Found 105 talks archived in Stars

Tuesday March 12, 2013
Dr. Nicolas Lodieu


The stellar spectroscopic sequence has now been extended into very cool objects bridging the gap between low-mass stars and classical planets. Those objects, called Y dwarfs, are the coolest substellar objects known to date with temperatures below 500 Kelvins. We obtained z-band far-red imaging for six Y dwarfs and a T9+Y0 binary with GTC/OSIRIS to characterise their spectral energy distribution. This photometric dataset represent the first optical detection of Y dwarfs. I will present the z-band photometry, optical-to-infrared colours, and proper motions of these Y dwarfs. I will discuss the larger dispersion in the optical-to-infrared colours of Y dwarfs than in warmer brown dwarfs, which may originate from presence of sulfide clouds, the depletion of alcalines, and/or gravity effects.

Wednesday March 6, 2013
Dr. Jesús Corral Santana


X-ray transients are binary systems composed by a 'normal' star which is transfering mass onto a compact object (either a black hole or a neutron star) through Roche lobe overflow. These systems show sporadic outburst episodes and long quiescence states, being ideal systems to search for stellar-mass black holes. Different studies predict a Galactic population of ~10^3-10^4 X-ray transients, however, there are only 18 stellar-mass black holes dynamically confirmed (and other ~32 candidates whichc share similar timing and spectral properties).

In this talk I'll present the case of Swift J1357.2-0933, a new X-ray transient discovered in 2011. Our analysis shows that Swift J1357.2-0933 is the first black hole transient seen at a large inclination (>75º). High time resolution lightcurves show dips or eclipses produced by a vertical structure present in the inner accretion rather than the companion star. Some dips display up to ~50% reduction of flux in ~2min (~30% reduction of flux in 7s). Moreover, the dips present a recurrence period of a few minutes which increases with time. This can only be explained by the expansion of the obscuring structure outward in the accretion. Swift J1357.2-0933 could be the prototype of an hytherto Galactic population of black hole transients with large inclinations.

Thursday February 21, 2013
Prof. Sally Oey
University of Michigan


The fate of ionizing radiation from massive stars has fundamental consequences on scales ranging from the physics of circumstellar disks to the ionization state of the entire universe. On galactic scales, the radiative feedback from massive stars is a major driver for the energetics and phase balance of the interstellar medium in star-forming galaxies. While even starburst galaxies appear to be largely optically thick in the Lyman continuum, ionization-parameter mapping shows that significant populations of HII regions within galaxies are optically thin, powering the diffuse, warm ionized medium. I will discuss our multi-faceted work to clarify our understanding of radiative feedback in star-forming galaxies from the Magellanic Clouds to starbursts.

Thursday January 31, 2013
Dr. Santi Cassisi
INAF - Osservatorio Astronomico di Teramo


In recent years accurate photometric and spectroscopic observations have
provided sound evidence that Galactic Globular Clusters can not be longer
considered the prototype of Simple Stellar Populations. In this talk we
present the most recent updates concerning the empirical evidence and
discuss the theoretical framework required for interpreting observations.
Current shortcomings in the interpretation of how the multiple stellar
populations formed in a given cluster are also discussed.

Thursday October 18, 2012
Dr. Miguel Cerviño


The basis of stellar population modeling was established around 40 years ago somehow
optimized to the technical facilities and observational data available at that epoch. Since then,
it has been used extensively in astronomy and there has been great improvements relating
their associated ingredients in concordance with the development of more powerful computational
and observational facilities.
However, there has been no similar improvements in the understanding about what is
actually modeling neither in improve the modeling itself to include the current technical advances
to obtain more accurate result in the physical inferences obtained from them.
In this talk I present some advances in the subject of stellar
population modeling and how to take advantage of current facilities to obtain more robust
and accurate inferences from stellar systems at different scales
covering the continuum between fully resolved populations to fully unresolved ones in a unified framework.

Tuesday October 9, 2012
Mr. Bartek Gauza


We report on the discovery of a fourth component in the HD 221356 star system, previously known to be formed by an F8V, slightly metal-poor primary ([Fe/H]= -0.26), and a distant M8V+L3V pair. In our ongoing common proper motion search based on VISTA Hemisphere Survey (VHS) and 2MASS catalogues, we have detected a faint (J = 13.76 ± 0.04 mag) co-moving companion of the F8 star located at a projected distance of ~312 AU. Near-infrared spectroscopy of the new companion indicates an L1±1 spectral type. Using evolutionary models the mass of the new companion is estimated at ~0.08 solar masses, which places the object close to the stellar-substellar borderline. This multiple system provides an interesting example of objects with masses slightly above and below the hydrogen burning mass limit.

Friday October 5, 2012
Dr. Jonay González Hernández


Type-Ia supernovae (SNIa) are believed to be thermonuclear explosions of accreting carbon-oxygen white dwarfs that reach the Chandrasekhar mass limit of about 1.39 solar masses. However, the nature of the companion star is still under debate, i.e. to be either a dwarf, a sub giant, or a giant star (single-degenerate channel), or another white dwarf (double-degenerate channel). Both channels have been proposed but their relative frequency remains unclear. We have been exploring regions close to the center of supernova remnants of Galactic SNe to search for the companion of these type-Ia SNe. I will show the very recent results we have found in two Galactic type-Ia SNe.

Tuesday May 29, 2012
Dr. Sebastián Ramírez Alegría
Instituto de Astrofísica de Canarias, Spain


I am going to present the new massive and obscured cluster Masgomas-1. This cluster, discovered by our group formed by astronomers from the IAC and CEFCA, is the
first candidate derived from the preliminary version of our systematic search algorithm for obscured (and young) massive clusters, and part of the MASGOMAS project  (MAssive Stars in Galactic Obscured MAssive clusterS). In this talk I will present the spectrophotometric observations obtained with LIRIS at William Herschel Telescope (ORM), which allowed the physical characterization of the massive stellar population of Masgomas-1, and the confirmation of the  cluster's massive nature
(i.e. Mcl > 10^4 Msun).

Thursday May 24, 2012
Dr. Miguel Urbaneja
Institute for Astronomy, University of Hawaii


Measuring distances to galaxies and determining their chemical compositions are two fundamental activities in modern extragalactic astronomy, in that they help characterizing the physical properties of their constituents and their evolutionary status. Ultimately, these measurements lead to stronger constraints on the cosmological parameters of an expanding universe and the history of cosmic chemical enrichment. Both these questions can be tackled afresh with the quantitative analysis of the absorption line spectra of individual massive and luminous, young B- and A-type supergiant stars. A spectroscopic distance determination method, the FGLR, can yield accurate distances up to several Mpc, extending to a local volume where the results can be compared with those obtained from Cepheids and other distance indicators. Moreover, and this being a unique advantage of the FGLR, reddening values and metallicities are simultaneously determined for each individual stellar target. These stellar metallicities are very accurate and can be used to constrain the formation and evolution of galaxies and to assess and overcome the systematic uncertainties of H II region strong-line abundances through a galaxy-by-galaxy comparison. Moreover, stellar spectroscopy provides fundamental complementary abundance information for star forming galaxies on additional atomic species such as iron-group elements. I will present recent results of our on-going efforts to study individual blue supergiant stars in galaxies within and beyond the Local Group based on medium and low resolution optical spectra collected with ESO VLT and the Keck telescopes. The promising perspectives of future work, based on the giant ground-based telescopes of the next generation (E-ELT, TMT) are also discussed.

Thursday May 17, 2012
Mr. Yuri Cavecchi
Universiteit van Amsterdam


I will review what we know about Type I Bursts (thermonuclear explosions on the surface of accreting Neutron Stars) and burst oscillations (fluctuations in the intensity of the burst lightcurves). I will describe the known problems in burst oscillation models and the various solutions that have been proposed. I will report recent progress made in the case of the pulsar IGR J17480-2446 in the Globular Cluster Terzan 5, where we were able to pin point the most likely mechanism responsible for the oscillations. I will explain whether this might be applicable to the other bursters and discuss future perspectives including current efforts to develop magneto-hydrodynamical simulations of the bursting process.

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