1,812 research outputs found
Evaluation of coatings for cobalt- and nickel-base superalloys, volume 2
The final results of an oxidation-reduction rig evaluation of aluminide coatings are presented, as applied to 1N-100 and B1900 nickel-base and X-10 and WI-52 cobalt-base alloys. Burner rigs were used which operated on JP-5 fuel and air, producing a gas velocity of 0.85 (2000 to 2500 ft/sec) at the specimen's leading edge. One-hour heating cycles were used with three minutes of air blast cooling. Results of testing at temperatures ranging from T sub max of 1850 to 2050 F indicate that coated B1900 has the longest oxidation life at all temperatures, followed by IN-100X-40WI-52, based on a weight change criterion. Coatings on nickel-base alloys provided more than twice the life of coating on cobalt-base alloys at comparable temperatures. The coatings with higher aluminum content and comparable thickness had longer lives. Silicon appeared to be a beneficial additive in the nickel-base alloy coatings for long term, low temperature life, but not for short term, high temperature performance. Chromium was identified in all coatings. Extrapolating the life results obtained to 1600 F, all the selected coatings on the four alloys would be protective for at least 10,000 hours
The Stellar Populations of the Carina Dwarf Spheroidal Galaxy: I. a New Color-Magnitude Diagram for the Giant and Horizontal Branches
We report on the first in a series of studies of the Carina dwarf spheroidal
galaxy, a nearby satellite of our Galaxy. Our two major results are: 1) precise
BI photometry (\sigma_{B-I} \simlt 0.05 for V \simlt 22) for 11,489 stars
in the Carina field, and 2) the detection of two, morphologically distinct,
horizontal branches, which confirms that star formation in Carina occurred in
two well-separated episodes. The old horizontal branch and RR Lyrae instability
strip belong to a > 10 Gyr stellar population, while the populous red-clump
horizontal branch belongs to an approximately 6 Gyr stellar population. We
derive a distance modulus for Carina from the apparent
magnitudes of the old horizontal branch and the tip of the red giant branch,
and discuss modifications to the previously estimated distance, total
magnitude, and stellar ages. Using the color of the red giant branch, we
estimate the metallicities of the younger and older populations to be [Fe/H] =
-2.0 and -2.2, respectively.Comment: 18 pages, 9 figures, uses AAS LaTex macros, PostScript figures
available through anonymous ftp, accepted for publication in the Astronomical
Journal, DAO-tsh94-
The HAL 9000 Space Operating System Real-Time Planning Engine Design and Operations Requirements
In support of future deep space manned missions, an autonomous/automated vehicle, providing crew autonomy and an autonomous response planning system, will be required due to the light time delays in communication. Vehicle capabilities as a whole must provide for tactical response to vehicle system failures and space environmental effects induced failures, for risk mitigation of permanent loss of communication with Earth, and for assured crew return capabilities. The complexity of human rated space systems and the limited crew sizes and crew skills mix drive the need for a robust autonomous capability on-board the vehicle. The HAL 9000 Space Operating System[2] designed for such missions and space craft includes the first distributed real-time planning / re-planning system. This paper will detail the software architecture of the multiple planning engine system, and the interface design for plan changes, approval and implementation that is performed autonomously. Operations scenarios will be defined for analysis of the planning engines operations and its requirements for nominal / off nominal activities. An assessment of the distributed realtime re-planning system, in the defined operations environment, will be provided as well as findings as it pertains to the vehicle, crew, and mission control requirements needed for implementation
Astrometry with MCAO: HST-GeMS proper motions in the globular cluster NGC 6681
Aims: for the first time the astrometric capabilities of the Multi-Conjugate
Adaptive Optics (MCAO) facility GeMS with the GSAOI camera on Gemini-South are
tested to quantify the accuracy in determining stellar proper motions in the
Galactic globular cluster NGC 6681. Methods: proper motions from HST/ACS for a
sample of its stars are already available, and this allows us to construct a
distortion-free reference at the epoch of GeMS observations that is used to
measure and correct the temporally changing distortions for each GeMS exposure.
In this way, we are able to compare the corrected GeMS images with a
first-epoch of HST/ACS images to recover the relative proper motion of the
Sagittarius dwarf spheroidal galaxy with respect to NGC 6681. Results: we find
this to be (\mu_{\alpha}cos\delta, \mu_{\delta}) = (4.09,-3.41) mas/yr, which
matches previous HST/ACS measurements with a very good accuracy of 0.03 mas/yr
and with a comparable precision (r.m.s of 0.43 mas/yr). Conclusions: this study
successfully demonstrates that high-quality proper motions can be measured for
quite large fields of view (85 arcsec X 85 arcsec) with MCAO-assisted,
ground-based cameras and provides a first, successful test of the performances
of GeMS on multi-epoch data.Comment: 5 pages, 4 figures. Accepted for publication by A&A Letter
A chemical trompe-l'\oe{}il: no iron spread in the globular cluster M22
We present the analysis of high-resolution spectra obtained with UVES and
UVES-FLAMES at the Very Large Telescope of 17 giants in the globular cluster
M22, a stellar system suspected to have an intrinsic spread in the iron
abundance. We find that when surface gravities are derived spectroscopically
(by imposing to obtain the same iron abundance from FeI and FeII lines) the
[Fe/H] distribution spans ~0.5 dex, according to previous analyses. However,
the gravities obtained in this way correspond to unrealistic low stellar masses
(0.1-0.5 Msun) for most of the surveyed giants. Instead, when photometric
gravities are adopted, the [FeII/H] distribution shows no evidence of spread at
variance with the [FeI/H] distribution. This difference has been recently
observed in other clusters and could be due to non-local thermodynamical
equilibrium effects driven by over-ionization mechanisms, that mainly affect
the neutral species (thus providing lower [FeI/H]) but leave [FeII/H]
unaltered. We confirm that the s-process elements show significant star-to-star
variations and their abundances appear to be correlated with the difference
between [FeI/H] and [FeII/H]. This puzzling finding suggests that the peculiar
chemical composition of some cluster stars may be related to effects able to
spuriously decrease [FeI/H]. We conclude that M22 is a globular cluster with no
evidence of intrinsic iron spread, ruling out that it has retained the
supernovae ejecta in its gravitational potential well.Comment: Accepted for publication to ApJ; 33 pages, 10 figures, 6 table
Towards Precision Photometry with Extremely Large Telescopes: the Double Subgiant Branch of NGC 1851
The Extremely Large Telescopes currently under construction have a collecting
area that is an order of magnitude larger than the present largest optical
telescopes. For seeing-limited observations the performance will scale as the
collecting area but, with the successful use of adaptive optics, for many
applications it will scale as (where is the diameter of the primary
mirror). Central to the success of the ELTs, therefore, is the successful use
of multi-conjugate adaptive optics (MCAO) that applies a high degree correction
over a field of view larger than the few arcseconds that limits classical
adaptive optics systems. In this letter, we report on the analysis of crowded
field images taken on the central region of the Galactic globular cluster NGC
1851 in band using GeMS at the Gemini South telescope, the only
science-grade MCAO system in operation. We use this cluster as a benchmark to
verify the ability to achieve precise near-infrared photometry by presenting
the deepest photometry in crowded fields ever obtained from the ground.
We construct a colour-magnitude diagram in combination with the F606W band from
HST/ACS. As well as detecting the "knee" in the lower main sequence at
, we also detect the double subgiant branch of NGC 1851, that
demonstrates the high photometric accuracy of GeMS in crowded fields.Comment: Accepted for publication in ApJL (3 Sep 2015). A version of the paper
with high-res images is available at
http://www.astro.uvic.ca/~alan/ms_arxiv_hr.pd
Structure and Dynamics of the Globular Cluster Palomar 13
We present Keck/DEIMOS spectroscopy and Canada-France-Hawaii Telescope/MegaCam photometry for the Milky Way globular cluster Palomar 13. We triple the number of spectroscopically confirmed members, including many repeat velocity measurements. Palomar 13 is the only known globular cluster with possible evidence for dark matter, based on a Keck/High Resolution Echelle Spectrometer 21 star velocity dispersion of Ï = 2.2 ± 0.4 km s^(â1). We reproduce this measurement, but demonstrate that it is inflated by unresolved binary stars. For our sample of 61 stars, the velocity dispersion is Ï = 0.7^(+0.6)_(â0.5) km s^(â1). Combining our DEIMOS data with literature values, our final velocity dispersion is Ï = 0.4^(+0.4)_( â0.3) km s^(â1). We determine a spectroscopic metallicity of [Fe/H] = â1.6 ± 0.1 dex, placing a 1Ï upper limit of Ï_([Fe/H]) ~ 0.2 dex on any internal metallicity spread. We determine Palomar 13's total luminosity to be M_V = â2.8 ± 0.4, making it among the least luminous known globular clusters. The photometric isophotes are regular out to the half-light radius and mildly irregular outside this radius. The outer surface brightness profile slope is shallower than typical globular clusters (ÎŁ α r^η, η = â2.8 ± 0.3). Thus at large radius, tidal debris is likely affecting the appearance of Palomar 13. Combining our luminosity with the intrinsic velocity dispersion, we find a dynamical mass of M_(1/2) = 1.3^(+2:7)_(â1.3) Ă 10^3 M_â and a mass-to-light ratio of M/L_V = 2.4^(+5.0)_(â2.4) M_â/L_â. Within our measurement errors, the mass-to-light ratio agrees with the theoretical predictions for a single stellar population. We conclude that, while there is some evidence for tidal stripping at large radius, the dynamical mass of Palomar 13 is consistent with its stellar mass and neither significant dark matter, nor extreme tidal heating, is required to explain the cluster dynamics
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