1,025 research outputs found
Star formation environments and the distribution of binary separations
We have carried out K-band speckle observations of a sample of 114 X-ray
selected weak-line T Tauri stars in the nearby Scorpius-Centaurus OB
association. We find that for binary T Tauri stars closely associated to the
early type stars in Upper Scorpius, the youngest subgroup of the OB
association, the peak in the distribution of binary separations is at 90 A.U.
For binary T Tauri stars located in the direction of an older subgroup, but not
closely associated to early type stars, the peak in the distribution is at 215
A.U. A Kolmogorov-Smirnov test indicates that the two binary populations do not
result from the same distibution at a significance level of 98%. Apparently,
the same physical conditions which facilitate the formation of massive stars
also facilitate the formation of closer binaries among low-mass stars, whereas
physical conditions unfavorable for the formation of massive stars lead to the
formation of wider binaries among low-mass stars. The outcome of the binary
formation process might be related to the internal turbulence and the angular
momentum of molecular cloud cores, magnetic field, the initial temperature
within a cloud, or - most likely - a combination of all of these. We conclude
that the distribution of binary separations is not a universal quantity, and
that the broad distribution of binary separations observed among main-sequence
stars can be explained by a superposition of more peaked binary distributions
resulting from various star forming environments. The overall binary frequency
among pre-main-sequence stars in individual star forming regions is not
necessarily higher than among main-sequence stars.Comment: 7 pages, Latex, 4 Postscript figures; also available at
http://spider.ipac.caltech.edu/staff/brandner/pubs/pubs.html ; accepted for
publication in ApJ Letter
NGC 3603 - a Local Template for Massive Young Clusters
We present a study of the star cluster associated with the massive Galactic
HII region NGC3603 based on near-IR broad-- and narrowband observations taken
with ISAAC/VLT under excellent seeing conditions (<0.4''). We discuss
color-color diagrams and address the impact of the high UV flux on the disk
evolution of the low-mass stars.Comment: 3 pages, 3 figures. To appear in the Proceedings of IAU Symposium 207
"Extragalactic Star Clusters", eds. E. Grebel, D. Geisler and D. Minitt
The search for planetary mass companions to field brown dwarfs with HST/NICMOS
We present the results of a high-resolution spectral differential imaging
survey of 12 nearby, relatively young field L dwarfs (<1 Gyr) carried out with
HST/NICMOS to search for planetary mass companions at small physical
separations from their host. The survey resolved two brown dwarf binaries: the
L dwarf system Kelu-1AB and the newly discovered L/T transition system 2MASS
J031059+164815AB. For both systems common proper motion has already been
confirmed in follow-up observations which have been published elsewhere. The
derived separations of the binaries are smaller than 6 AU and consistent with
previous brown dwarf binary statistics. Their mass ratios of q > 0.8 confirm
the preference for equal mass systems similar to a large number of other
surveys. Furthermore, we found tentative evidence for a companion to the L4
dwarf 2MASS W033703-175807, straddling the brown dwarf/planetary mass boundary
and revealing an uncommonly low mass ratio system (q ~ 0.2) compared to the
vast majority of previously found brown dwarf binaries. With a derived minimum
mass of 10 - 15 Mjup, a planetary nature of the secondary cannot be ruled out
yet. However, it seems more likely to be a very low mass brown dwarf secondary
at the border of the spectral T/Y transition regime, primarily due to its
similarities to recently found very cool T dwarfs. This would make it one of
the closest resolved brown dwarf binaries (0.087" 0.015", corresponding
to 2.52 0.44 AU at a distance of 29 pc) with the coolest (Teff ~ 600-630
K) and least massive companion to any L or T dwarf.Comment: 33 pages, 8 figures, 2 tables, accepted for publication by Ap
Computational and Experimental Investigation of Flow Around a 3-1 Prolate Spheroid
The flow around a 3-1 prolate spheroid near the critical
Reynolds number is investigated experimentally and numerically.
This work was conducted as part of a larger project to
examine the flow around Unmanned Underwater Vehicles. The
experimental investigation has been performed in a water tunnel
at the Australian Maritime College. Fast response pressure
probes and a 3-D automated traverse have been developed to
investigate the state of the boundary layer. A commercial CFD
code has been modified to allow the experimentally determined
boundary layer state to be included in the computation. Qualitative
and quantitative comparisons between the measured and
calculated results are discussed. The tests on the spheroid were
conducted within a Reynolds numbers range of 0 .6 ×106 to
4×106. The results presented here are for an incidence of 10
Multiple episodes of star formation in the CN15/16/17 molecular complex
We have started a campaign to identify massive star clusters inside bright
molecular bubbles towards the Galactic Center. The CN15/16/17 molecular complex
is the first example of our study. The region is characterized by the presence
of two young clusters, DB10 and DB11, visible in the NIR, an ultra-compact HII
region identified in the radio, several young stellar objects visible in the
MIR, a bright diffuse nebulosity at 8\mu m coming from PAHs and sub-mm
continuum emission revealing the presence of cold dust. Given its position on
the sky (l=0.58, b=-0.85) and its kinematic distance of ~7.5 kpc, the region
was thought to be a very massive site of star formation in proximity of the
CMZ. The cluster DB11 was estimated to be as massive as 10^4 M_sun. However the
region's properties were known only through photometry and its kinematic
distance was very uncertain given its location at the tangential point. We
aimed at better characterizing the region and assess whether it could be a site
of massive star formation located close to the Galactic Center. We have
obtained NTT/SofI JHKs photometry and long slit K band spectroscopy of the
brightest members. We have additionally collected data in the radio, sub-mm and
mid infrared, resulting in a quite different picture of the region. We have
confirmed the presence of massive early B type stars and have derived a
spectro-photometric distance of ~1.2 kpc, much smaller than the kinematic
distance. Adopting this distance we obtain clusters masses of M(DB10) ~ 170
M_sun and M(DB11) ~ 275 M_sun. This is consistent with the absence of any O
star, confirmed by the excitation/ionization status of the nebula. No HeI
diffuse emission is detected in our spectroscopic observations at 2.113\mu m,
which would be expected if the region was hosting more massive stars. Radio
continuum measurements are also consistent with the region hosting at most
early B stars.Comment: Accepted for publication in Astronomy and Astrophysics. Fig. 1 and 3
presented in reduced resolutio
Severe Hypothermia in a Patient withCerebral Relapse of Whipple's Disease
Abstract. : The diagnosis of cerebral relapse of Whipple's disease in a 67-year-old patient was made after he presented with somnolence and severe hypothermia 4 months after discontinuing treatment with cotrimoxazole. Hypothermia is a rare hypothalamic manifestation of cerebral Whipple's diseas
Deep near-infrared imaging of W3 Main: constraints on stellar cluster formation
Embedded clusters like W3 Main are complex and dynamically evolving systems
that represent an important phase of the star formation process. We aim at the
characterization of the entire stellar content of W3 Main in a statistical
sense to identify possible differences in evolutionary phase of the stellar
populations and find clues about the formation mechanism of this massive
embedded cluster. Methods. Deep JHKs imaging is used to derive the disk
fraction, Ks-band luminosity functions and mass functions for several
subregions in W3 Main. A two dimensional completeness analysis using artificial
star experiments is applied as a crucial ingredient to assess realistic
completeness limits for our photometry. We find an overall disk fraction of 7.7
2.3%, radially varying from 9.4 3.0 % in the central 1 pc to 5.6
2.2 % in the outer parts of W3 Main. The mass functions derived for three
subregions are consistent with a Kroupa and Chabrier mass function. The mass
function of IRSN3 is complete down to 0.14 Msun and shows a break at M
0.5 Msun. We interpret the higher disk fraction in the center as evidence for a
younger age of the cluster center. We find that the evolutionary sequence
observed in the low-mass stellar population is consistent with the observed age
spread among the massive stars. An analysis of the mass function variations
does not show evidence for mass segregation. W3 Main is currently still
actively forming stars, showing that the ionizing feedback of OB stars is
confined to small areas ( 0.5 pc). The FUV feedback might be influencing
large regions of the cluster as suggested by the low overall disk fraction.Comment: 15 pages, 13 figures, accepted by A&
An Investigation of Cloud Cavitation about a Sphere
Cloud cavitation occurrence about a sphere is investigated in a variable pressure water tunnel using still and high-speed photography. The model sphere, 0.15 m in diameter, was sting mounted within a 0.6 m square test section and tested at a constant Reynolds number of 1.5 x 106 with cavitation numbers varying between 0.36 and 1.0. High-speed photographic recordings were made at 6 kHz for several cavitation numbers. Shedding phenomena and frequency content is investigated by means of pixel intensity time series data using wavelet analysis. The boundary layer at cavity separation is shown to be laminar for all cavitation numbers, with Kelvin-Helmholtz instability the main mechanism for cavity break up and cloud formation at high cavitation numbers. At intermediate cavitation numbers, cavity lengths allow the development of re-entrant jet phenomena providing a mechanism for shedding of large scale Karman-type vortices similar to those for low mode shedding in single-phase subcritical flow. This shedding mode is eliminated at low cavitation numbers with the onset of supercavitation
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