Publication archive

Publication archive

Ordinary baryonic particles (such as protons and neutrons) account for only one-sixth of the total matter in the Universe. The remainder is a mysterious 'dark matter' component, which does not interact via electromagnetism and thus neither emits nor reflects light. As dark matter cannot be seen directly using traditional observations, very little is currently known about its properties. It does interact via gravity, and is most effectively probed through gravitational lensing: the deflection of light from distant galaxies by the gravitational attraction of foreground mass concentrations. This is a purely geometrical effect that is free of astrophysical assumptions and sensitive to all matter - whether baryonic or dark. Here we show high-fidelity maps of the large-scale distribution of dark matter, resolved in both angle and depth. We find a loose network of filaments, growing over time, which intersect in massive structures at the locations of clusters of galaxies. Our results are consistent with predictions of gravitationally induced structure formation, in which the initial, smooth distribution of dark matter collapses into filaments then into clusters, forming a gravitational scaffold into which gas can accumulate, and stars can be built.
Published: 07 January 2007

Context: In 2004 asteroid (2867) Steins has been selected as a flyby target for the Rosetta mission. Determination of its spin period and the orientation of its rotation axis are essential for optimization of the flyby planning.

Aims: Measurement of the rotation period and light curve of asteroid (2867) Steins at a phase angle larger than achievable from ground based observations, providing a high quality data set to contribute to the determination of the orientation of the spin axis and of the pole direction.

Methods: On March 11, 2006, asteroid (2867) Steins was observed continuously for 24 h with the scientific camera system OSIRIS onboard Rosetta. The phase angle was 41.7 degrees, larger than the maximum phase angle of 30 degrees when Steins is observed from Earth. A total of 238 images, covering four rotation periods without interruption, were acquired.

Results: The light curve of (2867) Steins is double peaked with an amplitude of 0.23 mag. The rotation period is 6.052 +- 0.007 h. The continuous observations over four rotation periods exclude the possibility of period ambiguities. There is no indication of deviation from a principal axis rotation state. Assuming a slope parameter of G = 0.15, the absolute visual magnitude of Steins is 13.05 +/- 0.03.

Published: 04 January 2007
The surface of Saturn's haze-shrouded moon Titan has long been proposed to have oceans or lakes, on the basis of the stability of liquid methane at the surface. Initial visible and radar imaging failed to find any evidence of an ocean, although abundant evidence was found that flowing liquids have existed on the surface. Here we provide definitive evidence for the presence of lakes on the surface of Titan, obtained during the Cassini Radar flyby of Titan on 22 July 2006 (T16). The radar imaging polewards of 70° north shows more than 75 circular to irregular radar-dark patches, in a region where liquid methane and ethane are expected to be abundant and stable on the surface. The radar-dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions. Some of the lakes do not completely fill the depressions in which they lie, and apparently dry depressions are present. We interpret this to indicate that lakes are present in a number of states, including partly dry and liquid-filled. These northern-hemisphere lakes constitute the strongest evidence yet that a condensable-liquid hydrological cycle is active in Titan's surface and atmosphere, in which the lakes are filled through rainfall and/or intersection with the subsurface 'liquid methane' table.
Published: 04 January 2007
Fast reconnection is crucial to magnetospheric substorms, solar and stellar flares and fusion plasmas. Ultimate confirmation of fast reconnection must be achieved by multi-spacecraft detections of the reconnection rate itself and associated dimensions of the diffusion region. Here we report a multi-spacecraft measurement of fast reconnection rate gammarec ~ Vin ~ (0.07-0.15)VA based on directly measurements of the plasma flow into the diffusion region, where Vin is the speed of reconnecting flux and VA the characteristic Alfvén speed. It falls in the range of ~(0.03-0.2)VA predicted in steady state reconnection simulation. The characteristic sizes for the diffusion region of the width Lz ~ 0.9 di (= 460 km) and the length Lx ~ (3.3-5.1)di (= 1680-2597 km) are measured as well. The length of the diffusion region Lx is determined for the first time based on the in situ observations. Furthermore, other features detected during the event also match the previous observation and simulation results.
Published: 03 January 2007
Contents:
  • Hubble's Bequest to Astronomy
  • PHLAG: Pipeline for Hubble Legacy Archive Grism Data
  • ESA-ESO Topical Science Working Groups
  • Solar System Bodies in Hubble Observations
  • Staff Update
  • Communication of the "Pluto Affair"
  • NASA Award for ST-ECF Staff
  • The International Year of Astronomy 2009
  • Servicing Mission 4
Published: 15 December 2006
E-print arXiv:astro-ph/0612384, for ApJS COSMOS Special Issue, 2007 in press

We present the first identification of large-scale structures (LSS) at z < 1.1 in the Cosmic Evolution Survey (COSMOS). The structures are identified from adaptive smoothing of galaxy counts in the pseudo-3d space (R.A., Dec., z) using the COSMOS photometric redshift catalog. The technique is tested on a simulation including galaxies distributed in model clusters and a field galaxy population - recovering structures on all scales from 1 to 202 without a priori assumptions for the structure size or density profile. Our procedure makes no a priori selection on galaxy spectral energy distribution (SED, for example the Red Sequence), enabling an unbiased investigation of environmental effects on galaxy evolution. The COSMOS photometric redshift catalog yields a sample of 1.5 × 105 galaxies with redshift accuracy, delta zFWHM/(1 + z) d 0.1 at z < 1.1 down to IAB d 25 mag. Using this sample of galaxies, we identify 42 large-scale structures and clusters. Projected surface-density maps for the structures indicate multiple peaks and internal structure in many of the most massive LSS. The stellar masses (determined from the galactic SEDs) for the LSS range from M* ~ 1011 up to ~ 3 × 1013 Msun. Five LSS have total stellar masses exceeding 1013 Msun. (Total masses including non-stellar baryons and dark matter are expected to be ~ 50 - 100 times greater.) The derived mass function for the LSS is consistent (within the expected Poisson and cosmic variances) with those derived from optical and X-ray studies at lower redshift.

Published: 14 December 2006
A hemispheric dichotomy on Mars is marked by the sharp contrast between the sparsely cratered northern lowland plains and the heavily cratered southern highlands. Mechanisms proposed to remove ancient crust or form younger lowland crust include one or more giant impacts, subcrustal transport by mantle convection, the generation of thinner crust by plate tectonics, and mantle overturn following solidification of an early magma ocean. The age of the northern lowland crust is a significant constraint on these models. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on the European Space Agency's Mars Express spacecraft is providing new constraints on the martian subsurface. Here we show evidence of buried impact basins ranging in diameter from about 130 km to 470 km found over 14 per cent of the northern lowlands. The number of detected buried basins >200 km in diameter indicates that the lowland crust is ancient, dating back to the Early Noachian epoch. This crater density is a lower limit because of the likelihood that not all buried basins in the area surveyed by MARSIS have been detected. An Early Noachian age for the lowland crust has been previously suggested on the basis of a large number of quasi-circular topographic depressions interpreted to be evidence of buried basins. Only a few of these depressions in the area surveyed by MARSIS, however, correlate with the detected subsurface echoes. On the basis of the MARSIS data, we conclude that the northern lowland crust is at least as old as the oldest exposed highland crust. This suggests that the crustal dichotomy formed early in the geologic evolution of Mars.
Published: 14 December 2006
XMM-Newton is a major X-ray observatory of the European Space Agency (ESA). Its observing time is open to astronomers from the whole scientific community on a peer reviewed competitive basis. The Science Operations Centre, located at ESAs premises in Villafranca del Castillo, Spain, is responsible for the instrument operations, as well as for all the tasks related to facilitating the scientific exploitation of the data which the mission has been producing since its launch in December 1999.
Published: 12 December 2006
The objective of the DARWIN System Assessment Study is the definition of the overall architecture and the preliminary design of the DARWIN mission. This includes an operational orbit at the Second Lagrange Point of the Sun-Earth system (L2), a launch and transfer scenario, and a spacecraft and payload design which ensure that the mission requirements can be fulfilled. The specifications of the subsystems have to be derived, critical items and drivers have to be identified, and required technology development activities have to be proposed in order to allow for establishing a roadmap towards the verification of the science performance feasibility. The DARWIN System Assessment Study is divided into two phases. Phase 1 is concerned with a review and trade-off of different concepts concerning the payload, the space segment, and the mission. Phase 2 is devoted to a detailed design of the payload and the space segment, as well with a consolidated mission design. A third phase is foreseen for design consolidation.
Published: 12 December 2006
Some examples of space-borne applications that require improvements in detector technology compared with conventional Si and Ge designs are described. Properties of compound semiconductors are noted, and a range of different detector developments are briefly reviewed. Material fabrication improvements for several compound semiconductors have resulted in near Fano-limited performance.
Published: 30 November 2006
Cluster crossed the magnetotail neutral sheet on four occasions between 16:38 and 16:43 UT on 08/17/2003. The four-spacecraft capabilities of Cluster are used to determine spatial gradients from the magnetic field vectors and, for the first time, full electron pressure tensors. We find that the contribution to the electric field from the Hall term (max of ~6 mV/m) pointed towards the neutral sheet, whereas that from the electron pressure divergence (max of ~1 mV/m) pointed away from the neutral sheet. The electric field contributions in this direction were closely anti-correlated. During this period Clusters 1 and 4 were sometimes above and below the neutral sheet respectively. This allowed the simultaneous observation of magnetic fields that are interpreted as two quadrants of the Hall magnetic field system. An associated field-aligned current system was detected using the curlometer and moments of the particle distributions.
Published: 29 November 2006
We present a statistical study of four years of Cluster crossings of the mid-altitude cusp. In this first part of the study, we start by introducing the method we have used a) to define the cusp properties, b) to sort the interplanetary magnetic field (IMF) conditions or behaviors into classes, c) to determine the proper time delay between the solar wind monitors and Cluster. Out of the 920 passes that we have analyzed, only 261 fulfill our criteria and are considered as cusp crossings. We look at the size, location and dynamics of the mid-altitude cusp under various IMF orientations and solar wind conditions. For southward IMF, Bz rules the latitudinal dynamics, whereas By governs the zonal dynamics, confirming previous works. We show that when |By| is larger than |Bz|, the cusp widens and its location decorrelates from By. We interpret this feature in terms of component reconnection occurring under By-dominated IMF. For northward IMF, we demonstrate that the location of the cusp depends primarily upon the solar wind dynamic pressure and upon the Y-component of the IMF. Also, the multipoint capability of Cluster allows us to conclude that the cusp needs typically more than ~20 min to fully adjust its location and size in response to changes in external conditions, and its speed is correlated to variations in the amplitude of IMF-Bz. Indeed, the velocity in °ILAT/min of the cusp appears to be proportional to the variation in Bz in nT: Vcusp=0.024 Delta-Bz. Finally, we observe differences in the behavior of the cusp in the two hemispheres. Those differences suggest that the cusp moves and widens more freely in the summer hemisphere.
Published: 22 November 2006
We report the observation of two stellar occultations by Titan on 14 November 2003, using stations in the Indian Ocean, southern Africa, Spain, and northern and southern Americas. These occultations probed altitudes between~550 and 250 km (~1 to 250 microbar) in Titan's upper stratosphere. The light curves reveal a sharp inversion layer near 515 ± 6 km altitude (1.5 microbar pressure level), where the temperature increases by 15 K in only 6 km. This layer is close to an inversion layer observed fourteen months later by the Huygens HASI instrument during the entry of the probe in Titan's atmosphere on 14 January 2005 [Fulchignoni et al., 2005]. Central flashes observed during the first occultation provide constraints on the zonal wind regime at 250 km, with a strong northern jet (~200 m s-1) around the latitude 55°N, wind velocities of ~150 m s-1 near the equator, and progressively weaker winds as more southern latitudes are probed. The haze distribution around Titan's limb at 250 km altitude is close to that predicted by the Global Circulation Model of Rannou et al. (2004) in the southern hemisphere, but a clearing north of 40°N is necessary to explain our data. This contrasts with Rannou et al.'s (2004) model, which predicts a very thick polar hood over Titan's northern polar regions. Simultaneous observations of the flashes at various wavelengths provide a dependence of tau ~ lambda-q , with q = 1.8 ± 0.5 between 0.51 and 2.2 micron for the tangential optical depth of the hazes at 250 km altitude.
Published: 18 November 2006
The work of ESA's INTEGRAL high-energy observatory usually follows a long-term plan that is established every year by selecting only the very best of the numerous observing proposals from the scientific community. However, nature does not always follow the same plan, so INTEGRAL and the people who keep it running have to react to unforeseen and sudden 'Targets of Opportunity'. The case here is neutron star IGR J00291+5934, an incredibly dense object with a mass similar to that of our Sun's compressed into a rapidly spinning sphere only a few kilometres across. Not only that, but it is busy swallowing its stellar companion.
Published: 15 November 2006
In 2008, an Ariane-5 will lift off from French Guiana carrying ESA's two pioneering Herschel and Planck deep space observatories to explore previously unknown regions of the Universe. Their target is the 'bright' part of the far-infrared spectrum that has tantalised scientists for decades. Until now, the technology has not existed to make precise observations of a distant domain that touches the very beginning of time.
Published: 15 November 2006
In 2008, an Ariane-5 will lift off from French Guiana carrying ESA's two pioneering Herschel and Planck deep space observatories to explore previously unknown regions of the Universe. Their target is the 'bright' part of the far-infrared spectrum that has tantalised scientists for decades. Until now, the technology has not existed to make precise observations of a distant domain that touches the very beginning of time.
Published: 15 November 2006

Venus Express was launched in late October, 2005, and arrived at the planet in April 2006, where it is now in orbit and the return to Earth of new information about Venus' atmosphere, surface, and space environment has begun. The purpose of this special issue of Planetary and Space Science is to lay out the background to the mission, in terms of the planet and its mysteries as well as the spacecraft, its instruments, and the planned observations, in order to review the context in which the new results will be analysed and interpreted.

- The remainder of the abstract is truncated -

Published: 15 November 2006
We examined the relationship between bursty bulk flow (BBF) events observed by Cluster between -19 RE < X < -12 RE and dipolarization events observed by Double Star TC1 between -13 RE < X < -6 RE. TC1 observed dipolarizations for ~33% of the cases when BBFs were observed by Cluster. During these dipolarization events the TC1 location was closer to the Cluster location and the local BZ at TC1 was smaller than during events where TC1 observed no clear dipolarization associated with BBFs at Cluster. This result suggests that (1) flow-associated activity dissipates within a limited spatial scale, 4-8 RE, and that (2) the initial magnetic topology in the inner magnetosphere can contribute strongly to fast flow penetration toward the Earth. The fact that there were no TC1 dipolarization events at X > -8 RE associated with BBFs at Cluster in our dataset suggests two possibilities: near-geosynchronous dipolarization needs another mechanism in addition to flux pile-up and braking, or during near-geosynchronous dipolarization the near-tail current sheet/plasma sheet is too thin to be observed by Cluster.
Published: 11 November 2006
Of all the non-terrestrial ionospheres and thermospheres in our solar system those of Venus have been explored and studied the most. This is mainly because of the 14 year exploration of the well instrumented Pioneer Venus spacecraft and the theoretical studies prompted by the resulting observational information. However, there are still a number of important scientific questions that remain unanswered. These include: i) dynamics of the thermosphere; ii) the energy mechanisms/sources responsible for maintaining the elevated plasma temperatures in the ionosphere; iii) airglow/aurora intensities and their sources; and iv) hot atom populations. Venus Express is likely to help address some of the issues listed under i), iii) and iv) above.
Published: 10 November 2006
The X-Ray Observatory (XRO), also known as XEUS (X-Ray Evolving-Universe Spectroscopy), is one of the potential future missions identified in the framework of the ESA Call for Themes issued in April 2004 [RD-CV1525].

A summary of the study evolution has been provided in the previous XRO status report [RSStRep] issued at the end of March 2006. The work of ESA and JAXA on the revised mission scenario has progressed further over the past 6 months, including internal as well as industrial activities and dedicated technology developments.

Published: 02 November 2006
4-Dec-2020 23:54 UT

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