1,676 research outputs found
The Classification of M1-78
The published properties of M1-78 are discussed with the purpose to resolve
the object's classification as either a planetary nebula or an ultracompact HII
region. A classification as a planetary nebula is rejected primarily because of
the high luminosity of the object, but because of the chemical composition and
expansion velocity of the nebula, a novel classification is proposed instead:
that of an ultracompact HII region with a post-main sequence central star
(possibly a WN star). It must therefore follow that observable ultracompact HII
regions persist beyond the main sequence lifetimes of at least some massive
stars, and so cannot be transient phenomena that are seen only during pre-main
sequence or early main sequence evolution.Comment: 16 pages, Postscrip
Low-luminosity Extragalactic Water Masers toward M82, M51, and NGC4051
Sub-arcsecond observations using the Very Large Array (VLA) are presented for
low-luminosity water maser in M82, M51, and NGC4051. New maser features have
been detected within the M82 starburst complex. They are largely associated
with star-forming activity, such as optically identified starburst-driven
winds, H II regions, or the early phase of star formation in the galaxy. The
water maser in M51 consists of blueshifted and redshifted features relative to
thesystemic velocity of the galaxy. The redshifted features are measured to the
northwest of the nuclear radio source, while the location of the blueshifted
counterpart is displaced by about 2" from the radio source. A small velocity
gradient closely aligned with the radio jet is detected from the redshifted
features. The redshifted maser most likely amplifies the background radio
continuum jet, while the blueshifted counterpart marks off-nuclear star
formation in the galaxy. All of the detected maser features in the narrow-line
Seyfert 1 galaxy NGC 4051 remain unresolved by new VLA observations. Due to the
low luminosity of the maser, the maser excitation is not directly related to
the active galactic nucleus.Comment: 20 pages, 10 figures. Accepted for publication in AJ (17/Oct/2006
Helium Ionization in the Diffuse Ionized Gas surrounding UCHII regions
We present measurements of the singly ionized helium to hydrogen ratio
() toward diffuse gas surrounding three Ultra-Compact HII
(UCHII ) regions: G10.15-0.34, G23.46-0.20 \& G29.96-0.02. We observe radio
recombination lines (RRLs) of hydrogen and helium near 5 GHz using the GBT to
measure the ratio. The measurements are motivated by the low
helium ionization observed in the warm ionized medium (WIM) and in the inner
Galaxy diffuse ionized regions (DIR). Our data indicate that the helium is not
uniformly ionized in the three observed sources. Helium lines are not detected
toward a few observed positions in sources G10.15-0.34 \& G23.46-0.20 and the
upper limits of the ratio obtained are 0.03 and 0.05
respectively. The selected sources harbor stars of type O6 or hotter as
indicated by helium line detection toward the bright radio continuum emission
from the sources with mean value 0.060.02. Our data
thus show that helium in diffuse gas located a few pc away from the young
massive stars embedded in the observed regions is not fully ionized.We
investigate the origin of the non-uniform helium ionization and rule out the
possibilities : (a) that the helium is doubly ionized in the observed regions
and (b) that the low values are due to additional hydrogen
ionizing radiation produced by accreting low-mass stars (Smith 2014). We find
that selective absorption of ionizing photons by dust can result in low helium
ionization but needs further investigation to develop a self-consistent model
for dust in HII regions.Comment: 43 pages, 11 figures, 5 tables accepted to Ap
On Radiation Pressure in Static, Dusty HII Regions
Radiation pressure acting on gas and dust causes HII regions to have central
densities that are lower than the density near the ionized boundary. HII
regions in static equilibrium comprise a family of similarity solutions,
parametrized by 3 parameters: beta, gamma, and the product (Q_0 n_rms); beta
characterizes the stellar spectrum, gamma characterizes the dust/gas ratio, Q_0
is the ionizing output from the star (photons/s), and n_rms is the rms density
within the ionized region. Adopting standard values for beta and gamma, varying
(Q_0 n_rms) generates a one-parameter family of density profiles, ranging from
nearly uniform density (small Q_0 n_rms), to hollow-sphere HII regions (large
Q_0 n_rms). When (Q_0 n_rms) exceeds 10^{52} cm^{-3} s^{-1}, dusty HII regions
have conspicuous central cavities, even if no stellar wind is present. For
given beta, gamma and (Q_0 n_rms), a fourth quantity, which can be Q_0,
determines the overall size and density of the HII region. Examples of density
and emissivity profiles are given. We show how quantities of interest -- such
as the peak-to-center emissivity ratio, the rms-to-mean density ratio, the
edge-to-rms density ratio, and the fraction of the ionizing photons absorbed by
the gas -- depend on the 3 parameters beta, gamma, and (Q_0 n_rms). For dusty
HII regions, compression of the gas and dust into an ionized shell results in a
substantial increase in the fraction of the >13.6 eV photons that actually
ionize H (relative to a uniform density HII region with the same dust/gas ratio
and density n=n_rms). We discuss the extent to which radial drift of dust
grains in HII regions can alter the dust-to-gas ratio. The applicability of
these solutions to real HII regions is discussed.Comment: New material and figures that were not in version 1. To appear in Ap
A review of the supply of liquid propellants and other fluids in support of the Space Shuttle Program
In this study, over twenty significant liquid propellants and other fluids were reviewed as to their supply in support of the Space Shuttle Program (SSP), primarily at KSC. The uniqueness of most of the products, either by their application or production characteristics, present a variety of supply issues to contend with. Each, however, is critical to the success of the SSP. It becomes necessary to formulate, and maintain, a logistic approach to assure a continued availability of each product. For convenience, two categories were established. One, labeled limited-availability, represents those products wherein they are single sourced, have production restrictions and/or there has been a history of supply problems. The other, labeled universally-available, is characteristic of those having several sources and/or having little, if any, historical supply problems. This last category was not examined in depth. Through concepts of establishing stockpile inventories, multiple supply contracts, or other arrangements, the supply of liquid propellants and other fluids can be assured
James Britt Howard in a Senior Trombone Recital
This is the program for the senior trombone recital of James Britt Howard. Mr. Howard was accompanied by John Churchwell on the piano. This recital took place on April 9, 1996, in the McBeth Recital Hall in the Mabee Recital Hall
Star formation associated with the infrared dust bubble N68
We investigated the environment of the infrared dust bubble N68 and searched
for evidence of triggered star formation in its surroundings. We performed a
multiwavelength study of the nebula with data taken from several large-scale
surveys: GLIMPSE, MIPSGAL, IRAS, NVSS, GRS, and JCMT. We analyzed the spectral
profile and the distribution of the molecular gas (^{13}CO J = 1 - 0 and J = 3
- 2), and the dust in the environment of the N68. The position-velocity diagram
clearly shows that the N68 may be expanding outward. We used two three-color
images of the mid-infrared emission to explore the physical environment, and
one color-color diagram to investigate the distribution of young stellar
objects (YSOs). We found that the 24 \mu m emission is surrounded by the 8.0
\mu m emission. Morphologically, the 1.4 GHz continuum correlates strongly with
the 24 \mu m emission, and the ^{13}CO J = 1 - 0 and J = 3 - 2 emissions
correlate well with the 8.0 \mu m emission. We investigated two compact cores
located at the shell of the N68. The spectral intensity ratios of ^{13}CO J = 3
- 2 to J = 1 - 0 range from 5 to 0.3. In addition, young star objects, masers,
IRAS, and UC HII regions distribute at the shell of bubble. The active region
may be triggered by the expanding of the bubble N68.Comment: 10 pages, 11 figures, submited to RA
Predictive Effects of District Characteristics on Arkansas Transportation Expense
The purpose of this dissertation was to examine the predictive effects of district and community characteristics on transportation expenses of school districts in Arkansas using the Kaldor-Hicks Pareto efficiency theory. The 2004 ruling in Lakeview School District No. 25 v. Mike Huckabee mandated adequate and equitable public school funding standards in Arkansas. As a result, school districts are funded using a foundation model where the prior year’s average daily membership is multiplied by the foundation amount set by the Arkansas General Assembly. Data were collected from state databases and each school district’s website. Of the 235 districts in the state, the 222 districts not receiving isolated transportation funding were analyzed using multiple regression. The results indicated that the district\u27s poverty percentage and square miles significantly predicted the percentage of transportation funded. Average daily membership and percentage of transportation funded significantly predicted the school district’s actual transportation expense. The district’s average daily membership and square miles did not significantly affect the average age of the bus fleet. Average daily membership and square miles of the district did significantly predict beginning bus driver salary. The results indicated that average daily membership was not the only predictor variable influencing transportation expenditures, suggesting that an improved transportation funding model could benefit school districts in Arkansas
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