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| Filaments and ridges in Vela C revealed by Herschel: from low-mass to high-mass star-forming sites |
| We present the first Herschel PACS and SPIRE results of the Vela C molecular complex in the far-infrared and submillimetre regimes at 70, 160, 250, 350, and 500 micron, spanning the peak of emission of cold prestellar or protostellar cores. Column density and multi-resolution analysis (MRA) differentiates the Vela C complex into five distinct sub-regions. Each sub-region displays differences in their column density and temperature probability distribution functions (PDFs), in particular, the PDFs of the "Centre-Ridge" and "South-Nest" sub-regions appear in stark contrast to each other. The Centre-Ridge displays a bimodal temperature PDF representative of hot gas surrounding the HII region RCW 36 and the cold neighbouring filaments, whilst the South-Nest is dominated by cold filamentary structure. The column density PDF of the Centre-Ridge is flatter than the South-Nest, with a high column density tail, consistent with formation through large-scale flows, and regulation by self-gravity. At small to intermediate scales MRA indicates the Centre-Ridge to be twice as concentrated as the South-Nest, whilst on larger scales, a greater portion of the gas in the South-Nest is dominated by turbulence than in the Centre-Ridge. In Vela C, high-mass stars appear to be preferentially forming in ridges, i.e., dominant high column density filaments. |
| Publication date: 07 Sep 2011 |
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| Herschel detects a massive dust reservoir in supernova 1987A |
| Published online in Science Express, 7 July 2011.
We report far-infrared and submillimeter observations of Supernova 1987A, the star that exploded on 23 February 1987 in the Large Magellanic Cloud, a galaxy located 160 000 light years away. The observations reveal the presence of a population of cold dust grains radiating with a temperature of about 17 to 23 K at a rate of about 220 LSun. The intensity and spectral energy distribution of the emission suggests a dust mass of about 0.4 to 0.7 MSun. The radiation must originate from the supernova ejecta and requires the efficient precipitation of all refractory material into dust. Our observations imply that supernovae can produce the large dust masses detected in young galaxies at very high redshifts. |
| Publication date: 02 Sep 2011 |
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| Diffuse emission measurement with the spectrometer on INTEGRAL as an indirect probe of cosmic-ray electrons and positrons |
Significant advances have been made in the understanding of the diffuse Galactic hard X-ray continuum emission using data from the INTEGRAL observatory. The diffuse hard power-law component seen with the SPectrometer on INTEGRAL (SPI) has been identified with inverse-Compton emission from relativistic (GeV) electrons on the cosmic microwave background and Galactic interstellar radiation field. In the present analysis, SPI data from 2003 to 2009, with a total exposure time of ~108 s, are used to derive the Galactic ridge hard X-ray spatial distribution and spectrum between 20 keV and 2.4 MeV. Both are consistent with predictions from the GALPROP code. The good agreement between measured and predicted emission from keV to GeV energies suggests that the correct production mechanisms have been identified. We discuss the potential of the SPI data to provide an indirect probe of the interstellar cosmic-ray electron distribution, in particular for energies below a few GeV. |
| Publication date: 01 Sep 2011 |
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| Spatium 27: Science First |
| This issue of Spatium is devoted to Professor Johannes Geiss, cofounder of PRO ISSI and spiritus
rector of ISSI, on the occasion of his 85th anniversary on 4 September 2011. |
| Publication date: 01 Sep 2011 |
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| Transient Pc3 wave activity generated by a hot flow anomaly: Cluster, Rosetta, and ground-based observations |
| Pc3 pulsations are observed in the magnetosphere with wave periods of 10-45 s. Two distinct populations have been observed; one exhibits a frequency dependence on the solar wind magnetic field strength, whereas the other does not. The first population is explained in terms of a model where the bow shock reflects ions which generate upstream foreshock ULF waves. These waves are convected through the shock to the dayside magnetopause and thus to the magnetosphere. The source of the second population is not well understood. In this paper we examine the generation of a transient patch of Pc3 wave activity due to a hot flow anomaly (HFA) using a unique spacecraft conjunction that occurred during the first Earth flyby of the Rosetta spacecraft. Cluster, upstream of the bow shock and close to the Sun-Earth line observed an HFA. At this time Rosetta was nearing closest approach and together with ground magnetometer stations, observed a transient interval of Pc3 wave activity. Analysis also shows that the Pc3 waves occurred in the absence of a ULF wavefield just upstream of the bow shock. This result shows that HFAs can be a source of Pc3 wave activity, and may explain in part the origin of the second population of Pc3 waves. It also demonstrates in new detail the manner in which kinetic physics at the bow shock, driven by structure in the solar wind, can influence magnetospheric dynamics. |
| Publication date: 27 Aug 2011 |
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| The origin of the Martian moons revisited |
| The origin of the Martian moons, Phobos and Deimos, is still an open issue: either they are asteroids captured by Mars or they formed in situ from a circum-Mars debris disk. The capture scenario mainly relies on the remote-sensing observations of their surfaces, which suggest that the moon material is similar to outer-belt asteroid material. This scenario, however, requires high tidal dissipation rates inside the moons to account for their current orbits around Mars. Although the in situ formation scenarios have not been studied in great details, no observational constraints argue against them. Little attention has been paid to the internal structure of the moons, yet it is pertinent for explaining their origin. The low density of the moons indicates that their interior contains significant amounts of porous material and/or water ice. The porous content is estimated to be in the range of 3060% of the volume for both moons. This high porosity enhances the tidal dissipation rate but not sufficiently to meet the requirement of the capture scenario. On the other hand, a large porosity is a natural consequence of re-accretion of debris at Mars' orbit, thus providing support to the in situ formation scenarios. The low density also allows for abundant water ice inside the moons, which might significantly increase the tidal dissipation rate in their interiors, possibly to a sufficient level for the capture scenario. Precise measurements of the rotation and gravity field of the moons are needed to tightly constrain their internal structure in order to help answering the question of the origin. |
| Publication date: 25 Aug 2011 |
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| Nonaxisymmetric Anisotropy of Solar Wind Turbulence |
| A key prediction of turbulence theories is frame-invariance, and in magnetohydrodynamic (MHD) turbulence, axisymmetry of fluctuations with respect to the background magnetic field. Paradoxically the power in fluctuations in the turbulent solar wind are observed to be ordered with respect to the bulk macroscopic flow as well as the background magnetic field. Here, nonaxisymmetry across the inertial and dissipation ranges is quantified using in situ observations from Cluster. The observed inertial range nonaxisymmetry is reproduced by a "fly through" sampling of a direct numerical simulation of MHD turbulence. Furthermore, fly through sampling of a linear superposition of transverse waves with axisymmetric fluctuations generates the trend in nonaxisymmetry with power spectral exponent. The observed nonaxisymmetric anisotropy may thus simply arise as a sampling effect related to Taylor's hypothesis and is not related to the plasma dynamics itself. |
| Publication date: 23 Aug 2011 |
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| Detection of Emerging Sunspot Regions in the Solar Interior |
| Sunspots are regions where strong magnetic fields emerge from the solar interior and where major eruptive events occur. These energetic events can cause power outages, interrupt telecommunication and navigation services, and pose hazards to astronauts. We detected subsurface signatures of emerging sunspot regions before they appeared on the solar disc. Strong acoustic travel-time anomalies of an order of 12 to 16 seconds were detected as deep as 65,000 kilometers. These anomalies were associated with magnetic structures that emerged with an average speed of 0.3 to 0.6 kilometer per second and caused high peaks in the photospheric magnetic flux rate 1 to 2 days after the detection of the anomalies. Thus, synoptic imaging of subsurface magnetic activity may allow anticipation of large sunspot regions before they become visible, improving space weather forecast. |
| Publication date: 19 Aug 2011 |
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| Magnetic signatures of a tenuous atmosphere at Dione |
| We present Cassini magnetic field observations from the only two close flybys (16DI and 129DI) of Saturn's icy satellite Dione which have been carried out so far. Data from 16DI show a weak field perturbation in the upstream region, indicative of a tenuous atmosphere around the satellite. By applying an analytical model of the perturbations caused by subalfvénic atmosphere-magnetosphere interactions, we demonstrate that an atmospheric column density of approximately 1x1017 m-2 would be able to sustain the observed field signature. Magnetic field data from 16DI also contain hints that Dione's gas envelope might possess a slight asymmetry between the Saturn-facing and the Saturn-averted hemisphere. The detection of a thin atmosphere at Dione might be correlated to the occurrence of a transient radiation belt near the moon's L-shell at the time of the 16DI flyby, as reported by Roussos et al. (2008b). On the other hand, magnetic field observations from the subsequent downstream encounter 129DI show no clear evidence of an atmosphere, probably due to the flyby trajectory being unsuitable for the detection of the associated perturbations. |
| Publication date: 12 Aug 2011 |
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| Comparative Planetology: Venus-Earth-Mars |
Comparative planetology has long been a field of general interest but with a fairly small number of scientists actively involved. During the last decade, until recently, there has been no significant growth, possibly much due to lack of new data from Venus; perhaps the most obvious planet to compare with the Earth. Availability of ample data of high quality is of paramount importance for proper comparisons. With the arrival of Venus Express at Venus in March 2006 a new impulse to the field has been injected. Venus Express addresses a large number of topics relevant to comparative planetology; in particular in the field of atmospheric dynamics and chemistry, clouds and atmospheresolar wind interaction.
Mars has been the subject of significant interest and many space missions in the recent years. Being smaller and cooler and in several aspects more evolved, Mars is still a planet of great interest for comparison with the Earth on the other end of the parameter space.
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| Publication date: 02 Aug 2011 |
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| Reflection in Seyfert Galaxies and the Unified Model of AGN |
We present a deep study of the average hard X-ray spectra of Seyfert galaxies. We aim to test the unified model of active galactic nuclei, and constrain differences and similarities between different classes of objects. We analyzed all public INTEGRAL IBIS/ISGRI data available on all the 165 Seyfert galaxies detected at z < 0.2. Our final sample consists of 44 Seyfert 1s, 29 Seyfert 1.5s, 78 Seyfert 2s, and 14 narrow-line Seyfert 1s. For each subsample, we stacked all the images, and derived their average hard X-ray spectra in the 17-250 keV energy range. We performed a detailed spectral analysis using both a model-independent and a model-dependent approach. All classes of Seyfert galaxies show on average the same nuclear continuum, as foreseen by the zeroth order unified model, with a cutoff energy of EC >~ 200 keV, and a photon index of Gamma~1.8. The average optical depth of the Comptonizing medium is consistent for the different classes (tau~0.8). Compton-thin Seyfert 2s show a reflection component stronger than Seyfert 1s and Seyfert 1.5s. Most of this reflection is due to mildly obscured (1023 cm-2 <= NH < 1024 cm-2) Seyfert2s, which have a significantly stronger reflection component (R = 2.2+4.5-1.1) than Seyfert 1s (R <= 0.4), Seyfert 1.5s (R <= 0.4) and lightly obscured (NH < 1023 cm-2) Seyfert 2s (R <= 0.5). This cannot be explained easily by the unified model. The absorber/reflector in mildly obscured Seyfert 2s might cover a large fraction of the X-ray source, and contain clumps of Compton-thick material. The large reflection found in the spectrum of mildly obscured Seyfert 2s reduces the amount of Compton-thick objects needed to explain the peak of the cosmic X-ray background. Our results are consistent with the fraction of Compton-thick sources being ~10 percent.
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| Publication date: 02 Aug 2011 |
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| Atmospheric angular momentum variations of Earth, Mars and Venus at seasonal time scales |
| Atmospheric angular momentum variations of a planet are associated with the global atmospheric mass redistribution and the wind variability. The exchange of angular momentum between the fluid layers and the solid planet is the main cause for the variations of the planetary rotation at seasonal time scales. In the present study, we investigate the angular momentum variations of the Earth, Mars and Venus, using geodetic observations, output of state-of-the-art global circulation models as well as assimilated data. We discuss the similarities and differences in angular momentum variations, planetary rotation and angular momentum exchange for the three terrestrial planets. We show that the atmospheric angular momentum variations for Mars and Earth are mainly annual and semi-annual whereas they are expected to be "diurnal" on Venus. The wind terms have the largest contributions to the LOD changes of the Earth and Venus whereas the matter term is dominant on Mars due to the CO2 sublimation/condensation. The corresponding LOD variations (DLOD) have similar amplitudes on Mars and Earth but are much larger on Venus, though more difficult to observe. |
| Publication date: 01 Aug 2011 |
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| Herschel measurements of molecular oxygen in Orion |
| We report observations of three rotational transitions of molecular oxygen (O2) in emission from the H2 Peak 1 position of vibrationally excited molecular hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using HIFI on the Herschel Space Observatory, having velocities of 11 km s-1 to 12 km s-1 and widths of 3 km s-1. The beam-averaged column density is N(O2) = 6.5×1016 cm-2, and assuming that the source has an equal beam filling factor for all transitions (beam widths 44, 28, and 19'), the relative line intensities imply a kinetic temperature between 65 K and 120 K. The fractional abundance of O2 relative to H2 is 0.3-7.3×10-6. The unusual velocity suggests an association with a ~5' diameter source, denoted Peak A, the Western Clump, or MF4. The mass of this source is ~10 MSun and the dust temperature is >= 150 K. Our preferred explanation of the enhanced O2 abundance is that dust grains in this region are sufficiently warm (T >= 100 K) to desorb water ice and thus keep a significant fraction of elemental oxygen in the gas phase, with a significant fraction as O2. For this small source, the line ratios require a temperature >= 180 K. The inferred O2 column density ~5×1018 cm-2 can be produced in Peak A, having N(H2) ~4×1024 cm-2. An alternative mechanism is a low-velocity (10 to 15 km s-1) C-shock, which can produce N(O2) up to 1017 cm-2. |
| Publication date: 01 Aug 2011 |
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| Super-Alfvénic Propagation of Substorm Reconnection Signature and Poynting Flux |
The propagation of reconnection signatures and their associated energy are examined using kinetic particle-in-cell simulations and Cluster satellite observations. It is found that the quadrupolar out-of-plane magnetic field near the separatrices is associated with a kinetic Alfvén wave. For magnetotail parameters, the parallel propagation of this wave is super-Alfvénic (V||<~1500-5500 km/s) and generates substantial Poynting flux (S~10-5-10-4 W/m2) consistent with Cluster observations of magnetic reconnection. This Poynting flux substantially exceeds that due to frozen-in ion bulk outflows and is sufficient to generate white light aurora in Earth's ionosphere. |
| Publication date: 01 Aug 2011 |
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| Reconnection and waves: a review with a perspective |
| This review is intended to help prepare a new stage of wave studies in the context of magnetic reconnection. Various results that have accumulated would not let the two-dimensional, steady and laminar magnetic reconnection to remain as the standard model. Emphasis on three-dimensional, temporally varying, and turbulent effects is growing and this fact tells that the effects of waves in various frequency ranges deserve further attention in the context of magnetic reconnection. In this review, by setting a perspective, selected recent topics are reviewed and the ways in which these can be viewed as the stepping stones towards a new research horizon of magnetic reconnection are discussed. |
| Publication date: 27 Jul 2011 |
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| PLATO Definition Study Report (Red Book) |
| Reference: ESA/SRE(2011)13
This report, the so-called Red Book, describes the outcome of the mission definition study (Phase A) for the PLATO mission. PLATO is a medium-size (M-class) mission of the ESA Cosmic Vision 2015-2025 programme, and competes for one of the two launch slots foreseen in 2017 and 2018. |
| Publication date: 25 Jul 2011 |
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| Solar Orbiter definition study report (Red Book) |
| Reference: ESA/SRE(2011)14
This report, the so-called Red Book, presents an overview of the Solar Orbiter mission in its present state of advanced definition. Solar Orbiter is a medium-size (M-class) mission of the ESA Cosmic Vision 2015-2025 programme, and competes for one of the two launch slots foreseen in 2017 and 2018. |
| Publication date: 25 Jul 2011 |
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| EChO External Final Presentation of the CDF study |
Summary of the study performed at ESA's Concurrent Design Facility (CDF), with the goals to:
- Assess the technical feasibility of the ECHO mission proposal
- Design an example mission compatible with achieving the science goals
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| Publication date: 22 Jul 2011 |
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| MarcoPolo-R near earth asteroid sample return mission |
Made available online before print publication MarcoPolo-R is a sample return mission to a primitive Near-Earth Asteroid (NEA) proposed in collaboration with NASA. It will rendezvous with a primitive NEA, scientifically characterize it at multiple scales, and return a unique sample to Earth unaltered by the atmospheric entry process or terrestrial weathering. MarcoPolo-R will return bulk samples (up to 2 kg) from an organic-rich binary asteroid to Earth for laboratory analyses, allowing us to: explore the origin of planetary materials and initial stages of habitable planet formation; identify and characterize the organics and volatiles in a primitive asteroid; understand the unique geomorphology, dynamics and evolution of a binary NEA. This project is based on the previous Marco Polo mission study, which was selected for the Assessment Phase of the first round of Cosmic Vision. Its scientific rationale was highly ranked by ESA committees and it was not selected only because the estimated cost was higher than the allotted amount for an M class mission. The cost of MarcoPolo-R will be reduced to within the ESA medium mission budget by collaboration with APL (John Hopkins University) and JPL in the NASA program for coordination with ESA's Cosmic Vision Call. The baseline target is a binary asteroid (175706) 1996 FG3, which offers a very efficient operational and technical mission profile. A binary target also provides enhanced science return. The choice of this target will allow new investigations to be performed more easily than at a single object, and also enables investigations of the fascinating geology and geophysics of asteroids that are impossible at a single object. Several launch windows have been identified in the time-span 2020-2024.
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| Publication date: 20 Jul 2011 |
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| Direct detection of the Enceladus water torus with Herschel |
| Cryovolcanic activity near the south pole of Saturn's moon Enceladus produces plumes of H2O-dominated gases and ice particles, which escape and populate a torus-shaped cloud. Using submillimeter spectroscopy with Herschel, we report the direct detection of the Enceladus water vapor torus in four rotational lines of water at 557, 987, 1113, and 1670 GHz, and probe its physical conditions and structure. We determine line-of-sight H2O column densities of ~4 × 1013 cm-2 near the equatorial plane, with a ~50 000 km vertical scale height. The water torus appears to be rotationally cold (e.g. an excitation temperature of 16 K is measured for the 1113 GHz line) but dynamically excited, with non-Keplerian dispersion velocities of ~2 kms-1, and appears to be largely shaped by molecular collisions. From estimates of the influx rates of torus material into Saturn and Titan, we infer that Enceladus' activity is likely to be the ultimate source of water in the upper atmosphere of Saturn, but not in Titan's. |
| Publication date: 14 Jul 2011 |
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