Found 108 talks archived in Stars

Wednesday April 9, 2014
Dr. Nicolas Fabas


To study the extended atmosphere of evolved stars such as Mira-type variables, spectropolarimetry is an innovative tool. For many kinds of stars, it has been used to measure global magnetic fields through circular polarization and the Zeeman effect. However, linear polarization has seldom been used in the past years even though phenomena such as scattering and the Hanle effect can definitely be studied as well, as it is done in solar physics. In this presentation, I am going to describe original results coming from a spectropolarimetric survey of Mira stars with NARVAL@TBL. Such results concern spectral lines like the Balmer lines of hydrogen and calcium lines. More specifically, I will focus on linear polarization and link this polarization to the propagation of the hypersonic radiative shock wave which is typical of Miras' atmospheres. In general, these environements are very dynamical and scattering in an aspherical atmosphere and velocity gradients can induce a strong linear polarization, likely to be further affected by weak magnetic fields. This analysis is very inspired of what is already done with solar spectra. In addition to that, I am going to present exclusive results about the first detection of a surface magnetic field in a Mira star and explain how the shock wave can impact this field. This work is likely to lead to collaborations with other disciplines such as interferometry (geometry of the scattering environement and characterization of the shock) and radio-astronomy (study of the polarization of masers).

Tuesday April 1, 2014
Dr. N. Kameswara Rao
Indian Institute of Astrophysics


R Coronae Borealis (RCB) stars are the more prominent group  of high luminosity hydrogen deficient stars that are rich in carbon  and helium. They also show characteristic irregular light drops of  several magnitudes (between 3 and 8 magnitudes) at unpredictable  times, caused by expulsion of self-made clouds of dust. They range in 
surface temperatures from 4500 K to  20000 K. Some of them seem to  have made even such complex molecules like fullerenes (C60) in their  circumstellar regions. Neither their evolutionary history nor the dust 
formation mechanism are well understood. Two scenarios that have been  suggested are that the present stars are a result of merger of two  white dwarfs (CO+He) or a post born-again (AGB) giant that is  surviving after a final helium shell flash. The talk would describe  the RCB properties and highlight the problems and challenges they pose 
in understanding their origins and dust production.

Tuesday March 25, 2014
Dr. Pieter Degroote
Institute for Astronomy- KU Leuven


The theory of stellar evolution is well developed over the past decades, and in particular the predictions of one dimensional numerical models have passed basic observational tests. With the advent of high precision astronomical observations, these tests can now be improved to fine tune the physics of the models. In particular, the combination of exploiting binary properties with the information obtained from asteroseismology, proves to provide a promising test framework. However, both binarity and seismology increase the complexity of the observational models and their relation to the stellar evolutionary model, and therefore require as many independent tests as possible.

Tuesday February 11, 2014
Dr. Jorge Casares


Stellar-mass black holes have all been discovered through X-ray emission, which arises from the accretion of gas from their binary companions. Currently known black holes are fed by material stripped from a low-mass star or by the wind of a massive companion. Binary evolution models also predict the existence of black holes accreting from the equatorial envelope of rapidly spinning Be-type stars. However, among the ~80 Be X-ray binaries known in the Galaxy (~150 including the Magellanic Clouds), only pulsating neutron stars have been found as companions, which is known as the missing Be/black-hole X-ray binary problem. In this talk I present the first dynamical evidence for a 3.8-6.9 Msun black hole orbiting the Be star and gamma-ray candidate MWC 656 (=AGL J2241+4454). This discovery has been allowed by the detection of a HeII emission line from an accretion disc encircling the black hole. We find the black hole is X-ray quiescent with Lx<1.6 × 10−7 times the Eddington luminosity. This implies that Be binaries with black-hole companions are difficult to detect by conventional X-ray surveys and may be more abundant than predicted by population synthesis models.


Thursday November 28, 2013
Prof. Michiel van der Klis
Astronomical Institute, University Amsterdam, the Netherlands


The matter within a few Schwarzschild radii of accreting neutron stars and black holes is moving under the influence of a strong gravitational field, and, in stellar mass compact objects, through strongly curved spacetime. The X-rays emitted in the accretion process can be used to diagnose this motion, using both spectroscopy and rapid time variability. Similarly, X-rays emitted from the surface of accreting neutron stars can be used to diagnose neutron star mass, radius and even internal structure. I discuss these ways to probe strong gravitational fields and ultradense matter from an empirical perspective and in the context of proposed future X-ray observatories, in particular, LOFT.

Wednesday November 13, 2013
Prof. Rudolf Kudritzki
Institute for Astronomy, University of Hawaii at Manoa


The determination of chemical composition and distances of galaxies is crucial for constraining the theory of galaxy formation and evolution in a dark energy and cold dark matter dominated universe. However, the standard technique using HII regions to determine the metallicity of star forming galaxies, nearby and at high redshift, is subject to large systematic uncertainties that are poorly understood and the determinination of accurate distances using Cepheids suffers from uncertainties caused by the metallicity dependence of the period luminosity relationship and extinction and crowding corrections. Multi-object spectroscopy of blue and red supergiant stars - the brightest stars in the universe at visual and NIR wavelengths - provides an attractive alternative. I will present results accumulated over recent years for galaxies in the Local Group and beyond out to a distance of 8 Mpc and will discuss the potential of future work with TMT and E-ELT. Combining the photon collecting power of these next generation telescopes with Adaptive Optics we will be able to study individual supergiant stars in galaxies as distant as the Coma cluster. With spectroscopy of the integrated light of young very massive Star Super Clusters and simple population synthesis techniques we can reach out ten times further.

Tuesday November 5, 2013
Dr. ZengHua Zhang



Studying the Halo via Its NEarby low-mass Dwarfs (SHINED) is an ongoing project aiming to understand the Galactic halo with metal-deficient low-mass stars (LMS) and brown dwarfs (BD). At the first stage of the project we are focusing on the discovery and characterisation of halo LMS and BD. I give an overview of the SHINED project then brief discuss the proprieties of halo LMS and BD. I introduce the main results of our work on red subdwarf binaries (Zhang et al. MNRAS, 434, 1005). I talk more about our work on the search of L subdwarfs from SDSS and UKIDSS. I discuss the spectral types of L subdwarfs, and spectral signature of halo BD. I also discuss the location of the Substellar Subdwarf Gap predicted by low-mass evolutionary models.

Thursday October 17, 2013
Dr. B-G Anderson
SOFIA Science Center-USRA


The Stratospheric Observatory for Infrared Astronomy (SOFIA) is now performing scientific observations and the results of the second open observing cycle solicitation is about to be announced.  With an available wavelength coverage from the visual to sub-mm wavelengths and a long life time - including planned instrument upgrades, SOFIA will provide critical resource for the astronomical community for the next decade and beyond.  Current and expected SOFIA instruments provide heterodyne spectroscopy in the THz band, including the line of [O I], [C II] and [N II] as well as OH, HD and many other hydrides, at high spectral resolution.  Echelle spectroscopy in the Mid-infrared (MIR) which will allow observations of e.g. fine-structure lines of and H2 pure rotational lines.  These will help address questions of interstellar chemistry and physics in star forming regions, PDRs and galaxies.  Mid-infrared (MIR) grism spectroscopy, of e.g. dust and ices, can be used to address questions of the freeze-out of molecules from the gas phase to better understand the formation, destruction and characteristics of interstellar ices.  Imaging in the MIR and FIR and FIR polarimetry can provide a more complete picture of the temperature, density and magnetic field structure of e.g. star forming cores. I will highlight the current and expected capabilities of SOFIA and some of the early science results achieved.

Thursday September 19, 2013
Dr. Pere Munar
Departamento de Astronomia y Meteorologia, Universidad de Barcelona


In the last years star-forming regions and massive protostars have been suggested to be gamma-ray emitters. Isolated massive protostars present powerful outflows interacting with the surrounding medium. Some of these sources power non-thermal radio jets, indicative of particle acceleration up to relativistic energies. At the jet-termination region strong shocks form which can lead to gamma-ray emission, as theoretical models predict. It has also been prognosticated that the combined effect of several low-mass protostellar objects may produce significant amount of gamma rays. We present here two studies: IRAS 16547- 4247, an isolated protostar showing non-thermal radio emission; and Monoceros R2, a star forming region coincident with a source of the 2nd Fermi-LAT catalog. In the first case, we analized archival X-ray data and detected the source. We also studied the system in a broad- band one-zone model context and tried to fit the X-ray detection with a non-thermal model. In the second case, we analyzed 3.5 years of Fermi-LAT data and confirmed the source with a detection above 12 sigma. Our results are compatible with the source being the result the combined effect of multiple young stellar objects in Monoceros R2.

Tuesday June 25, 2013
Dr. Elena Zaninoni
Astronomical Observatory of Brera, INAF


Gamma-Ray Bursts (GRBs) are the most powerful sources of electromagnetic radiation in the Universe. There are many open questions about their origin and their nature, and the answers should be searched in the large amount of data collected during these last years. We focused on the study of the their X-ray and optical afterglow properties, as observed by the Swift X-Ray Telescope (XRT) and ground-based optical telescopes. We investigated the observer and rest-frame properties of all GRBs observed by Swift between December 2004 and December 2010 with spectroscopic redshift through a comprehensive statistical analysis of the XRT light-curves of GRBs carried out in a model-independent way. We found out a three parameter correlation that is followed both by short and long GRBs. We also carried out a systematic analysis of the optical data available in literature for the same GRBs to investigate the GRB emission mechanisms and to study their environment properties. Our analysis shows that the gas-to-dust ratios of GRBs are larger than the values calculated for the Milky Way, the Large Magellanic Cloud, and the Small Magellanic Cloud. In this talk I will show the major results of the analysis of this large set of data.

Upcoming talks

  • TBD
    Donaji Esparza Arredondo
    Tuesday September 17, 2019 - 12:30  (Aula)
    Prof. Michael Kramer
    Thursday October 3, 2019 - 10:30  (Aula)

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