5,901 research outputs found
The return of the Andromedids meteor shower
The Andromedid meteor shower underwent spectacular outbursts in 1872 and
1885, producing thousands of visual meteors per hour and described as `stars
fell like rain' in Chinese records of the time. The shower originates from
comet 3D/Biela whose disintegration in the mid-1800's is linked to the
outbursts, but the shower has been weak or absent since the late 19th Century.
This shower returned in December 2011 with a zenithal hourly rate of
approximately 50, the strongest return in over a hundred years. Some 122
probable Andromedid orbits were detected by the Canadian Meteor Orbit Radar.
The shower outburst occurred during 2011 Dec 3-5. The radiant at RA
+ and Dec + is typical of the `classical' Andromedids of
the early 1800's, whose radiant was actually in Cassiopeia. The orbital
elements indicate that the material involved was released before 3D/Biela's
breakup prior to 1846. The observed shower in 2011 had a slow geocentric speed
(16 km s) and was comprised of small particles: the mean measured mass
from the radar is kg corresponding to radii of 0.5 mm at
a bulk density of 1000 kg/m.
Numerical simulations of the parent comet indicate that the meteoroids of the
2011 return of the Andromedids shower were primarily ejected during 3D/Biela's
1649 perihelion passage. The orbital characteristics, radiant, timing as well
as the absence of large particles in the streamlet are all consistent with
simulations. Predictions are made regarding other appearances of the shower in
the years 2000-2047 based on our numerical model. We note that the details of
the 2011 return can, in principle, be used to better constrain the orbit of
3D/Biela prior to the comets first recorded return in 1772.Comment: submitted to the Astronomical Journal Sep 22 201
The HI - Star Formation Connection: Open Questions
We show data from the Survey of Ionization in Neutral Gas Galaxies (SINGG)
and Survey of Ultraviolet emission in Neutral Gas Galaxies (SUNGG) which survey
the star formation properties of HI selected galaxies as traced by H-alpha and
ultraviolet emission, respectively. The correlations found demonstrate a strong
relationship between the neutral ISM, young massive stars, and the evolved
stellar populations. For example the correlation between R band surface
brightness and the HI cycling time is tighter than the Kennicutt-Schmidt Star
Formation Law. Other scaling relations from SINGG give strong direct
confirmation of the downsizing scenario: low mass galaxies are more gaseous and
less evolved into stars than high mass galaxies. There are strong variations in
the H-alpha to UV flux ratios within and between galaxies. The only plausible
explanations for this result are that either the escape fraction of ionizing
photons or the upper end of the IMF varies with galaxy mass. We argue for the
latter interpretation, although either result has major implications for
astrophysics. A detailed dissection of the massive star content in the extended
HI disk of NGC2915 provides a consistent picture of continuing star formation
with a truncated or steep IMF, while other GALEX results indicate that star
formation edges seen in Halpha are not always apparent in the UV. These and
other recent results settle some old questions but open many new questions
about star formation and its relation to the ISM.Comment: To appear in AIP Conference Proceedings, "The Evolution of Galaxies
through the Neutral Hydrogen Window", Feb 1-3 2008, Arecibo, Puerto Rico,
eds. R. Minchin & E. Momjian. 7 page
The Interstellar Medium and Star Formation in Edge-On Galaxies. II. NGC 4157, 4565, and 5907
We present a study of the vertical structure of the gaseous and stellar disks
in a sample of edge-on galaxies (NGC 4157, 4565, and 5907) using BIMA/CARMA
12CO (J = 1 --> 0), VLA H I, and Spitzer 3.6 micron data. In order to take into
account projection effects when we measure the disk thickness as a function of
radius, we first obtain the inclination by modeling the radio data. Using the
measurement of the disk thicknesses and the derived radial profiles of gas and
stars, we estimate the corresponding volume densities and vertical velocity
dispersions. Both stellar and gas disks have smoothly varying scale heights and
velocity dispersions, contrary to assumptions of previous studies. Using the
velocity dispersions, we find that the gravitational instability parameter Q
follows a fairly uniform profile with radius and is greater than or equal to 1
across the star forming disk. The star formation law has a slope that is
significantly different from those found in more face-on galaxy studies, both
in deprojected and pixel-by-pixel plots. Midplane gas pressure based on the
varying scale heights and velocity dispersions appears to roughly hold a
power-law correlation with the midplane volume density ratio.Comment: 26 pages, 26 figures, Accepted for publication in A
Intuitive Data-Driven Visualization of Food Relatedness via t-Distributed Stochastic Neighbor Embedding
The relationship between diet and health is important, yet difficultto study in practice. Dietary pattern analysis is one method forinvestigating this link; having more variety in diet tends to be bene-ficial and a score can be generated based on a heuristic approachto food intake habits. We aim to enhance the intuition behindthese food scores by creating an intuitive data-driven visualizationof food relatedness by leveraging t-distributed stochastic neighborembedding (t-SNE). More specifically, by performing t-SNE anal-ysis in a controlled manner to project the high-dimensional nutri-tional information of food items into a lower dimensional food sim-ilarity space, the natural clustering of foods based on the underly-ing nutritional composition becomes visually observable. The effi-cacy of this data-driven approach for visualizing food relatednesswas investigated on a total of 8549 food item entries in the USDAfood composition database, with the results showing considerablepromise as a tool for gaining important nutritional insights. This isthe first step toward providing a novel method to enhance dietarypattern analysis with additional context and insight into food intakehabits based on the inherent nutritional content of the foods con-sumed
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A Sec14 domain protein is required for photoautotrophic growth and chloroplast vesicle formation in Arabidopsis thaliana.
In eukaryotic photosynthetic organisms, the conversion of solar into chemical energy occurs in thylakoid membranes in the chloroplast. How thylakoid membranes are formed and maintained is poorly understood. However, previous observations of vesicles adjacent to the stromal side of the inner envelope membrane of the chloroplast suggest a possible role of membrane transport via vesicle trafficking from the inner envelope to the thylakoids. Here we show that the model plant Arabidopsis thaliana has a chloroplast-localized Sec14-like protein (CPSFL1) that is necessary for photoautotrophic growth and vesicle formation at the inner envelope membrane of the chloroplast. The cpsfl1 mutants are seedling lethal, show a defect in thylakoid structure, and lack chloroplast vesicles. Sec14 domain proteins are found only in eukaryotes and have been well characterized in yeast, where they regulate vesicle budding at the trans-Golgi network. Like the yeast Sec14p, CPSFL1 binds phosphatidylinositol phosphates (PIPs) and phosphatidic acid (PA) and acts as a phosphatidylinositol transfer protein in vitro, and expression of Arabidopsis CPSFL1 can complement the yeast sec14 mutation. CPSFL1 can transfer PIP into PA-rich membrane bilayers in vitro, suggesting that CPSFL1 potentially facilitates vesicle formation by trafficking PA and/or PIP, known regulators of membrane trafficking between organellar subcompartments. These results underscore the role of vesicles in thylakoid biogenesis and/or maintenance. CPSFL1 appears to be an example of a eukaryotic cytosolic protein that has been coopted for a function in the chloroplast, an organelle derived from endosymbiosis of a cyanobacterium
Analytical solution of the tooling/workpiece contact interface shape during a flow forming operation
Flow forming involves complicated tooling/workpiece interactions. Purely
analytical models of the tool contact area are difficult to formulate,
resulting in numerical approaches that are case-specific. Provided are the
details of an analytical model that describes the steady-state
tooling/workpiece contact area allowing for easy modification of the dominant
geometric variables. The assumptions made in formulating this analytical model
are validated with experimental results attained from physical modelling. The
analysis procedure can be extended to other rotary forming operations such as
metal spinning, shear forming, thread rolling and crankshaft fillet rolling.Comment: 28 pages, 11 figure
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