Found 105 talks archived in Stars

Video
Tuesday February 11, 2014
Dr. Jorge Casares
IAC

Abstract

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.

 






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

Abstract

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.


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

Abstract

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.


Video
Tuesday November 5, 2013
Dr. ZengHua Zhang
IAC

Abstract

 

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.

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

Abstract

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.


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

Abstract

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.


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

Abstract

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.


Video
Friday May 31, 2013
Dr. Steve Boudreault
IAC

Abstract

Fundamental properties of brown dwarfs, such as luminosity and effective temperature, evolve with age. Large samples of spectroscopically-confirmed substellar objects with well-determined ages and distances are needed to constrain those parameters. We are embarked in a spectroscopic follow-up with GTC/OSIRIS of low-mass member candidates selected in several open clusters to constrain their membership. Here I will present the first L dwarf member in Praesepe confirmed by photometry, astrometry, and spectroscopy. We derived an optical spectral type of L0.3+/-0.4 and a mass placing it at the hydrogen-burning boundary. Considering the measured equivalent width of the gravity-sensitive sodium doublet, and the derived membership probability of ~80% or higher, we conclude that this object is likely to be a true member of Praesepe, with evidence of being a binary system.


Video
Thursday May 16, 2013
Dr. Yakiv Pavlenko
Main Astronomical Observatory of the National Academy of Sciences of Ukraine

Abstract

We discuss the role and significance of molecules in the modern astrophysics. Molecular opacities govern the structure of model atmospheres of late-type stars and ultracool dwarfs. Some problems of computations of model atmosphere and synthetical spectra of cool stars are discussed. We present some successful attempts of the application of the molecular spectroscpy for the studies of late -type stars and ultracool dwarfs. Finally, some problems of fitting theoretical spectra to the observed SED are discussed.


Video
Tuesday April 9, 2013
Dr. Teodoro Muñoz Darias
University of Southampton

Abstract

X-ray observations performed by several missions during the last few decades have provided a very large data base on black hole X-ray binaries. Many of these objects are transient systems that spend most part of their lives in quiescence, showing occasional outburst where their luminosity increases up to eight orders of magnitude. I will review the state-of-the-art in the field, focussing on the different accretion regimes observed in these sources. In the second part of the talk I will concentrate on the influence that the orbital inclination (i.e., viewing angle) has in the spectral properties of black hole binaries, with emphasis on the detection of relativistic effects in the inner accretion flow surrounding the black hole.



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