1,305 research outputs found
Development of 2 underseat energy absorbers for application to crashworthy passenger seats for general aviation aircraft
This report presents the methodology and results of a program conducted to develop two underseat energy absorber (E/A) concepts for application to nonadjustable crashworthy passenger seats for general aviation aircraft. One concept utilizes an inflated air bag, and the other, a convoluted sheet metal bellows. Prototypes of both were designed, built, and tested. Both concepts demonstrated the necessary features of an energy absorber (load-limiter); however, the air bag concept is particularly encouraging because of its light weight. Several seat frame concepts also were investigated as a means of resisting longitudinal and lateral loads and of guiding the primary vertical stroke of the underseat energy absorber. Further development of a seat system design using the underseat energy absorbers is recommended because they provide greatly enhanced crash survivability as compared with existing general aviation aircraft seats
First detection of a low-mass stellar halo around the young open cluster Eta Chamaeleontis
We have identified several lithium-rich low-mass (0.08<M<0.3 Msun) stars
within 5.5 deg of the young open cluster Eta Chamaeleontis, nearly four times
the radius of previous search efforts. Of these stars we propose 4 new probable
cluster members, and 3 possible members requiring further investigation. These
findings are consistent with a dynamical origin for the current configuration
of the cluster, without the need to invoke an abnormal Initial Mass Function
deficient in low-mass objects. Candidates were selected on the basis of DENIS
and 2MASS photometry, NOMAD astrometry and extensive follow-up spectroscopy.Comment: 5 Pages. 5 Figures and 1 Table. Accepted for publication in MNRAS
Letters. Higher resolution figures available at
http://www.mso.anu.edu.au/~murphysj/
HST Observations of Gravitationally Lensed Features in the Rich Cluster Ac114
Deep Hubble Space Telescope images of superlative resolution obtained for the
distant rich cluster AC114 (z=0.31) reveal a variety of gravitational lensing
phenomena for which ground-based spectroscopy is available. We present a
luminous arc which is clearly resolved by HST and appears to be a lensed z=0.64
sub-L star spiral galaxy with a detected rotation curve. Of greatest interest
is a remarkably symmetrical pair of compact blue images separated by 10 arcsec
and lying close to the cluster cD. We propose that these images arise from a
single very faint background source gravitationally lensed by the cluster core.
Deep ground-based spectroscopy confirms the lensing hypothesis and suggests the
source is a compact star forming system at a redshift z=1.86. Taking advantage
of the resolved structure around each image and their very blue colours, we
have identified a candidate third image of the same source roughly 50 arcsec
away. The angular separation of the three images is much larger than previous
multiply-imaged systems and indicates a deep gravitational potential in the
cluster centre. Resolved multiply-imaged systems, readily recognised with HST,
promise to provide unique constraints on the mass distribution in the cores of
intermediate redshift clusters.Comment: submitted to ApJ, 6 pages (no figures), uuencoded Postscript,
compressed TAR of Postscript figures available via anonymous ftp in
users/irs/figs/ac114_figs.tar.gz on astro.caltech.edu. PAL-IRS-
A deep photometric survey of the eta Chamaeleontis cluster down to the brown dwarf - planet boundary
We report the outcome of the deep optical/infrared photometric survey of the
central region (33 X 33 arcmin or 0.9 pc^2) of the eta Chamaeleontis pre-main
sequence star cluster. The completeness limits of the photometry are I = 19.1,
J = 18.2 and H = 17.6; faint enough to reveal low mass members down to the
brown dwarf and planet boundary of ~ 13 M_Jup. We found no such low mass
members in this region. Our result combined with a previous shallower (I = 17)
but larger area survey indicates that low mass objects (0.013 < M/M(solar mass)
< 0.075) either were not created in the eta Cha cluster or were lost due to the
early dynamical history of the cluster and ejected to outside the surveyed
areas.Comment: 5 pages with 4 figures, accepted by MNRA
Episodic disk accretion in the halo of the 'old' Pre-Main Sequence cluster Eta Chamaeleontis
We present multi-epoch medium-resolution observations of two M4.5 candidate
members in the halo of the ~8 Myr Eta Chamaeleontis open cluster. Over six
months of observations both stars exhibited variations in their H-alpha line
profiles on timescales of days to months, with at least one episode of
substantial activity attributable to accretion from a circumstellar disk. We
derive an accretion rate ~10^-8.7 Msun/yr for this event, with a rate of
~10^-10.6 Msun/yr in quiescence. Episodic accretion like that observed here
means existing surveys of accreting Weak-lined T-Tauri Stars in young clusters
are likely incomplete and that gas dissipation timescales calculated from the
fraction of accreting objects are underestimates.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS
Letter
Potential formation sites of super star clusters in ultra-luminous infrared galaxies
Recent observational results on high spatial resolution images of
ultra-luminous infrared galaxies (ULIGs) have revealed very luminous, young,
compact, and heavily obscured super star clusters in their central regions,
suggested to be formed by gas-rich major mergers. By using stellar and gaseous
numerical simulations of galaxy mergers, we firstly demonstrate that the
central regions of ULIGs are the most promising formation sites of super star
clusters owing to the rather high gaseous pressure of the interstellar medium.
Based on simple analytical arguments, we secondly discuss the possibility that
super star clusters in an ULIG can be efficiently transferred into the nuclear
region owing to dynamical friction and consequently merge with one another to
form a single compact stellar nucleus with a seed massive black hole. We thus
suggest that multiple merging between super star clusters formed by nuclear
starbursts in the central regions of ULIGs can result in the formation of
massive black holes.Comment: 12 pages 4 figures, 2001, accepted by ApJ
On the Stellar Kinematics and Mass of the Virgo Ultra-Diffuse Galaxy VCC 1287
Here, we present a kinematical analysis of the Virgo cluster ultra-diffuse
galaxy (UDG) VCC 1287 based on data taken with the Keck Cosmic Web Imager
(KCWI). We confirm VCC 1287's association both with the Virgo cluster and its
globular cluster (GC) system, measuring a recessional velocity of $1116 \pm 2\
\mathrm{km\ s^{-1}}19 \pm 6\
\mathrm{km\ s^{-1}}1.11^{+0.81}_{-0.81}
\times 10^{9} \ \mathrm{M_{\odot}}13^{+11}_{-11}$)
within the half light radius (4.4 kpc). This places VCC 1287 slightly above the
well established relation for normal galaxies, with a higher mass to light
ratio for its dynamical mass than normal galaxies. We use our dynamical mass,
and an estimate of GC system richness, to place VCC 1287 on the GC number --
dynamical mass relation, finding good agreement with a sample of normal
galaxies. Based on a total halo mass derived from GC counts, we then infer that
VCC 1287 likely resides in a cored or low concentration dark matter halo. Based
on the comparison of our measurements to predictions from simulations, we find
that strong stellar feedback and/or tidal effects are plausibly the dominant
mechanisms in the formation of VCC 1287. Finally, we compare our measurement of
the dynamical mass with those for other UDGs. These dynamical mass estimates
suggest relatively massive halos and a failed galaxy origin for at least some
UDGs.Comment: 13 pages, 10 figures with an additional 5 pages and 5 figures in
appendices. Accepted for publication in MNRAS. v2: with small updates from
publication formatting and a minor plotting fix for Fig. 1
Abell 1201: a Minor merger at second core passage
We present an analysis of the structures and dynamics of the merging cluster
Abell~1201, which has two sloshing cold fronts around a cooling core, and an
offset gas core approximately 500kpc northwest of the center. New Chandra and
XMM-Newton data reveal a region of enhanced brightness east of the offset core,
with breaks in surface brightness along its boundary to the north and east.
This is interpreted as a tail of gas stripped from the offset core. Gas in the
offset core and the tail is distinguished from other gas at the same distance
from the cluster center chiefly by having higher density, hence lower entropy.
In addition, the offset core shows marginally lower temperature and metallicity
than the surrounding area. The metallicity in the cool core is high and there
is an abrupt drop in metallicity across the southern cold front. We interpret
the observed properties of the system, including the placement of the cold
fronts, the offset core and its tail in terms of a simple merger scenario. The
offset core is the remnant of a merging subcluster, which first passed
pericenter southeast of the center of the primary cluster and is now close to
its second pericenter passage, moving at ~1000 km/s. Sloshing excited by the
merger gave rise to the two cold fronts and the disposition of the cold fronts
reveals that we view the merger from close to the plane of the orbit of the
offset core.Comment: accepted by Ap
HST Observations of Giant Arcs: High Resolution Imaging Of Distant Field Galaxies.
We present HST imaging of eight spectroscopically-confirmed giant arcs, pairs
and arclets. These objects have all been extensively studied from the ground
and we demonstrate the unique advantages of HST imaging in the study of such
features by a critical comparison of our data with the previous observations.
In particular we present new estimates of the core radii of two clusters
(Cl0024+16, A370) determined from lensed features which are identifiable in our
HST images. Although our HST observations include both pre- and
post-refurbishment images, the depth of the exposures guarantees that the
majority of the arcs are detected with diffraction-limited resolution. A number
of the objects in our sample are multiply-imaged and we illustrate the ease of
identification of such features when working at high resolution. We discuss the
morphological and scale information on these distant field galaxies in the
light of HST studies of lower redshift samples. We conclude that the dominant
population of star-forming galaxies at z=1 is a factor of 1.5-2 times smaller
than the similar group in the local field. This implies either a considerable
evolution in the sizes of star-forming galaxies within the last 10 Gyrs
or a shift in the relative space densities of massive and dwarf star-forming
systems over the same timescale.Comment: 9 pages (no figures), uuencoded, compressed Postscript. Postscript
text, tables and figures (803 Kb) available via anonymous ftp in at
ftp://ociw.edu//pub/irs/pub/hstarcs.tar.
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