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| A salt-water reservoir as the source of a compositionally stratified plume on Enceladus |
Published online on 22 June 2011
The discovery of a plume of water vapour and ice particles emerging from warm fractures ('tiger stripes') in Saturn's small, icy moon Enceladus raised the question of whether the plume emerges from a subsurface liquid source or from the decomposition of ice. Previous compositional analyses of particles injected by the plume into Saturn's diffuse E ring have already indicated the presence of liquid water, but the mechanisms driving the plume emission are still debated. Here we report an analysis of the composition of freshly ejected particles close to the sources. Salt-rich ice particles are found to dominate the total mass flux of ejected solids (more than 99 per cent) but they are depleted in the population escaping into Saturn's E ring. Ice grains containing organic compounds are found to be more abundant in dense parts of the plume. Whereas previous Cassini observations were compatible with a variety of plume formation mechanisms, these data eliminate or severely constrain non-liquid models and strongly imply that a salt-water reservoir with a large evaporating surface provides nearly all of the matter in the plume. |
| Publication date: 30 Jun 2011 |
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| STE-QUEST CDF study - External Final Presentation |
Summary of the study performed at ESA's Concurrent Design Facility (CDF) into the M-class mission Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST). Contents of the presentation:
- Study: goals, tasks, organization
- Drivers and constraints
- Configuration and payload accomodation
- Instruments
- Mission analysis
- Ground stations and operations
- Attitude and orbit control
- System design
- Communication
- Budgets
- Risk, programmatics, cost
- Requirements review
- Conclusions
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| Publication date: 30 Jun 2011 |
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| Constraints on Lorentz Invariance Violation using integral/IBIS observations of GRB041219A |
| One of the experimental tests of Lorentz invariance violation is to measure the helicity dependence of the propagation velocity of photons originating in distant cosmological obejcts. Using a recent determination of the distance of the gamma-ray burst GRB 041219A, for which a high degree of polarization is observed in the prompt emission, we are able to improve by four orders of magnitude the existing constraint on Lorentz invariance violation, arising from the phenomenon of vacuum birefringence. |
| Publication date: 28 Jun 2011 |
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| XMM-Newton observations of IGR J18410-0535: the ingestion of a clump by a supergiant fast X-ray transient |
Context. IGR J18410-0535 is one of the supergiant fast X-ray transients. This subclass of supergiant X-ray binaries typically under- goes few-hour long outbursts reaching luminosities of 1036-1037 erg/s, the occurrence of which has been ascribed to the combined effect of the intense magnetic field and rotation of the compact object hosted in them and/or the presence of dense structures ("clumps") in the wind of their supergiant companion.
Aims. IGR J18410-0535 was observed for 45 ks by XMM-Newton as part of a program aimed at studying the quiescent emission of supergiant fast X-ray transients and clarifying the origin of their peculiar X-ray variability.
Methods. We carried out an in-depth spectral and timing analysis of the XMM-Newton data.
Results. IGR J18410-0535 underwent a bright X-ray flare that started about 5 ks after the beginning of the observation and lasted for ~15 ks. Thanks to the capabilities of the instruments on-board XMM-Newton, the whole event could be followed in great detail. The results of our analysis provide strong convincing evidence that the flare was due to the accretion of matter from a massive clump onto the compact object hosted in this system.
Conclusions. By assuming that the clump is spherical and is moving at the same velocity as the homogeneous stellar wind, we estimate a mass and radius of Mcl~1.4×1022 g and Rcl~8×1011 cm. These are in qualitative agreement with values expected from theoretical calculations. No evidence for pulsations at ~4.7 s was found (we investigated coherent modulations in the range 3.5 ms-100 s). A reanalysis of the archival ASCA and Swift data of IGR J18410-0535, where such pulsations were previously detected, revealed that they were likely due to a statistical fluctuation and to an instrumental effect, respectively. |
| Publication date: 28 Jun 2011 |
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| X-ray illumination of the ejecta of supernova 1987A |
| When a massive star explodes as a supernova, substantial amounts of radioactive elements - primarily 56Ni, 57Ni and 44Ti - are produced. After the initial flash of light from shock heating, the fading light emitted by the supernova is due to the decay of these elements. However, after decades, the energy powering a supernova remnant comes from the shock interaction between the ejecta and the surrounding medium. The transition to this phase has hitherto not been observed: supernovae occur too infrequently in the Milky Way to provide a young example, and extragalactic supernovae are generally too faint and too small. Here we report observations that show this transition in the supernova SN 1987A in the Large Magellanic Cloud. From 1994 to 2001, the ejecta faded owing to radioactive decay of 44Ti as predicted. Then the flux started to increase, more than doubling by the end of 2009. We show that this increase is the result of heat deposited by X-rays produced as the ejecta interacts with the surrounding material. In time, the X-rays will penetrate farther into the ejecta, enabling us to analyse the structure and chemistry of the vanished star. |
| Publication date: 23 Jun 2011 |
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| Thermal Structure and Dynamics of Saturn's Northern Springtime Disturbance |
| Saturn's slow seasonal evolution was disrupted in 2010-2011 by the eruption of a bright storm in its northern spring hemisphere. Thermal infrared spectroscopy showed that within a month, the resulting planetary-scale disturbance had generated intense perturbations of atmospheric temperatures, winds, and composition between 20° and 50°N over an entire hemisphere (140,000 kilometers). The tropospheric storm cell produced effects that penetrated hundreds of kilometers into Saturn's stratosphere (to the 1-millibar region). Stratospheric subsidence at the edges of the disturbance produced "beacons" of infrared emission and longitudinal temperature contrasts of 16 kelvin. The disturbance substantially altered atmospheric circulation, transporting material vertically over great distances, modifying stratospheric zonal jets, exciting wave activity and turbulence, and generating a new cold anticyclonic oval in the center of the disturbance at 41°N. |
| Publication date: 17 Jun 2011 |
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| Transiting exoplanets from the CoRoT space mission - XVI. The hot Jupiter CoRoT-17b: a very old planet |
| We report the detection of a hot Jupiter-type exoplanet, named CoRoT-17b, detected by the CoRoT satellite. It has a mass of 2.43 ± 0.30MJup and a radius of 1.02 ± 0.07RJup, while its mean density is 2.82 ± 0.38 g/cm3. The orbital period is 3.768125 ± 0.000257 days and the orbit is circular. It orbits an old (10.7 ± 1.0 Gyrs) main-sequence star making it an intriguing object for planetary evolution studies. Its internal structure is not well constrained yet and it can be a pure H/He giant as well as it can contain ~320 earth masses of heavier elements. |
| Publication date: 14 Jun 2011 |
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| Structure and Dynamics of the 2010 July 11 Eclipse White-light Corona |
| The white-light corona (WLC) during the total solar eclipse on 2010 July 11 was observed by several teams in the Moon's shadow stretching across the Pacific Ocean and a number of isolated islands. We present a comparison of the WLC as observed by eclipse teams located on the Tatakoto Atoll in French Polynesia and on Easter Island, 83 minutes later, combined with near-simultaneous space observations. The eclipse was observed at the beginning of the solar cycle, not long after solar minimum. Nevertheless, the solar corona shows a plethora of different features (coronal holes, helmet streamers, polar rays, very faint loops and radial-oriented thin streamers, a coronal mass ejection, and a puzzling "curtain-like" object above the north pole). Comparing the observations from the two sites enables us to detect some dynamic phenomena. The eclipse observations are further compared with a hairy-ball model of the magnetic field and near-simultaneous images from the Atmospheric Imaging Assembly on NASA's Solar Dynamics Observatory, the Extreme Ultraviolet Imager on NASA's Solar Terrestrial Relations Observatory, the Sun Watcher, using Active Pixel System Detector and Image Processing on ESA's PRoject for Onboard Autonomy, and the Naval Research Laboratory's Large Angle and Spectrometric Coronagraph on ESA's Solar and Heliospheric Observatory. The Ludendorff flattening coefficient is 0.156, matching the expected ellipticity of coronal isophotes at 2 solar radii, for this rising phase of the solar-activity cycle. |
| Publication date: 06 Jun 2011 |
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| Thermodynamics of the Solar Corona and Evolution of the Solar Magnetic Field as Inferred from the total Solar Eclipse Observations of 2010 July 11 |
| We report on the first multi-wavelength coronal observations, taken simultaneously in white light, H alpha 656.3 nm, Fe IX 435.9 nm, Fe X 637.4 nm, Fe XI 789.2 nm, Fe XIII 1074.7 nm, Fe XIV 530.3 nm, and Ni XV 670.2 nm, during the total solar eclipse of 2010 July 11 from the atoll of Tatakoto in French Polynesia. The data enabled temperature differentiations as low as 0.2 × 10^6 K. The first-ever images of the corona in Fe IX and Ni XV showed that there was very little plasma below 5 × 10^5 K and above 2.5 × 10^6 K. The suite of multi-wavelength observations also showed that open field lines have an electron temperature near 1× 10^6 K, while the hottest, 2× 10^6 K, plasma resides in intricate loops forming the bulges of streamers, also known as cavities, as discovered in our previous eclipse observations. The eclipse images also revealed unusual coronal structures, in the form of ripples and streaks, produced by the passage of coronal mass ejections and eruptive prominences prior to totality, which could be identified with distinct temperatures for the first time. These trails were most prominent at 106 K. Simultaneous Fe X 17.4 nm observations from Proba2/SWAP provided the first opportunity to compare Fe X emission at 637.4 nm with its extreme-ultraviolet (EUV) counterpart. This comparison demonstrated the unique diagnostic capabilities of the coronal forbidden lines for exploring the evolution of the coronal magnetic field and the thermodynamics of the coronal plasma, in comparison with their EUV counterparts in the distance range of 1-3 solar radii. These diagnostics are currently missing from present space-borne and ground-based observatories. |
| Publication date: 06 Jun 2011 |
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