Found 108 talks archived in Stars
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
- The First Black HolesProf. Andrea FerraraThursday October 18, 2018 - 10:30 (Aula)
- Problems with the dark matter and dark energy hypotheses, and alternative ideasDr. Martin Lopez CorredoiraThursday October 25, 2018 - 10:30 (Aula)