9,134 research outputs found
Engineering Guidelines for Clean Assembly and Sterilization of Spaceflight Hardware
Engineering specifications for clean assembly and sterilization of space flight hardware to prevent contamination of planetary environment
CF6 Jet Engine Performance Improvement: High Pressure Turbine Active Clearance Control
An active clearance control system was developed which reduces fuel consumption and performance degradation. This system utilizes compressor discharge air during takeoff and fan discharge air during cruise to impinge on the shroud structure to improve the thermal response. The system was evaluated in component and engine tests. The test results demonstrated a performance improvement of 0.7 percent in cruise SFC
Lunar analogs of fluvial landscapes - Possible implications, 1 March 1968 - 1 February 1970
Geomorphic approach to possibility of fluid erosion on moo
High-resolution CRIRES spectra of Terzan1: a metal-poor globular cluster toward the inner bulge
Containing the oldest stars in the Galaxy, globular clusters toward the bulge
can be used to trace its dynamical and chemical evolution. In the bulge
direction, there are ~50 clusters, but only about 20% have been subject of
high-resolution spectroscopic investigations. So far, the sample observed at
high resolution spans a moderate-to-high metallicity regime. In this sample,
however, very few are located in the innermost region (1.5\,Kpc and
). To constrain the chemical evolution enrichment of the
innermost region of Galaxy, accurate abundances and abundance patterns of key
elements based on high-resolution spectroscopy are necessary. Here we present
the results we obtained for Terzan 1, a metal-poor cluster located in the
innermost bulge region. Using the near-infrared spectrograph CRIRES at ESO/VLT,
we obtained high-resolution (R50,000) H-band spectra of 16 bright
giant stars in the innermost region () of Terzan1. Full spectral
synthesis techniques and equivalent width measurements of selected lines,
isolated and free of significant blending and/or contamination by telluric
lines, allowed accurate chemical abundances and radial velocities to be
derived. Fifteen out of 16 observed stars are likely cluster members, with an
average heliocentric radial velocity of +571.8\,km/s and mean iron
abundance of [Fe/H]=--1.260.03\,dex. For these stars we measured some
[/Fe] abundance ratios, finding average values of
[O/Fe]=+0.390.02\,dex, [Mg/Fe]=+0.420.02\,dex,
[Si/Fe]=+0.310.04\,dex, and [Ti/Fe]=+0.150.04\,dex The
enhancement (\,dex) found in the observed giant stars of Terzan1
is consistent with previous measurements on other, more metal-rich bulge
clusters, which suggests a rapid chemical enrichment.Comment: 7, pages, 6 figures, accepted for publication on A&
Automated Microbial Metabolism Laboratory Final report
Automated microbial metabolism life detection experiments for exobiological studie
Can realistic nuclear interactions tolerate a resonant tetraneutron ?
The possible existence of four-neutron resonances close to the physical
energy region is explored. Faddeev-Yakubovsky equations have been solved in
configuration space using realistic nucleon-nucleon interaction models. Complex
Scaling and Analytical Continuation in the Coupling Constant methods were used
to follow the resonance pole trajectories, which emerge out of artificially
bound tetraneutron states. The final pole positions for four-neutron states lie
in the third energy quadrant with negative real energy parts and should thus
not be physically observable.Comment: 10 pages, 6 figs, 2 tables. To be published in Phys. Rev.
A Slowly Precessing Disk in the Nucleus of M31 as the Feeding Mechanism for a Central Starburst
We present a kinematic study of the nuclear stellar disk in M31 at infrared
wavelengths using high spatial resolution integral field spectroscopy. The
spatial resolution achieved, FWHM = 0."12 (0.45 pc at the distance of M31), has
only previously been equaled in spectroscopic studies by space-based long-slit
observations. Using adaptive optics-corrected integral field spectroscopy from
the OSIRIS instrument at the W. M. Keck Observatory, we map the line-of-sight
kinematics over the entire old stellar eccentric disk orbiting the supermassive
black hole (SMBH) at a distance of r<4 pc. The peak velocity dispersion is
381+/-55 km/s , offset by 0.13 +/- 0.03 from the SMBH, consistent with previous
high-resolution long-slit observations. There is a lack of near-infrared (NIR)
emission at the position of the SMBH and young nuclear cluster, suggesting a
spatial separation between the young and old stellar populations within the
nucleus. We compare the observed kinematics with dynamical models from Peiris &
Tremaine (2003). The best-fit disk orientation to the NIR flux is [,
, ] = [-33 +/- 4, 44 +/- 2, -15 +/-
15], which is tilted with respect to both the larger-scale galactic
disk and the best-fit orientation derived from optical observations. The
precession rate of the old disk is = 0.0 +/- 3.9 km/s/pc, lower than
the majority of previous observations. This slow precession rate suggests that
stellar winds from the disk will collide and shock, driving rapid gas inflows
and fueling an episodic central starburst as suggested in Chang et al. (2007).Comment: accepted by Ap
The Terzan 5 puzzle: discovery of a third, metal-poor component
We report on the discovery of 3 metal-poor giant stars in Terzan 5, a complex
stellar system in the the Galactic bulge, known to have two populations at
[Fe/H]=-0.25 and +0.3. For these 3 stars we present new echelle spectra
obtained with NIRSPEC at Keck II, which confirm their radial velocity
membership and provide average [Fe/H]=-0.79 dex iron abundance and
[alpha/Fe]=+0.36 dex enhancement. This new population extends the metallicity
range of Terzan~5 0.5 dex more metal poor, and it has properties consistent
with having formed from a gas polluted by core collapse supernovae.Comment: Accepted for publication on ApJ Lette
Revised metallicity classes for low-mass stars: dwarfs (dM), subdwarfs (sdM), extreme subdwarfs (esdM), and ultra subdwarfs (usdM)
The current classification system of M stars on the main sequence
distinguishes three metallicity classes (dwarfs - dM, subdwarfs - sdM, and
extreme subdwarfs - esdM). The spectroscopic definition of these classes is
based on the relative strength of prominent CaH and TiO molecular absorption
bands near 7000A, as quantified by three spectroscopic indices (CaH2, CaH3, and
TiO5). We re-examine this classification system in light of our ongoing
spectroscopic survey of stars with proper motion \mu > 0.45 "/yr, which has
increased the census of spectroscopically identified metal-poor M stars to over
400 objects. Kinematic separation of disk dwarfs and halo subdwarfs suggest
deficiencies in the current classification system. Observations of common
proper motion doubles indicates that the current dM/sdM and sdM/esdM boundaries
in the [TiO5,CaH2+CaH3] index plane do not follow iso-metallicity contours,
leaving some binaries inappropriately classified as dM+sdM or sdM+esdM. We
propose a revision of the classification system based on an empirical
calibration of the TiO/CaH ratio for stars of near solar metallicity. We
introduce the parameter \zeta_{TiO/CaH} which quantifies the weakening of the
TiO bandstrength due to metallicity effect, with values ranging from
\zeta_{TiO/CaH}=1 for stars of near-solar metallicity to \zeta_{TiO/CaH}~0 for
the most metal-poor (and TiO depleted) subdwarfs. We redefine the metallicity
classes based on the value of the parameter \zeta_{TiO/CaH}; and refine the
scheme by introducing an additional class of ultra subdwarfs (usdM). We
introduce sequences of sdM, esdM, and usdM stars to be used as formal
classification standards.Comment: 15 pages, accepted for publication in the Astrophysical Journa
Proper motions in Terzan 5: membership of the multi-iron sub-populations and first constrain to the orbit
By exploiting two sets of high-resolution images obtained with HST ACS/WFC
over a baseline of ~10 years we have measured relative proper motions of
~70,000 stars in the stellar system Terzan 5. The results confirm the
membership of the three sub-populations with different iron abudances
discovered in the system. The orbit of the system has been derived from a first
estimate of its absolute proper motion, obtained by using bulge stars as
reference. The results of the integration of this orbit within an axisymmetric
Galactic model exclude any external accretion origin for this cluster. Terzan 5
is known to have chemistry similar to the Galactic bulge; our findings support
a kinematic link between the cluster and the bulge, further strengthening the
possibility that Terzan 5 is the fossil remnant of one of the pristine clumps
that originated the bulge.Comment: 25 pages, 14 figures, accepted for publication by Ap
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