Garching bei München, Germany
Garching bei München, Germany

Time filter

Source Type

Schleicher D.R.G.,ESO Garching | Schleicher D.R.G.,Leiden Observatory | Banerjee R.,University of Heidelberg | Sur S.,University of Heidelberg | And 4 more authors.
AIP Conference Proceedings | Year: 2010

We discuss the formation of supermassive black holes in the early universe, and how to probe their subsequent evolution with the upcoming mm/sub-mm telescope ALMA. We first focus on the chemical and radiative conditions for black hole formation, in particular considering radiation trapping and molecular dissociation effects. We then turn our attention towards the magnetic properties in the halos where the first black holes form, and show that the presence of turbulence may lead to a magnetic dynamo, which could support the black hole formation process by providing an efficient means of transporting the angular momentum.We finally focus on observable properties of high-redshift black holes with respect to ALMA, and discuss how to distinguish between chemistry driven by the starburst and chemistry driven by X-rays from the black hole. © 2010 American Institute of Physics.

Uttenthaler S.,Catholic University of Leuven | Pontoppidan K.M.,California Institute of Technology | Seifahrt A.,University of California at Davis | Kendrew S.,Leiden University | And 8 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

METIS is a mid-infrared instrument proposed for the European Extremely Large Telescope. It is designed to provide imaging and spectroscopic capabilities in the 3 - 14 micron region up to a spectral resolution of 100000. One of the novel concepts of METIS is that of a high-resolution integral field spectrograph for a diffraction-limited mid-IR instrument. While this concept has many scientific and operational advantages over a long-slit spectrograph, one drawback is that the spectral resolution changes over the field of view. This has an impact on the procedures to correct for telluric absorption lines imprinted on the science spectra. They are a major obstacle in the quest to maximize spectral fidelity, the ability to distinguish a weak spectral feature from the continuum. The classical technique of division by a standard star spectrum, observed in a single IFS spaxel, cannot simply be applied to all spaxels, because the spectral resolution changes from spaxel to spaxel. Here we present and discuss possible techniques of telluric line correction of METIS IFS spectra, including the application of synthetic model spectra of telluric transmission, to maximize spectral fidelity. © 2010 Copyright SPIE - The International Society for Optical Engineering.

Schleicher D.R.G.,ESO Garching | Spaans M.,Leiden University | Glover S.C.O.,University of Groningen | Glover S.C.O.,University of Heidelberg
Astrophysical Journal Letters | Year: 2010

In massive primordial galaxies, the gas may directly collapse and form a single central massive object if cooling is suppressed. H2 line cooling can be suppressed in the presence of a strong soft-ultraviolet radiation field, but the role played by other cooling mechanisms is less clear. In optically thin gas, Lyα cooling can be very effective, maintaining the gas temperature below 104 K over many orders of magnitude in density. However, the large neutral hydrogen column densities present in primordial galaxies render them highly optically thick to Lyα photons. In this paper, we examine in detail the effects of the trapping of these Lyα photons on the thermal and chemical evolution of the gas. We show that despite the high optical depth in the Lyman series lines, cooling is not strongly suppressed, and proceeds via other atomic hydrogen transitions. At densities larger than ∼ 109 cm-3, collisional dissociation of molecular hydrogen becomes the dominant cooling process and decreases the gas temperature to about 5000 K. The gas temperature evolves with density as T α ρ γ eff -1, with γeff = 0.97-0.98. The evolution is thus very close to isothermal, and so fragmentation is possible, but unlikely to occur during the initial collapse. However, after the formation of a massive central object, we expect that later-infalling, higher angular momentum material will form an accretion disk that may be unstable to fragmentation, which may give rise to star formation with a top-heavy initial mass function. © 2010. The American Astronomical Society. All rights reserved.

Sur S.,University of Heidelberg | Schleicher D.R.G.,ESO Garching | Schleicher D.R.G.,Leiden University | Banerjee R.,University of Heidelberg | And 3 more authors.
Astrophysical Journal Letters | Year: 2010

Cosmological hydrodynamical simulations of primordial star formation suggest that the gas within the first starforming halos is turbulent. This has strong implications on the subsequent evolution, in particular on the generation of magnetic fields. Using high-resolution numerical simulations, we show that in the presence of turbulence, weak seed magnetic fields are exponentially amplified by the small-scale dynamo during the formation of the first stars. We conclude that strong magnetic fields are generated during the birth of the first stars in the universe, potentially modifying the mass distribution of these stars and influencing the subsequent cosmic evolution. We find that the presence of the small-scale turbulent dynamo can only be identified in numerical simulations in which the turbulent motions in the central core are resolved with at least 32 grid cells. © 2010. The American Astronomical Society. All rights reserved.

Hudepohl G.,ESO Chile | Haddad J.-P.,ESO Chile | Lucuix C.,ESO Garching
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2016

The ESO Very Large Telescope Observatory (VLT) at Cerro Paranal in Chile had its first light in 1998. Most of the telescopes' electronics components were chosen and designed in the mid 1990s and are now around 20 years old. As a consequence we are confronted with increasing failure rates due to aging and lack of spare parts, since many of the components are no longer available on the market. The lifetime of large telescopes is generally much beyond 25 years. Therefore the obsolescence of electronics components and modules becomes an issue sooner or later and forces the operations teams to upgrade the systems to new technology in order to avoid that the telescope becomes inoperable. Technology upgrade is a time and money consuming process, which in many cases is not straightforward and has various types of complications. This paper shows the strategy, analysis, approach, timeline, complications and progress in obsolescence driven electronics upgrades at the ESO Very Large Telescope (VLT) at the Paranal Observatory. © 2016 SPIE.

Peng T.-C.,ESO Garching | Humphreys E.M.L.,ESO Garching | Testi L.,ESO Garching | Testi L.,Excellence Cluster Universe | And 11 more authors.
Astronomy and Astrophysics | Year: 2013

Aims. Galactic chemical evolution (GCE) is important for understanding the composition of the present-day interstellar medium (ISM) and of our solar system. In this paper, we aim to track the GCE by using the 29Si/ 30Si ratios in evolved stars and tentatively relate this to presolar grain composition. Methods. We used the APEX telescope to detect thermal SiO isotopologue emission toward four oxygen-rich M-type stars. Together with the data retrieved from the Herschel science archive and from the literature, we were able to obtain the 29Si/30Si ratios for a total of 15 evolved stars inferred from their optically thin 29SiO and 30SiO emission. These stars cover a range of masses and ages, and because they do not significantly alter 29Si/30Si during their lifetimes, they provide excellent probes of the ISM metallicity (or 29Si/30Si ratio) as a function of time. Results. The 29Si/30Si ratios inferred from the thermal SiO emission tend to be lower toward low-mass oxygen-rich stars (e.g., down to about unity for W Hya), and close to an interstellar or solar value of 1.5 for the higher-mass carbon star IRC+10216 and two red supergiants. There is a tentative correlation between the 29Si/30Si ratios and the mass-loss rates of evolved stars, where we take the mass-loss rate as a proxy for the initial stellar mass or current stellar age. This is consistent with the different abundance ratios found in presolar grains. Before the formation of the Sun, the presolar grains indicate that the bulk of presolar grains already had 29Si/30Si ratios of about 1.5, which is also the ratio we found for the objects younger than the Sun, such as VY CMa and IRC+10216. However, we found that older objects (up to possibly 10 Gyr old) in our sample trace a previous, lower 29Si/30Si value of about 1. Material with this isotopic ratio is present in two subclasses of presolar grains, providing independent evidence of the lower ratio. Therefore, the 29Si/30Si ratio derived from the SiO emission of evolved stars is a useful diagnostic tool for the study of the GCE and presolar grains. © ESO, 2013.

Wittkowski M.,ESO Garching | Arroyo-Torres B.,DIPC | Marcaide J.M.,University of Valencia | Abellan F.J.,University of Nice Sophia Antipolis | And 5 more authors.
Proceedings of the International Astronomical Union | Year: 2014

We present near-infrared spectro-interferometric studies of red supergiant (RSG) stars using the VLTI/AMBER instrument, which are compared to previously obtained similar observations of AGB stars. Our observations indicate spatially extended atmospheric molecular layers of water vapor and CO, similar as previously observed for Mira stars. Data of VY∼CMa indicate that the molecular layers are asymmetric, possibly clumpy. Thanks to the spectro-interferometric capabilities of the VLTI/AMBER instrument, we can isolate continuum bandpasses, estimate fundamental parameters of our sources, locate them in the HR diagram, and compare their positions to recent evolutionary tracks. For the example of VY CMa, this puts it close to evolutionary tracks of initial mass 25-32 M S™. Comparisons of our data to hydrostatic model atmospheres, 3d simulations of convection, and 1d dynamic model atmospheres based on self-excited pulsation models indicate that none of these models can presently explain the observed atmospheric extensions for RSGs. The mechanism that levitates the atmospheres of red supergiant is thus a currently unsolved problem. Copyright © International Astronomical Union 2015.

Manara C.F.,ESO Garching | Testi L.,ESO Garching | Alcala J.M.,National institute for astrophysics | Covino E.,National institute for astrophysics | And 4 more authors.
Astrophysics and Space Science Proceedings | Year: 2014

The knowledge of the photospheric parameters and the level of chromospheric activity in young pre-main sequence stars is one of the main limitations when trying to measure mass accretion rates in Class II YSOs. A detailed characterization of photospheres and chromospheric activities in low-mass, young stars without disks (late K and M type Class III YSOs) is still missing. Using VLT/XShooter spectra we have analyzed a sample of Class III in the Spectral Type range between K5 and M9.5. We report on the characterization of the chromospheric emission in the stars in our sample and on the implications that our work has on accurate measurements of mass accretion rates in YSOs. © Springer International Publishing Switzerland 2014.

Federrath C.,Ecole Normale Superieure de Lyon | Federrath C.,University of Heidelberg | Sur S.,University of Heidelberg | Schleicher D.R.G.,ESO Garching | And 3 more authors.
Astrophysical Journal | Year: 2011

Cosmic structure formation is characterized by the complex interplay between gravity, turbulence, and magnetic fields. The processes by which gravitational energy is converted into turbulent and magnetic energies, however, remain poorly understood. Here, we show with high-resolution, adaptive-mesh simulations that MHD turbulence is efficiently driven by extracting energy from the gravitational potential during the collapse of a dense gas cloud. Compressible motions generated during the contraction are converted into solenoidal, turbulent motions, leading to a natural energy ratio of E sol/E tot ≈ 2/3. We find that the energy injection scale of gravity-driven turbulence is close to the local Jeans scale. If small seeds of the magnetic field are present, they are amplified exponentially fast via the small-scale dynamo process. The magnetic field grows most efficiently on the smallest scales, for which the stretching, twisting, and folding of field lines, and the turbulent vortices are sufficiently resolved. We find that this scale corresponds to about 30 grid cells in the simulations. We thus suggest a new minimum resolution criterion of 30 cells per Jeans length in (magneto)hydrodynamical simulations of self-gravitating gas, in order to resolve turbulence on the Jeans scale, and to capture minimum dynamo amplification of the magnetic field. Due to numerical diffusion, however, any existing simulation today can at best provide lower limits on the physical growth rates. We conclude that a small, initial magnetic field can grow to dynamically important strength on timescales significantly shorter than the free-fall time of the cloud. © 2011. The American Astronomical Society. All rights reserved.

Navarrete J.,ESO LaSilla Paranal Observatory | Kolb J.,ESO Garching | Lombardi G.,ESO LaSilla Paranal Observatory | Noethe L.,ESO Garching | Sarazin M.,ESO Garching
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2014

A one year database has been gathered from the VLT active optics Shack-Hartmann (S-H) wavefront sensor images taken at each operating focus about every 30 seconds. The VLT telescope control software includes a dedicated code to extract the median full width at half maximum of the unvignetted S-H spots which is used for this study. This code applies a 1-D fit, assuming circular Hartmann spots, which allows to work only on foci equipped with atmospheric dispersion correction, or when the telescopes are observing close to zenith. The S-H image size measured inside the 30m enclosures is compared the outside seeing measured at 6m above ground by the VLT Astronomical Site Monitor (DIMM). A method for correcting DIMM measurements from surface layer turbulence contamination is proposed. © 2014 SPIE.

Loading ESO Garching collaborators
Loading ESO Garching collaborators