4,053 research outputs found
Modeling and Comparing Vertical Density Profiles
The vertical density profile of pressed wood panels is influenced by the manufacturing process and is important to panel end-users. Modeling the vertical density profile and making statistical comparisons among profiles resulting from different manufacturing treatments are critical to understanding and improving panel properties. Nonparametric regression analysis was used to model the vertical density profile of aspen (Populus tremuloides) oriented strandboard panels. A methodology is presented to compare vertical density profile curves. Twenty-seven laboratory panels were manufactured at 608 kg/m3 incorporating three levels of furnish moisture content (4%, 8%, 12%) and three levels of press closure rate (20 s, 60 s, 100 s) in a replicated, experimental design.The nonparametric regression technique called cubic splines was used to fit the data, R2 ranged from 0.985 to 0.998. Detailed discussion is presented that describes the method and interpretation of the nonparametric regression analysis. Statistical comparison of vertical density profile curves among treatment levels revealed that the 4% furnish moisture content level was significantly different (P = 0.015) from the 8% and 12% levels; the 8% level was not significantly different (P > 0.99) from the 12% level. Vertical density profile curves for all press closure rate treatments were significantly different (P < 0.003)
Fundamentals of Vertical Density Profile Formation in Wood Composites. Part III. MDF Density Formation During Hot-Pressing
The formation characteristics of the vertical density profile of MDF are presented herein. Results of laboratory studies indicate that the vertical density profile of MDF is formed from a combination of actions that occur both during compaction and also after the press has reached final position. The methodology, which was used to describe the formation of the density profile for OSB that used two periods and five stages, can also be used to describe the density profile formation of MDF mats. There was a clearer surface layer consolidation stage for MDF mats when compared to OSB mats. At least 58% of the area in an MDF mat was in "spring status" when the press reached 120% of final panel thickness. The observed stress-strain response of MDF mats in hot-pressing with the one-step closing schedule was similar to OSB pressing. This was characterized by a long stress plateau followed by a rapid increase in stress, followed by an immediate reduction in stress after the press reached final position. Compared to OSB hot-pressing, the same pressing schedules (traditional or step-closure) did not result in similar MDF density profiles. It appears that high-density surface layers are easier to create in MDF than in OSB
Metal-Poor Stars Observed with the Magellan Telescope. III. New Extremely and Ultra Metal-Poor Stars from SDSS/SEGUE and Insights on the Formation of Ultra Metal-Poor Stars
We report the discovery of one extremely metal-poor (EMP; [Fe/H]<-3) and one
ultra metal-poor (UMP; [Fe/H]<-4) star selected from the SDSS/SEGUE survey.
These stars were identified as EMP candidates based on their medium-resolution
(R~2,000) spectra, and were followed-up with high-resolution (R~35,000)
spectroscopy with the Magellan-Clay Telescope. Their derived chemical
abundances exhibit good agreement with those of stars with similar
metallicities. We also provide new insights on the formation of the UMP stars,
based on comparison with a new set of theoretical models of supernovae
nucleosynthesis. The models were matched with 20 UMP stars found in the
literature, together with one of the program stars (SDSS J1204+1201), with
[Fe/H]=-4.34. From fitting their abundances, we find that the supernovae
progenitors, for stars where carbon and nitrogen are measured, had masses
ranging from 20.5 M_sun to 28 M_sun and explosion energies from 0.3 to
0.9x10^51 erg. These results are highly sensitive to the carbon and nitrogen
abundance determinations, which is one of the main drivers for future
high-resolution follow-up of UMP candidates. In addition, we are able to
reproduce the different CNO abundance patterns found in UMP stars with a single
progenitor type, by varying its mass and explosion energy.Comment: 15 pages, 12 figures; accepted for publication in Ap
Population Studies. XIII. A New Analysis of the Bidelman-MacConnell "Weak-Metal" Stars - Confirmation of Metal-Poor Stars in the Thick Disk of the Galaxy
A new set of very high signal-to-noise (S/N > 100/1), medium-resolution
(R~3000) optical spectra have been obtained for 302 of the candidate
"weak-metal" stars selected by Bidelman & MacConnell. We use these data to
calibrate the recently developed generalization of the SEGUE Stellar Parameter
Pipeline, and obtain estimates of the atmospheric parameters (Teff, log g , and
[Fe/H]) for these non-SDSS/SEGUE data; we also obtain estimates of [C/Fe]. The
new abundance measurements are shown to be consistent with available
high-resolution spectroscopic determinations, and represent a substantial
improvement over the accuracies obtained from the previous photometric
estimates reported in Paper I of this series. The apparent offset in the
photometric abundances of the giants in this sample noted by several authors is
confirmed by our new spectroscopy; no such effect is found for the dwarfs. The
presence of a metal-weak thick-disk (MWTD) population is clearly supported by
these new abundance data. Some 25% of the stars with metallicities -1.8 <
[Fe/H] <= -0.8 exhibit orbital eccentricities e < 0.4, yet are clearly
separated from members of the inner-halo population with similar metallicities
by their location in a Lindblad energy vs. angular momentum diagram. A
comparison is made with recent results for a similar-size sample of RAVE stars
from Ruchti et al. We conclude, based on both of these samples, that the MWTD
is real, and must be accounted for in discussions of the formation and
evolution of the disk system of the Milky Way.Comment: 45 pages, 14 figures; accepted for publication in Ap
Insight Into the Formation of the Milky Way Through Cold Halo Substructure. III. Statistical Chemical Tagging in the Smooth Halo
We find that the relative contribution of satellite galaxies accreted at high
redshift to the stellar population of the Milky Way's smooth halo increases
with distance, becoming observable relative to the classical smooth halo about
15 kpc from the Galactic center. In particular, we determine
line-of-sight-averaged [Fe/H] and [alpha/Fe] in the metal-poor main-sequence
turnoff (MPMSTO) population along every Sloan Extension for Galactic
Understanding and Exploration (SEGUE) spectroscopic line of sight. Restricting
our sample to those lines of sight along which we do not detect elements of
cold halo substructure (ECHOS), we compile the largest spectroscopic sample of
stars in the smooth component of the halo ever observed in situ beyond 10 kpc.
We find significant spatial autocorrelation in [Fe/H] in the MPMSTO population
in the distant half of our sample beyond about 15 kpc from the Galactic center.
Inside of 15 kpc however, we find no significant spatial autocorrelation in
[Fe/H]. At the same time, we perform SEGUE-like observations of N-body
simulations of Milky Way analog formation. While we find that halos formed
entirely by accreted satellite galaxies provide a poor match to our
observations of the halo within 15 kpc of the Galactic center, we do observe
spatial autocorrelation in [Fe/H] in the simulations at larger distances. This
observation is an example of statistical chemical tagging and indicates that
spatial autocorrelation in metallicity is a generic feature of stellar halos
formed from accreted satellite galaxies.Comment: 27 pages, 8 figures, and 7 tables in emulateapj format; accepted for
publication in ApJ. Full tables can be extracted from LaTeX sourc
Evidence of boosted 13CO/12CO ratio in early-type galaxies in dense environments
We present observations of CO(1-0) in 17 Combined Array for Research
in Millimeter Astronomy (CARMA) Atlas3D early-type galaxies (ETGs), obtained
simultaneously with CO(1-0) observations. The CO in six ETGs is
sufficiently bright to create images. In these 6 sources, we do not detect any
significant radial gradient in the CO/CO ratio between the
nucleus and the outlying molecular gas. Using the CO channel maps as 3D
masks to stack the CO emission, we are able to detect 15/17 galaxies to
(and 12/17 to at least 5) significance in a spatially
integrated manner. Overall, ETGs show a wide distribution of
CO/CO ratios, but Virgo cluster and group galaxies preferentially
show a CO/CO ratio about 2 times larger than field galaxies,
although this could also be due to a mass dependence, or the CO spatial extent
(). ETGs whose gas has a morphologically-settled
appearance also show boosted CO/CO ratios. We hypothesize that
this variation could be caused by (i) the extra enrichment of gas from
molecular reprocessing occurring in low-mass stars (boosting the abundance of
C to C in the absence of external gas accretion), (ii) much
higher pressure being exerted on the midplane gas (by the intracluster medium)
in the cluster environment than in isolated galaxies, or (iii) all but the
densest molecular gas clumps being stripped as the galaxies fall into the
cluster. Further observations of CO in dense environments, particularly
of spirals, as well as studies of other isotopologues, should be able to
distinguish between these hypotheses.Comment: 13 pages, 3 tables, 7 figures, accepted by MNRA
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