Seminars and Colloquia at ESO Santiago

Be stars are rapidly rotating stars with circumstellar envelopes in the form of equatorial disks. The close and bright B8Ve star beta CMi represents a perfect laboratory to study the detailed structure of a stable, unperturbed disk which is not truncated by a close binary companion. The NLTE Monte Carlo radiative transfer code HDUST is used to model the SED from UV up to cm wavelengths along with visual spectroscopy, polarimetry and near-IR interferometry. Special focus is put on testing the viscous decretion disk (VDD) scenario in the outer parts of the disk, which are observed in the far-IR and radio wavelengths. With the available data we are able to put constrains on the physical extent of the disk for the first time.

We present optical to near-infrared transmission spectra of HAT-P-1b and WASP-6b, part of a Large HST/STIS hot Jupiter transmission spectral survey (P.I. David Sing). The spectra for each target cover the regimes 2900−5700Å and 5240−10270Å, with resolving power of R = 500. The HAT-P-1b data is coupled with a recent HST/WFC3 transit, spanning the wavelength range 1.087-1.687microns (R=130), acquired in spatial scan mode. The WASP-6b data is complemented with Spritzer/IRAC 3.6 and 4.5 micron transit observations, part of a comparative exoplanetology program (P.I. Jean-Michel Desert). The transmission spectrum of HAT-P-1b shows a strong absorption signature shortward of 5500Å, with a strong blueward slope into the near-UV. We detect atmospheric sodium absorption at a 3.3σ significance level, but see no evidence for the potassium feature. The red data implies a marginally flat spectrum with a tentative absorption enhancement at wavelength longer than ~8500Å. The combined STIS and WFC3 optical to NIR spectra differ significantly in absolute radius level (4.3+/-1.6 pressure scale heights), implying strong optical absorption in the atmosphere of HAT-P-1b. The optical to near-infrared difference cannot be explained by stellar activity, as simultaneous stellar activity monitoring of the G0V HAT-P-1b host star and its identical companion show no significant activity that could explain the result. The transmission spectrum of WASP-6b shows evidence for strong Daylight scattering with tentative detection of sodium (~1σ) and potassium (~2σ). We compare both spectra with theoretical models including cloud-free, haze dominated and models with extra optical absorbers. We find that both an optical absorber and a super-solar sodium to water abundance ratio might be a scenario explaining the HAT-P-1b observations. Both Rayleigh scattering and a cloud deck in the optical might be possible scenarios for the atmosphere of WASP-6b. Our results suggest that strong optical absorbers/hazes may be a dominant atmospheric features shaping the transmission spectra in some hot Jupiter exoplanets.

In recent years, strong evidence for starburst-like star formation close to the base of the Scutum arm has emerged. The birth of several massive young clusters and a spatially-extended associated population of red supergiants is thought to have been triggered by the interaction of the Long Galactic Bar, whose near tip is found along this line of sight, with the spiral arm. If this view is correct, the opposite side of the Bar should be giving rise to similarly vigorous star formation. Unfortunately, the location of the far end of the Galactic Bar is unsure, with different models making quite diverse predictions. I will report on the identification of two clusters of red supergiants towards galactic longitude 350 deg, with kinematic and photometric distances from the Sun around 10 kpc. One of the clusters is likely to have a mass above 20000 Msun, based on the presence of a large number of blue and red supergiants. The other cluster is surrounded by a large number of red supergiants with identical radial velocity, pointing to the existence of a rather extended star-forming region that could mirror the Scutum arm Complex. The clusters are seen through a gap in the extinction that will allow for an accurate determination of the extinction law towards the inner Milky Way.

I will review the discovery of significant structure in the diffuse interstellar medium (ISM) on scales down to ~10 AU (10^-4 pc). This contradicts standard theories of the ISM, which indicate that it should be homogeneous on scales below about a parsec. Evidence will be presented from observations of the ISM at UV, optical, far-IR and radio wavelengths. I will conclude by examining two competing interpretations: fluctuations in ionisation, or fluctuations in density.

The presence of a large number of bulgeless or pure-disc galaxies in the universe challenges the LCDM model of galaxy-formation. We have studied their luminosity and size evolution in comparison to disc-galaxies with bulges upto z~1. Our results strongly suggest that a fraction of the bulgeless galaxy population might be acquiring a bulge with time. We are currently trying to ascertain this conjecture. To that end, we have computed the concentration, asymmetry and clumpiness parameters for our sample of 567 disc-galaxies, obtained from HST and SDSS, both with and without bulges, at three redshift-bins. In this talk, I will share some of our recent findings.

Our location among billions of stars of our disky Galaxy makes the studies on its structure and evolution extremely difficult. Ongoing large-scale optical and infrared galaxy surveys, such as OGLE and VVV, are on a good way to find answers to key questions.

The presence of magnetic fields in a small subset of main sequence intermediate-mass (A and late B-type) stars has been well-known for over 60 years. In the last 10 years, a new generation of spectropolarimeters has furthered our knowledge with the systematic discovery of magnetic fields among the pre-main sequence progenitors of the A and B-type stars (the Herbig Ae/Be stars) and among high-mass stars (main sequence early B and O-type stars). It is now well-established that ~10% of all stars with radiative envelopes (spanning ~1.5 decades of mass and at different evolutionary phases) host strong (surface polar field strengths of ~1 kG), stable (on the order of at least decades), globally-order (mainly dipolar) magnetic fields despite the fact that these stars lack a significant convective envelope - a key ingredient in driving any contemporaneous dynamo that is believed to be responsible for generating magnetic fields in a large range of astrophysical objects. Despite all our knowledge about magnetism in these stars, the fundamental question regarding the origin of their magnetic fields is still highly debated. In this talk I will discuss several theories that endeavour to explain the origin of magnetism in these stars and the observational evidence that may or may not support these ideas, including preliminary results from several ongoing programs that try to address this fundamental question.

Herbig Ae/Be (HAeBe) stars are a class of early-type pre-main sequence objects whose stellar mass corresponds to the transition regime between Solar-type stars and high-mass stars. They are generally bright at most wavelengths allowing a detailed view of their evolving environment and are pivotal objects for the formation of both stars and planets. The high-resolution observational and theoretical advances of the past 10 to 15 years provide the primary motivation and the evolution of the circumstellar disk material constitutes the main, but not the exclusive topic, for the workshop. Areas to be addressed include circumstellar disk structure, transition and debris disks, disk dispersal, jets and outflows, young clusters and the impact of future instrumentation.

This workshop will commemorate the life and work of George H. Herbig (2 January 1920 - 12 October 2013). Herbig pioneered the field of star formation, especially that of young stars and their nebulous surroundings. A brief appreciation of his life and work can be found at http://www.ifa.hawaii.edu/info/press-releases/Herbig/

Systemic mass loss in interacting binaries such as of the Algol-type has been inferred since the 50s. There is indeed gathering indirect evidence indicating that some Algols follow non-conservative evolutions but still no direct detection of large mass outflows. As a result, little is known about the eventual ejection mechanism. In order to reconcile stellar models with observations, we compute typical Algol models with the state-of-the-art binary star evolution code Binstar. We investigate systemic mass losses within the hotspot paradigm where large outflows of material forms from the accretion impact during the mass transfer phase. We then study the impact of this outflow on the spectral emission distribution of the system with the radiative transfer codes Cloudy and Skirt.

The combination of condensate clouds and rapid rotation has long motivated searches for weather phenomena in ultracool (late-M, L and T) dwarf (UCD) atmospheres. Pioneering work in this field dating back as early as 1999 suggested that variability is quite common for UCDs. Yet these early studies were ambiguous: detections were often low-amplitude and/or lacking periodicity, and the mechanisms responsible remained unclear. Observations made in the past 5 years, utilizing continuous monitoring strategies, better instruments, and larger telescopes have demonstrated conclusive and surprisingly large near-infrared variability for a subset of brown dwarfs at the transition between L and T spectral types, suggesting a patchy distribution of silicate clouds in their atmospheres. Brightness variations as large as 25% on readily observable rotational timescales allow light curves of exquisite precision, worthy of detailed analysis, to be obtained from both ground and space based facilities. While the L/T transition is the realm of spectacular variability, recent space-based efforts have confirmed lower levels of variability for a substantial fraction of UCDs at all spectral types. I will describe how such multi-wavelength, multi-epoch observations are contributing to an emerging picture of cloud structure in brown dwarf and exoplanet atmospheres.

The so-called dusty torus is a key component in our current picture of AGN: It is thought to be responsible for the orientation-dependent obscuration of the central engine and to play an important role in fuelling the AGN. However, little is known about the physics of the torus, as it is very small and is essentially unresolved by single dish measurements. Only by employing interferometric methods, is it possible to observe the torus directly.

I will present the results of our observations of the dust distributions in nearby AGN using infrared interferometry. I will focus on the results for the Circinus galaxy. Being the closest Seyfert 2 galaxy and the second brightest AGN in the mid-infrared, it is an ideal target for interferometric studies of its AGN-heated dust distribution. Its dust distribution is resolved well by our measurements. We find a two component structure consisting of a dense inner disk surrounded by a less dense dust distribution extended in polar direction.

The extended dust most likely traces the inner filamentary funnel of the torus, which is directly lluminated by the accretion disk. While in general agreement with the idea of a dusty torus, our observations are nevertheless puzzling: We find no temperature gradient in the dust distribution, as would be expected for centrally heated dust.

Gravitational microlensing is a powerful tool which allows astronomers to study the most inner parts of QSO that can’t be spatially resolved by current telescopes. Indeed, stellar-mass objects contained in the lensing galaxy can typically substantially magnify regions of the source plane on scales of a few nano- or micro-arcseconds, which interestingly correspond to the size of QSO unresolved inner regions. Among other things, microlensing can give rise to spectral differences between the multiple images of a same background quasar which constitute a cosmic mirage. A lot of information about the physical properties of the accretion disk and the geometry of the broad line region can be retrieved from these spectral differences. (e.g., Hutsemékers et al., 2010, Sluse et al., 2012). Using the decomposition method described in Sluse et al. (2007) and Hutsemékers et al. (2010), which allows to disentangle the part of the quasar spectrum affected by microlensing, we have studied three quadruple gravitational lenses which are known or suspected to be affected by microlensing: H1413+117, HE0435-1223 and PG1115+080

Star formation is a fundamental process in the universe, shaping the structure of our and other galaxies. The birth of stars is triggered by the gravitational collapse and fragmentation of cold molecular clouds. The talk will summarize the general physical principles of the star formation process. It will discuss global properties such as the star formation efficiency and the initial mass function. In addition, it will also demonstrate the power of adaptive optics in revealing the structure of star-forming regions and will show new exciting results obtained with the Herschel Space Observatory.

Covering the 50 square degrees of Spitzer SWIRE Legacy Fields, the Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) is one of the largest surveys of its kind. It has detected hundreds of galaxy clusters up to z=1.7. Over the past few years, the SpARCS team has been examining the properties of galaxies in these clusters though multi-wavelength imaging and multi-object spectroscopy. In this talk, I will discuss what we are learning about galaxy evolution in these the densest of environments. I will also discuss our plans to push these studies to even higher redshifts where it seems that drastic changes to the galaxy population in the cores of clusters is occurring.

We systematically search for discrete absorption events in the vast archive of the Rossi X-ray Timing Explorer. This includes dozens of nearby type I and Compton-thin type II AGN and covers timescales from days to over a decade for individual objects. We are sensitive to discrete absorption events due to clouds of full-covering, neutral or mildly-ionized gas with columns 1022-25 cm-2 transiting the line of sight. We detect 13 eclipse events in 8 objects, roughly tripling the number of previously published events from this archive. Despite sensitivity to events with NH up to 1024-25 cm-2, we measured no Compton-thick eclipses in our sample. Peak column densities span 2.5 - 19 x 1022 cm-2. Event durations span hours to months. We infer the clouds’ distances from the black hole, assuming Keplerian motion, to span 0. 2 - 80 x 104 Schwarzschild radii. We find no statistically significant difference between the individual cloud properties of type I and II objects. The presence of eclipses in both type Is and IIs argues against sharp-edged cloud distributions. The type II AGN show a level of “base-line” X-ray absorption that is consistent with being constant over timescales from 0.6 to 8.4 yr. This can either be explained by a homogeneous medium, or by X-ray-absorbing clouds that each have NH << 1022 cm-2. Considering the "selection function" of the monitoring, we derive the probability of cloud occultation events. Finally, we derive the first X-ray statistical constraints for clumpy-torus models.

After a brief introduction I review some of the highlights of the GRBs afterglow evidencing the observations obtained using the ESO facilities. With the discovery that also short GRBs are extragalactic we got a reasonable understanding of the energy and the likely progenitor identified as a NS+NS merging. This leads to the need of demonstrating it via gravitational waves detection. Long GRBs show evidence of early (prompt emission) variability all over the electromagnetic spectrum and the effect the radiation has on the Interstellar Medium. This call for high speed pointing capability telescopes. One of the strongest clue for modeling the collapse is the connection GRBs ­ SNe. ESO did an excellent work observing all events with z <0.2. Long GRBs have been detected at high redshifts so that GRBs may be a way to better understand the Cosmic Star Formation Rate (CSFR) that is poorly understood at high z. One of the main tasks for the future will be the understanding of GRBs magnetic fields, their formation and related particle acceleration.