214 research outputs found
A preferred vision for administering schools: A reflective essay
My first encounter with the prospect of becoming a principal happened quite by chance in the spring of 1988, during a post-evaluation conference with my principal. After our formal discussion, we talked about the future. He commented to me that I should go to graduate school and enter the principalship. I immediately laughed his suggestion off, first by making light of principals in general, then letting him know that I could not imagine myself sitting in the principal\u27s chair. He ended our conversation by mentioning why I would be right for the principalship and to give his suggestion serious consideration. His confidence in me really left an impression, but at the time the classroom was where I wanted to be
Spatial Distributions of Multiple Dust Components in the PPN/PN Dust Shells
We investigate spatial distributions of specific dust components in the
circumstellar shells of a proto-planetary nebula candidate, HD 179821, and a
planetary nebula, BD3639, by means of spectral imaging. With
high-resolution ground-based images and ISO spectra in the mid-infrared, we can
derive ``dust feature only'' maps by subtracting synthesized continuum maps
from the observed images at the feature wavelength. Such spatially detailed
information will help to develop models for the evolution of dust grains around
evolved stars.Comment: 4 pages + 7 figures, to appear in the proceedings of the conference,
"Post-AGB Objects (proto-planetary nebulae) as a Phase of Stellar Evolution",
Torun, Poland, July 5-7, 2000, eds. R. Szczerba, R. Tylenda, and S.K. Gorny.
Figures have been degraded to minimize the total file siz
Mid-infrared imaging of the massive young star AFGL 2591: Probing the circumstellar environment of an outflow source
Most, if not all, stars are now believed to produce energetic outflows during
their formation. Yet, almost 20 years after the discovery of bipolar outflows
from young stars, the origins of this violent phenomenon are not well
understood. One of the difficulties of probing the outflow process,
particularly in the case of massive embedded stars, is a deficit of high
spatial resolution observations. Here, we present sub-arcsecond-resolution
mid-infrared images of one massive young stellar object, AFGL 2591, and its
immediate surroundings. Our images, at 11.7, 12.5 and 18.0 microns, reveal a
knot of emission ~6'' SW of the star, which may be evidence for a recent
ejection event or an embedded companion star. This knot is roughly coincident
with a previously seen near-infrared reflection nebula and a radio source, and
lies within the known large-scale CO outflow. We also find a new faint NW
source which may be another embedded lower-luminosity star. The IRAS
mid-infrared spectrum of AFGL 2591 shows a large silicate absorption feature at
10 microns, implying that the primary source is surrounded by an optically
thick dusty envelope. We discuss the interrelationship of these phenomena and
suggest that mid-infrared imaging and spectroscopy provide powerful tools for
probing massive star birth.Comment: 14 pages, 3 PostScript figures, accepted for publication in The
Astrophysical Journal Letter
Discovery of an Extended Dust Emission around IRAS 18576+0341 (AFGL 2298) at 10.3 and 18.0 microns: a New Luminous Blue Variable Candidate?
We report detection of an extended mid-infrared emission from \iras
18576+0341 (AFGL 2298). The object shows a dusty circumstellar shell that has
diameter of \age 7\arcsec at 10.3 and 18.0 \um. The dust nebula shows two
emission peaks concentrically elongated and symmetrically oriented on the
opposite sides of the third, central peak, which appears to be the central star
of the system. The observed mid-infrared morphology indicates that the
circumstellar dust shell has an equatorially-enhanced material distribution,
which is a common signature of stellar objects that have experienced mass loss.
Radiative transfer model calculations suggest that the central star is an
extremely bright (\lstar = 10^{6.4} \lsun) star at a distance of about 10
kpc: this object is best described as a new luminous blue variable candidate.
The circumstellar dust shell seems to have been generated by an
equatorially-enhanced mass loss process with {\dot M} \ge 6.8 \times 10^{-6}
\msun yr and .Comment: 21 pages, 4 figures, to appear in Ap
A Mid-Infrared Imaging Survey of Proto-Planetary Nebula Candidates
We present the data from a mid-infrared imaging survey of 66 proto-planetary
nebula candidates using two mid-IR cameras (MIRAC2 and Berkcam) at the NASA
Infrared Telescope Facility and the United Kingdom Infrared Telescope. The goal
of this survey is to determine the size, flux, and morphology of the mid-IR
emission regions, which sample the inner regions of the circumstellar dust
shells of proto-planetary nebulae. We imaged these proto-planetary nebulae with
narrow-band filters () at wavelengths of
notable dust features. With our typical angular resolution of 1\arcsec, we
resolve 17 sources, find 48 objects unresolved, and do not detect 1 source. For
several sources, we checked optical and infrared associations and positions of
the sources. In table format, we list the size and flux measurements for all
the detected objects and show figures of all the resolved sources. Images for
all the detected objects are available on line in FITS format from the
Astronomy Digital Image Library at the National Center for Supercomputing
Application. The proto-planetary nebula candidate sample includes, in addition
to the predominant proto-planetary nebulae, extreme asymptotic giant branch
stars, young planetary nebulae, a supergiant, and a luminous blue variable. We
find that dust shells which are cooler ( K) and brighter in the
infrared are more easily resolved. Eleven of the seventeen resolved sources are
extended and fall into one of two types of mid-IR morphological classes:
core/elliptical or toroidal. Core/elliptical structures show unresolved cores
with lower surface brightness elliptical nebulae. Toroidal structures show
limb-brightened peaks suggesting equatorial density enhancements. We argue that
core/ellipticals have denser dust shells than toroidals.Comment: 32 pages, 5 tables, 2 e/ps figures (fig3 is available through ADIL
[see text]), to be published in ApJS May 1999 issu
Mid-IR Observations of Mira Circumstellar Environment
This paper presents results from high-angular resolution mid-IR imaging of
the Mira AB circumbinary environment using the MIRAC3 camera at the NASA
Infrared Telescope Facility (IRTF). We resolved the dusty circumstellar
envelope at 9.8, 11.7 and 18 micron around Mira A (o Ceti), and measured the
size of the extended emission. Strong deviations from spherical symmetry are
detected in the images of Mira AB system, including possible dust clumps in the
direction of the companion (Mira B). These observations suggest that Mira B
plays an active role in shaping the morphology of the circumstellar environment
of Mira A as it evolves toward the Planetary Nebula phase.Comment: 11 pages, 2 PostScript figures, accepted for publication in the
Astrophysical Journal Letter
Earnings Functions and the Measurement of the Determinants of Wage Dispersion: Extending Oaxaca's Approach
Simulating the global distribution of nitrogen isotopes in the ocean
We present a new nitrogen isotope model incorporated into the three-dimensional ocean component of a global Earth system climate model designed for millennial timescale simulations. The model includes prognostic tracers for the two stable nitrogen isotopes, 14N and 15N, in the nitrate (NO3â), phytoplankton, zooplankton, and detritus variables of the marine ecosystem model. The isotope effects of algal NO3â uptake, nitrogen fixation, water column denitrification, and zooplankton excretion are considered as well as the removal of NO3â by sedimentary denitrification. A global database of ÎŽ15NO3â observations is compiled from previous studies and compared to the model results on a regional basis where sufficient observations exist. The model is able to qualitatively and quantitatively reproduce many of the observed patterns such as high subsurface values in water column denitrification zones and the meridional and vertical gradients in the Southern Ocean. The observed pronounced subsurface minimum in the Atlantic is underestimated by the model presumably owing to too little simulated nitrogen fixation there. Sensitivity experiments reveal that algal NO3â uptake, nitrogen fixation, and water column denitrification have the strongest effects on the simulated distribution of nitrogen isotopes, whereas the effect from zooplankton excretion is weaker. Both water column and sedimentary denitrification also have important indirect effects on the nitrogen isotope distribution by reducing the fixed nitrogen inventory, which creates an ecological niche for nitrogen fixers and, thus, stimulates additional N2 fixation in the model. Important model deficiencies are identified, and strategies for future improvement and possibilities for model application are outlined
Modelling distributions of Aedes aegypti and Aedes albopictus using climate, host density and interspecies competition.
Florida faces the challenge of repeated introduction and autochthonous transmission of arboviruses transmitted by Aedes aegypti and Aedes albopictus. Empirically-based predictive models of the spatial distribution of these species would aid surveillance and vector control efforts. To predict the occurrence and abundance of these species, we fit a mixed-effects zero-inflated negative binomial regression to a mosquito surveillance dataset with records from more than 200,000 trap days, representative of 53% of the land area and ranging from 2004 to 2018 in Florida. We found an asymmetrical competitive interaction between adult populations of Aedes aegypti and Aedes albopictus for the sampled sites. Wind speed was negatively associated with the occurrence and abundance of both vectors. Our model predictions show high accuracy (72.9% to 94.5%) in validation tests leaving out a random 10% subset of sites and data since 2017, suggesting a potential for predicting the distribution of the two Aedes vectors
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