71 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
The nature of the Lyman-alpha emission region of FDF-4691
In order to study the origin of the strong Lyman-alpha emission of
high-redshift starburst galaxies we observed and modeled the emission of the z
= 3.304 galaxy FDF-4691 (rest-frame EW = 103 Angstroem). The observations show
that FDF-4691 is a young starburst galaxy with a (for this redshift) typical
metallicity. The broad, double-peaked profile of the Lyman-alpha emission line
can be explained assuming a highly turbulent emission region in the inner part
of the starburst galaxy, and a surrounding extended shell of low-density
neutral gas with a normal dust/gas ratio and with Galactic dust properties. The
detection of the Lyman-alpha emission line is explained by the intrinsic broad
Lyman-alpha emission and a low HI column density of the neutral shell. A low
dust/gas ratio in the neutral shell is not needed to explain the strong
Lyman-alpha line.Comment: Accepted for publication in A&A Letter
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
Lyman alpha emission in high-redshift galaxies
A significant fraction of the high-redshift galaxies show strong Lyman
emission lines. For redshifts z>5, most known galaxies belong to this class.
However, so far not much is known about the physical structure and nature of
these objects. Our aim is to analyse the Lyman alpha emission in a sample of
high-redshift UV-continuum selected galaxies and to derive the physical
conditions that determine the Lyman alpha profile and the line strength.
VLT/FORS spectra with a resolution of R ~ 2000 of 16 galaxies in the redshift
range of z = 2.7 to 5 are presented. The observed Lyman alpha profiles are
compared with theoretical models. The Lyman alpha lines range from pure
absorption (EW = -17 Angstroem) to strong emission (EW = 153 Angstroem). Most
Lyman alpha emission lines show an asymmetric profile, and three galaxies have
a double-peaked profile. Both types of profiles can be explained by a uniform
model consisting of an expanding shell of neutral and ionised hydrogen around a
compact starburst region. The broad, blueshifted, low-ionisation interstellar
absorption lines indicate a galaxy-scale outflow of the ISM. The strengths of
these lines are found to be determined in part by the velocity dispersion of
the outflowing medium. We find star-formation rates of these galaxies ranging
from SFR(UV) = 1.2 to 63.2 Msun uncorrected for dust absorption. The Lyman
alpha emission strength of our target galaxies is found to be determined by the
amount of dust and the kinematics of the outflowing material.Comment: 11 pages, 6 figures. A&A accepte
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
On the formation of massive stars
We calculate numerically the collapse of slowly rotating, non-magnetic,
massive molecular clumps, which conceivably could lead to the formation of
massive stars. Because radiative acceleration on dust grains plays a critical
role in the clump's dynamical evolution, we utilize a wavelength-dependent
radiation transfer and a three component dust model: amorphous carbon
particles, silicates and "dirty ice"-coated silicates. We do not spatially
resolve the innermost regions of the molecular clump and assume that all
material in the innermost grid cell accretes onto a single object. We introduce
a semi-analytical scheme for augmenting existing evolution tracks of pre-main
sequence protostars by including the effects of accretion. By considering an
open outermost boundary, an arbitrary amount of material could, in principal,
be accreted onto this central star. However, for the three cases considered
(30, 60, and 120 solar masses originally within the computation grid),
radiation acceleration limited the final masses to 31.6, 33.6, and 42.9 solar
masses, respectively, for wavelength-dependent radiation transfer and to 19.1,
20.1, and 22.9 solar masses for comparison simulations with grey radiation
transfer. We demonstrate that massive stars can in principle be formed via
accretion through a disk. We conclude with the warning that a careful treatment
of radiation transfer is a mandatory requirement for realistic simulations of
the formation of massive stars.Comment: 39 pages, 13 figures, 4 tables, AASTEX v5.0, accepted by Ap
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
A highly efficacious pediculicide based on dimeticone: Randomized observer blinded comparative trial
BACKGROUND: Infestation with the human head louse (Pediculus humanus capitis) occurs worldwide. Existing treatment options are limited, and reports of resistance to commonly used pediculicides have been increasing. In this trial we assessed the efficacy of a product containing a high (92%) concentration of the silicone oil dimeticone (identical in composition to NYDA(R)), as compared to a 1% permethrin lotion. METHODS: Randomized, controlled, observer blinded clinical trial. Participants were recruited from a poor urban neighbourhood in Brazil where pediculosis capitis was highly prevalent. To minimize reinfestation during the trial, participants (145 children aged 5-15 years with head lice infestations) were transferred to a holiday resort outside the endemic area for a period of 9 days. Two applications of dimeticone or 1% permethrin were done, seven days apart. Outcome measures were defined as cure (absence of vital head lice) after first application and before and after second applications, degree of itching, cosmetic acceptability, and clinical pathology. RESULTS: Overall cure rates were: day 2 - dimeticone 94.5% (95% CI: 86.6% - 98.5%) and permethrin 66.7% (95% CI: 54.6% - 77.3%; p < 0.0001); day 7 - dimeticone 64.4% (95% CI: 53.3% - 75.3%) and permethrin 59.7% (95% CI: 47.5% - 71.1%; p = 0.5); day 9 - dimeticone 97.2% (95% CI: 90.3% - 99.7%) and permethrin 67.6% (95% CI: 55.4%-78.2%); p < 0.0001). Itching was reduced similarly in both groups. Cosmetic acceptability was significantly better in the dimeticone group as compared to the permethrin group (p = 0.01). Two mild product-related incidents occurred in the dimeticone group. CONCLUSION: The dimeticone product is a safe and highly efficacious pediculicide. Due to its physical mode of action (interruption of the lice's oxygen supply of the central nervous system), development of resistance is unlikely. TRIAL REGISTRATION: Current Controlled Trials ISRCTN15117709
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
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