365 research outputs found
Mass Flows in Cometary UCHII Regions
High spectral and spatial resolution, mid-infrared fine structure line
observations toward two ultracompact HII (UCHII) regions (G29.96 -0.02 and Mon
R2) allow us to study the structure and kinematics of cometary UCHII regions.
In our earlier study of Mon R2, we showed that highly organized mass motions
accounted for most of the velocity structure in that UCHII region. In this
work, we show that the kinematics in both Mon R2 and G29.96 are consistent with
motion along an approximately paraboloidal shell. We model the velocity
structure seen in our mapping data and test the stellar wind bow shock model
for such paraboloidal like flows. The observations and the simulation indicate
that the ram pressures of the stellar wind and ambient interstellar medium
cause the accumulated mass in the bow shock to flow along the surface of the
shock. A relaxation code reproduces the mass flow's velocity structure as
derived by the analytical solution. It further predicts that the pressure
gradient along the flow can accelerate ionized gas to a speed higher than that
of the moving star. In the original bow shock model, the star speed relative to
the ambient medium was considered to be the exit speed of ionized gas in the
shell.Comment: 34 pages, including 14 figures and 1 table, to be published in ApJ,
September 200
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Language support for immigrant children: a study of state schools in the UK and US
In recent decades, immigrants, refugees and asylum seekers have sought a new way of life in large numbers, often leaving their countries of origin behind in search of places that offer a better way of life. The purpose of this study was to investigate how elementary and middle school students in state schools in Reading, England (primarily speakers of Asian languages), and Richmond, Virginia (primarily speakers of Spanish), were supported academically, when most childrenâs first language was not English. The authors were interested in exploring whether or not there were cultural or structural differences in the way each country helped or hindered these students as they progressed through the school systems. Three UK schools in a district of approximately 100,000 and three US schools in a district of approximately 250,000 were the focus of this exploration from 2000 to 2003. Findings indicated that there were cultural and legislative differences and similarities. Teachers and administrators in both countries attempted to provide services with limited and sometimes diminishing resources. Community support varied based on resources, attitudes toward various ethnic groups, and the coping strategies adopted by these groups in their new environments. Marked differences appeared with regard to the manner in which assessments took place and how the results were made available to the public
Trends in stream nitrogen concentrations for forested reference catchments across the USA
To examine whether stream nitrogen concentrations in forested reference catchments have changed over time and if patterns were consistent across the USA, we synthesized up to 44 yr of data collected from 22 catchments at seven USDA Forest Service Experimental Forests. Trends in stream nitrogen presented high spatial variability both among catchments at a site and among sites across the USA. We found both increasing and decreasing trends in monthly flow-weighted stream nitrate and ammonium concentrations. At a subset of the catchments, we found that the length and period of analysis influenced whether trends were positive, negative or non-significant. Trends also differed among neighboring catchments within several Experimental Forests, suggesting the importance of catchment-specific factors in determining nutrient exports. Over the longest time periods, trends were more consistent among catchments within sites, although there are fewer long-term records for analysis. These findings highlight the critical value of long-term, uninterrupted stream chemistry monitoring at a network of sites across the USA to elucidate patterns of change in nutrient concentrations at minimally disturbed forested sites
Effects of Ralgro Implants on Growth, Sexual Development, Carcass Characteristics, and Eating Quality of Bulls Implanted From Birth to Slaughter
Growth, performance, sexual development, carcass characteristics, and eating quality were evaluated on 40 fall-born Angus bulls. Twenty of the bulls were implanted five times with Ralgro at an average interval of 106 days, beginning near birth. The other 20 bulls served as nonimplanted controls. Bull calves remained with their dams on native southeast Kansas pasture for an average of 320 days; they were then allotted to drylot pens (feedlot beginning) and placed on a 75% concentrate ration. Bulls from each treatment were fed to target weights of 1000 and 1100 pounds, and then slaughtered. Ralgro implanting increased average daily gain 6.6% over that of the controls from birth to the feedlot beginning, and 9.4% from the feedlot beginning to the first slaughter endpoint (196 days on feed). Feed efficiency was improved 8.2%, with implanting when the first group of implanted and control bulls were slaughtered (196 and 231 days on feed, respectively). Implanting reduced semen quality, reduced the number of bulls producing semen, and depressed the development of reproductive organs. Sex drive was unaffected by implanting. Implanting resulted in fatter carcasses and tended to increase yield grades, but did not affect final quality grades. Lean from control carcasses tended to be firmer and have a finer texture, but color was not affected by implanting. Loineye steaks from implanted bulls were significantly more tender, as judged by taste panel ratings and by Warner-Bratzler shear forces
Regulation of Sodium Channel Function by Bilayer Elasticity: The Importance of Hydrophobic Coupling. Effects of Micelle-forming Amphiphiles and Cholesterol
Membrane proteins are regulated by the lipid bilayer composition. Specific lipidâprotein interactions rarely are involved, which suggests that the regulation is due to changes in some general bilayer property (or properties). The hydrophobic coupling between a membrane-spanning protein and the surrounding bilayer means that protein conformational changes may be associated with a reversible, local bilayer deformation. Lipid bilayers are elastic bodies, and the energetic cost of the bilayer deformation contributes to the total energetic cost of the protein conformational change. The energetics and kinetics of the protein conformational changes therefore will be regulated by the bilayer elasticity, which is determined by the lipid composition. This hydrophobic coupling mechanism has been studied extensively in gramicidin channels, where the channelâbilayer hydrophobic interactions link a âconformationalâ change (the monomerâdimer transition) to an elastic bilayer deformation. Gramicidin channels thus are regulated by the lipid bilayer elastic properties (thickness, monolayer equilibrium curvature, and compression and bending moduli). To investigate whether this hydrophobic coupling mechanism could be a general mechanism regulating membrane protein function, we examined whether voltage-dependent skeletal-muscle sodium channels, expressed in HEK293 cells, are regulated by bilayer elasticity, as monitored using gramicidin A (gA) channels. Nonphysiological amphiphiles (β-octyl-glucoside, Genapol X-100, Triton X-100, and reduced Triton X-100) that make lipid bilayers less âstiffâ, as measured using gA channels, shift the voltage dependence of sodium channel inactivation toward more hyperpolarized potentials. At low amphiphile concentration, the magnitude of the shift is linearly correlated to the change in gA channel lifetime. Cholesterol-depletion, which also reduces bilayer stiffness, causes a similar shift in sodium channel inactivation. These results provide strong support for the notion that bilayerâprotein hydrophobic coupling allows the bilayer elastic properties to regulate membrane protein function
Toward Rational Design of Metal-Organic Frameworks for Sensing Applications: Efficient Calculation of Adsorption Characteristics in Zero Loading Regime
The TEXES Survey For H2 Emission From Protoplanetary Disks
We report the results of a search for pure rotational molecular hydrogen
emission from the circumstellar environments of young stellar objects with
disks using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASA
Infrared Telescope Facility and the Gemini North Observatory. We searched for
mid-infrared H2 emission in the S(1), S(2), and S(4) transitions. Keck/NIRSPEC
observations of the H2 S(9) transition were included for some sources as an
additional constraint on the gas temperature. We detected H2 emission from 6 of
29 sources observed: AB Aur, DoAr 21, Elias 29, GSS 30 IRS 1, GV Tau N, and HL
Tau. Four of the six targets with detected emission are class I sources that
show evidence for surrounding material in an envelope in addition to a
circumstellar disk. In these cases, we show that accretion shock heating is a
plausible excitation mechanism. The detected emission lines are narrow (~10
km/s), centered at the stellar velocity, and spatially unresolved at scales of
0.4 arcsec, which is consistent with origin from a disk at radii 10-50 AU from
the star. In cases where we detect multiple emission lines, we derive
temperatures > 500 K from ~1 M_earth of gas. Our upper limits for the
non-detections place upper limits on the amount of H2 gas with T > 500 K of
less than a few Earth masses. Such warm gas temperatures are significantly
higher than the equilibrium dust temperatures at these radii, suggesting that
the gas is decoupled from the dust in the regions we are studying and that
processes such as UV, X-ray, and accretion heating may be important.Comment: 24 pages, 16 figures, 5 tables, ApJ accepte
Water Vapor on Betelgeuse as Revealed by TEXES High-Resolution 12 Micron Spectra
The outer atmosphere of the M supergiant Betelgeuse is puzzling. Published
observations of different kinds have shed light on different aspects of the
atmosphere, but no unified picture has emerged. They have shown, for example,
evidence of a water envelope (MOLsphere) that in some studies is found to be
optically thick in the mid-infrared. In this paper, we present high-resolution,
mid-infrared spectra of Betelgeuse recorded with the TEXES spectrograph. The
spectra clearly show absorption features of water vapor and OH. We show that a
spectrum based on a spherical, hydrostatic model photosphere with T_eff = 3600
K, an effective temperature often assumed for Betelgeuse, fails to model the
observed lines. Furthermore, we show that published MOLspheres scenarios are
unable to explain our data. However, we are able to model the observed spectrum
reasonably well by adopting a cooler outer photospheric structure corresponding
to T_mod = 3250 K. The success of this model may indicate the observed
mid-infrared lines are formed in cool photospheric surface regions. Given the
uncertainties of the temperature structure and the likely presence of
inhomogeneities, we cannot rule out the possibility that our spectrum could be
mostly photospheric, albeit non-classical. Our data put new, strong constraints
on atmospheric models of Betelgeuse and we conclude that continued
investigation requires consideration of non-classical model photospheres as
well as possible effects of a MOLsphere. We show that the mid-infrared
water-vapor features have great diagnostic value for the environments of K and
M (super-) giant star atmospheres.Comment: Accepted by Ap
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