7,665 research outputs found
Software requirements: Guidance and control software development specification
The software requirements for an implementation of Guidance and Control Software (GCS) are specified. The purpose of the GCS is to provide guidance and engine control to a planetary landing vehicle during its terminal descent onto a planetary surface and to communicate sensory information about that vehicle and its descent to some receiving device. The specification was developed using the structured analysis for real time system specification methodology by Hatley and Pirbhai and was based on a simulation program used to study the probability of success of the 1976 Viking Lander missions to Mars. Three versions of GCS are being generated for use in software error studies
Wide Complex Tachycardia in a Critically Ill Patient:
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72775/1/j.1540-8167.1997.tb01025.x.pd
An Age Difference of 2 Gyr between a Metal-Rich and a Metal-Poor Globular Cluster
Globular clusters trace the formation history of the spheroidal components of
both our Galaxy and others, which represent the bulk of star formation over the
history of the universe. They also exhibit a range of metallicities, with
metal-poor clusters dominating the stellar halo of the Galaxy, and higher
metallicity clusters found within the inner Galaxy, associated with the stellar
bulge, or the thick disk. Age differences between these clusters can indicate
the sequence in which the components of the Galaxy formed, and in particular
which clusters were formed outside the Galaxy and later swallowed along with
their original host galaxies, and which were formed in situ. Here we present an
age determination of the metal-rich globular cluster 47 Tucanae by fitting the
properties of the cluster white dwarf population, which implies an absolute age
of 9.9 (0.7) Gyr at 95% confidence. This is about 2.0 Gyr younger than inferred
for the metal-poor cluster NGC 6397 from the same models, and provides
quantitative evidence that metal-rich clusters like 47 Tucanae formed later
than the metal-poor halo clusters like NGC 6397.Comment: Main Article: 10 pages, 4 figures; Supplementary Info 15 pages, 5
figures. Nature, Aug 1, 201
An Empirical Measure of the Rate of White Dwarf Cooling in 47 Tucanae
We present an empirical determination of the white dwarf cooling sequence in
the globular cluster 47 Tucanae. Using spectral models, we determine
temperatures for 887 objects from Wide Field Camera 3 data, as well as 292
objects from data taken with the Advanced Camera for Surveys. We make the
assumption that the rate of white dwarf formation in the cluster is constant.
Stellar evolution models are then used to determine the rate at which objects
are leaving the main sequence, which must be the same as the rate at which
objects are arriving on the white dwarf sequence in our field. The result is an
empirically derived relation between temperature () and time () on
the white dwarf cooling sequence. Comparing this result to theoretical cooling
models, we find general agreement with the expected slopes between 20,000K and
30,000K and between 6,000K and 20,000K, but the transition to the Mestel
cooling rate of is found to occur at hotter
temperatures, and more abruptly than is predicted by any of these models.Comment: 10 pages, 16 figures, accepted for publication in Ap
Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low Mass White Dwarfs
We present the first detailed study of the properties (temperatures,
gravities, and masses) of the NGC 6791 white dwarf population. This unique
stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~
0.4) open clusters in our Galaxy, and has a color-magnitude diagram (CMD) that
exhibits both a red giant clump and a much hotter extreme horizontal branch.
Fitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS
spectra, suggests that most of these stars are undermassive, = 0.43 +/-
0.06 Msun, and therefore could not have formed from canonical stellar evolution
involving the helium flash at the tip of the red giant branch. We show that at
least 40% of NGC 6791's evolved stars must have lost enough mass on the red
giant branch to avoid the flash, and therefore did not convert helium into
carbon-oxygen in their core. Such increased mass loss in the evolution of the
progenitors of these stars is consistent with the presence of the extreme
horizontal branch in the CMD. This unique stellar evolutionary channel also
naturally explains the recent finding of a very young age (2.4 Gyr) for NGC
6791 from white dwarf cooling theory; helium core white dwarfs in this cluster
will cool ~3 times slower than carbon-oxygen core stars and therefore the
corrected white dwarf cooling age is in fact ~7 Gyr, consistent with the well
measured main-sequence turnoff age. These results provide direct empirical
evidence that mass loss is much more efficient in high metallicity environments
and therefore may be critical in interpreting the ultraviolet upturn in
elliptical galaxies.Comment: 15 pages, 9 figures, 2 tables. Accepted for publication in Astrophys.
J. Very minor changes from first versio
The Space Motion of the Globular Cluster NGC 6397
As a by-product of high-precision, ultra-deep stellar photometry in the
Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able
to measure a large population of background galaxies whose images are nearly
point-like. These provide an extragalactic reference frame of unprecedented
accuracy, relative to which we measure the most accurate absolute proper motion
ever determined for a globular cluster. We find mu_alpha = 3.56 +/- 0.04 mas/yr
and mu_delta = -17.34 +/- 0.04 mas/yr. We note that the formal statistical
errors quoted for the proper motion of NGC 6397 do not include possible
unavoidable sources of systematic errors, such as cluster rotation. These are
very unlikely to exceed a few percent. We use this new proper motion to
calculate NGC 6397's UVW space velocity and its orbit around the Milky Way, and
find that the cluster has made frequent passages through the Galactic disk.Comment: 5 pages including 3 figures, accepted for publication in the
Astrophysical Journal Letters. Very minor changes in V2. typos fixe
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