List of all the talks in the archive, sorted by date.
In symbiotic stars, two different physical regimes of circumstellar
material exist side by side. Around the donor red giant star, there is a
cool and dense conical region of neutral wind. During quiescent phases,
the rest of the wind from the donor is ionized by its companion, in most
cases, a very hot and luminous white dwarf powered by accretion from the
giant's wind. Mass outflow from the majority of cool components in
symbiotic binaries is still not understood well. Some information about
the distribution of circumstellar matter can be obtained by measuring the
neutral hydrogen column densities from Rayleigh scattering along the
multiple lines of sight. I will present the wind velocity profiles derived
from the measured column densities of neutral hydrogen for two quiet
high-inclination symbiotic systems, EG And and SY Mus. The column density
models indicate the wind focusing towards the orbital plane and allow to
investigate the origin of the asymmetric UV continuum light curve profiles
of symbiotic stars.
A new era of observational surveys that are both deep and wide is poised to revolutionise our understanding of galaxy evolution, by enabling, for the first time, statistical studies of the low-surface-brightness (LSB) Universe. While largely inaccessible in past wide-area surveys like the SDSS (due to their lack of depth), the uncharted LSB regime holds the key to a complete understanding of galaxy evolution. While small, deep surveys and new instruments have long hinted at the existence of a rich population of LSB galaxies below the surface-brightness limits of surveys like the SDSS, the mechanisms that create these galaxies remain unexplored. We use, Horizon-AGN, a cosmological hydrodynamical simulation to study how and why low-surface-brightness galaxies (LSBGs; mu > 23 mag arcsec^-2), and in particular, the recently studied population of ultra-diffuse galaxies, form and evolve over time. For stellar masses greater than 10^7 MSun, LSBGs contribute 85, 10 and 11 per cent of the local number, mass and luminosity densities respectively. When controlled for stellar mass, today's LSBGs have similar dark-matter fractions and angular momenta to their high-surface-brightness (HSB) counterparts but larger (x 2.5) effective radii and lower (< 5% vs 30%) star-forming gas fractions. Interestingly, LSBGs originate from the same progenitors as HSB systems at high redshift (z~3). However, LSBG progenitors form stars more rapidly at early epochs. The higher resultant supernova energy injection flattens their gas-density profiles which, in turn, creates shallow stellar profiles that are more susceptible to tidal processes. After z~1, harassment and tidal heating steadily expand LSBG stellar distributions and quench star formation by heating cold gas, creating the population of diffuse, gas-poor LSB systems seen today. In clusters, ram-pressure stripping provides an additional mechanism that assists in gas removal in LSBG progenitors. The study of LSBGs will be one of most exciting advances in galaxy evolution in the coming years. This study offers insights into the demographics and properties of a population of galaxies that will have a transformational impact on our understanding of galaxy evolution.
I will present a story of how a chance observing run kicked off more than a decade of exploration of the dynamics of the Magellanic Clouds, leading to the discovery, from line-of-sight velocities and Ca triplet abundances, that ~5% of the stars in the inner LMC actually appear to belong to the SMC. The existence of this debris agrees well with a scenario in which the Clouds collided directly with each other, and provides a natural explanation for the star formation activity in 30 Doradus and the LMC microlensing signal, and may be linked with star clusters with multiple populations. I will show how Gaia DR2 resolves the ambiguity present in our line-of-sight velocity data and allows us to consider the geometry of the debris. These results were motivation for the SMASH survey, with which we are mapping the debris from the LMC/SMC interaction, exploring their star formation histories, and have discovered new structures around the Clouds and potential companion dwarf galaxies.
Understanding formation and evolution of galaxies on the galactic and sub-galactic scales is a key question to modern astrophysics. The L-CDM concordant cosmology paradygm, sucessful in predicting many large scale observables of the Universe, starts to fail at the galactic or sub-galactic scales (e.g., missing satellites problems, planes of satellites, central dark matter density profiles of galaxies, etc.). The Milky Way, with its system of dwarf galaxy satelites, is the environment in which we can hope to constrain in most details the physical processes that play a role in the formation and evolution of galaxies, encoded in the location, kinematics and chemistry of individual stars, a field often referred to as Galactic Archaeology. Taking the example of the Sculptor dwarf galaxy, for which a wealth of complementary data are available, from wide field photometry to sizeable spectroscopic samples, and now also astrometric Gaia data, I will discuss our current observational understanding of how chemical enrichment proceeds at the smalest scales.
In the context of the Gaia space mission and ground based large spectroscopic surveys such as WEAVE@WHT, Galactic Archaeology, is living a revolution. I will review some of the most prominent science cases for a Galactic Archaeology survey with the WEAVE wide field multi-object facility for the WHT, and highlight how this complements the Gaia astrometric mission.
By providing information on distances and proper motions for one billion stars, the Gaia satellite allows us to investigate the major unsolved challenges in galaxy formation: the nature of dark matter, the origin of Galactic spiral activity and its relation to the bar, and more generally the history of the Milky Way.
My research aims to develop a theoretical approach to modeling and exploiting the big data and address problems at the forefront of Galactic Dynamics at various scales. What is the origin of the spiral activity in the Milky Way? How are all of these perturbations to the structure of the Galaxy coupled to each other directly and through the dark-matter halo? I will also present my ongoing work on statistical techniques of big-data analysis and advanced numerical simulations used to interpret the evolution of star clusters and discover streams in the stellar disk of the Milky Way.
Principios del siglo XXI, en una fecha por determinar. Una desconocida deja en la biblioteca del IAC siete cuadernos de observaciones astronómicas realizadas más de cien años antes (1886 – 1891), en Madrid y Santa Cruz de Tenerife, por un tal Juan Valderrama y Aguilar (JVyA). Los cuadernos duermen en la biblioteca otros diez años hasta que los descubrimos por casualidad. Nos sorprende la calidad y meticulosidad de los dibujos que ilustran los cuadernos, el hecho de que trabajos de Juan Valderrama son citados en el ADS (Astrophysics Data System) y, sobre todo, el que siendo nosotros astrónomos profesionales del IAC, jamás hubiéramos oído hablar de él. ¿Podría haber vivido en Santa Cruz un astrónomo del que la historia oficial de la astronomía Canaria no tuviera noticia alguna? La respuesta a la pregunta anterior es "sí". JVyA es un completo desconocido para la historia oficial, pero tiene una biografía científica digna de ser recordada. Fuera de los ambientes universitarios y de la ciencia oficial, JVyA estaba, sin embargo, bien conectado con el mundo de la astronomía moderna de la época. Suyo es el que puede ser considerado el primer artículo de astronomía publicado por un canario en una revista internacional (Valderrama, 1886, L’Astronomie, Vol. 5, Pág. 388), y lo hace cuando tiene sólo 17 años. Hemos escrito una breve biografía de JVyA que reivindica su figura de astrónomo, y que será publicado por el Cabildo de Tenerife. En la charla queremos presentar a JVyA así como alguna de sus contribuciones astronómicas.
I will review the properties of nearby early-type galaxies and galaxy clusters showing the most interesting observed parallelisms, such as the Sersic behavior of the light profiles, the color-magnitude diagram and the main scaling relations. I will show a comparison with the data extracted from the ILLUSTRIS simulation and present the constraints to the star formation history that are necessary to obtain a reliable distribution of galaxies in the color-magnitude diagram.
The very metal-poor (VMP; [Fe/H] < –2.0) and extremely metal-poor (EMP; [Fe/H] < –3.0) stars provide a direct view of Galactic chemical and dynamical evolution; detailed spectroscopic studies of these objects are the best way to identify and distinguish between various scenarios for the enrichment of early star-forming gas clouds soon after the Big Bang. It has been recognized that a large fraction of VMP (15-20%) and EMP stars (30-40%) possess significant over-abundances of carbon relative to iron, [C/Fe] > +0.7. This fraction rises to at least 80% for stars with [Fe/H] < –4.0. Recent studies show that the majority of CEMP stars with [Fe/H] < –3.0 belong to the CEMP-no sub-class, characterized by the lack of strong enhancements in the neutron-capture elements (e.g., [Ba/Fe] < 0.0). The brightest EMP star in the sky, BD+44:493, with [Fe/H] = –3.8 and V = 9.1, is a CEMP-no star. It shares a common elemental-abundance signature with the recently discovered CEMP-no star having [Fe/H] < –7.8. The distinctive CEMP-no pattern has also been identified in high-z damped Lyman-alpha systems, and is common among stars in the ultra-faint dwarf spheroidal galaxies, such as SEGUE-1. These observations suggest that CEMP-no stars exhibit the nucleosynthesis products of the VERY first generation of stars. We discuss the multiple lines of evidence that support this hypothesis, and describe current efforts to identify the nature of the massive stellar progenitors that produced these signatures.
Turbulent convection in stellar envelopes is critical to heat transport and dynamo activity. Modeling it well has proven surprisingly difficult, and recent solar and stellar observations have raised questions about our understanding of the dynamics of both the deep solar convection and the mean structure of the upper layers of convective stellar envelopes. In particular, the amplitude of low wavenumber convective motions in both local area radiative magnetohydrodynamic and global spherical shell magnetohydrodynamic simulations of the Sun appear to be too high. In global simulations this results in weaker than needed rotational constraint and consequent non solar-like differential rotation profiles. In deep local area simulations it yields strong horizontal flows in the photosphere on scales much larger than the observed supergranulation, leaving the origin of the solar supergranular scale enigmatic. The problem is not confined to the Sun. When comparing computed oscillation frequencies to observations, mixing length models of stellar convection show too sharp a transition to the interior adiabatic gradient. This contributes to what asteroseismologists call the `surface effect' correction.
We suggest that there is a common solution to these problems: convective motions in stellar envelopes are even more nonlocal than numerical models suggest. Small scale photospherically driven motions dominate convective transport even at depth, descending through a very nearly adiabatic or possible even somewhat subadiabatic deep convection zone. Convection of this form may meet Rossby number constraints set by global scale motions, and implies that the solar supergranulation is the largest buoyantly driven scale of motion in the Sun. We test this hypothesis using a suite of three-dimensional stellar atmosphere models, and can use it to both recover their mean stratification and estimate the supergranular scale on other stars
Charla divulgativa sobre cultura preventiva en el IAC
- Big data, Big responsability: reproducible, archivable and branchable pipelinesDr. Mohammad Akhlaghi, Raúl Infante-SainzThursday February 20, 2020 - 10:30 (Aula)
- Observing magnetic activity with giant stars: between magnetic braking and revivalProf. Klaus-Peter SchroederFriday February 21, 2020 - 10:30 (Aula)