News Article | May 22, 2017
Artist's impression of a solar flare and the twisted magnetic field that carries away the ejected solar material. Credit: G.Valori, M. Berger & NASA SDO The emerging discipline of space meteorology aims to reliably predict solar flares so that we may better guard against their effects. Using 3-D numerical models, an international team headed by Etienne Pariat, a researcher at LESIA (Observatoire de Paris / CNRS / Université Paris Diderot / UPMC), has discovered a proxy that could be used to forecast an eruptive event. The proxy is associated with magnetic helicity, which reflects the extent of twist and entanglement of the magnetic field. The study is published in the journal Astronomy and Astrophysics dated 17 May 2017. Solar flares or eruptions are one of the most violent phenomena in the Solar System. They coincide with a sudden, violent reconfiguration of the magnetic field, releasing huge amounts of energy that can eject billions of tons of solar material into space at speeds of over a thousand kilometers per second. Although numerous parameters have been studied, the probability of forecasting a major flare one day in advance is currently no greater than 40%. And yet the most powerful flares can lead to major disruptions on Earth, causing interference with telecommunications or knocking out electrical power grids across entire regions of the world. Our technologies, which are increasingly dependent on electrical components and on satellites (GPS, telephony, etc), are thus ever more sensitive to solar activity, while such flares can even put astronauts' lives in danger. One of the aims of space meteorology is to forecast solar flares, in the same way as meteorological services forecast storms on Earth. Looking for a predictive parameter, the astrophysicists based their work on 3-D numerical simulations, which use computers to reproduce the behavior of the magnetic field in the Sun's atmosphere as well as the formation of sunspots, where eruptions take place. The researchers tested various parametric simulations and analyzed changes in magnetic energy and magnetic helicity, a quantity that measures the extent of entanglement and twist of the magnetic field. For their study, the researchers carried out computer simulations of two scenarios, one with an eruption and the other without. Their initial calculations confirmed that neither magnetic energies nor the total helicity of the magnetic field fulfilled the criteria for a predictive factor. Using a complex mathematical approach based on the separation of the magnetic field into several components, the researchers successfully obtained a proxy capable of predicting eruptions. The proxy (which compares two helicities in the potentially eruptive region) remains low in non-eruptive scenarios; whereas in every other case it increases significantly before the eruption (see Figures). The study, carried out as part of the HéliSol program, thus opens the way to more effective forecasting of solar flares. The theoretical findings now need to be confirmed by analyzing observations of active solar regions. This is currently being done as part of the European Flarecast project, which aims to set up an automatic system for forecasting flares. Explore further: Igniting a solar flare in the corona with lower-atmosphere kindling More information: E. Pariat et al. Relative magnetic helicity as a diagnostic of solar eruptivity, Astronomy & Astrophysics (2017). DOI: 10.1051/0004-6361/201630043
News Article | May 25, 2017
The emerging discipline of space meteorology aims to reliably predict solar flares so that we may better guard against their effects. Using 3D numerical models , an international team headed by Etienne Pariat, a researcher at LESIA (Observatoire de Paris / CNRS / Université Paris Diderot / UPMC), has discovered a proxy that could be used to forecast an eruptive event. The proxy is associated with magnetic helicity, which reflects the extent of twist and entanglement of the magnetic field. The study is published in the journal Astronomy and Astrophysics dated 17 May 2017. Solar flares or eruptions are one of the most violent phenomena in the Solar System. They coincide with a sudden, violent reconfiguration of the magnetic field, releasing huge amounts of energy that can eject billions of tons of solar material into space at speeds of over a thousand kilometers per second. Although numerous parameters have been studied, the probability of forecasting a major flare one day in advance is currently no greater than 40%. And yet the most powerful flares can lead to major disruptions on Earth, causing interference with telecommunications or knocking out electrical power grids across entire regions of the world. Our technologies, which are increasingly dependent on electrical components and on satellites (GPS, telephony, etc), are thus ever more sensitive to solar activity, while such flares can even put astronauts' lives in danger. One of the aims of space meteorology is to forecast solar flares, in the same way as meteorological services forecast storms on Earth. Looking for a predictive parameter, the astrophysicists based their work on 3D numerical simulations, which use computers to reproduce the behavior of the magnetic field in the Sun's atmosphere as well as the formation of sunspots, where eruptions take place. The researchers tested various parametric simulations and analyzed changes in magnetic energy and magnetic helicity, a quantity that measures the extent of entanglement and twist of the magnetic field. For their study, the researchers carried out computer simulations of two scenarios, one with an eruption and the other without. Their initial calculations confirmed that neither magnetic energies nor the total helicity of the magnetic field fulfilled the criteria for a predictive factor. Using a complex mathematical approach based on the separation of the magnetic field into several components, the researchers successfully obtained a proxy capable of predicting eruptions. The proxy (which compares two helicities in the potentially eruptive region) remains low in non-eruptive scenarios; whereas in every other case it increases significantly before the eruption. The study, carried out as part of the HéliSol program, thus opens the way to more effective forecasting of solar flares. The theoretical findings now need to be confirmed by analyzing observations of active solar regions. This is currently being done as part of the European Flarecast project, which aims to set up an automatic system for forecasting flares.
News Article | October 23, 2015
Coralie Neiner, from the Observatoire de Paris (LESIA/CNRS/UPMC/Université Paris Diderot) and Patricia Lampens, from the Royal Observatory of Belgium discovered the first magnetic delta Scuti star through spectropolarimetric observations at the CFHT telescope in Hawaii. The delta Scuti stars are pulsating stars some of which show signatures attributed to a second type of pulsations. The discovery shows that it is actually the signature of a magnetic field. This has important implications for understanding the interior of such stars.
News Article | November 12, 2015
To the naked eye comet 67P/Churyumov-Gerasimenko, destination and by now longtime companion of ESA's Rosetta spacecraft, is rather unremarkably colored: black as a piece of coal all over. However, with the help of OSIRIS, Rosetta's onboard scientific imaging system, scientists can make visible subtle, yet comprehensive differences in surface reflectivity. The newest analysis, presented today at the annual meeting of the Division for Planetary Sciences (DPS) of the American Astronomical Society (AAS) in National Harbor (Maryland, USA), thus paints a much more diverse picture of 67P. Not only is the neck region between the comet's two lobes apparently richer in frozen water than surrounding areas. OSIRIS data also show the body to be covered by a porous layer of fine grains and suggest the presence of frozen sulfur dioxide. Cometary nuclei and other primitive bodies populating the outer regions of the solar system commonly reflect red light slightly more effectively than light of other wavelengths. The effect is believed to be one of the results of space weathering. Images obtained during and shortly after Rosetta's approach phase in July and August of last year with different color filters have now been extensively analyzed and confirm this effect also for 67P. "Like most cometary nuclei, 67P's reflectivity spectrum is rather smooth and featureless," says OSIRIS team member Sonia Fornasier from the LESIA-Observatoire de Paris/University of Paris Diderot in France, who presented the new results today. Characteristic fingerprints of certain chemical compounds, so-called absorption bands, cannot be found in the wavelengths sampled by OSIRIS—except for a feature centered around 290 nanometers. "This feature lies in the ultraviolet range where instrument calibrations tend to be tricky and need still to be confirmed," says Fornasier. If the feature proves to be real, it is compatible with the presence of frozen sulfur dioxide on the comet's surface," she adds. The gaseous products of sulfur dioxide have been detected in several cometary comae including 67P. Many of the OSIRIS images analyzed in the new study offer a high spatial resolution of up to almost one meter per pixel. Rosetta can therefore observe differences in surface reflectivity in far greater detail than previous cometary missions. "Using the reflectivity in different wavelengths as a criterion, we were able to identify three different groups of terrains on 67P," Fornasier sums up the extensive analyzes. All three terrains occur on both the comet's lobes, but are often clustered in certain regions. These sometimes, but not always roughly coincide with the 25 different morphological regions so far identified on the comet's surface. For example, terrain with a slightly suppressed reflectivity of red light can be found mainly in the regions Hapi, Hathor, and in parts of Seth. "Especially the Hapi region on the comet's neck is slightly more bluish than other regions," says Fornasier. This points to a higher abundance of frozen water, as recently confirmed by measurements of the VIRTIS instrument. Terrain reflecting red light most efficiently is concentrated around the Imhotep depression on the large lobe and around the Hatmehit depression on the small lobe. "The three groups of terrain we identified are not correlated to a particular morphology that may expose material from deeper inside the nucleus," says Fornasier. Therefore, the reflectivity variations of the surface do not show evidence of vertical diversity in the nucleus composition, at least for the first tens of meters. Apart from composition, reflectivity data can also give insights into the fine structure of surface material. "Between July and August 2014, the Sun, the comet, and Rosetta were often arranged in very different observing geometries. This can change the amount of light reaching the OSIRIS camera and allows inferences on surface structure," Fornasier explains the basic idea behind this type of analysis. When all three bodies were almost aligned, the measured reflectivity proved to be high. With increasing deviations from this geometry, the surface showed itself darker and darker. This phenomenon referred to as an opposition surge is known from other bodies in the solar system such as the Moon. It is due to a combination of back scattering and shadow hiding processes in the particulate medium. "Studying this behavior in detail allows us to understand photometric properties of the surface material," says Fornasier. Again, 67P proves to resemble its cometary siblings such as Wild 2 and Tempel 1 which were visited by previous space missions. Data modeling indicates that the surface is covered by a porous layer of regolith with grains that reflect light in a back scattering manner. The inferred value of porosity of 87 percent is compatible with fractal aggregates which are believed to be the best analogs of cometary dust. ESA's Rosetta spacecraft arrived at comet 67P/Churyumov-Gerasimenko in August, 2014 after a ten year journey through space. Since then, it has been orbiting the comet at distances varying between six and several hundreds of kilometers. On 12 November, 2014 Rosetta deployed a lander to the comet's surface. Explore further: New images of comet 67P/Churyumov-Gerasimenko reveal an irregular shape More information: "Spectrophotometric Properties of the Nucleus of Comet 67P/Churyumov-Gerasimenko from the OSIRIS Instrument Onboard the ROSETTA Spacecraft," S. Fornasier et al., 2015 October 30, Astronomy & Astrophysics dx.doi.org/10.1051/0004-6361/201525901
News Article | December 8, 2016
Composite view of L2 Puppis in visible light (from the VLT/SPHERE instrument, blue colors) and ALMA continuum (orange colors). The central star light has been subtracted from the ALMA image to better show the companion object. Credit: © P. Kervella et al. (CNRS / U. de Chile / Observatoire de Paris / LESIA / ESO / ALMA) What will happen to Earth when, in a few billion years' time, the sun is a hundred times bigger than it is today? Using the most powerful radio telescope in the world, an international team of astronomers has set out to look for answers in the star L2 Puppis. Five billion years ago, this star was very similar to the sun as it is today. "Five billion years from now, the sun will have grown into a red giant star, more than a hundred times larger than its current size," says Professor Leen Decin from the KU Leuven Institute of Astronomy. "It will also experience an intense mass loss through a very strong stellar wind. The end product of its evolution, 7 billion years from now, will be a tiny white dwarf star. This will be about the size of the Earth, but much heavier: one tea spoon of white dwarf material weighs about 5 tons." This metamorphosis will have a dramatic impact on the planets of our solar system. Mercury and Venus, for instance, will be engulfed in the giant star and destroyed. "But the fate of the Earth is still uncertain," continues Decin. "We already know that our sun will be bigger and brighter, so that it will probably destroy any form of life on our planet. But will the Earth's rocky core survive the red giant phase and continue orbiting the white dwarf?" To answer this question, an international team of astronomers observed the evolved star L2 Puppis. This star is 208 light years away from Earth - which, in astronomy terms, means nearby. The researchers used the ALMA radio telescope, which consists of 66 individual radio antennas that together form a giant virtual telescope with a 16-kilometre diameter. "We discovered that L2 Puppis is about 10 billion years old," says Ward Homan from the KU Leuven Institute of Astronomy. "Five billion years ago, the star was an almost perfect twin of our sun as it is today, with the same mass. One third of this mass was lost during the evolution of the star. The same will happen with our sun in the very distant future." 300 million kilometres from L2 Puppis—or twice the distance between the sun and the Earth—the researchers detected an object orbiting the giant star. In all likelihood, this is a planet that offers a unique preview of our Earth five billion years from now. A deeper understanding of the interactions between L2 Puppis and its planet will yield valuable information on the final evolution of the sun and its impact on the planets in our solar system. Whether the Earth will eventually survive the sun's expansion or be destroyed is still uncertain. L2 Puppis may be the key to answering this question. More information: P. Kervella et al, ALMA observations of the nearby AGB star L Puppis, Astronomy & Astrophysics (2016). DOI: 10.1051/0004-6361/201629877
Proceedings of the International Astronomical Union | Year: 2011
Comets are made of ices, organics and minerals that record the chemistry of the outer regions of the primitive solar nebula where they agglomerated 4.6 Gyr ago. Compositional analyses of comets can provide important clues on the chemical and physical processes that occurred in the early phases of Solar System formation, and possibly in the natal molecular cloud that predated the formation of the solar nebula. This paper presents a short review of our present knowledge of the composition of comets. Implications for the origin of cometary materials are discussed. © 2011 International Astronomical Union.
Bourouaine S.,Max Planck Institute for Solar System Research |
Alexandrova O.,LESIA |
Marsch E.,Max Planck Institute for Solar System Research |
Astrophysical Journal | Year: 2012
We analyze the radial variation of the power spectra of the magnetic field from 0.3 to about 0.9AU, using Helios 2 spacecraft measurements in the fast solar wind. The time resolution of the magnetic field data allows us to study the power spectra up to 2Hz. Generally, the corresponding spectral break frequency fb and the Doppler-shifted frequencies, which are related to the proton gyroradius and inertial scales, are close to a frequency f of about 0.5Hz at a distance of 1AU from the Sun. However, studying the radial evolution of the power spectra offers us the possibility to distinguish between those scales. Recent Ulysses observations show that, while the proton scales vary, fb stays nearly constant with the heliocentric distance R. In our study we confirm that fb varies within a small interval of [0.2, 0.4]Hz only, as R varies from 0.3 to 0.9AU. Moreover, if we assume parallel propagating fluctuations (with respect to the solar wind flow or background magnetic field), we can show that none of the proton scales are coincident with the break scale. If, however, we take into account the two-dimensional nature of the turbulent fluctuations, then we can show that the spatial scale corresponding to fb (R) does follow the proton inertial scale, λp(R), but not the proton gyroradius scale, ρp(R), as a function of heliocentric distance. These observations indicate that the spectral break at the proton inertial scale might be related to the Hall effect, or be controlled by the ion-cyclotron damping of obliquely propagating fluctuations or the formation of current sheets scaling like λp, which could be responsible for ion heating through magnetic reconnection. © 2012 The American Astronomical Society. All rights reserved.
Celestial Mechanics and Dynamical Astronomy | Year: 2011
HD 196885 Ab is the most "extreme" planet-in-a-binary discovered to date, whose orbit places it at the limit for orbital stability. The presence of a planet in such a highly perturbed region poses a clear challenge to planet-formation scenarios. We investigate this issue by focusing on the planet-formation stage that is arguably the most sensitive to binary perturbations: the mutual accretion of kilometre-sized planetesimals. To this effect we numerically estimate the impact velocities dv amongst a population of circumprimary planetesimals. We find that most of the circumprimary disc is strongly hostile to planetesimal accretion, especially the region around 2.6 AU (the planet's location) where binary perturbations induce planetesimal-shattering dv of more than 1 kms-1. Possible solutions to the paradox of having a planet in such accretion-hostile regions are (1) that initial planetesimals were very big, at least 250 km (2) that the binary had an initial orbit at least twice the present one, and was later compacted due to early stellar encounters (3) that planetesimals did not grow by mutual impacts but by sweeping of dust (the "snowball" growth mode identified by Xie et al., in Astrophys J 724:1153, 2010b), or (4) that HD 196885 Ab was formed not by core-accretion but by the concurrent disc instability mechanism. All of these 4 scenarios remain however highly conjectural. © 2011 Springer Science+Business Media B.V.
Lovekin C.C.,LESIA |
Astronomy and Astrophysics | Year: 2010
Context. Recent work on several Β Cephei stars has succeeded in constraining both their interior rotation profile and their convective core overshoot. In particular, a recent study focusing on θ Ophiuchi has shown that a convective core overshoot parameter of αov = 0.44 is required to model the observed pulsation frequencies, significantly higher than for other stars of this type. Aims.We investigate the effects of rotation and overshoot in early type main sequence pulsators, such as Β Cephei stars, and attempt to use the low order pulsation frequencies to constrain these parameters. This will be applied to a few test models and the Β Cephei star θ Ophiuchi. Methods. We use the 2D stellar evolution code ROTORC and the 2D linear adiabatic pulsation code NRO to calculate pulsation frequencies for 9.5 M⊙ models evolved to an age of 15.6 Myr. We calculate low order p-modes (l ≤ 2) for models with a range of rotation rates and convective core overshoot parameters. These low order modes are the same range of modes observed in. Ophiuchi. Results.Using these models, we find that the convective core overshoot has a larger effect on the pulsation frequencies than the rotation, except in the most rapidly rotating models considered. When the differences in radii are accounted for by scaling the frequencies by v(GM/R(40°)3), the effects of rotation diminish, but are not entirely accounted for. Thus, this scaling emphasizes the differences produced by changing the convective core overshoot. We find that increasing the convective core overshoot decreases the large separation, while producing a slight increase in the small separations. We created a model frequency grid which spanned several rotation rates and convective core overshoot values. We used this grid to define a modified x2 statistic in order to determine the best fitting parameters from a set of observed frequencies. Using this statistic, we are able to recover the rotation velocity and convective core overshoot for a few test models. We have also performed a "hare and hound" exercise to see how well 1D models can recover these parameters. Finally, we discuss the case of the Β Cephei star θ Oph. Using the observed frequencies and a fixed mass and metallicity, we find a lower overshoot than previously determined, with αov = 0.28 ± 0.05. Our determination of the rotation rate agrees well with both previous work and observations, around 30 km s-1. © ESO, 2010.
News Article | November 18, 2016
The carbon dioxide ice layer covered an area comparable to the size of a football pitch, while the two water ice patches were each larger than an Olympic swimming pool and much larger than any signs of water ice previously spotted at the comet. The three icy layers were all found in the same region, on the comet's southern hemisphere. A combination of the complex shape of the comet, its elongated path around the Sun and the substantial tilt of its spin, seasons are spread unequally between the two hemispheres of the double-lobed Comet 67P/Churyumov-Gerasimenko. When Rosetta arrived in August 2014, the northern hemisphere was still undergoing its 5.5 year summer, while the southern hemisphere was in winter and much of it was shrouded in darkness. However, shortly before the comet's closest approach to the Sun in August 2015, the seasons changed and the southern hemisphere experienced a brief but intense summer, exposing this region to sunlight again. In the first half of 2015, as the comet steadily became more active, Rosetta observed water vapour and other gases pouring out of the nucleus, lifting its dusty cover and revealing some of the comet's icy secrets. In particular, on two occasions in late March 2015, Rosetta's visible, infrared and thermal imaging spectrometer, VIRTIS, found a very large patch of carbon dioxide ice in the Anhur region, in the comet's southern hemisphere. This is the first detection of solid carbon dioxide on any comet, although it is not uncommon in the Solar System – it is abundant in the polar caps of Mars, for example. "We know comets contain carbon dioxide, which is one of the most abundant species in cometary atmospheres after water, but it's extremely difficult to observe it in solid form on the surface," explains Gianrico Filacchione from Italy's INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, who led the study. In the comet environment, carbon dioxide freezes at -193°C, much below the temperature where water turns into ice. Above this temperature, it changes directly from a solid to a gas, hampering its detection in ice form on the surface. By contrast, water ice has been found at various comets, and Rosetta detected plenty of small patches on several regions. "We hoped to find signs of carbon dioxide ice and had been looking for it for quite a while, but it was definitely a surprise when we finally detected its unmistakable signature," adds Gianrico. The patch, consisting of a few percent of carbon dioxide ice combined with a darker blend of dust and organic material, was observed on two consecutive days in March. This was a lucky catch: when the team looked at that region again around three weeks later, it was gone. Assuming that all of the ice had turned into gas, the scientists estimated that the 80 m × 60 m patch contained about 57 kg of carbon dioxide, corresponding to a 9 cm-thick layer. Its presence on the surface is likely an isolated rare case, with the majority of carbon dioxide ice being confined to deeper layers of the nucleus. Gianrico and his collaborators believe the icy patch dates back a few years, when the comet was still in the cold reaches of the outer Solar System and the southern hemisphere was experiencing its long winter. At that time, some of the carbon dioxide still outgassing from the interior of the nucleus condensed on the surface, where it remained frozen for a very long while, and vaporised only as the local temperature finally rose again in April 2015. This reveals a seasonal cycle of carbon dioxide ice, which unfolds over the comet's 6.5 year orbit, as opposed to the daily cycle of water ice, also spotted by VIRTIS shortly after Rosetta's arrival. Interestingly, shortly after the carbon dioxide ice had disappeared, Rosetta's OSIRIS narrow-angle camera detected two unusually large patches of water ice in the same area, between the southern regions of Anhur and Bes. "We had already seen many metre-sized patches of exposed water ice in various regions of the comet, but the new detections are much larger, spanning some 30 m × 40 m each, and they persisted for about 10 days before they completely disappeared," says Sonia Fornasier from LESIA–Observatoire de Paris and Université Paris Diderot, France, lead scientist of the study focusing on seasonal and daily surface colour variations. These ice-rich areas appear as very bright portions of the comet surface reflecting light that is bluer in colour compared with the redder surroundings. Scientists have experimented with mixtures of dust and water ice to show that, as the concentration of ice in them increases, the reflected light becomes gradually bluer in colour, until reaching a point where equal amounts of light are reflected in all colours. The two newly detected patches contain 20–30% of water ice mixed with darker material, forming a layer up to 30 cm thick of solid ice. One of them was likely lurking underneath the carbon dioxide ice sheet revealed by VIRTIS about a month before. "On a global scale, we also found that the entire comet surface turned increasingly bluer in colour as it approached the Sun and the intense activity lifted off large amounts of dust, exposing more of the ice-rich terrain underneath," explains Sonia. As the comet moved away from the Sun, the scientists observed the overall colour of the comet surface gradually turning redder again. They also revealed local variations of colour, indicative of the daily cycle of water ice. Quickly turning into water vapour when exposed to sunlight during the local daytime, it condensed back into thin layers of frost and ice as the temperature decreases after sunset, only to vaporise again on the following day. The distribution of water ice beneath the dusty surface of the comet seems widely but not uniformly spread, with small patches punctuating the nucleus, appearing and disappearing as a result of the comet's activity. Occasionally, larger and thicker portions of ice are also uncovered, dating back to a previous approach to the Sun. "These two studies of the comet's icy content are revealing new details about the composition and history of the nucleus," says Matt Taylor, ESA Rosetta project scientist. "While the flight part of the mission is now over, the scientific exploitation of the enormous quantity of data collected by Rosetta continues." "Seasonal exposure of carbon dioxide ice on the nucleus of comet 67P/Churyumov-Gerasimenko" by G. Filacchione et al. and "Rosetta's comet 67P/Churyumov-Gerasimenko sheds its dusty mantle to reveal its icy nature" by S. Fornasier et al. are published in the journal Science. Explore further: Far away, so close More information: G. Filacchione et al. Seasonal exposure of carbon dioxide ice on the nucleus of comet 67P/Churyumov-Gerasimenko, Science (2016). DOI: 10.1126/science.aag3161 S. Fornasier et al. Rosettas comet 67P/Churyumov-Gerasimenko sheds its dusty mantle to reveal its icy nature, Science (2016). DOI: 10.1126/science.aag2671