125 research outputs found
HST/WFPC2 and VLT/ISAAC observations of PROPLYDS in the giant HII region NGC 3603
We report the discovery of three proplyd-like structures in the giant HII
region NGC 3603. The emission nebulae are clearly resolved in narrow-band and
broad-band HST/WFPC2 observations in the optical and broad-band VLT/ISAAC
observations in the near-infrared. All three nebulae are tadpole shaped, with
the bright ionization front at the head facing the central cluster and a
fainter ionization front around the tail pointing away from the cluster.
Typical sizes are 6,000 A.U. x 20,000 A.U. The nebulae share the overall
morphology of the proplyds (``PROto PLanetarY DiskS'') in Orion, but are 20 to
30 times larger in size. Additional faint filaments located between the nebulae
and the central ionizing cluster can be interpreted as bow shocks resulting
from the interaction of the fast winds from the high-mass stars in the cluster
with the evaporation flow from the proplyds. The striking similarity of the
tadpole shaped emission nebulae in NGC 3603 to the proplyds in Orion suggests
that the physical structure of both types of objects might be the same. We
present 2D radiation hydrodynamical simulations of an externally illuminated
star-disk-envelope system, which was still in its main accretion phase when
first exposed to ionizing radiation from the central cluster. The simulations
reproduce the overall morphology of the proplyds in NGC 3603 very well, but
also indicate that mass-loss rates of up to 10^-5 Mo/yr are required in order
to explain the size of the proplyds. (abbreviated)Comment: 10 pages, 4 Postscript figures, uses emulateapj.sty and psfig.tex.
Astronomical Journal, in press (January 2000 issue
Massive star formation via high accretion rates and early disk-driven outflows
We present an investigation of massive star formation that results from the
gravitational collapse of massive, magnetized molecular cloud cores. We
investigate this by means of highly resolved, numerical simulations of initial
magnetized Bonnor-Ebert-Spheres that undergo collapse and cooling. By comparing
three different cases - an isothermal collapse, a collapse with radiative
cooling, and a magnetized collapse - we show that massive stars assemble
quickly with mass accretion rates exceeding 10^-3 Msol/yr. We confirm that the
mass accretion during the collapsing phase is much more efficient than
predicted by selfsimilar collapse solutions, i.e. dM/dt ~ c^3/G. We find that
during protostellar assembly the mass accretion reaches 20 - 100 c^3/G.
Furthermore, we determined the self-consistent structure of bipolar outflows
that are produced in our three dimensional magnetized collapse simulations.
These outflows produce cavities out of which radiation pressure can be
released, thereby reducing the limitations on the final mass of massive stars
formed by gravitational collapse. Moreover, we argue that the extraction of
angular momentum by disk-threaded magnetic fields and/or by the appearance of
bars with spiral arms significantly enhance the mass accretion rate, thereby
helping the massive protostar to assemble more quickly.Comment: 22 pages, 12 figures, aastex style, accepted for publication in ApJ,
see http://www.ita.uni-heidelberg.de/~banerjee/publications/MassiveStars.pdf
for high resolution figure
High performance liquid chromatography electrospray ionization tandem mass spectrometry (HPLC-ESI-MS-MS) for the quantification of L-kynurenine and indole-3-acetic acid in grape must by isotope dilution assay
Isotope dilution assay was used for the qualitative and quantitative analysis of L-kynurenine and indole-3-acetic acid in grape must. After solid phase extraction, highest selectivity and sensitivity was achieved by high performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-ESI-MS-MS) using selected reaction monitoring (SRM). In the 24 samples under study, the amounts of L-kynurenine and indole-3-acetic acid ranged from 0 to 94 mu g.l-1 and from 20 to 380 mu g.l-1, respectively. These compounds are considered as potential precursors of 2-aminoacetophenone, causing the ''untypical aging off-flavour'' in Vitis vinifera white wines
Planet Formation in the Outer Solar System
This paper reviews coagulation models for planet formation in the Kuiper
Belt, emphasizing links to recent observations of our and other solar systems.
At heliocentric distances of 35-50 AU, single annulus and multiannulus
planetesimal accretion calculations produce several 1000 km or larger planets
and many 50-500 km objects on timescales of 10-30 Myr in a Minimum Mass Solar
Nebula. Planets form more rapidly in more massive nebulae. All models yield two
power law cumulative size distributions, N_C propto r^{-q} with q = 3.0-3.5 for
radii larger than 10 km and N_C propto r^{-2.5} for radii less than 1 km. These
size distributions are consistent with observations of Kuiper Belt objects
acquired during the past decade. Once large objects form at 35-50 AU,
gravitational stirring leads to a collisional cascade where 0.1-10 km objects
are ground to dust. The collisional cascade removes 80% to 90% of the initial
mass in the nebula in roughly 1 Gyr. This dust production rate is comparable to
rates inferred for alpha Lyr, beta Pic, and other extrasolar debris disk
systems.Comment: invited review for PASP, March 2002. 33 pages of text and 12 figure
A Keck High Resolution Spectroscopic Study of the Orion Nebula Proplyds
We present the results of spectroscopy of four bright proplyds in the Orion
Nebula obtained at a velocity resolution of 6 km/s. After careful isolation of
the proplyd spectra from the confusing nebular radiation, the emission line
profiles are compared with those predicted by realistic dynamic/photoionization
models of the objects. The spectral line widths show a clear correlation with
ionization potential, which is consistent with the free expansion of a
transonic, ionization-stratified, photoevaporating flow. Fitting models of such
a flow simultaneously to our spectra and HST emission line imaging provides
direct measurements of the proplyd size, ionized density and outflow velocity.
These measurements confirm that the ionization front in the proplyds is
approximately D-critical and provide the most accurate and robust estimate to
date of the proplyd mass loss rate. Values of 0.7E-6 to 1.5E-6 Msun/year are
found for our spectroscopic sample, although extrapolating our results to a
larger sample of proplyds implies that 0.4E-6 Msun/year is more typical of the
proplyds as a whole. In view of the reported limits on the masses of the
circumstellar disks within the proplyds, the length of time that they can have
been exposed to ionizing radiation should not greatly exceed 10,000 years - a
factor of 30 less than the mean age of the proplyd stars. We review the various
mechanisms that have been proposed to explain this situation, and conclude that
none can plausibly work unless the disk masses are revised upwards by a
substantial amount.Comment: 23 pages, 8 figures, uses emulateapj.sty, accepted for publication in
The Astronomical Journal (scheduled November 1999
Increasing compliance with wearing a medical device in children with autism
Health professionals often recommend the use of medical devices to assess the health, monitor
the well-being, or improve the quality of life of their patients. Children with autism may present
challenges in these situations as their sensory peculiarities may increase refusals to wear such
devices. To address this issue, we systematically replicated prior research by examining the
effects of differential reinforcement of other behavior (DRO) to increase compliance with
wearing a heart rate monitor in 2 children with autism. The intervention increased compliance to
100% for both participants when an edible reinforcer was delivered every 90 s. The results
indicate that DRO does not require the implementation of extinction to increase compliance with
wearing a medical device. More research is needed to examine whether the reinforcement
schedule can be further thinned
Massive Stars: Their Environment and Formation
Cloud environment is thought to play a critical role in determining the
mechanism of formation of massive stars. In this contribution we review the
physical characteristics of the environment around recently formed massive
stars. Particular emphasis is given to recent high angular resolution
observations which have improved our knowledge of the physical conditions and
kinematics of compact regions of ionized gas and of dense and hot molecular
cores associated with luminous O and B stars. We will show that this large body
of data, gathered during the last decade, has allowed significant progress in
the understanding of the physical processes that take place during the
formation and early evolution of massive stars.Comment: Pub. Astron. Soc. of Pacific (Invited Review), 95 pages (Latex), 5
pages (tables, Latex), 11 postscript or gif figure
Theory of Star Formation
We review current understanding of star formation, outlining an overall
theoretical framework and the observations that motivate it. A conception of
star formation has emerged in which turbulence plays a dual role, both creating
overdensities to initiate gravitational contraction or collapse, and countering
the effects of gravity in these overdense regions. The key dynamical processes
involved in star formation -- turbulence, magnetic fields, and self-gravity --
are highly nonlinear and multidimensional. Physical arguments are used to
identify and explain the features and scalings involved in star formation, and
results from numerical simulations are used to quantify these effects. We
divide star formation into large-scale and small-scale regimes and review each
in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and
their substructures. Important problems include how GMCs form and evolve, what
determines the star formation rate (SFR), and what determines the initial mass
function (IMF). Small scales range from dense cores to the protostellar systems
they beget. We discuss formation of both low- and high-mass stars, including
ongoing accretion. The development of winds and outflows is increasingly well
understood, as are the mechanisms governing angular momentum transport in
disks. Although outstanding questions remain, the framework is now in place to
build a comprehensive theory of star formation that will be tested by the next
generation of telescopes.Comment: 120 pages, to appear in ARAA. No changes from v1 text; permission
statement adde
Toward Understanding Massive Star Formation
Although fundamental for astrophysics, the processes that produce massive
stars are not well understood. Large distances, high extinction, and short
timescales of critical evolutionary phases make observations of these processes
challenging. Lacking good observational guidance, theoretical models have
remained controversial. This review offers a basic description of the collapse
of a massive molecular core and a critical discussion of the three competing
concepts of massive star formation:
- monolithic collapse in isolated cores
- competitive accretion in a protocluster environment
- stellar collisions and mergers in very dense systems
We also review the observed outflows, multiplicity, and clustering properties
of massive stars, the upper initial mass function and the upper mass limit. We
conclude that high-mass star formation is not merely a scaled-up version of
low-mass star formation with higher accretion rates, but partly a mechanism of
its own, primarily owing to the role of stellar mass and radiation pressure in
controlling the dynamics.Comment: 139 pages, 18 figures, 5 tables, glossar
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