Publication archive

Publication archive

We present radio and plasma wave science (RPWS) Langmuir probe (LP) observations that give evidence for a population of heavy, negative ions at altitudes below 900 km in Titan's ionosphere during the Cassini T70 flyby. The negative ion density in this region is comparable to, or higher than, the electron density of 760 cm-3. Both positive and negative ions are moving with a velocity of at least a few hundred m s-1 relative to Titan. We show two limiting cases where we have analysed RPWS/LP ion measurements. The data can be interpreted as either that a population of negative ions with density comparable to the electron density is present, moving at a very high (>2 km s-1) velocity, or that the ion population is moving at a few hundred m s-1, but with a density an order of magnitude larger than the electron density in the same region.
Published: 17 May 2012
We analyze night-time near-infrared (NIR) thermal emission images of the Venus surface obtained with the 1-micron channel of the Venus Monitoring Camera onboard Venus Express. Comparison with the results of the Magellan radar survey and the model NIR images of the Beta-Phoebe region show that the night-time VMC images provide reliable information on spatial variations of the NIR surface emission. In this paper we consider if tessera terrain has the different NIR emissivity (and thus mineralogic composition) in comparison to the surrounding basaltic plains. This is done through the study of an area SW of Beta Regio where there is a massif of tessera terrain, Chimon-mana Tessera, surrounded by supposedly basaltic plains. Our analysis showed that 1-micron emissivity of tessera surface material is by 15-35% lower than that of relatively fresh supposedly basaltic lavas of plains and volcanic edifices. This is consistent with hypothesis that the tessera material is not basaltic, maybe felsic, that is in agreement with the results of analyses of VEX VIRTIS and Galileo NIMS data. If the felsic nature of venusian tesserae will be confirmed in further studies this may have important implications on geochemical environments in early history of Venus. We have found that the surface materials of plains in the study area are very variegated in their 1-micron emissivity, which probably reflects variability of degree of their chemical weathering. We have also found a possible decrease of the calculated emissivity at the top of Tuulikki Mons volcano which, if real, may be due to different (more felsic?) composition of volcanic products on the volcano summit.
Published: 17 May 2012
The Tyrrhena Terra region of Mars is studied with the imaging spectrometers OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) onboard Mars Express and CRISM (Compact Reconnaissance Infrared Spectrometer for Mars) onboard Mars Reconnaissance Orbiter, through the observation of tens of craters that impacted into this part of the martian highlands. The 175 detections of hydrated silicates are reported, mainly associated with ejecta blankets, crater walls and rims, and central up-lifts. Sizes of craters where hydrated silicates are detected are highly variable, diameters range from less than 1 km to 42 km. We report the presence of zeolites and phyllosilicates like prehnite, Mg-chlorite, Mg-rich smectites and mixed-layer chlorites-smectites and chlorite-vermiculite from comparison of hyperspectral infrared observations with laboratory spectra. These minerals are associated with fresh craters post-dating any aqueous activity. They likely represent ancient hydrated terrains excavated by the crater-forming impacts, and hence reveal the composition of the altered Noachian crust, although crater-related hydrothermal activity may have played a minor role for the largest craters (>20 km in diameter). Most detected minerals formed over relatively high temperatures (100-300 °C), likely due to aqueous alteration of the Noachian crust by regional low grade metamorphism from the Noachian thermal gradient and/or by extended hydrothermal systems associated with Noachian volcanism and ancient large impact craters. [Abstract abbreviated due to character limitations.]
Published: 16 May 2012
Context. The Carina Nebula represents one of the most massive star forming regions known in our Galaxy and displays a high level of feedback from the large number of very massive stars. While the stellar content is now well known from recent deep X-ray and near-infrared surveys, the properties of the clouds remained rather poorly studied until today. Methods. We used SPIRE and PACS onboard of Herschel to map the full spatial extent (~5.3 square-degrees) of the clouds in the Carina Nebula complex at wavelengths between 70 and 500 micron. We used here the 70 micron and 160 micron far-infrared maps to determine color temperatures and column densities, and to investigate the global properties of the gas and dust clouds in the complex. Results. Our Herschel maps show the far-infrared morphology of the clouds at unprecedented high angular resolution. The clouds show a very complex and filamentary structure that is dominated by the radiation and wind feedback from the massive stars. In most locations, the column density of the clouds is NH < 2 × 1022 cm-2 (corresponding to visual extinctions of AV < 10 mag); denser cloud structures are restricted to the massive cloud west of Tr 14 and the innermost parts of large pillars. Our temperature map shows a clear large-scale gradient from ~35-40 K in the central region to <20 K at the periphery and in the densest parts of individual pillars. The total mass of the clouds seen by Herschel in the central (1 degree radius) region is ~656 000 M_Sun. We also derive the global spectral energy distribution in the mid-infrared to mm wavelength range. A simple radiative transfer model suggests that the total mass of all the gas (including a warmer component that is not well traced by Herschel) in the central 1 degree radius region is ~890 000 M_Sun. [Abstract abbreviated due to character limitations.]
Published: 15 May 2012
Published online 9 May 2012

The old, red stars that constitute the bulges of galaxies, and the massive black holes at their centres, are the relics of a period in cosmic history when galaxies formed stars at remarkable rates and active galactic nuclei (AGN) shone brightly as a result of accretion onto black holes. It is widely suspected, but unproved, that the tight correlation between the mass of the black hole and the mass of the stellar bulge results from the AGN quenching the surrounding star formation as it approaches its peak luminosity. X-rays trace emission from AGN unambiguously, whereas powerful star-forming galaxies are usually dust-obscured and are brightest at infrared and submillimetre wavelengths. Here we report submillimetre and X-ray observations that show that rapid star formation was common in the host galaxies of AGN when the Universe was 2-6 billion years old, but that the most vigorous star formation is not observed around black holes above an X-ray luminosity of 1044 ergs per second. This suppression of star formation in the host galaxy of a powerful AGN is a key prediction of models in which the AGN drives an outflow, expelling the interstellar medium of its host and transforming the galaxy's properties in a brief period of cosmic time.

Published: 11 May 2012
We present a local mesoscale model of the magnetospheric cusp region with high resolution (up to 300 km). We discuss the construction and implementation of the initial configuration and give a detailed description of the numerical simulation. An overview of simulation results for the case of strongly northward interplanetary magnetic field (IMF) is then presented and compared with data from Cluster 2 spacecraft from 14 February 2003. Results show a cusp diamagnetic cavity (CDC) with depth normal to the magnetospheric boundary on the order of 1-2 RE and a much larger extent of ~5-9 RE tangential to the boundary, bounded by a gradual inner boundary with the magnetospheric lobe and a more distinct exterior boundary with the magnetosheath. These results are qualitatively consistent with observational data.
Published: 07 May 2011

Published online 5 April 2012, in Science Express

Observations with the Venus Express magnetometer and low-energy particle detector revealed magnetic field and plasma behaviour in the near-Venus wake symptomatic of magnetic reconnection, a process that occurs in the Earth's magnetotail but is not expected in the magnetotail of a non-magnetized planet like Venus. On 15 May 2006, the plasma flow in this region was toward the planet and the magnetic field component transverse to the flow was reversed. Magnetic reconnection is a plasma process that changes the topology of the magnetic field and results in energy exchange between the magnetic field and the plasma. Thus, the energetics of the Venus magnetotail resembles that of the terrestrial tail where energy is stored and later released from the magnetic field to the plasma.

Published: 05 May 2012
This document lists the JUICE mission's science objectives and the corresponding investigations, plus the necessary measurements (and instruments) to achieve these objectives.
Published: 03 May 2012
This issue of Spatium is based on a presentation by Prof. Peter Wurz of the Physics Institute of the University of Bern, reporting on the status of Mercury research.
Published: 02 April 2012
Ontario Lacus is the largest lake of the whole southern hemisphere of Titan, Saturn's major moon. It has been imaged twice by each of the Cassini imaging systems (Imaging Science Subsystem (ISS) in 2004 and 2005, Visual and Infrared Mapping Spectrometer (VIMS) in 2007 and 2009 and RADAR in 2009 and 2010). We compile a geomorphological map and derive a "hydrogeological" interpretation of Ontario Lacus, based on a joint analysis of ISS, VIMS and RADAR SAR datasets, along with the T49 altimetric profile acquired in December 2008. The morphologies observed on Ontario Lacus are compared to landforms of a semi-arid terrestrial analog, which resembles Titan's lakes: the Etosha Pan, located in the Owambo Basin (Namibia). The Etosha Pan is a flat-floored depression formed by dissolution, under semi-arid conditions, of a surface evaporitic layer (calcretes) controlled by groundwater vertical motions. We infer that Ontario Lacus is an extremely flat and shallow depression lying in an alluvial plain surrounded by small mountain ranges under climatic conditions similar to those of terrestrial semi-arid regions. Channels are seen in the southern part of Ontario Lacus in VIMS and RADAR data, acquired at a 2-years time interval. Their constancy in location with time implies that the southern portion of the depression is probably not fully covered by a liquid layer at the time of the observations, and that they most probably run on the floor of the depression. A shallow layer of surface liquids, corresponding to the darkest portions of the RADAR images, would thus cover about 53% of the surface area of the depression, of which almost 70% is located in its northern part. These liquid-covered parts of the depression, where liquid ethane was previously identified, are interpreted as topographic lows where the "alkanofer" raises above the depression floor. [Abstract abbreviated due to character limitations.]
Published: 20 April 2012
Magnetospheres of neutron stars are anchored in the rigid crust and can be twisted by sudden crustal motions ("starquakes"). The twisted magnetosphere does not remain static and gradually untwists, dissipating magnetic energy and producing radiation. The equation describing this evolution is derived, and its solutions are presented.
Published: 05 September 2009
The magnetar 1E1547.0-5408 exhibited outbursts in October 2008 and January 2009. In this paper we present in great detail the evolution of the temporal and spectral characteristics of the persistent total and pulsed emission of 1E1547.0-5408 between <1 and 300 keV starting in October 3, 2008, and ending in January 2011. We analyzed data collected with the Rossi X-ray Timing Explorer, the International Gamma-Ray Astrophysics Laboratory and the Swift satellite. We report the evolution of the pulse frequency, and the measurement at the time of the onset of the January 2009 outburst of an insignificant jump in frequency, but a major frequency derivative jump. Before this glitch, a single broad pulse is detected, mainly for energies below ~10 keV. Surprisingly, ~11 days after the glitch a new transient high-energy (up to ~150 keV) pulse appears with a Gaussian shape and width 0.23, shifted in phase by ~0.31 compared to the low-energy pulse, which smoothly fades to undetectable levels in ~350 days. We report the evolution of the pulsed-emission spectra. For energies 2.5-10 keV all pulsed spectra are very soft with photon indices between -4.6 and -3.9. For ~10-150 keV, after the glitch, we report hard non-thermal pulsed spectra, similar to what has been reported for the persistent pulsed emission of some anomalous X-ray pulsars. This pulsed hard X-ray emission reached maximal luminosity 70 ± 30 days after the glitch epoch, followed by a gradual decrease by more than a factor of 10 over ~300 days. These characteristics differ from those of the total emission. [Abstract abbreviated due to character limitations.] We discuss these findings in the framework of the magnetar model.
Published: 02 March 2012
Context. Fomalhaut is a young (2 ± 1 × 108 years), nearby (7.7 pc), 2 MSun star that is suspected to harbor an infant planetary system, interspersed with one or more belts of dusty debris.
Aims. We present far-infrared images obtained with the Herschel Space Observatory with an angular resolution between 5.7" and 36.7" at wavelengths between 70 um and 500 um. The images show the main debris belt in great detail. Even at high spatial resolution, the belt appears smooth. The region in between the belt and the central star is not devoid of material; thermal emission is observed here as well. Also at the location of the star, excess emission is detected. We aim to construct a consistent image of the Fomalhaut system.
Methods. We use a dynamical model together with radiative-transfer tools to derive the parameters of the debris disk. We include detailed models of the interaction of the dust grains with radiation, for both the radiation pressure and the temperature determination. Comparing these models to the spatially resolved temperature information contained in the images allows us to place strong constraints on the presence of grains that will be blown out of the system by radiation pressure. We use this to derive the dynamical parameters of the system.
Results. The appearance of the belt points toward a remarkably active system in which dust grains are produced at a very high rate by a collisional cascade in a narrow region filled with dynamically excited planetesimals. Dust particles with sizes below the blow-out size are abundantly present.
- The remainder of the abstract is truncated -
Published: 12 April 2012
We report observations of quasi-periodic pulsations (QPPs) during the X2.2 flare of 2011 February 15, observed simultaneously in several wavebands. We focus on fluctuations on timescale 1-30 s and find different time lags between different wavebands. During the impulsive phase, the Reuven Ramaty High Energy Solar Spectroscopic Imager channels in the range 25-100 keV lead all the other channels. They are followed by the Nobeyama RadioPolarimeters at 9 and 17 GHz and the extreme-ultraviolet (EUV) channels of the Euv SpectroPhotometer (ESP) on board the Solar Dynamic Observatory. The zirconium and aluminum filter channels of the Large Yield Radiometer on board the Project for On-Board Autonomy satellite and the soft X-ray (SXR) channel of ESP follow. The largest lags occur in observations from the Geostationary Operational Environmental Satellite, where the channel at 1-8 Å leads the 0.5-4 Å channel by several seconds. The time lags between the first and last channels is up to approximately 9 s. We identified at least two distinct time intervals during the flare impulsive phase, during which the QPPs were associated with two different sources in the Nobeyama RadioHeliograph at 17 GHz. The radio as well as the hard X-ray channels showed different lags during these two intervals. To our knowledge, this is the first time that time lags are reported between EUV and SXR fluctuations on these timescales. We discuss possible emission mechanisms and interpretations, including flare electron trapping.
Published: 11 April 2012
Radio science tracking of Mars Express (MaRS experiment) and Mars Reconnaissance Orbiter produced high resolution Martian gravity data. Applying localized spectral analysis on the new gravity data sets, we study the surface density and lithospheric elastic thickness in the Tharsis province. The gravity signal is predicted with geophysical models either including bottom loading (top/bottom model) or taking into account the loading history (top/top model). Volcanic shields are mainly composed of high-density lava but their construction could have begun with lower density lava, with the exception of Ascraeus Mons which has a top of lower density. Buoyant bottom loading may have been present in the form of a mantle plume under Olympus Mons. The elastic thickness was much larger at Olympus Mons than at other volcanoes, suggesting large spatial variations of heat flux during the Hesperian. Alternatively, small elastic thicknesses could be artifacts reflecting the presence of very localized high-density crustal intrusions beneath the volcanoes. Thaumasia highlands were probably supported by a mantle plume at the time of formation. In Valles Marineris, top/bottom models predict low densities and large elastic thicknesses, in conflict with the basaltic rock composition and Hesperian age of the valley. Dense mafic dikes underlie the western part of the valley. The top/top model serves to test another scenario in which the trough is formed with sedimentary infilling removed much later by erosion, the elastic thickness increasing in between. At the large volcanoes, the relation between gravity and topography is anisotropic probably because of density variations.
Published: 05 April 2012
We present photometry of the nearby galaxy NGC 5128 (Centaurus A) observed with the Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) instruments on board the Herschel Space Observatory, at 70, 160, 250, 350 and 500 micron, as well as new CO J=3-2 observations taken with the HARP-B instrument on the James Clerk Maxwell Telescope (JCMT). Using a single-component modified blackbody, we model the dust spectral energy distribution within the disc of the galaxy using all five Herschel wavebands and find dust temperatures of ~30 K towards the centre of the disc and a smoothly decreasing trend to ~20 K with increasing radius. We find a total dust mass of (1.59 ± 0.05) × 107 MSun and a total gas mass of (2.7 ± 0.2) × 109 MSun. The average gas-to-dust mass ratio is 103 ± 8, but we find an interesting increase in this ratio to approximately 275 towards the centre of Cen A. We discuss several possible physical processes that may be causing this effect, including dust sputtering, jet entrainment and systematic variables such as the XCO factor. Dust sputtering by X-rays originating in the active galactic nucleus or the removal of dust by the jets is our most favoured explanation.

Published online on 27 March 2012, to appear in forthcoming issue of MNRAS
Published: 28 March 2012
We present the first results of a multiwavelength survey, incorporating Herschel-inline image, Spitzer, GALEX and Australian Telescope Compact Array (ATCA) observations, of a 1° × 1° field centred on Centaurus A. As well as detecting the inner lobes of the active galactic nucleus (AGN) jet and counterjet, we have found two clouds, bright at submillimetre wavelengths, ~15 kpc from the centre of Cen A that are co-aligned with the jets. Flux measurements at Herschel wavelengths have proved vital in constraining fits to the spectral energy distributions (SEDs). The clouds are well fitted by a single-temperature, modified blackbody spectrum (beta=2) indicating that we are looking at two cold dust clouds on the outskirts of Cen A. The temperature and masses of the clouds are T_north=12.6(+1.1, -1.2) K, T_south=15.1(+1.7, -1.6) K; log(M_north/M_Sun)=5.8(+0.2, -0.2); log(M_south/M_Sun)=5.6(+0.2, -0.2) and the gas-dust ratio for both clouds is ~100. The measured values for the northern dust cloud are consistent with previous measurements from ISO while the southern cloud is a new submillimetre detection. The two dust clouds are located at the termini of the partial HI ring that surrounds Cen A which is also where the gas column density peaks. The Herschel survey encompasses the partial HI ring yet we find no evidence of dust emission in any other part of the ring. Assuming that the gas-dust ratio is the same in the rest of the ring, dust mass upper limits in the HI ring are consistent with low column density dust being present but falling below the SPIRE detection limit. We have discussed the origin of these clouds and various possible heating mechanisms. The observations favour a scenario in which the gas and dust were once part of a late-type galaxy, which has since merged with Cen A. The dominant heating mechanism which adequately explains the observed temperatures in both clouds is heating from the evolved stellar population within Cen A.
Published: 15 March 2012
The nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is investigated extensively nowadays, both theoretically and observationally. Although recent observations gave evidence of the dominance of kinetic Alfvén waves (KAWs) at sub-ion scales with w < w_ci, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., w > w_ci) is more relevant. Here we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically b_i > b_e ~1 and for high oblique angles of propagation 80° d theta_kB d 90° as observed from Cluster spacecraft data. The linear properties of plasma modes under these conditions are poorly known, which contrasts with the well-documented cold plasma limit and/or moderate oblique angles of propagation (theta_kB < 80°). Based on linear solutions of the Vlasov kinetic theory, we discuss the relevance of each plasma mode (fast, Bernstein, KAW, whistler) in carrying the energy cascade down to electron scales. We show, in particular, that the shear Alfvén mode (known in the magnetohydrodynamic limit) extends at scales k-rho_i e1 to frequencies either larger or smaller than w_ci, depending on the anisotropy k_para/k_perp. This extension into small scales is more readily called whistler (w > w_ci) or KAW (w < w_ci), although the mode is essentially the same. This contrasts with the well-accepted idea that the whistler branch always develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter the more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small-scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW.
Published: 13 March 2012
Aims: As part of the Herschel guaranteed time key programme "HOBYS", we present the PACS and SPIRE photometric survey of the star-forming region Vela-C, one of the nearest sites of low-to-high-mass star formation in the Galactic plane. Our main objectives are to take a census of the cold sources and to derive their mass distribution down to a few solar masses.
Methods: Vela-C was observed with PACS and SPIRE in parallel mode at five wavelengths between 70 micron and 500 micron over an area of about 3 square degrees. A photometric catalogue was extracted from the detections in each of the five bands, using a threshold of 5sigma over the local background. Out of this catalogue we selected a robust sub-sample of 268 sources, of which ~75 per cent are cloud clumps (diameter between 0.05 pc and 0.13 pc) and 25 per cent are cores (diameter between 0.025 pc and 0.05 pc). Their spectral energy distributions (SEDs) were fitted with a modified black body function. We classify 48 sources as protostellar, based on their detection at 70 um or at shorther wavelengths, and 218 as starless, because of non-detections at 70 micron. For two other sources, we do not provide a secure classification, but suggest they are Class 0 protostars.
Results: From the SED fitting we derived key physical parameters (i.e. mass, temperature, bolometric luminosity). Protostellar sources are in general warmer (< T > = 12.8 K) and more compact (< diameter > = 0.040 pc) than starless sources (< T > = 10.3 K, < diameter > = 0.067 pc). Both these findings can be ascribed to the presence of an internal source(s) of moderate heating, which also causes a temperature gradient and hence a more peaked intensity distribution. Moreover, the reduced dimensions of protostellar sources may indicate that they will not fragment further.
The remainder of the abstract is truncated.
Published: 12 March 2012
Solar wind controls non-thermal escape of planetary atmospheric volatiles, regardless of the strength of planetary magnetic fields. For both Earth with a strong dipole and Mars with weak remnant fields, the oxygen ion (O+) outflow has been separately found to be enhanced during corotating interaction region (CIR) passage. Here we compared the enhancements of O+ outflow on Earth and Mars driven by a CIR in January, 2008 when Sun, Earth and Mars were approximately aligned. The CIR propagation was recorded by STEREO, ACE, Cluster and Mars Express (MEX). During the CIR passage, Cluster observed enhanced flux of upwelling oxygen ions above the Earth's polar region, while MEX detected an increased escape flux of oxygen ions in the Martian magnetosphere. We found that, (1) under a solar wind dynamic pressure increase by 2-3 nPa, the rate of increase in Martian O+ outflow flux was one order higher than those on Earth; (2) as response to the same part of the CIR body, the rate of increase in Martian O+ outflow flux was on the same order as for Earth. The comparison results imply that the dipole effectively prevents coupling of solar wind kinetic energy to planetary ions, and the distance to the Sun is also crucially important for planetary volatile loss in our inner solar system.
Published: 09 March 2012
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