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Publication archive

A&A doi http://dx.doi.org/10.1051/0004-6361/200912855

We present a system-level description of the Low Frequency Instrument (LFI) considered as a differencing polarimeter, and evaluate its expected performance. The LFI is one of the two instruments on board the ESA Planck mission to study the cosmic microwave background. It consists of a set of 22 radiometers sensitive to linear polarisation, arranged in orthogonally-oriented pairs connected to 11 feed horns operating at 30, 44 and 70 GHz. In our analysis, the generic Jones and Mueller-matrix formulations for polarimetry are adapted to the special case of the LFI. Laboratory measurements of flight components are combined with optical simulations of the telescope to investigate the values and uncertainties in the system parameters affecting polarisation response. Methods of correcting residual systematic errors are also briefly discussed.

Published: 09 July 2010

Context. In July 2010 the ESA spacecraft Rosetta will fly by the main belt asteroid 21 Lutetia. Several observations of this asteroid have been performed so far, but its surface composition and nature are still a matter of debate. For a long time Lutetia was supposed to have a metallic nature due to its high IRAS albedo. Later on it has been suggested that the asteroid has a surface composition similar to primitive carbonaceous chondrite meteorites, while further observations proposed a possible genetic link with more evolved enstatite chondrite meteorites.

Aims. We performed visible spectroscopic observations of 21 Lutetia in November 2008 at the Telescopio Nazionale Galileo (TNG, La Palma, Spain) to make a decisive contribution to solving the conundrum of its nature.

Methods. Thirteen visible spectra were acquired at different rotational phases and subsequently analyzed.

Results. We confirm a narrow spectral feature at about 0.47-0.48 microns which was already found by Lazzarin et al. (2004, A&A, 425, L25) in the spectra of Lutetia. We also confirm an earlier find of Lazzarin et al. (2004), who detected a spectral feature at about 0.6 microns in one of their Lutetia's spectra. More remarkable is the difference of our spectra though, which exhibit different spectral slopes between 0.6 and 0.75 microns and, in particular, we found that up to 20% of the Lutetia surface could have flatter spectra.

Conclusions. We detected a variation of the spectral slopes at different rotational phases that could be interpreted as possibly due to differences in the chemical/mineralogical composition as well as to inhomogeneities of the structure of the Lutetia's surface (e.g., to craters or albedo spots) in the southern hemisphere.

Published: 16 April 2010
We seek the best size estimates of the asteroid (21) Lutetia, the direction of its spin axis, and its bulk density, assuming its shape is well described by a smooth featureless triaxial ellipsoid, and to evaluate the deviations from this assumption. Methods. We derive these quantities from the outlines of the asteroid in 307 images of its resolved apparent disk obtained with adaptive optics (AO) at Keck II and VLT, and combine these with recent mass determinations to estimate a bulk density. Our best triaxial ellipsoid diameters for Lutetia, based on our AO images alone, are a x b x c = 132 x 101 x 93 km, with uncertainties of 4 x 3 x 13 km including estimated systematics, with a rotational pole within 5 deg. of ECJ2000 [long,lat] = [45, -7], or EQJ2000 [RA, DEC] = [44, +9]. The AO model fit itself has internal precisions of 1 x 1 x 8 km, but it is evident, both from this model derived from limited viewing aspects and the radius vector model given in a companion paper, that Lutetia has significant departures from an idealized ellipsoid. In particular, the long axis may be overestimated from the AO images alone by about 10 km. Therefore, we combine the best aspects of the radius vector and ellipsoid model into a hybrid ellipsoid model, as our final result, of 124 +/- 5 x 101 +/- 4 x 93 +/- 13 km that can be used to estimate volumes, sizes, and projected areas. The adopted pole position is within 5 deg. of [long, lat] = [52, -6] or[RA DEC] = [52, +12]. Using two separately determined masses and the volume of our hybrid model, we estimate a density of 3.5 +/- 1.1 or 4.3 +/- 0.8 g cm-3 . From the density evidence alone, we argue that this favors an enstatite-chondrite composition, although other compositions are formally allowed at the extremes (low-porosity CV/CO carbonaceous chondrite or high-porosity metallic). We discuss this in the context of other evidence.
Published: 09 July 2010
A taxonomic system was introduced by C. R. Chapman, D. Morrison, and B. Zellner [Icarus 25, 104 - 130 (1975)], in which minor planets are classified according to a few readily observable optical properties, independent of specific mineralogical interpretations. That taxonomy is here augmented to five classes, now precisely defined in terms of seven parameters obtained from polarimetry, spectrophotometry, radiometry, and UBV photometry of 523 objects. We classify 190 asteroids as type C, 141 as type S, 13 as type M, 3 as type E, and 3 as type R; 55 objects are shown to fall outside these five classes and are designated U (unclassifiable). For the remaining 118, the data exclude two or more types but are insufficient for unambiguous classification. Reliable diameters, from radiometry or polarimetry or else from albedos adopted as typical of the types, are listed for 396 objects. We also compare our taxonomy with other ones and discuss how classification efforts are related to the interpretation of asteroid mineralogies.
Published: 09 July 2010

Aims.The aim of this paper is to investigate the surface composition of the two asteroids 21 Lutetia and 2867 Steins, targets of the Rosetta space mission.

Methods.We observed the two asteroids through their full rotational periods with the Infrared Spectrograph of the Spitzer Space Telescope to investigate the surface properties. The analysis of their thermal emission spectra was carried out to detect emissivity features that diagnose the surface composition.

Results. For both asteroids, the Christiansen peak, the Reststrahlen, and the Transparency features were detected. The thermal emissivity shows a clear analogy to carbonaceous chondrite meteorites, in particular to the CO-CV types for 21 Lutetia, while for 2867 Steins, already suggested as belonging to the E-type asteroids, the similarity to the enstatite achondrite meteorite is confirmed.

Published: 15 January 2008
(Abbreviated abstract)
New large-scale CO surveys of the first and second Galactic quadrants and the nearby molecular cloud complexes in Orion and Taurus, obtained with the CfA 1.2 m telescope, have been combined with 31 other surveys obtained over the past two decades with that instrument and a similar telescope on Cerro Tololo in Chile, to produce a new composite CO survey of the entire Milky Way. The survey consists of 488,000 spectra that Nyquist or beamwidth (1/8 °) sample the entire Galactic plane over a strip 4°-10° wide in latitude, and beamwidth 1/4 ° sample nearly all large local clouds at higher latitudes. Compared with the previous composite CO survey of Dame et al. (1987), the new survey has 16 times more spectra, up to 3.4 times higher angular resolution, and up to 10 times higher sensitivity per unit solid angle.
Published: 01 February 2001
This publication, often referred to as the Blue Book, provides a introduction to the Planck mission and a comprehensive overview of the scientific capabilities of the mission.
Published: 01 January 2005

The assessment study of Planetary Entry Probes (PEP) for Venus and three of the outer planets (Saturn, Uranus, and Neptune) has been performed at ESA's Concurrent Design Facility (CDF) and ran from 14 April to 30 June 2010.

The internal final presentation has been prepared by the PEP/CDF team and summarizes the outcome of the PEP assessment study. This presentation can be downloaded below as a PDF (link to publication).

Contents of the presentation:

Introduction
Systems
Mission analysis
Aerothermodynamics
Thermal
Entry, Descent & Landing
Payload
Configuration
Structures
Mechanisms
Guidance & Navigation Control
Power
Communications
Data Handling System
Ground Segment & Operations
Programmatics/ AIV
Risk
Annex I: Atmospheric Models
for Outer Planets

A summary of the PEP study is on the "Planetary Entry Probes (PEP)" page, linked form the right-hand menu.

Published: 01 July 2010
The composition of the ancient martian crust is a key ingredient in deciphering the environment and evolution of early Mars. We present an analysis of the composition of large craters in the martian northern plains based on data from spaceborne imaging spectrometers. Nine of the craters have excavated assemblages of phyllosilicates from ancient, Noachian crust buried beneath the plains' cover. The phyllosilicates are indistinguishable from those exposed in widespread locations in the southern highlands, demonstrating that liquid water once altered both hemispheres of Mars.
Published: 25 June 2010
This paper is the second in a series devoted to the hard X-ray (17-60 keV) whole sky survey performed by the INTEGRAL observatory over seven years. Here we present a catalog of detected sources which includes 521 objects, 449 of which exceed a 5 sigma detection threshold on the time-averaged map of the sky, and 53 were detected in various subsamples of exposures. Among the identified sources with known and suspected nature, 262 are Galactic (101 low-mass X-ray binaries, 95 high-mass X-ray binaries, 36 cataclysmic variables, and 30 of other types) and 219 are extragalactic, including 214 active galactic nuclei (AGNs), 4 galaxy clusters, and galaxy ESO 389-G 002. The extragalactic (|b|>5 deg) and Galactic (|b|<5 deg) persistently detected source samples are of high identification completeness (respectively ~96% and ~94%) and valuable for population studies.
Published: 24 June 2010
A general circulation model (GCM) has been developed for the Venus atmosphere, from the surface up to 100 km altitude, based on the GCM developed for Earth at our laboratory. Key features of this new GCM include topography, diurnal cycle, dependence of the specific heat on temperature, and a consistent radiative transfer module based on net exchange rate matrices. This allows a consistent computation of the temperature field, in contrast to previous GCMs of Venus atmosphere that used simplified temperature forcing. The circulation is analyzed after 350 Venus days (111 Earth years). Superrotation is obtained above roughly 40 km altitude. Below, the zonal wind remains very small compared to observed values, which is a major pending question. The meridional circulation consists of equator-to-pole cells, the dominant one being located within the cloud layers. The modeled temperature structure is globally consistent with observations, though discrepancies persist in the stability of the lowest layers and equator-pole temperature contrast within the clouds (10 K in the model compared to the observed 40 K). In agreement with observational data, a convective layer is found between the base of the clouds (around 47 km) and the middle of the clouds (55-60 km altitude). The transport of angular momentum is analyzed, and comparison between the reference simulation and a simulation without diurnal cycle illustrates the role played by thermal tides in the equatorial region. Without diurnal cycle, the Gierasch-Rossow-Williams mechanism controls angular momentum transport. The diurnal tides add a significant downward transport of momentum in the equatorial region, causing low latitude momentum accumulation.
Published: 13 June 2010

The mapping IR channel of the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS-M) on board the Venus Express spacecraft observes the CO2 band at 4.3 Œm at a spectral resolution adequate to retrieve the atmospheric temperature profiles in the 65-96 km altitude range.

Observations acquired in the period June 2006 - July 2008 were used to derive average temperature fields as a function of latitude, subsolar longitude (i.e.: local time, LT) and pressure. Coverage presented here is limited to the nighttime because of the adverse effects of daytime non-LTE emission on the retrieval procedure, and to southernmost latitudes because of the orientation of the Venus-Express orbit. Maps of air temperature variability are also presented as the standard deviation of the population included in each averaging bin.

At the 100 mbar level (about 65 km above the reference surface) temperatures tend to decrease from the evening to the morning side, despite a local maximum observed around 20-21LT. The cold collar is evident around 65S, with a minimum temperature at 3LT. Moving to higher altitudes, local time trends become less evident at 12.6 mbar (about 75 km) where the temperature monotonically increases from middle-latitudes to the southern pole. Nonetheless, at this pressure level, two weaker local time temperature minima are observed at 23LT and 2LT equatorward of 60S. Local time trends in temperature reverse about 85 km, where the morning side is the warmer.

The variability at the 100 mbar level is maximum around 80S and stronger toward the morning side. Moving to higher altitudes, the morning side always shows the stronger variability. Southward of 60S, standard deviation presents minimum values around 12.6 mbar for all the local times.

Published: 23 June 2010
Ultra-compact X-ray binaries consist of a neutron star or black hole that accretes material from a white dwarf-donor star. The ultra-compact nature is expressed in very short orbital periods of less than 1 hour. In the case of 4U 0614+091 oxygen-rich material from a CO or ONe white dwarf is flowing to the neutron star. This oxygen-rich disc can reflect X-rays emitted by the neutron star giving a characteristic emission spectrum. We have analyzed high-resolution RGS and broad band EPIC spectra of 4U 0614+091 obtained by the XMM-Newton satellite. We detect a broad emission feature at ~0.7 keV in both instruments, which cannot be explained by unusual abundances of oxygen and neon in the line of sight, as proposed before in the literature. We interpret this feature as O VIII Ly-alpha emission caused by reflection of X-rays off highly ionized oxygen, in the strong gravitational field close to the neutron star.
Published: 22 June 2010
Aims. A wide observational campaign was carried out in 2004-2009 that aimed to complete the ground-based investigation of Lutetia prior to the Rosetta fly-by in July 2010.

Methods. We obtained BVRI photometric and V-band polarimetric measurements over a wide range of phase angles, and visible and infrared spectra in the 0.4-2.4 micron range. We analyze them with previously published data to retrieve information about Lutetia's surface properties.

Results. Values of lightcurve amplitudes, absolute magnitude, opposition effect, phase coefficient, and BVRI colors of Lutetia surface seen at near pole-on aspect are determined. We define more precisely parameters of polarization phase curve and show their distinct deviation from any other moderate-albedo asteroid. An indication of possible variations in both polarization and spectral data across the asteroid surface are found. To explain features found by different techniques, we propose that (i) Lutetia has a non-convex shape, probably due to a large crater, and heterogeneous surface properties probably related to surface morphology; (ii) at least part of the surface is covered by a fine-grained regolith of particle size smaller than 20 micron; (iii) the closest meteorite analogues of Lutetia's surface composition are particular types of carbonaceous chondrites, or Lutetia has specific surface composition that is not representative among studied meteorites.

Published: 04 June 2010
We report on the discovery of two galaxy clusters, SPT-CL J2332-5358 and SPT-CL J2342-5411, in X-rays. These clusters were also independently detected through their Sunyaev-Zel'dovich effect by the South Pole Telescope, and in the optical band by the Southern Cosmology Survey. They are thus the first clusters detected under survey conditions by all major cluster search approaches. The X-ray detection is made within the frame of the XMM-BCS cluster survey utilizing a novel XMM-Newton mosaic-mode of observations.
The present study makes the first scientific use of this operation mode. We estimate the X-ray spectroscopic temperature of SPT-CL J2332-5358 (at redshift z = 0.32) to be T = 9.3 [+3.3][-1.9] keV, implying a high mass, M500 = 8.8 ± 3.8 × 1014 solar masses. For SPT-CL J2342-5411, at z = 1.08, the available X-ray data do not allow us to directly estimate the temperature with good confidence. However, using our measured luminosity and scaling relations we estimate that T = 4.5 ± 1.3 keV and M500 = 1.9 ± 0.8 × 1014 solar masses. We find a good agreement between the X-ray masses and those estimated from the Sunyaev-Zel'dovich effect.
Published: 19 May 2010
This paper is the first in a series devoted to the hard X-ray whole sky survey performed by the INTEGRAL observatory over seven years. Here we present an improved method for image reconstruction with the IBIS coded mask telescope. The main improvements are related to the suppression of systematic effects which strongly limit sensitivity in the region of the Galactic Plane (GP), especially in the crowded field of the Galactic Center (GC). We extended the IBIS/ISGRI background model to take into account the Galactic Ridge X-ray Emission (GRXE). To suppress residual systematic artifacts on a reconstructed sky image we applied nonparametric sky image filtering based on wavelet decomposition. The implemented modifications of the sky reconstruction method decrease the systematic noise in the ~20 Ms deep field of GC by ~44%, and practically remove it from the high-latitude sky images. New observational data sets, along with an improved reconstruction algorithm, allow us to conduct the hard X-ray survey with the best currently available minimal sensitivity 3.7E-12 erg/s/cm2 ~0.26 mCrab in the 17-60 keV band at a 5 sigma detection level. The survey covers 90% of the sky down to the flux limit of 6.2E-11 erg/s/cm2 (~4.32 mCrab) and 10% of the sky area down to the flux limit of 8.6E-12 erg/s/cm2 (~0.60 mCrab).
Published: 21 May 2010

The May 2010 issue of Astronomy & Astrophysics is a special feature devoted to the new results obtained with the infrared satellite AKARI, a JAXA project with the participation of ESA. It includes 17 articles dealing with various subjects. Some papers are based on the AKARI all-sky survey, which has just been released. Others are dedicated to pointed observations of many astronomical targets from solar system bodies to distant galaxies.

Published: 02 April 2010
In a previous paper, we reported a 3 sigma detection of an absorption line from the warm-hot intergalactic medium (WHIM) using the Chandra and XMM X-ray grating spectra of the blazar H2356-309, the sight line of which intercepts the Sculptor Wall, a large-scale superstructure of galaxies at z ~ 0.03. To verify our initial detection, we obtained a deep (500 ks), follow-up exposure of H2356-309 as part of the Cycle-10 Chandra Large Project Program. From a joint analysis of the Cycle-10 and previous (Cycle-8) Chandra grating data we detect the redshifted O VII WHIM line at a significance level of 3.4 sigma, a substantial improvement over the 1.7 sigma level reported previously when using only the Cycle-8 data. The significance increases to 4.0 sigma when the existing XMM grating data are included in the analysis, thus confirming at higher significance the existence of the line at the redshift of the Sculptor Wall with an equivalent width of 28.5 ± 10.5 mÅ (90% confidence). We obtain a 90% lower limit on the O VII column density of 0.8 × 1016 cm-2 and a 90% upper limit on the Doppler b parameter of 460 km s-1. Assuming the absorber is uniformly distributed throughout the ~15 Mpc portion of the blazar's sight line that intercepts the Sculptor Wall, that the O VII column density is 2 × 1016 cm-2 (corresponding to b >~150 km-1where the inferred column density is only weakly dependent on b), and that the oxygen abundance is 0.1 solar, we estimate a baryon over-density of ~30 for the WHIM, which is consistent with the peak of the WHIM mass fraction predicted by cosmological simulations. The clear detection of O VII absorption in the Sculptor Wall demonstrates the viability of using current observatories to study WHIM in the X-ray absorption spectra of blazars behind known large-scale structures.
Published: 12 May 2010
In June 2006 Venus Express crossed several times the outer boundary of Venus induced magnetosphere, its magnetosheath and its bow shock. During the same interval the Cluster spacecraft surveyed the dawn flank of the terrestrial magnetosphere, intersected the Earth's magnetopause and spent long time intervals in the magnetosheath. This configuration offers the opportunity to perform a joint investigation of the interface between Venus and Earth's outer plasma layers and the shocked solar wind. We discuss the kinetic structure of the magnetopause of both planets, its global characteristics and the effects on the interaction between the planetary plasma and the solar wind. A Vlasov equilibrium model is constructed for both planetary magnetopauses and provides good estimates of the magnetic field profile across the interface. The model is also in agreement with plasma data and evidence the role of planetary and solar wind ions on the spatial scale of the equilibrium magnetopause of the two planets. The main characteristics of the two magnetopauses are discussed and compared.
Published: 07 May 2010
The downstream region of a collisionless quasiparallel shock is structured containing bulk flows with high kinetic energy density from a previously unidentified source. We present Cluster multispacecraft measurements of this type of supermagnetosonic jet as well as of a weak secondary shock front within the sheath, that allow us to propose the following generation mechanism for the jets: The local curvature variations inherent to quasiparallel shocks can create fast, deflected jets accompanied by density variations in the downstream region. If the speed of the jet is super(magneto)sonic in the reference frame of the obstacle, a second shock front forms in the sheath closer to the obstacle. Our results can be applied to collisionless quasiparallel shocks in many plasma environments.
Published: 20 April 2010
3-Apr-2020 18:27 UT

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