8,493 research outputs found
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
An empirical mass-loss law for Population II giants from the Spitzer-IRAC survey of Galactic globular clusters
The main aim of the present work is to derive an empirical mass-loss (ML) law
for Population II stars in first and second ascent red giant branches. We used
the Spitzer InfraRed Array Camera (IRAC) photometry obtained in the 3.6-8
micron range of a carefully chosen sample of 15 Galactic globular clusters
spanning the entire metallicity range and sampling the vast zoology of
horizontal branch (HB) morphologies. We complemented the IRAC photometry with
near-infrared data to build suitable color-magnitude and color-color diagrams
and identify mass-losing giant stars. We find that while the majority of stars
show colors typical of cool giants, some stars show an excess of mid-infrared
light that is larger than expected from their photospheric emission and that is
plausibly due to dust formation in mass flowing from them. For these stars, we
estimate dust and total (gas + dust) ML rates and timescales. We finally
calibrate an empirical ML law for Population II red and asymptotic giant branch
stars with varying metallicity. We find that at a given red giant branch
luminosity only a fraction of the stars are losing mass. From this, we conclude
that ML is episodic and is active only a fraction of the time, which we define
as the duty cycle. The fraction of mass-losing stars increases by increasing
the stellar luminosity and metallicity. The ML rate, as estimated from
reasonable assumptions for the gas-to-dust ratio and expansion velocity,
depends on metallicity and slowly increases with decreasing metallicity. In
contrast, the duty cycle increases with increasing metallicity, with the net
result that total ML increases moderately with increasing metallicity, about
0.1 Msun every dex in [Fe/H]. For Population II asymptotic giant branch stars,
we estimate a total ML of <0.1 Msun, nearly constant with varying metallicity.Comment: 17 pages, 9 figures, in press on A&
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
IUE observations of the clusters of the Magellanic Clouds
IUE observations of 17 clusters in the Large Magellanic Cloud are presented. The derived ultraviolet colors become progressively redder as the cluster's classification in the scheme of Searle, Wilkinson, and Bagnuolo increases. With the aid of theoretical models of the integrated light of coeval clusters of varying ages, it is shown that the SWB classification is one of increasing age accompanied by decreasing metallicity toward later SWB classes. The behavior of the UV absorption line features in the integrated light of the LMC clusters supports the contention
Notes on Visit to Moore School, University of Pennsylvania on September 9, 1947
Memorandum on a visit to Moore School, University of Pennsylvania on September 9, 1947. Topics include the purpose of the visit, magnetic recording, and mercury delay lines
Conference On Vacuum-tube Problems At Sylvania - Emporium
Memorandum on a conference held at the Sylvania plant in Emporium, Pa., October 6, 1949, to discuss problems relating to the design and manufacture of vacuum tubes for use in electronic commuters
The Carnegie Hubble Program: The Distance and Structure of the SMC as Revealed by Mid-infrared Observations of Cepheids
Using Spitzer observations of classical Cepheids we have measured the true
average distance modulus of the SMC to be mag (corresponding to kpc), which is
mag more distant than the LMC. This is in agreement with previous results from
Cepheid observations, as well as with measurements from other indicators such
as RR Lyrae stars and the tip of the red giant branch.
Utilizing the properties of the mid--infrared Leavitt Law we measured precise
distances to individual Cepheids in the SMC, and have confirmed that the galaxy
is tilted and elongated such that its eastern side is up to 20 kpc closer than
its western side. This is in agreement with the results from red clump stars
and dynamical simulations of the Magellanic Clouds and Stream.Comment: Accepted for publication in ApJ. 38 Pages, 11 figures. Figure 9 is
interactive. Spitzer photometry for all Cepheids available as online tabl
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