there may be multiple emitting sources, this considerably complicates the interpretation of the observations. To avoid this ambiguity there are several tools, including the widely used Differential Emission Measure (DEM) analysis and the tomography reconstruction technique. However, both the derivation and the interpretation of the DEM from observations are difficult mainly due to the inverse nature of the problem. I will present a new strategy to evaluate the robustness of the DEM inversion problem. An application of the DEM formalism will be presented, allowing us to measure the relative abundances in both interplumes and plumes using Hinode/Extreme Ultraviolet Imaging Spectrometer (EIS) data. Finally, I will present the inversion code I developed, able to perform the coupling of the DEM formalism and the tomography, providing a three dimensional diagnostic in temperature and density of the solar corona.

The lower solar atmosphere is very weakly ionized, and by conductivity it is comparable to the sea water. The collisional frequency for electrons and ions can be over 10^10 Hz and 10^9 Hz, respectively. This implies that particles may not be magnetized and are thus unaffected by the magnetic field. In this talk I shall present accurate collision cross sections and collision frequencies for electrons, protons and hydrogen atoms, and the corresponding transport coefficients for layers with both unmagnetized and magnetized particles. The cross sections include many essential effects like charge exchange, quantum-mechanical in-distinguishability at low energies, polarization of neutral atoms by external charges, and dependence on energy of colliding particles. The effects of collisions on Alfven waves will also be discussed.

I will report on the results of our paper published in Nature this week, outlining the discovery of a super-Chandrasekhar double-degenerate binary system at the heart of the planetary nebula Hen 2-428.  Planetary nebulae (PNe) represent the final stage in the evolution of low- and intermediate-mass stars, forming from the mass ejected by the star during its AGB evolution before being ionised by the star's, now exposed, core.  As binarity is expected to play a key role in the formation of aspherical PN morphologies, we have been intensively searching for new binary central stars in a push towards a statistical sample.  One of our newly-discovered binary systems, lying at the heart of Hen 2-428, had a further surprise to reveal, with observations and modelling showing the system to consist of twin evolved stars with a total mass greater than the Chandrasekhar limit.  The short period of the system, only 4.2 hours, means that the two stars will merge together in approximately 700 Myr, resulting in a Supernova Type Ia.  While the super-Chandrasekhar merger of two white dwarfs has long been considered a formation pathway for SN Ia, this is the first system found that is confirmed to be both massive enough and in a tight enough orbit to merge in less than a Hubble time.

Detection and characterisation of weak periodic signals from noisy time series is a common problem in many different fields of astrophysics. Here I detail one approach for testing whether a signal with roughly known characteristics exists in the data, using a search of secondary eclipses from Kepler-observed photometric time series as an example. The method is based on Bayesian model selection and uses Gaussian processes to model the stochastic variability in the data in non-parametric fashion.

The solar abundance of chemical elements play an important role in addressing such important issues as the formation, structure, and evolution of the Sun and the solar system, the origin of the chemical elements, the evolution of stars and galaxies. Despite the large number of papers published on this issue, debates about the solar composition of the Sun continue. In this talk we start summarizing the current understanding of the solar abundances of iron and CNO elements, which play a crucial role on the determination of the solar metallicity. We then pay especial attention to the impact of the quiet Sun magnetism on the determination of the abundances of these elements. The solar photosphere is significantly magnetized, due to the ubiquitous presence of a small-scale magnetic field whose mean strength is thought to be of the order of 100 gauss. Here we address the problem of the determination of the abundances of chemical elements taking into account the significant magnetization of the quiet Sun photosphere. To this end, we use 3D models of the quiet solar photosphere resulting from a state-of-the-art magneto-convection simulation with small-scale dynamo action where the net magnetic flux is zero. We conclude that if the magnetism of the quiet solar photosphere is mainly produced by a small-scale dynamo,then its impact on the determination of the solar abundance of iron and CNO elements is negligible.

Understanding the atmospheric and evolutive properties of very low mass stars, brown dwarfs, and gas giant exoplanets have been important challenges for modelers around the world since the discovery of the first brown dwarfs in the Pleiades cluster (Rebolo et al. 1995) and in the field (Nakajima et al. 1995). The early studies of brown dwarfs have provided rich insights into atmospheric physics, with discoveries ranging from cloud formation (Tsuji et al. 1996), methane bands (Oppenheimer et al. 1995) and ammonia bands (Delorme et al. 2008), to the formation of wasi-molecular KI-H2 absorption (Allard et al. 2007), and to disequilibrium chemistry (Yelle & Griffith 2001). New classical 1D models yield spectral energy distribution (SED) that match relatively well despite these complexities. These models have for instance explained the spectral transition from M to L, T and now Y brown dwarf spectral types (Allard et al. 2013). However, in presence of surface inhomogeneities revealed recently for a nearby (2 pc) brown dwarf (Crossfield et al. 2014), the SED may well fit even exactly, but the model parameters could be far from exact, e.g. with the effective temperature by several hundred kelvins too cool in the case of dusty brown dwarfs and young gas giant exoplanets! I will review the progress achieved in reproducing the spectral properties of very low mass stars, brown dwarfs and gas giant exoplanets, and review progress in modeling more accurately their atmospheres using Radiation HydroDynamical (RHD) simulations.