Recent Talks

Gamma Ray Bursts (GRBs) are among the most energetic transient phenomena frequently followed up by different observatories and yet several fundamental questions are still open. Fermi and MAGIC are continuing their observations of GRBs since several years, giving highest priorities to the most interesting events. This effort led to remarkable discoveries in the High Energy regime, showing potential for even more meaningful achievements in the Very High Energy (VHE) regime. Enhanced follow up strategies of MAGIC and soon to come CTA Large Size Telescopes (LST) observations create unique opportunities for the detection of GRBs at VHE. In this talk I will give an overview of the high energy GRB properties as seen by Fermi and show the potential for the first VHE detection with MAGIC and CTA LSTs.

The identification and investigation of red supergiants (RSGs) in the Local Group and beyond are extremely important for understanding massive star evolution and mass-loss.  Star-forming dwarf irregular (dIrr) galaxies serve as ideal laboratories for investigating physics of red supergiants within the context of different metallicities of host galaxies. Also, RSGs may be used as tracers for abundance determinations and star formation history of dIrrs. I will present a systematic survey of RSGs and luminous blue variables (LBVs) in nearby dIrr galaxies with the goal to complete the census of these objects in the Local Group. Using the fact that RSGs and LBVs are bright in mid-infrared colors due to dust, we applied a technique that allows us to select dusty massive stars based on their [3.6] and [4.5] Spitzer photometry. I applied our criteria to 7 dIrr galaxies: Pegasus, Phoenix, Sextans A, Sextans B, WLM, IC 10 and IC 1613 selecting 124 point sources, which we observed with the VLT/FORS2, GTC/OSIRIS and duPont/WFCCD spectrographs. In total, we identified 28 RSGs (21 are new discoveries) and 2 new emission line objects in these galaxies. These new discoveries are statistically significant and this sample increased the number of spectroscopically confirmed RSGs in dIrrs by 50%. Moreover, for the newly identified RSGs we measured the fundamental physical parameters by fitting their observational spectral energy distributions with MARCS stellar atmosphere models. This work serves as a basis for further investigation (also in a framework of my activity in IAC) of the newly discovered dusty massive stars and their host galaxies.

After a basic introduction into asteroseismology for the non-expert, we emphasize how to achieve practical applications of the technique based on uninterrupted high-precision data from space. We show how series of detected and identified oscillation modes allow to deduce details of the interior physics of stars that are impossible to unravel in any other way. We highlight the most recent findings on the interior rotation and chemical mixing of stars with a convective core and illustrate how these affect stellar evolution theory. We reveal the power of combining Gaia and asteroseismic data for stellar physics, galactic archeology, and exoplanet studies. Finally, we provide an outlook for future projects in asteroseismology to illustrate the bright future of this research domain.

I give an overview of our spectroscopic work on the old open cluster M67 and what it may tell us about the origin of the Sun, the existence of terrestrial planets around solar twins and effects that change the surface composition of stars. I will argue that much remains to be learned from studies of stars in different environments (globular clusters, open clusters, associations).

The standard cosmological model has been established  and its parameters are now measured with unprecedented precision. However, there is a big difference between modelling and understanding.  The next decade  will see the era of large surveys; a large coordinated effort of the scientific community in the field  is on-going to map the cosmos producing an  exponentially growing amount of data. This will shrink the statistical errors. But precision is not enough: accuracy is also crucial.   Systematic effects may be in the data but may also be in the model used in their interpretation.  I will present a small selection of examples where I explore  approaches to help the  transition from precision to accurate cosmology. This selection is not meant to be exhaustive or representative, it just cover some of the problems I have been working on recently.

Time is one of the least explored dimensions of exoplanet research; most 
stars targeted by large surveys are middle-aged by  necessity or statistics. 
Yet the first few hundred million years of a planetary system are probably 
the most formative and include  accretion, migration, and escape of atmospheres.  
While the Kepler  prime field included a small number of young stars by chance, the 
K2  mission is deliberately selecting some target stars by age, and  previews the 
potential of TESS and PLATO.  The Zodical Exoplanets in  Time (ZEIT) project studies 
K2 systems in stellar clusters of established ages.  Transiting planets as small as 
Earth-size have been  detected in the Upper Scorpius, Pleiades, Hyades, and Praesepe 
clusters.  Mysterious aperiodic signals related to circumstellar disks  were found in 
Upper Scorpius, these may be related to planet  formation. We are also investigating 
planets around evolved stars and  report a Jupiter-mass planet inflated by irradiation 
from its host  star. Gaia distances, proper motions, and spectra can identify large numbers 
of young stars for observation by the TESS and PLATO missions,  enabling robust comparisons 
across a range of ages to understand  evolutionary trends, and select propitious targets for 
follow-up by  ELTs and space observatories such as JWST.

Red supergiant stars (RSGs) are among brightest stars in the Universe. Their extreme luminosities and young ages make them excellent candidates to probe young stellar populations of external galaxies. The problem is that their spectra are typically dominated by molecular absorption features, which makes a chemical analysis almost impossible.

Using state-of-the-art stellar model atmospheres I'll discuss how a new analysis method allows us to probe the chemical evolution of external galaxies using RSGs as abundance tracers. I'll detail why near-IR multi-object spectrographs are vital to this work, highlight some of our recent results and compare these results with other abundance indicators (i.e BSGs, HII regions). Finally, I'll outline the potential of this technique on future facilities such as JWST-NIRspec and a MOS on the E-ELT.

This talk shows an spectrophotometric analysis of HD 209458 and 18 A type stars. We have used absolute flux spectrophotometry obtained with the Hubble Space Telescope  and Kurucz model atmospheres to ascertain the angular diameters (precision of about 1%) and effective temperatures (similar or better than 1%). The same analysis also permits the determination of other stellar parameters such as metallicity and surface gravity. The A type stars were selected from the STIS Next Generation Spectral Library because they match our criteria that their absolute fluxes in the optical are reliable to within 3 per cent by comparing their HST spectra with Kurucz theoretical models. HD 209458 harbors the first transiting planet discovered and it is one of only two systems with absolute stellar and planetary masses derived from high-resolution spectroscopy. We updated the stellar radius by combining the angular diameter with the Gaia parallax and then the planetary radius with unprecedented precision. We have also determined the stellar parameters from stellar evolution models. Finally, our results are compared with those from the literature.

During the summer of 2013, a 4-month spectroscopic campaign took place at Teide IAC 80 and other professional and amateur observatories to observe the variabilities in the wind of the Wolf-Rayet star WR 134. The spectroscopic data have been analyzed to better understand its wind and long-term periodicity, which can now be interpreted as arising from corotating interaction regions (CIRs) in the wind coming from the hidden surface of the star.

We are currently conducting a survey of 25 Galactic globular clusters with MUSE, the panoramic integral-field spectrograph at the ESO VLT.

Although the observations are still ongoing, we already secured about one million spectra for more than 300000 individual stars. This is possible thanks to a novel method for the optimal extraction of stellar spectra, which in combination with MUSE provides unique multiplexing capabilities in crowded stellar fields.

In my talk, I want to highlight some of the science cases that we pursue with the MUSE data. I will put emphasis on the internal dynamics of the clusters. They are for example key to answer the question whether massive black holes reside in the centres of some clusters. Such intermediate-mass black holes would bridge the gap from stellar-mass black holes to the supermassive ones found in the bulges of galaxies. I will also touch upon other science cases, such as binary studies, chemical analyses, or the search for peculiar stars.