1,890 research outputs found
Demonstration test of burner liner strain measurement systems: Interim results
Work is in progress to demonstrate two techniques for static strain measurements on a jet engine burner liner. Measurements are being made with a set of resistance strain gages made from Kanthal A-1 wire and via heterodyne speckle photogrammetry. The background of the program is presented along with current results
Does Time Really Slow Down during a Frightening Event?
Observers commonly report that time seems to have moved in slow motion during a life-threatening event. It is unknown whether this is a function of increased time resolution during the event, or instead an illusion of remembering an emotionally salient event. Using a hand-held device to measure speed of visual perception, participants experienced free fall for 31 m before landing safely in a net. We found no evidence of increased temporal resolution, in apparent conflict with the fact that participants retrospectively estimated their own fall to last 36% longer than others' falls. The duration dilation during a frightening event, and the lack of concomitant increase in temporal resolution, indicate that subjective time is not a single entity that speeds or slows, but instead is composed of separable subcomponents. Our findings suggest that time-slowing is a function of recollection, not perception: 1a richer encoding of memory may cause a salient event to appear, retrospectively, as though it lasted longer
Galactic Globular Cluster Metallicity Scale from the Calcium Triplet. II. Rankings, Comparisons and Puzzles
We compare our compilation of the W' calcium index for 71 Galactic globular
clusters to the widely used Zinn and West (1984 ApJS, 55, 45) [Fe/H] scale and
to Carretta and Gratton's (1997 A&A Supplement 121, 95) scale from
high-dispersion spectra analyzed with Kurucz (1992, private communication)
model atmospheres. We find our calcium ranking to be tightly correlated with
each comparison set, in a non-linear and a linear fashion, respectively. By
combining our calcium index information with the Zinn and West ranking, we are
able to rank the globular clusters in our sample with a typical precision of
+/- 0.05 dex for [Fe/H] < -0.5 on the Zinn and West scale; for clusters more
metal rich than this, the ranking is less precise. The significant differences
between these metallicity scales raise important questions about our
understanding of Galactic formation and chemical enrichment processes.
Furthermore, in spite of the apparent improvement in metallicity ranking for
the Galactic globular clusters that results from our addition of information
from the Ca II triplet lines to the potpourri of other metallicity indicators,
caution -- perhaps considerable -- may be advisable when using W' as a
surrogate for metallicity, especially for systems where ranges in age and
metallicity are likely.Comment: To appear in the August 1997 issue of PASP Also available at
http://www.hia.nrc.ca/eprints.htm
Time-Series BVI Photometry for the Globular Cluster NGC 6981 (M72)
We present new BVI photometry of the globular cluster NGC 6981 (M72), based
mostly on ground-based CCD archive images. We present a new color-magnitude
diagram (CMD) that reaches almost four magnitudes below the turn-off level. We
performed new derivations of metallicity and morphological parameters of the
evolved sequences, in good agreement with previous authors, obtaining a value
of [Fe/H] ~ -1.50 in the new UVES scale. We also identify the cluster's blue
straggler population. Comparing the radial distribution of these stars with the
red giant branch population, we find that the blue stragglers are more
centrally concentrated, as found in previous studies of blue stragglers in
globular clusters. Taking advantage of the large field of view covered by our
study, we analyzed the surface density profile of the cluster, finding
extratidal main sequence stars out to r ~ 14.1 arcmin or about twice the tidal
radius. We speculate that this may be due to tidal disruption in the course of
M72's orbit, in which case tidal tails associated with the cluster may exist.
We also take a fresh look at the variable stars in the cluster, recovering all
previous known variables, including three SX Phoenicis stars, and adding three
previously unknown RR Lyrae (1 c-type and 2 ab-type) to the total census.
Finally, comparing our CMD with unpublished data for M3 (NGC 5272), a cluster
with similar metallicity and horizontal branch morphology, we found that both
objects are essentially coeval.Comment: Accepted for publication in A
Faint star counts in the near-infrared
We discuss near-infrared star counts at the Galactic pole with a view to
guiding the NGST and ground-based NIR cameras. Star counts from deep K-band
images from the CFHT are presented, and compared with results from the 2MASS
survey and some Galaxy models. With appropriate corrections for detector
artifacts and galaxies, the data agree with the models down to K~18, but
indicate a larger population of fainter red stars. There is also a significant
population of compact galaxies that extend to the observational faint limit of
K=20.5. Recent Galaxy models agree well down to K19, but diverge at
fainter magnitudes.Comment: 14 pages and 4 diagrams; to appear in PAS
Surface Brightness and Stellar Populations at the Outer Edge of the Large Magellanic Cloud: No Stellar Halo Yet
We present a high quality CMD for a 36'x 36' field located 8 degrees (7 kpc)
from the LMC center, as well as a precise determination of the LMC surface
brightness derived from the resolved stellar population out to this large
galactocentric radius. This deep CMD shows for the first time the detailed age
distribution at this position, where the surface brightness is V=26.5 mag/sq".
At a radius R=474' the main sequence is well populated from the oldest turnoff
at I=21.5 to the 2.5 Gyr turnoff at I=19.5. Beyond this radius, a relatively
strong gradient in the density of stars with ages in the 2.5-4 Gyr range is
apparent. There are some stars brighter and bluer than the main population,
quite uniformly distributed over the whole area surveyed, which are well
matched by a 1.5 Gyr isochrone and may be indicative of a relatively recent
star formation, or merger, event. The surface brightness profile of the LMC
remains exponential to this large galactocentric radius and shows no evidence
of disk truncation. Combining the information on surface brightness and stellar
population we conclude that the LMC disk extends (and dominates over a possible
stellar halo) out to a distance of at least 7 kpc. These results confirm that
the absence of blue stars in the relatively shallow off-center CMDs of dIrr
galaxies is not necessarily evidence for an exclusively old stellar population
resembling the halo of the Milky Way.Comment: ApJLett, in press 13 pages including 3 color figure
The influence of the C+N+O abundances on the determination of the relative ages of Globular Clusters: the case of NGC 1851 and NGC 6121 (M4)
The color magnitude diagram (CMD) of NGC 1851 presents two subgiant branches
(SGB), probably due the presence of two populations differing in total CNO
content. We test the idea that a difference in total CNO may simulate an age
difference when comparing the CMD of clusters to derive relative ages. We
compare NGC 1851 with NGC 6121 (M4), a cluster of very similar [Fe/H]. We find
that, with a suitable shift of the CMDs that brings the two red horizontal
branches at the same magnitude level, the unevolved main sequence and red giant
branch match, but the SGB of NGC 6121 and its red giant branch "bump" are
fainter than in NGC 1851. In particular, the SGB of NGC 6121 is even slightly
fainter than the the faint SGB in NGC 1851. Both these features can be
explained if the total CNO in NGC 6121 is larger than that in NGC 1851, even if
the two clusters are coeval. We conclude by warning that different initial
C+N+O abundances between two clusters, otherwise similar in metallicity and
age, may lead to differences in the turnoff morphology that can be easily
attributed to an age difference.Comment: accepted for publication in MNRA
The origin of the LMC stellar bar: clues from the SFH of the bar and inner disk
We discuss the origin of the LMC stellar bar by comparing the star formation
histories (SFH) obtained from deep color-magnitude diagrams (CMDs) in the bar
and in a number of fields in different directions within the inner disk. The
CMDs, reaching the oldest main sequence turnoffs in these very crowded fields,
have been obtained with VIMOS on the VLT in service mode, under very good
seeing conditions. We show that the SFHs of all fields share the same patterns,
with consistent variations of the star formation rate as a function of time in
all of them. We therefore conclude that no specific event of star formation can
be identified with the formation of the LMC bar, which instead likely formed
from a redistribution of disk material that occurred when the LMC disk became
bar unstable, and shared a common SFH with the inner disk thereafter. The
strong similarity between the SFH of the center and edge of the bar rules out
significant spatial variations of the SFH across the bar, which are predicted
by scenarios of classic bar formation through buckling mechanisms.Comment: MNRAS Letters, accepte
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