43 research outputs found
Can we trace very cold dust from its emission alone ?
Context. Dust is a good tracer of cold dark clouds but its column density is
difficult to quantify. Aims. We want to check whether the far-infrared and
submillimeter high-resolution data from Herschel SPIRE and PACS cameras
combined with ground-based telescope bolometers allow us to retrieve the whole
dust content of cold dark clouds. Methods. We compare far-infrared and
submillimeter emission across L183 to the 8 m absorption map from Spitzer
data and fit modified blackbody functions towards three different positions.
Results. We find that none of the Herschel SPIRE channels follow the cold dust
profile seen in absorption. Even the ground-based submillimeter telescope
observations, although more closely following the absorption profile, cannot
help to characterize the cold dust without external information such as the
dust column density itself. The difference in dust opacity can reach up to a
factor of 3 in prestellar cores of high extinction. Conclusions. In dark
clouds, the amount of very cold dust cannot be measured from its emission
alone. In particular, studies of dark clouds based only on Herschel data can
miss a large fraction of the dust content. This has an impact on core and
filament density profiles, masse and stability estimates.Comment: Letter to A&A (accepted for publication). must be viewed with ACROBAT
READER for full enhancement. Otherwise, check images in Appendix
Standing wave vs Green's function approach to the Casimir force problem
After a short recall of our previous standing wave approach to the Casimir
force problem, we consider Lifshitz's temperature Green's function method and
its virtues from a physical point of view. Using his formula, specialized for
perfectly reflecting mirrors, we present a quantitative discussion of the
temperature effect on the attractive force
Fabrication and structural characterization of highly ordered sub-100-nm planar magnetic nanodot arrays over 1 cm2 coverage area
Porous alumina masks are fabricated by anodization of aluminum films grown on both semiconducting and insulating substrates. For these self-assembled alumina masks, pore diameters and periodicities within the ranges of 10–130 and 20–200nm, respectively, can be controlled by varying anodization conditions. 20nm periodicities correspond to pore densities in excess of 1012 per square inch, close to the holy grail of media with 1Tbit∕in.2 density. With these alumina masks, ordered sub-100-nm planar ferromagnetic nanodot arrays covering over 1cm2 were fabricated by electron beam evaporation and subsequent mask lift-off. Moreover, exchange-biased bilayer nanodots were fabricated using argon-ion milling. The average dot diameter and periodicity are tuned between 25 and 130nm and between 45 and 200nm, respectively. Quantitative analyses of scanning electron microscopy (SEM) images of pore and dot arrays show a high degree of hexagonal ordering and narrow size distributions. The dot periodicity obtained from grazi..
The Millimeter Astronomy Legacy Team 90 GHz (MALT90) Pilot Survey
We describe a pilot survey conducted with the Mopra 22-m radio telescope in
preparation for the Millimeter Astronomy Legacy Team Survey at 90 GHz (MALT90).
We identified 182 candidate dense molecular clumps using six different
selection criteria and mapped each source simultaneously in 16 different lines
near 90 GHz. We present a summary of the data and describe how the results of
the pilot survey shaped the design of the larger MALT90 survey. We motivate our
selection of target sources for the main survey based on the pilot detection
rates and demonstrate the value of mapping in multiple lines simultaneously at
high spectral resolution.Comment: Accepted to ApJS. 23 pages and 16 figures. Full resolution version
with an appendix showing all the data (12.1 MB) is available at
http://malt90.bu.edu/publications/Foster_2011_Malt90Pilot.pd
Intrastrain genomic and phenotypic variability of the commercial Saccharomyces cerevisiae strain Zymaflore VL1 reveals microevolutionary adaptation to vineyard environments
The maintenance of microbial species in different environmental conditions is associated with adaptive microevolutionary changes that are shown here to occur within the descendants of the same strain in comparison with the commercial reference strain. However, scarce information is available regarding changes that occur among strain descendants during their persistence in nature. Herein we evaluate genome variations among four isolates of the commercial winemaking strain Saccharomyces cerevisiae Zymaflore VL1 that were re-isolated from vineyards surrounding wineries where this strain was applied during several years, in comparison with the commercial reference strain. Comparative genome hybridization showed amplification of 14 genes among the recovered isolates being related with mitosis, meiosis, lysine biosynthesis, galactose and asparagine catabolism, besides 9 Ty elements. The occurrence of microevolutionary changes was supported by DNA sequencing that revealed 339-427 SNPs and 12-62 indels. Phenotypic screening and metabolic profiles also distinguished the recovered isolates from the reference strain. We herein show that the transition from nutrient-rich musts to nutritionally scarce natural environments induces adaptive responses and microevolutionary changes promoted by Ty elements and by nucleotide polymorphisms that were not detected in the reference strain.Ricardo Franco-Duarte and Ines Mendes are recipients of a fellowship from the Portuguese Science Foundation, FCT (SFRH/BD/48591/2008, SFRH/BD/74798/2010, respectively). Financial support was obtained from FEDER funds through the program COMPETE, by national funds through FCT by the projects FCOMP-01-0124-008775 (PTDC/AGR-ALI/103392/2008) and PTDC/AGR-ALI/121062/2010, and through the strategic funding UID/BIA/04050/2013.info:eu-repo/semantics/publishedVersio
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Identifying the energy release site in a solar microflare with a jet
Context. One of the main science questions of the Solar Orbiter and Parker Solar Probe missions deals with understanding how electrons in the lower solar corona are accelerated and how they subsequently access interplanetary space. Aims. We aim to investigate the electron acceleration and energy release sites as well as the manner in which accelerated electrons access the interplanetary space in the case of the SOL2021-02-18T18:05 event, a GOES A8 class microflare associated with a coronal jet. Methods. This study takes advantage of three different vantage points, Solar Orbiter, STEREO-A, and Earth, with observations drawn from eight different instruments, ranging from radio to X-ray. Multi-wavelength timing analysis combined with UV/EUV imagery and X-ray spectroscopy by Solar Orbiter/STIX (Spectrometer/Telescope for Imaging X-rays) is used to investigate the origin of the observed emission during different flare phases. Results. The event under investigation satisfies the classical picture of the onset time of the acceleration of electrons coinciding with the jet and the radio type III bursts. This microflare features prominent hard X-ray (HXR) nonthermal emission down to at least 10 keV and a spectrum that is much harder than usual for a microflare with γ = 2.9 ± 0.3. From Eartha's vantage point, the microflare is seen near the limb, revealing the coronal energy release site above the flare loop in EUV, which, from STIX spectroscopic analysis, turns out to be hot (i.e., at roughly the same temperature of the flare). Moreover, this region is moving toward higher altitudes over time (∼30akmas-1). During the flare, the same region spatially coincides with the origin of the coronal jet. Three-dimensional (3D) stereoscopic reconstructions of the propagating jet highlight that the ejected plasma moves along a curved trajectory. Conclusions. Within the framework of the interchange reconnection model, we conclude that the energy release site observed above-The-loop corresponds to the electron acceleration site, corroborating that interchange reconnection is a viable candidate for particle acceleration in the low corona on field lines open to interplanetary space
Fabrication and structural characterization of highly ordered sub-100-nm planar magnetic nanodot arrays over 1cm2 coverage area
High precision astrometry mission for the detection and characterization of nearby habitable planetary systems with the Nearby Earth Astrometric Telescope (NEAT)
(abridged) A complete census of planetary systems around a volume-limited
sample of solar-type stars (FGK dwarfs) in the Solar neighborhood with uniform
sensitivity down to Earth-mass planets within their Habitable Zones out to
several AUs would be a major milestone in extrasolar planets astrophysics. This
fundamental goal can be achieved with a mission concept such as NEAT - the
Nearby Earth Astrometric Telescope. NEAT is designed to carry out space-borne
extremely-high-precision astrometric measurements sufficient to detect
dynamical effects due to orbiting planets of mass even lower than Earth's
around the nearest stars. Such a survey mission would provide the actual
planetary masses and the full orbital geometry for all the components of the
detected planetary systems down to the Earth-mass limit. The NEAT performance
limits can be achieved by carrying out differential astrometry between the
targets and a set of suitable reference stars in the field. The NEAT instrument
design consists of an off-axis parabola single-mirror telescope, a detector
with a large field of view made of small movable CCDs located around a fixed
central CCD, and an interferometric calibration system originating from
metrology fibers located at the primary mirror. The proposed mission
architecture relies on the use of two satellites operating at L2 for 5 years,
flying in formation and offering a capability of more than 20,000
reconfigurations (alternative option uses deployable boom). The NEAT primary
science program will encompass an astrometric survey of our 200 closest F-, G-
and K-type stellar neighbors, with an average of 50 visits. The remaining time
might be allocated to improve the characterization of the architecture of
selected planetary systems around nearby targets of specific interest (low-mass
stars, young stars, etc.) discovered by Gaia, ground-based high-precision
radial-velocity surveys.Comment: Accepted for publication in Experimental Astronomy. The full member
list of the NEAT proposal and the news about the project are available at
http://neat.obs.ujf-grenoble.fr. The final publication is available at
http://www.springerlink.co
La formation d'étoiles massives dans la Galaxie vue par le relevé infrarouge ISOGAL
Président du jury : Pierre Encrenaz Rapporteurs : Alain Abergel, Philippe AndréThis thesis deals with the study of recent star formation in the Galaxy and is based on the ISOGAL infrared imaging survey, which contains about 400 observations of the Galactic Bulge and Disk at 7 or 15 microns. The critical data reduction and the use of a PSF fitting procedure to extract the point sources enabled the publication of a catalogue containing 100,000 sources. The bulk of these sources are interpreted as evolved stars in the RGB or AGB stage, but a few thousand of them are likey to be young stellar objects enshrouded in dust envelopes. In the inner Galactic Bulge, 300 ISOGAL sources are considered as forming stars, and more than 200 ISO or MSX bright sources are interpreted as M supergiant or massive AGB stars. A rough average star formation rate over the last million years in this central area of the Galaxy is derived from this sample of young stars. An approximate estimate for the past ~50 Myr is also inferred from the inventory of sources interpreted as M supergiants. Currently, about 700 YSO candidates are extracted from the complete ISOGAL catalogue. Outside the inner bulge, ~100 of them show all the features of enshrouded young stellar objects. The nature of the others is still uncertain and would be clarified by complementary observations. The nature of a large number of other sources, including faint ones that were only detected at 15 microns, is also uncertain.Cette thèse porte sur l'étude de la formation stellaire récente dans la Galaxie à l'aide du relevé d'imagerie infrarouge ISOGAL, constitué de ~400 observations ISOCAM à 7 ou 15 microns réparties dans le Bulbe et le Disque Galactiques. La réduction critique de ces données et l'extraction des sources ponctuelles par une procédure d'ajustement de PSF a permis la publication d'un catalogue d'environ 100 000 sources. On interprète la plupart de ces sources comme des étoiles en fin d'évolution, dans le stade RGB ou AGB, mais quelques milliers d'entre elles correspondent probablement à des objets stellaires jeunes enfouis dans des enveloppes de poussière. Dans le bulbe Galactique interne, 300 sources ISOGAL sont considérées comme des étoiles en formation, et plus de 200 sources brillantes détectées par ISO ou par MSX sont interprétées comme des supergéantes M ou des AGB massives. Un taux moyen approximatif de formation stellaire au cours du dernier million d'années dans cette région est déduit de cet échantillon de sources jeunes. Son estimation approchée au cours des ~50 derniers millions d'années est également déduit de l'inventaire des sources considérées comme supergéantes M. La sélection des objets jeunes dans l'ensemble du relevé ISOGAL conduit actuellement à une liste de ~700 candidats, dont une centaine - en dehors du bulbe interne - montrent toutes les caractéristiques des objets jeunes enfouis. La nature des autres est plus incertaine, et ne pourra être déterminée que par des observations complémentaires. La détermination de la nature d'un grand nombre d'autres sources, en particulier faibles et détectées seulement à 15 microns, reste également incertaine