379 research outputs found
New Perspective on TPACK Framework in the Context of Early Childhood Education: The âAâ Stands for Affective
The purpose of this exploratory single-case study is to investigate the affordances of iPad transpired within a technological pedagogical and content knowledge (TPACK) framework by four early childhood educators with varying Technological Knowledge (TK) at a low-income preschool. Pre/post and follow-up survey, group and follow-up interviews, classroom observations and document of iPad workshop data were analyzed using coding methods in two cycles. The exploration in how teachers discovered the iPad affordances indicated parallel progression in TK and change in their value system. The exploration in the progression of TK and change in their value system suggest a relationship between progression of TK towards TPACK and of affective-valuing (AV) towards affective-characterization (AC)
Deep imaging of Eridanus II and its lone star cluster
We present deep imaging of the most distant dwarf discovered by the Dark
Energy Survey, Eridanus II (Eri II). Our Magellan/Megacam stellar photometry
reaches mag deeper than previous work, and allows us to confirm the
presence of a stellar cluster whose position is consistent with Eri II's
center. This makes Eri II, at , the least luminous galaxy known to
host a (possibly central) cluster. The cluster is partially resolved, and at
it accounts for of Eri II's luminosity. We derive
updated structural parameters for Eri II, which has a half-light radius of
pc and is elongated (), at a measured
distance of kpc. The color-magnitude diagram displays a blue,
extended horizontal branch, as well as a less populated red horizontal branch.
A central concentration of stars brighter than the old main sequence turnoff
hints at a possible intermediate-age ( Gyr) population; alternatively,
these sources could be blue straggler stars. A deep Green Bank Telescope
observation of Eri II reveals no associated atomic gas.Comment: 7 pages, 4 figures; ApJL accepte
Antlia B: A faint dwarf galaxy member of the NGC 3109 association
We report the discovery of Antlia B, a faint dwarf galaxy at a projected
distance of 72 kpc from NGC 3109 (15 mag), the primary
galaxy of the NGC 3109 dwarf association at the edge of the Local Group. The
tip of the red giant branch distance to Antlia B is =1.290.10 Mpc,
which is consistent with the distance to NGC 3109. A qualitative analysis
indicates the new dwarf's stellar population has both an old, metal poor red
giant branch (10 Gyr, [Fe/H]2), and a younger blue population
with an age of 200-400 Myr, analogous to the original Antlia dwarf,
another likely satellite of NGC 3109. Antlia B has \ion{H}{1} gas at a velocity
of =376 km s, confirming the association with NGC 3109
(=403 km s). The HI gas mass
(M=2.80.210 M), stellar luminosity
(=9.70.6 mag) and half light radius (=27329 pc) are
all consistent with the properties of dwarf irregular and dwarf spheroidal
galaxies in the Local Volume, and is most similar to the Leo P dwarf galaxy.
The discovery of Antlia B is the initial result from a Dark Energy Camera
survey for halo substructure and faint dwarf companions to NGC 3109 with the
goal of comparing observed substructure with expectations from the
+Cold Dark Matter model in the sub-Milky Way regime.Comment: 7 pages, 3 figures. Submitted to ApJ
Antlia B: A Faint Dwarf Galaxy Member of the NGC 3109 Association
We report the discovery of Antlia B, a faint dwarf galaxy at a projected distance of similar to 72 kpc from NGC 3109 (M-V similar to -15 mag), the primary galaxy of the NGC 3109 dwarf association at the edge of the Local Group. The tip of the red giant branch distance to Antlia B is D = 1.29 +/- 0.10 Mpc, which is consistent with the distance to NGC 3109. A qualitative analysis indicates the new dwarf \u27s stellar population has both an old, metal-poor red giant branch (greater than or similar to 10 Gyr, [Fe/H] similar to -2), and a younger blue population with an age of similar to 200-400 Myr, analogous to the original Antlia dwarf, another likely satellite of NGC 3109. Antlia B has H I gas at a velocity of v(helio, H I) = 376 kms(-1), confirming the association with NGC 3109 (v(helio) = 403 km s(-1)). The H I gas mass (M-H I = 2.8 +/- 0.2 x 10(5) M-circle dot), stellar luminosity (M-V = -9.7 +/- 0.6 mag) and half light radius (rh = 273 +/- 29 pc) are all consistent with the properties of dwarf irregular and dwarf spheroidal galaxies in the Local Volume, and is most similar to the Leo P dwarf galaxy. The discovery of Antlia B is the initial result from a Dark Energy Camera survey for halo substructure and faint dwarf companions to NGC 3109 with the goal of comparing observed substructure with expectations from the Lambda+Cold Dark Matter model in the sub-Milky Way regime
Factors influencing the distribution of overwintered bullfrog tadpoles (Rana catesbeiana) in two small ponds
Competitive exclusion of Salmonella enteritidis by Salmonella gallinarum in poultry.
Salmonella Enteritidis emerged as a major egg-associated pathogen in the late 20th century. Epidemiologic data from England, Wales, and the United States indicate that S. Enteritidis filled the ecologic niche vacated by eradication of S. Gallinarum from poultry, leading to an epidemic increase in human infections. We tested this hypothesis by retrospective analysis of epidemiologic surveys in Germany and demonstrated that the number of human S. Enteritidis cases is inversely related to the prevalence of S. Gallinarum in poultry. Mathematical models combining epidemiology with population biology suggest that S. Gallinarum competitively excluded S. Enteritidis from poultry flocks early in the 20th century
Vertical pairing of identical particles suspended in the plasma sheath
It is shown experimentally that vertical pairing of two identical
microspheres suspended in the sheath of a radio-frequency (rf) discharge at low
gas pressures (a few Pa), appears at a well defined instability threshold of
the rf power. The transition is reversible, but with significant hysteresis on
the second stage. A simple model, which uses measured microsphere resonance
frequencies and takes into account besides Coulomb interaction between
negatively charged microspheres also their interaction with positive ion wake
charges, seems to explain the instability threshold quite well.Comment: 4 pages, 6 figures. to appear in Phys. Rev. Lett. 86, May 14th (2001
Hubble Space Telescope Imaging of Antlia B: Star Formation History and a New Tip of the Red Giant Branch Distance
A census of the satellite population around dwarf galaxy primary hosts in environments outside the Local Group is essential to understanding Î cold dark matter galaxy formation and evolution on the smallest scales. We present deep optical Hubble Space Telescope imaging of the gas-rich, faint dwarf galaxy Antlia B (M_V = â9.4)âa likely satellite of NGC 3109 (D = 1.3 Mpc)âdiscovered as part of our ongoing survey of primary host galaxies similar to the Magellanic Clouds. We derive a new tip of the red giant branch distance of D = 1.35 ± 0.06 Mpc (m â M = 25.65 ± 0.10), consistent with membership in the nearby NGC 3109 dwarf association. The colorâmagnitude diagram (CMD) shows both a prominent old, metal-poor stellar component and confirms a small population of young, blue stars with ages âŸ1 Gyr. We use the CMD fitting algorithm MATCH to derive the star formation history (SFH) and find that it is consistent with the typical dwarf irregular or transitional dwarf galaxy (dTrans) in the Local Group. Antlia B shows relatively constant stellar mass growth for the first ~10â11 Gyr and almost no growth in the last ~2â3 Gyr. Despite being gas-rich, Antlia B shows no evidence of active star formation (i.e., no Hα emission) and should therefore be classified as a dTrans dwarf. Both Antlia B and the Antlia dwarf (dTrans) are likely satellites of NGC 3109, suggesting that the cessation of ongoing star formation in these galaxies may be environmentally driven. Future work studying the gas kinematics and distribution in Antlia B will explore this scenario in greater detail. Our work highlights the fact that detailed studies of nearby dwarf galaxies in a variety of environments may continue to shed light on the processes that drive the SFH and evolution of dwarf galaxies more generally
Ligand-Induced Proton Transfer and Low-Barrier Hydrogen Bond Revealed by X-ray Crystallography
Ligand binding can change the pKa of protein residues and influence enzyme catalysis. Herein, we report three sub-Angstrom resolution X-ray crystal structures of CTX-M \u3b2-lactamase, representing three stages of the enzymatic pathway, apo protein (0.79 \uc5), pre-covalent complex (0.89 \uc5), and acylation transition state analog (0.84 \uc5). The binding of a non-covalent ligand induces a proton transfer from the catalytic Ser70 to the general base Glu166, and the formation of a low-barrier hydrogen bond (LBHB) between Ser70 and Lys73. QM/MM reaction path calculations determined the proton transfer barrier between Ser70 and Lys73 to be 1.53 kcal/mol, further confirming the presence of a LBHB. This LBHB is absent in the other two structures. Our data represents the first evidence of a direct and transient LBHB stabilizing a nucleophilic serine, as hypothesized by Cleland and Kreevoy. These results have important implications for the study of enzyme mechanisms as well as protein-inhibitor interactions
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