Recent Talks

KIC 8462852, also known as the Boyajian's star, was observed by the Kepler mission during 4 years. During that time, this star underwent several events of strong brightness variations that are unlike that of any other known astronomical object. Since 2017, an intense ground-based observing campaign has been under way to closely survey this target for further brightness variations. The coverage of several minor such variations from both multicolor photometry and spectrophotometry showed that these events have a color-signature. This observation is compatible with the hypothesis by Neslu?an & Budaj of a pool of dusty planetesimals that may occasionally pass close to the star. Their  optically thin dust shroud might then generate the observed colors. However, while current observations support this scenario, they do not prove it. Alternative explanations for the star’s behavior will be revised, and in particular, variations in the temperature of the stellar surface will be evaluated for their compatibility with the observations. Furthermore, an update on the observing campaign of Boyajian’s star will be given, possibly with input from new data acquired in the first half of 2018.

This paper puts forward a possible new indicator for the presence of moderately advanced civilizations on transiting exoplanets. The idea is to examine the region of space around a planet where potential geostationary or geosynchronous satellites would orbit (herafter, the Clarke exobelt). Civilizations with a high density of devices and/or space junk in that region, but otherwise similar to ours in terms of space technology (our working definition of “moderately advanced”), may leave a noticeable imprint on the light curve of the parent star. The main contribution to such signature comes from the exobelt edge, where its opacity is maximum due to geometrical projection. Numerical simulations have been conducted for a variety of possible scenarios. In some cases, a Clarke exobelt with a fraction alface-on opacity of ∼10^{-4} would be easily observable with existing instrumentation. Simulations of Clarke exobelts and natural rings are used to quantify how they can be distinguished by their light curve.

To appear in ApJ.

En esta charla hablaremos sobre el proceso de actualización
del telescopio IAC80 en los últimos años, con el objetivo de llevar a cabo
observaciones muy exigentes en cuanto a número de imágenes, precisión temporal y astrométrica.
Se presentará la nueva cámara de gran campo CAMELOT-2
y su interfaz inteligente de usuario, que permite una observación cuasi-robótica por medio de macros
automáticas. Por último, se presentará el diseño del nuevo sistema de control, base de una futura robotización.

In a framework where galaxies form hierarchically, extended stellar haloes are predicted to be an ubiquitous feature around Milky Way-like galaxies and to consist mainly of the shredded stellar component of smaller galactic systems. The type of accreted stellar systems are expected to vary according to the specific accretion and merging history of a given galaxy, and so is the fraction of stars formed in situ versus accreted. Analysis of the chemical properties of Milky Way halo stars out to large Galactocentric radii can provide important insights into the properties of the environment in which the stars that contributed to the build-up of different regions of the Milky Way stellar halo formed. In this talk I will first give an overview of some of the main properties of the Milky Way stellar halo based on literature studies. I will then present results concerning the chemical properties of the outer regions of the Milky Way stellar halo, based on the elemental abundances of halo stars with large present-day Galactocentric distances, >15 kpc. The data-set we acquired consists of high resolution HET/HRS, Magellan/MIKE and VLT/UVES spectra for 28 red giant branch stars covering a wide metallicity range, -3.1 ≲ [Fe/H] ≲-0.6. We show that the ratio of α-elements over Fe as a function of [Fe/H] for our sample of outer halo stars is not dissimilar from the pattern shown by MW halo stars from solar neighborhood samples. On the other hand, significant differences appear at [Fe/H] ≳-1.5 when considering chemical abundance ratios such as [Ba/Fe], [Na/Fe], [Ni/Fe], [Eu/Fe], [Ba/Y]. Qualitatively, this type of chemical abundance trends are observed in massive dwarf galaxies, such as Sagittarius and the Large Magellanic Cloud. This appears to suggest a larger contribution in the outer halo of stars formed in an environment with high initial star formation rate and already polluted by asymptotic giant branch stars with respect to inner halo samples. 

Galaxy groups and clusters are believed to influence galaxy evolution. It has been shown that the groups with early formation epoch, also known as fossil groups, differ in halo and IGM properties, compared to the general population of galaxy groups. However, there is a controversy over the properties of the brightest group galaxies which may have been affected by the group's dynamical state. I will focus on two properties of the brightest group galaxies, the AGN activity and the stellar population. The groups with early formation epoch, or dynamically old, host under luminous AGNs in radio relative to those hosted by dynamically young groups. There is no evidence that such a distinction exists in the stellar population of these galaxies, leaving the debate open whether this is an observational limitation. 

Molecular hydrogen (H2) is a fundamental component of galaxies, being the most abundant element in molecular clouds, where stars form, and an important source of radiative cooling at low temperature. With the advent of the ALMA telescope, a large amount of data about the distribution of H2 in galaxies has become available. However, the large majority of numerical simulations on galactic and cosmological scales still lacks the ability to directly follow the formation and dissociation of H2, and must rely on pre-calibrated sub-grid models to compare the results with observations. I will present a new model to self-consistently track the evolution of H2, including gas and dust shielding, H2 self-shielding, star formation (SF), supernova feedback, and extragalactic and local stellar radiation. I will discuss the results of a suite of hydrodynamic simulations of an isolated gas-rich galaxy at z=3, showing that the model can naturally reproduce the observed correlation between SF and H2 surface densities, without assuming any a priori dependence of SF on the H2 abundance. I will also present a study of the kinematics and dynamics of molecular gas in high-redshift quasars (z=6), where we investigate whether a central accreting black hole (BH) can significantly affect the H2 distribution in the host galaxy and generate molecular outflows.

Lecture 3 by George

More than 40 years ago, Skumanich (1972) showed how rotation and magnetic activity decreased with the age of a solar-like star. While this result was based on the study of young cluster stars, later observations  of other clusters, still younger than the Sun, agreed with this “gyrochronology” relationship.

With the high-quality photometric data collected by the Kepler mission, we have the opportunity to test and study the evolution of stellar dynamics to older field stars. While for clusters, the determination of stellar ages is eased by the fact that the stars were born from the same molecular cloud, it gets trickier and less precise for field stars. This is where asteroseismology plays an important role by providing more precise ages than any other classical methods.

In this talk I will mostly focus on asteroseismic targets from solar-like stars to red giants where we could measure surface rotation, core rotation, and magnetic activity. I will show how the photometric data of Kepler is providing key information in the understanding of angular momentum transport in stars and of magnetic activity at different evolutionary stages of a star like the Sun.

The most metal-poor stars in the Galaxy are relics from the first generations of star formation, and their properties can reveal key information about the formation and evolution of the Milky Way. However, only a small number of these extremely rare stars are currently known, due to the difficulty in finding them amongst the overwhelmingly more abundant stars of higher metallicity. In this talk, I will present the Pristine survey, a narrow-band photometric survey in the wavelength region around the Ca H&K absorption lines designed to efficiently search for extremely metal-poor (EMP) stars. In the first three years of the survey, we have covered ~2,500 square degrees of sky in the Northern hemisphere using the CFHT on Mauna Kea in Hawaii, as well as a sizeable spectroscopic follow-up sample using mostly the INT and WHT in La Palma. With this data, we have demonstrated success rates of 70% for finding stars with [Fe/H] < -2.5, and 22% for stars with [Fe/H] < -3.0. This represents a significant improvement upon previous searches for EMP stars, which have reported success rates of 3-4%. With this efficiency, the Pristine survey is poised to make a significant contribution to constraining the metal-poor tail of the metallicity distribution function, as well as increasing the number of known ultra metal-poor (UMP) stars in the literature. In addition, I will discuss how the Pristine survey is being used to characterise the faint dwarf galaxy population, and analyse substructure in the Galactic Halo.