M. F. Sterzik

European Southern Observatory, Arching, Bavaria, Germany

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Publications (218)446.46 Total impact

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    ABSTRACT: T and Y-dwarfs are among the coolest and least luminous objects detected, and they can help to understand the properties of giant planets. Their multiplicity properties can shed light on the formation process. We observed a sample six T dwarfs and one L9 dwarf with the Laser Guide Star (LGS) and NAOS-CONICA, the adaptive optics (AO) facility, and the near infrared camera at the ESO Very Large Telescope. From the seven observed objects, we have identified a subarcsecond binary system, WISE~J0612-3036, composed of two similar components with spectral types of T6. We measure a separation of $\rho$ = 350$\pm$5 mas and a position angle of $PA = 235\pm1^{\circ}$. Using the mean absolute magnitudes of T6 dwarfs in the 2MASS $JHK_s$ bands, we estimate a distance of $d$=31$\pm$6 pc and derive a projected separation of $\rho$ = 11$\pm$2 au. Another target, WISE J2255-3118, shows a very faint object at 1.3 arcsec in the $K_{\rm s}$ image. The object is marginally detected in $H$, and we derive a near infrared color of $H-K_{\rm s}$$>$ 0.1\,mag. $HST/WFC3$ public archival data reveals that the companion candidate is an extended source.Together with the derived color, this suggests that the source is most probably a background galaxy. The five other sources are apparently single, with 3-$\sigma$ sensitivity limits between $H$=19-21 for companions at separations $\geq$ 0.5 arcsec. WISE 0612-3036 is probably a new T-dwarf binary composed of two T6 dwarfs. As in the case of other late T-dwarf binaries, it shows a mass ratio close to 1, although its projected separation, $\sim$11 au, is larger than the average ($\sim$ 5 au). Additional observations are needed to confirm that the system is bound.
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    S. Bagnulo, A. Cellino, M. Sterzik
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    ABSTRACT: We explore the use of spectro-polarimetry as a remote sensing tool for asteroids in addition to traditional reflectance measurements. In particular we are interested in possible relationships between the wavelength-dependent variation of linear polarization and the properties of the surfaces, including albedo and composition. We have obtained optical spectro-polarimetric measurements of a dozen asteroids of different albedo and taxonomic classes and of two small regions at the limb of the Moon. We found that objects with marginally different relative reflectance spectra (in the optical) may have totally different polarization spectra. This suggests that spectro-polarimetry may be used to refine the classification of asteroids. We also found that in some cases the Umov law may be violated, that is, in contrast to what is expected from basic physical considerations, the fraction of linear polarization and the reflectance may be positively correlated. In agreement with a few previous studies based on multi-colour broadband polarimetry, we found that the variation of linear polarization with wavelength and with phase-angle is correlated with the albedo and taxonomic class of the objects. Finally, we have serendipitously discovered that spinel-rich asteroid (599) Luisa, located very close to the Watsonia family, is a member of the rare class of Barbarian asteroids. We suggest that future modelling attempts of the surface structure of asteroids should be aimed at explaining both reflectance and polarization spectra.
  • Binary Systems, their Evolution and Environments; 09/2014
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    ABSTRACT: Context. Our study is a follow-up of the SACY project, an extended high spectral resolution survey of more than two thousand optical counterparts to X-ray sources in the southern hemisphere targeted to search for young nearby association. Nine associations have either been newly identified, or have had their member list revised. Groups belonging to the Sco-Cen-Oph complex are not considered in the present study.Aims. These nine associations, with ages of between about 6 Myr and 70 Myr, form an excellent sample to study the Li depletion in the pre-main sequence (PMS) evolution. In the present paper, we investigate the use of Li abundances as an independent clock to constrain the PMS evolution.Methods. Using our measurements of the equivalent widths of the Li resonance line and assuming fixed metallicities and microturbulence, we calculated the LTE Li abundances for 376 members of various young associations. In addition, we considered the effects of their projected stellar rotation.Results. We present the Li depletion as a function of age in the first hundred million years for the first time for the most extended sample of Li abundances in young stellar associations.Conclusions. A clear Li depletion can be measured in the temperature range from 5000 K to 3500 K for the age span covered by the nine associations studied in this paper. The age sequence based on the Li-clock agrees well with the isochronal ages, the $\epsilon$Cha association being the only possible exception. The lithium depletion patterns for the associations presented here resemble those of the young open clusters with similar ages, strengthening the notion that the members proposed for these loose young associations have indeed a common physical origin. The observed scatter in the Li abundances hampers the use of Li in determining reliable ages for individual stars. For velocities above 20 km s$^{-1}$, rotation seems to play an important role in inhibiting the Li depletion.
    Astronomy and Astrophysics 08/2014; DOI:10.1007/1-4020-2600-5_9 · 4.48 Impact Factor
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    ABSTRACT: Context: Dynamically undisrupted, young populations of stars are crucial to study the role of multiplicity in relation to star formation. Loose nearby associations provide us with a great sample of close ($<$150 pc) Pre-Main Sequence (PMS) stars across the very important age range ($\approx$5-70 Myr) to conduct such research. Aims: We characterize the short period multiplicity fraction of the SACY (Search for Associations Containing Young stars) accounting for any identifiable bias in our techniques and present the role of multiplicity fractions of the SACY sample in the context of star formation. Methods: Using the cross-correlation technique we identified double-lined spectroscopic systems (SB2), in addition to this we computed Radial Velocity (RV) values for our subsample of SACY targets using several epochs of FEROS and UVES data. These values were used to revise the membership of each association then combined with archival data to determine significant RV variations across different data epochs characteristic of multiplicity; single-lined multiple systems (SB1). Results: We identified 7 new multiple systems (SB1s: 5, SB2s: 2). We find no significant difference between the short period multiplicity fraction ($F_\mathrm{m}$) of the SACY sample and that of nearby star forming regions ($\approx$1-2 Myr) and the field ($F_\mathrm{m}\leq$10%) both as a function of age and as a function of primary mass, $M_1$, in the ranges $P$ [1:200 day] and $M_2$ [0.08 $M_{\odot}$-$ M_1$]. Conclusions: Our results are consistent with the picture of universal star formation, when compared to the field and nearby star forming regions (SFRs). We comment on the implications of the relationship between increasing multiplicity fraction with primary mass, within the close companion range, in relation to star formation.
    Astronomy and Astrophysics 06/2014; 568. DOI:10.1051/0004-6361/201423856 · 4.48 Impact Factor
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    ABSTRACT: We explore the possible connection between the open cluster IC 2391 and the unbound Argus association identified by the search for associations containing young stars survey. In addition to common kinematics and ages between these two systems, here we explore their chemical abundance patterns to confirm if the two substructures shared a common origin. We carry out a homogeneous high-resolution elemental abundance study of eight confirmed members of IC 2391 as well as six members of the Argus association using UVES spectra. We derive spectroscopic stellar parameters and abundances for Fe, Na, Mg, Al, Si, Ca, Ti, Cr, Ni and Ba.All stars in the open cluster and Argus association were found to share similar abundances with the scatter well within the uncertainties, where [Fe/H]=-0.04+/-0.03 for cluster stars and [Fe/H]=-0.06+/-0.05 for Argus stars. Effects of overionization/excitation were seen for stars cooler than roughly 5200K as previously noted in the literature. Also, enhanced Ba abundances of around 0.6dex were observed in both systems. The common ages, kinematics and chemical abundances strongly support the fact that the Argus association stars originated from the open cluster IC 2391. Simple modelling of this system finds this dissolution to be consistent with two-body interactions.(4 data files).
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    ABSTRACT: Stars in young nearby associations are the only targets allowing giant planet searches at all separations in the near future, by coupling indirect techniques such as radial velocity and deep imaging. These stars are first priorities targets for the forthcoming planets imagers on 10-m class telescopes. Young stars rotate more rapidly and are more active than their older counterparts. Both effects can limit the capability to detect planets using RV. We wish to explore the planet detection capabilities of a representative sample of stars in close and young associations with radial velocity data and explore the complementarity between this technique and direct imaging. We observed 26 such targets with spectral types from A to K and ages from
    Astronomy and Astrophysics 04/2013; 559. DOI:10.1051/0004-6361/201220770 · 4.48 Impact Factor
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    ABSTRACT: We explore the possible connection between the open cluster IC 2391 and the unbound Argus association identified by the SACY survey. In addition to common kinematics and ages between these two systems, here we explore their chemical abundance patterns to confirm if the two substructures shared a common origin. We carry out a homogenous high-resolution elemental abundance study of eight confirmed members of IC 2391 as well as six members of the Argus association using UVES spectra. We derive spectroscopic stellar parameters and abundances for Fe, Na, Mg, Al, Si, Ca, Ti, Cr, Ni and Ba. All stars in the open cluster and Argus association were found to share similar abundances with the scatter well within the uncertainties, where [Fe/H] = -0.04 +/-0.03 for cluster stars and [Fe/H] = -0.06 +/-0.05 for Argus stars. Effects of over-ionisation/excitation were seen for stars cooler than roughly 5200K as previously noted in the literature. Also, enhanced Ba abundances of around 0.6 dex were observed in both systems. The common ages, kinematics and chemical abundances strongly support that the Argus association stars originated from the open cluster IC 2391. Simple modeling of this system find this dissolution to be consistent with two-body interactions.
    Monthly Notices of the Royal Astronomical Society 01/2013; 431(2). DOI:10.1093/mnras/stt153 · 5.23 Impact Factor
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    ABSTRACT: Since there are several ways planets can survive the giant phase of the host star, we examine the habitability and detection of planets orbiting white dwarfs. As a white dwarf cools from 6000 K to 4000 K, a planet orbiting at 0.01 AU would remain in the Continuous Habitable Zone (CHZ) for ~8 Gyr. We show that photosynthetic processes can be sustained on such planets. The DNA-weighted UV radiation dose for an Earth-like planet in the CHZ is less than the maxima encountered on Earth, hence non-magnetic white dwarfs are compatible with the persistence of complex life. Polarisation due to a terrestrial planet in the CHZ of a cool white dwarf is 10^2 (10^4) times larger than it would be in the habitable zone of a typical M-dwarf (Sun-like star). Polarimetry is thus a viable way to detect close-in rocky planets around white dwarfs. Multi-band polarimetry would also allow reveal the presence of a planet atmosphere, providing a first characterisation. Planets in the CHZ of a 0.6 M_sun white dwarf will be distorted by Roche geometry, and a Kepler-11d analogue would overfill its Roche lobe. With current facilities a Super-Earth-sized atmosphereless planet is detectable with polarimetry around the brightest known cool white dwarf. Planned future facilities render smaller planets detectable, in particular by increasing the instrumental sensitivity in the blue.
    The Astrophysical Journal Letters 07/2012; 757(1). DOI:10.1088/2041-8205/757/1/L15 · 5.60 Impact Factor
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    ABSTRACT: We used high-precision Harps data collected over eight years since 2003 to measure and analyse β Pic radial velocities. (1 data file).
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    ABSTRACT: This second edition of the Observing Planetary Systems workshop was aimed at bringing together the two communities of Solar System and exoplanetary system scientists to review the recent progress made in our understanding of the formation of the Solar System and its early chemistry, and how this picture fits with our current knowledge of the formation and evolution of planetary systems in general.
    06/2012;
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    ABSTRACT: We present a study of the large-scale spatial distribution of 6482 RASS X-ray sources in approximately 5000 deg^2 in the direction of Orion. We examine the astrophysical properties of a sub-sample of ~100 optical counterparts, using optical spectroscopy. This sub-sample is used to investigate the space density of the RASS young star candidates by comparing X-ray number counts with Galactic model predictions. We characterize the observed sub-sample in terms of spectral type, lithium content, radial and rotational velocities, as well as iron abundance. A population synthesis model is then applied to analyze the stellar content of the RASS in the studied area. We find that stars associated with the Orion star-forming region do show a high lithium content. A population of late-type stars with lithium equivalent widths larger than Pleiades stars of the same spectral type (hence younger than ~70-100 Myr) is found widely spread over the studied area. Two new young stellar aggregates, namely "X-ray Clump 0534+22" (age~2-10 Myr) and "X-ray Clump 0430-08" (age~2-20 Myr), are also identified. The spectroscopic follow-up and comparison with Galactic model predictions reveal that the X-ray selected stellar population in the direction of Orion is characterized by three distinct components, namely the clustered, the young dispersed, and the widespread field populations. The clustered population is mainly associated with regions of recent or ongoing star formation and correlates spatially with molecular clouds. The dispersed young population follows a broad lane apparently coinciding spatially with the Gould Belt, while the widespread population consists primarily of active field stars older than 100 Myr. We expect the "bi-dimensional" picture emerging from this study to grow in depth as soon as the distance and the kinematics of the studied sources will become available from the future Gaia mission.
    Astronomy and Astrophysics 04/2012; 542. DOI:10.1051/0004-6361/201118379 · 4.48 Impact Factor
  • S. Bagnulo, M. F. Sterzik, E. Palle
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    ABSTRACT: One of the next most important goals of astronomy is the characterization of exo-solar planets and the search for extra terrestrial life. Traditional spectroscopic measurements cannot be easily applied to the study of the atmospheres of the exo-solar planets, because the light reflected by the planet is overwhelmed by the radiation of the hosting star. Polarimetric techniques offer an attractive solution to this problem. Since the light reflected by the planet is highly polarised, it can be distinguished from the nearly unpolarised stellar radiation, provided that the observations are obtained with sufficiently high signal-to-noise ratio. Theoretical models have been developed to predict what the polarised spectrum of an Earth-like planet would look like if observed in linear polarization from space [1]. Model predictions appear to be very sensitive to the percentage of ocean, vegetation, and clouds that cover the visible area of the planet. Here we present polarised spectra of the Earthshine, which simulate the observations of the planet Earth as seen from space. Interpreted with theoretical models, our observations clearly reveal bio-markers, and practically demonstrate that spectro-polarimetry may be a key diagnostic tool not only for the study of the exoplanets, but also for the search of extra-terrestrial life. [1] Stam, D.M., 2008, Astronomy and Astrophysics, 484, 989.
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    ABSTRACT: The formation of spectroscopic binaries (SB) may be a natural byproduct of star formation. The early dynamical evolution of multiple stellar systems after the initial fragmentation of molecular clouds leaves characteristic imprints on the properties of young, multiple stars. The discovery and the characterization of the youngest SB will allow us to infer the mechanisms and timescales involved in their formation. Our work aims to find spectroscopic companions around young stellar objects (YSO). We present a near-IR high-resolution (R ~ 60 000) multi-epoch radial velocity survey of seven YSO in the star-forming region (SFR) ρ Ophiuchus. The radial velocities of each source were derived using a two-dimensional cross-correlation function, using the zero-point established by the Earth's atmosphere as reference. More than 14 spectral lines in the CO Δν = (0-2) bandhead window were used in the cross-correlation against LTE atmospheric models to compute the final results. We found that the spectra of the protostars in our sample agree well with the predicted stellar photospheric profiles, indicating that the radial velocities derived are indeed of stellar nature. Three of the targets analyzed exhibit large radial velocity variations during the three observation epochs. These objects - pending further confirmation and orbital characteristics - may become the first evidence for proto-spectroscopic binaries, and will provide important constraints on their formation. Our preliminary binary fraction (BF) of ~71% (when merging our results with those of previous studies) is in line with the notion that multiplicity is very high at young ages and therefore a byproduct of star formation. Based on observations collected with the CRIRES spectrograph at the VLT/UT1 8.2-m Antu Telescope (ESO run ID. 081.C-0395(A)) at the Paranal Observatory, Chile.Table 1 is available in electronic form at http://www.aanda.org
    Astronomy and Astrophysics 03/2012; 539:62-. DOI:10.1051/0004-6361/201117703 · 4.48 Impact Factor
  • Michael F Sterzik, Stefano Bagnulo, Enric Palle
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    ABSTRACT: Low-resolution intensity spectra of Earth's atmosphere obtained from space reveal strong signatures of life ('biosignatures'), such as molecular oxygen and methane with abundances far from chemical equilibrium, as well as the presence of a 'red edge' (a sharp increase of albedo for wavelengths longer than 700 nm) caused by surface vegetation. Light passing through the atmosphere is strongly linearly polarized by scattering (from air molecules, aerosols and cloud particles) and by reflection (from oceans and land). Spectropolarimetric observations of local patches of Earth's sky light from the ground contain signatures of oxygen, ozone and water, and are used to characterize the properties of clouds and aerosols. When applied to exoplanets, ground-based spectropolarimetry can better constrain properties of atmospheres and surfaces than can standard intensity spectroscopy. Here we report disk-integrated linear polarization spectra of Earthshine, which is sunlight that has been first reflected by Earth and then reflected back to Earth by the Moon. The observations allow us to determine the fractional contribution of clouds and ocean surface, and are sensitive to visible areas of vegetation as small as 10 per cent. They represent a benchmark for the diagnostics of the atmospheric composition, mean cloud height and surfaces of exoplanets.
    Nature 03/2012; 483(7387):64-6. DOI:10.1038/nature10778 · 42.35 Impact Factor
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    ABSTRACT: We present the first observational evidence for disk photoevaporation driven by the central star and discuss the implications of star-driven photoevaporation on the architecture of planetary systems.
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    ABSTRACT: Context. The {\beta} Pictoris system with its debris disk and a massive giant planet orbiting at \simeq 9 AU represents an ideal laboratory to study giant planet formation and evolution as well as planet-disk interactions. {\beta} Pic b can also help testing brightness-mass relations at young ages. Other planets, yet undetected, may of course be present in the system. Aims. We aim at putting direct constrains on the mass of {\beta} Pic b and at searching for additional jovian planets on orbits closer than typically 2 AU. Methods. We use high precision Harps data collected over 8 years since 2003 to measure and analyse {\beta} Pic radial velocities. Results. We show that the true mass of {\beta} Pic b is less than 10, 12, 15.5, 20 and 25 MJup if orbiting respectively at 8, 9, 10, 11 and 12 AU. This is the first direct constraint on the mass of an imaged planet. The upper mass found is well in the range predicted by brightness-mass relations provided by current "hot start" models. We also exclude the presence of giant planets more massive than 2.5 MJup with periods less than 100 days (hot Jupiters), more massive than 9 MJup for periods in the range 100-500 days. In the 500-1000 day range, the detection limit is in the brown dwarf domain. Beyond the intrinsic interest for {\beta} Pic, these results show the possibilities of precise RV measurements of early type, rapidly rotating stars.
    Astronomy and Astrophysics 02/2012; 542. DOI:10.1051/0004-6361/201117985 · 4.48 Impact Factor
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    ABSTRACT: Constraining the spatial and thermal structure of the gaseous component of circumstellar disks is crucial to understand star and planet formation. Models predict that the [Ne II] line at 12.81 {\mu}m detected in young stellar objects with Spitzer traces disk gas and its response to high energy radiation, but such [Ne II] emission may also originate in shocks within powerful outflows. To distinguish between these potential origins for mid-infrared [Ne II] emission and to constrain disk models, we observed 32 young stellar objects using the high resolution (R~30000) mid-infrared spectrograph VISIR at the VLT. We detected the 12.81 {\mu}m [Ne II] line in 12 objects, tripling the number of detections of this line in young stellar objects with high spatial and spectral resolution spectrographs. We obtain the following main results: a) In Class I objects the [Ne II] emission observed from Spitzer is mainly due to gas at a distance of more than 20-40 AU from the star, where neon is, most likely, ionized by shocks due to protostellar outflows. b) In transition and pre-transition disks, most of the emission is confined to the inner disk, within 20-40 AU from the central star. c) Detailed analysis of line profiles indicates that, in transition and pre-transition disks, the line is slightly blue-shifted (2-12 km s{^-1}) with respect to the stellar velocity, and the line width is directly correlated with the disk inclination, as expected if the emission is due to a disk wind. d) Models of EUV/X-ray irradiated disks reproduce well the observed relation between the line width and the disk inclination, but underestimate the blue-shift of the line.
    The Astrophysical Journal 12/2011; 747(2). DOI:10.1088/0004-637X/747/2/142 · 6.28 Impact Factor
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    ABSTRACT: The formation of spectroscopic binaries (SB) may be a natural byproduct of star formation. The early dynamical evolution of multiple stellar systems after the initial fragmentation of molecular clouds leaves characteristic imprints on the properties of young, multiple stars. The discovery and the characterization of the youngest SB will allow us to infer the mechanisms and timescales involved in their formation. Our work aims to find spectroscopic companions around young stellar objects (YSO). We present a near-IR high-resolution (R ~ 60000) multi-epoch radial velocity survey of 7 YSO in the star forming region (SFR) rho-Ophiuchus. The radial velocities of each source were derived using a two-dimensional cross-correlation function, using the zero-point established by the Earth's atmosphere as reference. More than 14 spectral lines in the CO (0-2) bandhead window were used in the cross-correlation against LTE atmospheric models to compute the final results. We found that the spectra of the protostars in our sample agree well with the predicted stellar photospheric profiles, indicating that the radial velocities derived are indeed of stellar nature. Three of the targets analyzed exhibit large radial velocity variations during the three observation epochs. These objects - pending further confirmation and orbital characteristics - may become the first evidence for proto-spectroscopic binaries, and will provide important constraints on their formation. Our preliminary binary fraction (BF) of ~71% (when merging our results with those of previous studies) is in line with the notion that multiplicity is very high at young ages and therefore a byproduct of star formation
    Astronomy and Astrophysics 12/2011; 539:62. · 4.48 Impact Factor
  • S. Bagnulo, M. Sterzik
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    ABSTRACT: Astronomical polarimetric techniques are routinely used to characterize the surface structure of the atmosphere-less bodies of our solar system. Efforts are also dedicated to the study of the features of the atmospheres of Venus and of the giant planets. One of the most exciting subjects of Astronomy is the search and study of exo-solar planets, and in particular the search for extra-terrestrial life. In this kind of research, polarimetric techniques will play a fundamental role, and current polarimetric studies of our solar system represent the obvious reference point for future exo-solar observations. Of special interest are polarimetric observations of the planet Earth, its atmosphere and its surface characteristics (e.g., vegetation, oceans) through astronomical instrumentation. These "home-made" observations will ultimately represent a benchmark for future observations of Earth-like planets of other solar systems. In this talk we will review the polarimetric observations of our solar system, and we will present some of our new spectro-polarimetric observations of terrestrial material, organic and inorganic, obtained with the same astronomical instrumentation used to study the objects of our solar system. We will compare our observational results to models of polarized spectra expected from Earth-like extrasolar planets, and demonstrate the advantages of spectrapolarimetry to probe their atmospheric composition and surface properties.

Publication Stats

2k Citations
446.46 Total Impact Points

Institutions

  • 1998–2014
    • European Southern Observatory
      Arching, Bavaria, Germany
    • University of Hamburg
      • Hamburg Observatory
      Hamburg, Hamburg, Germany
  • 1970–2014
    • University of Santiago, Chile
      CiudadSantiago, Santiago Metropolitan, Chile
  • 2010
    • Pontifical Catholic University of Chile
      CiudadSantiago, Santiago, Chile
  • 1997–2003
    • Indiana University Bloomington
      • Department of Astronomy
      Bloomington, IN, United States
  • 2000
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, England, United Kingdom
    • University of Vienna
      Wien, Vienna, Austria
  • 1995–1998
    • Max Planck Institute for Extraterrestrial Physics
      Arching, Bavaria, Germany
    • University of California, Santa Barbara
      Santa Barbara, California, United States
  • 1994–1995
    • University of Tuebingen
      Tübingen, Baden-Württemberg, Germany