4,189 research outputs found
High-pressure annealing of a prestructured nanocrystalline precursor to obtain tetragonal and orthorhombic polymorphs of Hf3N4
Transition metal nitrides containing metal ions in high oxidation states are a significant goal for the discovery of new families of semiconducting materials. Most metal nitride compounds prepared at high temperature and high pressure from the elements have metallic bonding. However amorphous or nanocrystalline compounds can be prepared via metal-organic chemistry routes giving rise to precursors with a high nitrogen:metal ratio. Using X-ray diffraction in parallel with high pressure laser heating in the diamond anvil cell this work highlights the possibility of retaining the composition and structure of a metastable nanocrystalline precursor under high pressure-temperature conditions. Specifically, a nanocrystalline Hf3N4 with a tetragonal defect-fluorite structure can be crystallized under high-P,T conditions. Increasing the pressure and temperature of crystallization leads to the formation of a fully recoverable orthorhombic (defect cottunite-structured) polymorph. This approach identifies a novel class of pathways to the synthesis of new crystalline nitrogen-rich transition metal nitrides
Sur l'origine de l'augmentation apparente des inondations en région méditerranéenne
En septembre 2002, les rĂ©gions mĂ©diterranĂ©ennes françaises et notamment le dĂ©partement du Gard ont Ă©tĂ© affectĂ©es par des prĂ©cipitations d'une extrĂȘme intensitĂ©. On estime que 80% de ce dĂ©partement a Ă©tĂ© inondĂ©, on dĂ©nombre 23 victimes et les dĂ©gĂąts ont Ă©tĂ© Ă©valuĂ©s Ă 1.2 milliards d'euros. Cette catastrophe hydrologique soulĂšve Ă nouveau les problĂšmes de la frĂ©quence de ces Ă©vĂ©nements et de l'augmentation des forts cumuls de pluie ces derniĂšres annĂ©es. L'objet de cet article est d'apporter quelques Ă©lĂ©ments de rĂ©ponse, notamment Ă travers l'analyse rĂ©gionale des pluies extrĂȘmes journaliĂšres ayant affectĂ© la rĂ©gion Languedoc-Roussillon de 1958 Ă 2002.La frĂ©quence rĂ©gionale des pluies extrĂȘmes est estimĂ©e en prenant en compte la superficie couverte par ces Ă©vĂ©nements en fonction des hauteurs pluviomĂ©triques. A l'Ă©chelle rĂ©gionale la pĂ©riode de retour de l'Ă©vĂ©nement varie entre 80 ans pour la superficie touchĂ©e par au moins 200 mm Ă 140 ans pour celle couverte par 300 mm.La stationnaritĂ© des frĂ©quences des pluies extrĂȘmes est analysĂ©e Ă partir des chroniques du nombre annuel d'Ă©vĂ©nements pluvieux dĂ©passant 200 mm, 250 mm et 300 mm en 24h maximum, entre 1958 et 2002 sur la rĂ©gion. Les tests de stationnaritĂ© ne rĂ©vĂšlent pas de tendance significative Ă l'augmentation de ces frĂ©quences. Les donnĂ©es historiques aboutissent aux mĂȘmes conclusions. L'augmentation rĂ©elle des inondations est en fait principalement liĂ©e Ă l'augmentation de la vulnĂ©rabilitĂ© des bassins.In September 2002, the Gard department in the South of France was affected by heavy precipitation that covered a broad geographical area. It was estimated that 80% of the department was flooded; there were 23 victims and the damage was evaluated to be 1.2 billion euros. This hydrological catastrophe raised questions about a possible increase in the frequency of these events during recent years, since several other severe flooding events have been observed in the region over the last 15 years. The aim of this article is to explore these questions through a regional analysis of the extreme daily rainfall that affected the Languedoc-Roussillon region between 1958 and 2002. The daily rain data were used because they are the most available type of information over the observation period. Usually, the rainfall hazard description is based on statistical analysis of the maximum rainfall depth observed at a given rain gauge. However, because the spatial variability of rainfall in the Mediterranean region, such results are only representative of local rainfall conditions. Moreover, this type of analysis does not take into account the spatial coverage of the precipitation, which is another factor influencing the resulting floods. Thus, the regional frequency of extreme rainfall was estimated by taking into account the area covered according to a given rainfall depth. For each rainfall event, a rain field was built using a kriging interpolation (NEPPEL et al., 1997). The isohyet area defined a rainfall threshold from 10 to 300 mm with a step of 10 mm calculated for each rainfall event. For each rainfall depth from 10 to 300 mm with a step of 10 mm, the probability distribution of the isohyet area was estimated. The regional rainfall hazards were described with the Depth-Area-Frequency curves (DAF) for 24-h periods. It was shown that at on regional scale, the return period of the last event varied between 80 years for the surface affected by at least 200 mm and 140 years for the surface covered by 300 mm. Compared with other major events that have occurred in the region, it appears that the September 2002 event one was characterized by :1. the spatial extension of the heavy rainfall, for example more than 1800 kmÂČ were affected by at least 400 mm in less than 24 h;2. the spatial localisation of the heaviest rainfall depths, which were measured over the highest relief (1000 m to 1500 m) as usual in the 'cĂ©venols' meteorological situation, but rather in the plain where the altitude lies between 200 m and 300 m.The stationnarity analysis of the extreme rainfall frequency was based on the annual number of events exceeding 200 mm, 250 mm and 300 mm over a 24 h maximum duration, between 1958 and 2002. The hypothesis of random events against the hypothesis of a trend or a sudden break in the mean was examined through several statistical tests. The procedures used were the rank correlation test, PETTITT's test, BUISHAND's test, HUBERT's segmentation procedure, a linear regression procedure, and the turning points procedure. Detailed descriptions of these tests can be found in KENDALL and STUART (1977), LUBES-NIEL et al. (1998) and WMO (2000). Except for the rank correlation test, all the procedures led to the conclusion that the three series are randomly distributed at the level of significance 1%, 5% and 10% respectively. Thus no significant increase in extreme rainfall frequency seems to appear. Although the study period was short, 45 years, compared with climatological variability, LUBES-NIEL et al. (1998) show that the procedures used were adapted in detecting trends in 50-yr time series. In considering historical rainfall data before 1958 in the same region, at least two extreme rainfall events could be compared with the event on 8-9 September 2002: in October 1940, 840 mm of rainfall were measured during 24 h in the PyrĂ©nĂ©es-Orientales district and in September 1900, 940 mm were observed over 24 h in Valleraugue, upstream in the Herault catchment. Furthermore, if the evolution of the rain gauge network density is taken into account, one can argue that such an event could have occurred more frequently. Indeed, the number of rain gauges has varied from 162 gauges in 1900 to 330 today. It has been shown that the number of observed rainfall events varied according to the area of the events and the network density (NEPPEL et al., 1998b). For example, an event of 150 km2 (corresponding to the area covered by more than 600 mm in September 2002) had a probability of 70% to be observed by the network between 1958 and 1993. If one considers the period 1920-1939, this probability decreases to 30%.In addition, the basin vulnerability has increased. The regional population has grown from 1,460,000 inhabitants in 1949 to 2,300,000 in 2000. At the same time, urbanization has expanded widely. Moreover, this new population came from other districts, and they are not familiar with the Mediterranean rainfall regime and the resulting flash floods. Buildings have often been constructed near rivers, which are attractive building sites, and sometimes even in the river's main channel, increasing the flooding risk and the flood damages. Thus, rather than climate change, for which the effect on extreme rainfalls cannot be proved, the development of basin urbanisation and vulnerability could explain the apparent increase in floods. As the regional population is expected to reach more than 3,000,000 by 2030, it is necessary to take into account the flood risk in future urban planning
A Hubble Space Telescope ACS Search for Brown Dwarf Binaries in the Pleiades Open Cluster
We present the results of a high-resolution imaging survey for brown dwarf
binaries in the Pleiades open cluster. The observations were carried out with
the Advance Camera for Surveys onboard the Hubble Space Telescope. Our sample
consists of 15 bona-fide brown dwarfs. We confirm 2 binaries and detect their
orbital motion, but we did not resolve any new binary candidates in the
separation range between 5.4AU and 1700AU and masses in the range
0.035--0.065~Msun. Together with the results of our previous study (Martin et
al., 2003), we can derive a visual binary frequency of 13.3\%
for separations greater than 7~AU masses between 0.055--0.065~M_{\sun} and
mass ratios between 0.45--0.91.0. The other observed properties of
Pleiades brown dwarf binaries (distributions of separation and mass ratio)
appear to be similar to their older counterparts in the field.Comment: 29 pages, 7 figures, 6 tables, accepted for publication in Ap
Constraints on the Stellar/Sub-stellar Mass Function in the Inner Orion Nebula Cluster
We present the results of a 0.5-0.9" FWHM imaging survey at K (2.2 micron)
and H (1.6 micron) covering 5.1' x 5.1' centered on Theta 1C Ori, the most
massive star in the Orion Nebula Cluster (ONC). At the age and distance of this
cluster, and in the absence of extinction, the hydrogen burning limit (0.08 Mo)
occurs at K~13.5 mag while an object of mass 0.02 Mo has K~16.2 mag. Our
photometry is complete for source detection at the 7 sigma level to K~17.5 mag
and thus is sensitive to objects as low-mass as 0.02 Mo seen through visual
extinction values as high as 10 magnitudes. We use the observed magnitudes,
colors, and star counts to constrain the shape of the inner ONC stellar mass
function across the hydrogen burning limit. After determining the stellar age
and near-infrared excess properties of the optically visible stars in this same
inner ONC region, we present a new technique that incorporates these
distributions when extracting the mass function from the observed density of
stars in the K-(H-K) diagram. We find that our data are inconsistent with a
mass function that rises across the stellar/sub-stellar boundary. Instead, we
find that the most likely form of the inner ONC mass function is one that rises
to a peak around 0.15 Mo, and then declines across the hydrogen-burning limit
with slope N(log M) ~ M^(0.57+/-0.05). We emphasize that our conclusions apply
to the inner 0.71 pc x 0.71 pc of the ONC only; they may not apply to the ONC
as a whole where some evidence for general mass segregation has been found.Comment: Accepted for publication in the Astrophysical Journal.
Preprints/tables also available at http://phobos.caltech.edu/~jmc/papers/onc
The T Tauri Phase Down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs
Using the largest high-resolution spectroscopic sample to date of young, very
low mass stars (VLMS) and brown dwarfs (BDs), we investigate disk accretion in
objects ranging from just above the hydrogen-burning limit all the way to
nearly planetary masses. Our 82 targets span spectral types from M5 to M9.5, or
masses from 0.15 Msun down to ~15 Jupiters. They are confirmed members of the
rho Oph, Taurus, Cha I, IC 348, R CrA, Upper Sco and TW Hydrae regions, with
ages = M6.5). We
find that: (1) classical T Tauri-like disk-accretion persists in the BD domain
down to nearly the deuterium-burning limit; (2) in addition to H-alpha,
permitted emission lines of CaII, OI and HeI are also good accretion
indicators, as in CTTs; (3) the CaII 8662A flux is an excellent quantitative
measure of the accretion rate (Mdot) in VLMS and BDs(as in CTTs); (4) Mdot
diminishes as M^2 -- our measurements support previous findings of this
correlation, and extend it to the entire range of sub-stellar masses; (5) the
accretor fraction among VLMS and BDs decreases substantially with age, as in
higher-mass stars; (6) at any given age, the VLMS and BD accretor fraction is
comparable to that in higher-mass stars; and (7) a number of sources with IR
disk excesses do not evince measurable accretion, with the incidence of such a
mismatch increasing with age: this implies that disks in the low mass regime
can persist beyond the main accretion phase, and parallels the transition from
the classical to post-T Tauri stage in more massive stars. These strong
similarities at young ages, between higher-mass stars and low-mass bodies close
to and below the hydrogen-burning limit, are consistent with a common formation
mechanism in the two mass regimes. (abridged)Comment: 64 pages, 7 figures. ApJ accepte
Low-Mass Star Formation and the Initial Mass Function in the Rho Ophiuchi Cloud Core
We have obtained moderate-resolution (R=800-1200) K-band spectra for ~100
stars within and surrounding the cloud core of rho Oph. We have measured
spectral types and continuum veilings and have combined this information with
results from new deep imaging. The IMF peaks at about 0.4 M_sun and slowly
declines to the hydrogen burning limit with a slope of ~-0.5 in logarithmic
units (Salpeter is +1.35). Our lower limits on the numbers of substellar
objects demonstrate that the IMF probably does not fall more steeply below the
hydrogen burning limit, at least down to ~0.02 M_sun. We then make the first
comparison of mass functions of stars and pre-stellar clumps (Motte, Andre, &
Neri) measured in the same region. The similar behavior of the two mass
functions in rho Oph supports the suggestion of Motte et al. and Testi &
Sargent that the stellar mass function in young clusters is a direct product of
the process of cloud fragmentation. After considering the effect of extinction
on the SED classifications of the sample, we find that ~17% of the rho Oph
stars are Class I, implying ~0.1 Myr for the lifetime of this stage. In spectra
separated by two years, we observe simultaneous variability in the Br gamma
emission and K-band continuum veiling for two stars, where the hydrogen
emission is brighter in the more heavily veiled data. This behavior indicates
that the disk may contribute significantly to continuous K-band emission, in
contrast to the proposal that the infalling envelope always dominates. Our
detection of strong 2 micron veiling (r_K=1-4) in several Class II and III
stars, which should have disks but little envelope material, further supports
this proposition.Comment: 35 pages, 14 figures, accepted to Ap
The Magnetic Fields of Classical T Tauri Stars
We report new magnetic field measurements for 14 classical T Tauri stars
(CTTSs). We combine these data with one previous field determination in order
to compare our observed field strengths with the field strengths predicted by
magnetospheric accretion models. We use literature data on the stellar mass,
radius, rotation period, and disk accretion rate to predict the field strength
that should be present on each of our stars according to these magnetospheric
accretion models. We show that our measured field values do not correlate with
the field strengths predicted by simple magnetospheric accretion theory. We
also use our field strength measurements and literature X-ray luminosity data
to test a recent relationship expressing X-ray luminosity as a function of
surface magnetic flux derived from various solar feature and main sequence star
measurements. We find that the T Tauri stars we have observed have weaker than
expected X-ray emission by over an order of magnitude on average using this
relationship. We suggest the cause for this is actually a result of the very
strong fields on these stars which decreases the efficiency with which gas
motions in the photosphere can tangle magnetic flux tubes in the corona.Comment: 25 pages, 5 figure
Modelling of anisotropic work-hardening behaviour of metallic materials subjected to strain-path changes
The present paper aims at reviewing some recent progress in developing advanced constitutive models which are devoted to the description of the anisotropic work-hardening behaviour under strain-path changes at large strains of metallic materials. After reviewing some microscopic and macroscopic experimental evidence, a physically-based phenomenological model using four internal state tensor variables is presented. This model can be simplified into several classical phenomenological models in order to take into account either the isotropic or the kinematic hardening or both. The implementation of the proposed models in the in-house finite element code DD3IMP is briefly recalled. Numerical simulations of the stamping of a curved rail are carried out in order to evaluate the accuracy and the efficiency of the proposed models in modelling the springback.http://www.sciencedirect.com/science/article/B6TWM-4DPCBFG-5/1/69adb1186eb64eca13df008f0bfcfcd
Four Brown Dwarfs in the Taurus Star-Forming Region
We have identified four brown dwarfs in the Taurus star-forming region. They
were first selected from and CCD photometry of 2.29 square degrees
obtained at the Canada-France-Hawaii Telescope. Subsequently, they were
recovered in the 2MASS second incremental data release point source catalog.
Low-resolution optical spectra obtained at the William Herschel telescope allow
us to derive spectral types in the range M7--M9. One of the brown dwarfs has
very strong H emission (EW=-340 \AA). It also displays Br
emission in an infrared spectrum obtained with IRCS on the Subaru telescope,
suggesting that it is accreting matter from a disk. The \ion{K}{1} resonance
doublet and the \ion{Na}{1} subordinate doublet at 818.3 and 819.5 nm in these
Taurus objects are weaker than in field dwarfs of similar spectral type,
consistent with low surface gravities as expected for young brown dwarfs. Two
of the objects are cooler and fainter than GG Tau Bb, the lowest mass known
member of the Taurus association. We estimate masses of only 0.03 M for
them. The spatial distribution of brown dwarfs in Taurus hints to a possible
anticorrelation between the density of stars and the density of brown dwarfs.Comment: ApJ Letters (in press
- âŠ