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Saint-Martin-Vésubie, France

Crawled News Article
Site: http://www.rdmag.com/rss-feeds/all/rss.xml/all

On the outskirts of the Rho Ophiuchi cloud complex, which is about 400 light-years from Earth, a Flying Saucer glows. Okay, the Flying Saucer isn’t a spaceship. It’s a young star officially known as 2MASS J16281370-2431391; its colloquially name is inspired by its appearance in visible pictures. But just because it isn’t an alien spacecraft doesn’t mean something interesting isn’t happening. In fact, an international team of scientists with the European Southern Observatory (ESO) recently glimpsed something they thought was physically impossible. Viewing the star’s protoplanetary disk—a collection of gas and dust associated with early planet formation—with the Atacama Large Millimeter/submillimeter Array (ALMA), the researchers observed and imaged the glow emanating from the disk’s carbon monoxide molecules. Surprisingly, the team observed a negative signal. “This disk is not observed against a black and empty night sky,” said Stephane Guilloteau, who is the lead author of a Letter to the Editor appearing in Astronomy & Astrophysics. “Instead it’s seen in silhouette in front of the glow of the Rho Ophiuchi Nebula. This diffuse glow is too extended to be detected by ALMA, but the disk absorbs it. The resulting negative signal means that parts of the disk are colder than the background. The Earth is quite literally in the shadow of the Flying Saucer!” The ALMA data was combined with other observations of the background glow from Spain’s IRAM 30-m telescope. With the two datasets, the researchers discovered the disk dust grain temperature was -266 C, colder than previous temperature models predicted. “Although dust is the main agent to control the protoplanetary disk temperature, our knowledge of dust temperatures essentially relies on modeling of disk images and (Spectral Energy Distribution),” the researchers write in Astronomy & Astrophysics. “Despite (or even because of) their sophistication, these models suffer from many uncertainties because of the large number of assumed properties: radial distribution, dust grain growth, dust settling, composition and porosity, disk flaring geometry, etc.” According to the ESO, most current models predict temperatures between -258 and -253 C. While the researchers haven’t pinpointed the exact reason behind the low temperature, they have a few ideas. One idea postulated is the temperature may depend on the disk’s grain sizes. The larger grains being cooler, and the smaller ones warmer. “It is too early to be sure,” said coauthor Emmanuel di Folco. Further observations are needed to understand the role temperature plays in planet formation.

Crawled News Article
Site: http://phys.org/space-news/

This star is surrounded by a disc of gas and dust—such discs are called protoplanetary discs as they are the early stages in the creation of planetary systems. This particular disc is seen nearly edge-on, and its appearance in visible light pictures has led to its being nicknamed the Flying Saucer. The astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the glow coming from carbon monoxide molecules in the 2MASS J16281370-2431391 disc. They were able to create very sharp images and found something strange—in some cases they saw a negative signal! Normally a negative signal is physically impossible, but in this case there is an explanation, which leads to a surprising conclusion. Lead author Stephane Guilloteau takes up the story: "This disc is not observed against a black and empty night sky. Instead it's seen in silhouette in front of the glow of the Rho Ophiuchi Nebula. This diffuse glow is too extended to be detected by ALMA, but the disc absorbs it. The resulting negative signal means that parts of the disc are colder than the background. The Earth is quite literally in the shadow of the Flying Saucer!" The team combined the ALMA measurements of the disc with observations of the background glow made with the IRAM 30-metre telescope in Spain. They derived a disc dust grain temperature of only -266 degrees Celsius (only 7 degrees above absolute zero, or 7 Kelvin) at a distance of about 15 billion kilometres from the central star. This is the first direct measurement of the temperature of large grains (with sizes of about one millimetre) in such objects. This temperature is much lower than the -258 to -253 degrees Celsius (15 to 20 Kelvin) that most current models predict. To resolve the discrepancy, the large dust grains must have different properties than those currently assumed, to allow them to cool down to such low temperatures. "To work out the impact of this discovery on disc structure, we have to find what plausible dust properties can result in such low temperatures. We have a few ideas—for example the temperature may depend on grain size, with the bigger grains cooler than the smaller ones. But it is too early to be sure," adds co-author Emmanuel di Folco (Laboratoire d'Astrophysique de Bordeaux). If these low dust temperatures are found to be a normal feature of protoplanetary discs this may have many consequences for understanding how they form and evolve. For example, different dust properties will affect what happens when these particles collide, and thus their role in providing the seeds for planet formation. Whether the required change in dust properties is significant or not in this respect cannot yet be assessed. Low dust temperatures can also have a major impact for the smaller dusty discs that are known to exist. If these discs are composed of mostly larger, but cooler, grains than is currently supposed, this would mean that these compact discs can be arbitrarily massive, so could still form giant planets comparatively close to the central star. Further observations are needed, but it seems that the cooler dust found by ALMA may have significant consequences for the understanding of protoplanetary discs. This research was presented in a paper entitled "The shadow of the Flying Saucer: A very low temperature for large dust grains", by S. Guilloteau et al., published in Astronomy & Astrophysics Letters.

Gallerani S.,Normal School of Pisa | Ferrara A.,Normal School of Pisa | Neri R.,IRAM | Maiolino R.,University of Cambridge
Monthly Notices of the Royal Astronomical Society

We report the serendipitous detection of the CO(17-16) emission line towards the quasar sloan digital sky survey J114816.64+525150.3 (J1148) at redshift z ≃ 6.4 obtained with the Plateau de Bure Interferometer. The CO(17-16) line is possibly contaminated by OH+ emission, that may account for ∼35-60 per cent of the total flux observed. Photodissociation and X-ray-dominated regions (PDRs and XDRs) models show that PDRs alone cannot reproduce the high luminosity of the CO(17-16) line relative to low-J CO transitions and that XDRs are required. By adopting a composite PDR+XDR model, we derive molecular cloud and radiation field properties in the nuclear region of J1148. Our results show that highly excited CO lines represent a sensitive and possibly unique tool to infer the presence of X-ray faint or obscured supermassive black hole progenitors in high-z galaxies. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Cabrit S.,Cergy-Pontoise University | Codella C.,National institute for astrophysics | Codella C.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Gueth F.,IRAM | Gusdorf A.,Cergy-Pontoise University
Astronomy and Astrophysics

Context. Previous SiO maps of the innermost regions of HH212 set strong constraints on the structure and origin of this jet. They rule out a fast wide-angle wind, and tentatively favor a magneto-centrifugal disk wind launched out to 0.6 AU. Aims. We aim to assess the SiO content at the base of the HH212 jet to set an independent constraint on the location of the jet launch zone with respect to the dust sublimation radius. Methods. We present the first sub-arcsecond (0.″44× 0.″96) CO map of the HH212 jet base, obtained with the IRAM Plateau de Bure Interferometer. Combining this with previous SiO(5-4) data, we infer the CO(2-1) opacity and mass-flux in the high-velocity jet and arrive at a much tighter lower limit to the SiO abundance than possible from the (optically thick) SiO emission alone. Results. Gas-phase SiO at high velocity contains at least 10% of the elemental silicon if the jet is dusty, and at least 40% if the jet is dust-free, if CO and SiO have similar excitation temperatures. Such a high SiO content is challenging for current chemical models of both dust-free winds and dusty interstellar shocks. Conclusions. Updated chemical models (equatorial dust-free winds, highly magnetized dusty shocks) and observations of higher J CO lines are required to elucidate the dust content and launch radius of the HH212 high-velocity jet. © ESO, 2012. Source

Guilloteau S.,University of Bordeaux 1 | Guilloteau S.,French National Center for Scientific Research | Dutrey A.,University of Bordeaux 1 | Dutrey A.,French National Center for Scientific Research | And 3 more authors.
Astronomy and Astrophysics

Context. Proto-planetary disks are thought to provide the initial environment for planetary system formation. The dust and gas distribution and its evolution with time is one of the key elements in the process. Aims. We attempt to characterize the radial distribution of dust in disks around a sample of young stars from an observational point of view, and, when possible, in a model-independent way, by using parametric laws. Methods. We used the IRAM PdBI interferometer to provide very high angular resolution (down to 0.4′′ in some sources) observations of the continuum at 1.3 mm and 3 mm around a sample of T Tauri stars in the Taurus-Auriga region. The sample includes single and multiple systems, with a total of 23 individual disks. We used track-sharing observing mode to minimize the biases. We fitted these data with two kinds of models: a "truncated power law" model and a model presenting an exponential decay at the disk edge ("viscous" model). Results. Direct evidence for tidal truncation is found in the multiple systems. The temperature of the mm-emitting dust is constrained in a few systems. Unambiguous evidence for large grains is obtained by resolving out disks with very low values of the dust emissivity index β. In most disks that are sufficiently resolved at two different wavelengths, we find a radial dependence of β, which appears to increase from low values (as low as 0) at the center to about 1.7-2 at the disk edge. The same behavior could apply to all studied disks. It introduces further ambiguities in interpreting the brightness profile, because the regions with apparent β â‰̂ 0 can also be interpreted as being optically thick when their brightness temperature is high enough. Despite the added uncertainty on the dust absorption coefficient, the characteristic size of the disk appears to increase with a higher estimated star age. Conclusions. These results provide the first direct evidence of the radial dependence of the grain size in proto-planetary disks. Constraints of the surface density distributions and their evolution remain ambiguous because of a degeneracy with the β(r) law. © 2011 ESO. Source

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