2,151 research outputs found
Rewriteable optical disk recorder development
A NASA program to develop a high performance (high rate, high capability) rewriteable optical disk recorder for spaceflight applications is presented. An expandable, adaptable system concept is proposed based on disk Drive modules and a modular Controller. Drive performance goals are 10 gigabyte capacity are up to 1.8 gigabits per second rate with concurrent I/O, synchronous data transfer, and 2 to 5 years operating life in orbit. Technology developments, design concepts, current status, and future plans are presented
The Local Ly-alpha Forest IV: STIS G140M Spectra and Results on the Distribution and Baryon Content of HI Absorbers
We present HST STIS/G140M spectra of 15 extragalactic targets, which we
combine with GHRS/G160M data to examine the statistical properties of the low-z
Ly-alpha forest. We evaluate the physical properties of these Ly-alpha
absorbers and compare them to their high-z counterparts. We determine that the
warm, photoionized IGM contains 29+/-4% of the total baryon inventory at z = 0.
We derive the distribution in column density, N_HI^(1.65+/-0.07) for 12.5 < log
[N_HI] 14.5. The slowing
of the number density evolution of high-W Ly-alpha clouds is not as great as
previously measured, and the break to slower evolution may occur later than
previously suggested (z~1.0 rather than 1.6). We find a 7.2sigma excess in the
two-point correlation function (TPCF) of Ly-alpha absorbers for velocity
separations less than 260 km/s, which is exclusively due to the higher column
density clouds. From our previous result that higher column density Ly-alpha
clouds cluster more strongly with galaxies, this TPCF suggests a physical
difference between the higher and lower column density clouds in our sample.Comment: 71 pages, 6 tables, 26 EPS figures, to appear in ApJ Supplemen
The extent of the local hi halo
Forty-five high-latitude, OB stars have been observed in the Ly alpha and 21 cm lines of HI in an effort to map out the vertical distribution and extent of the local HI halo. The 25 stars for which a reliable HI colum density can be obtained from Ly alpha lie between 60 and 3100 pc from the plane. The principal result is that the total column density of HI at z 1 kpc is, on the average, 5 + or - 3 x 10 the 19th power/sq cm, or 15% of the total sub HI. At relatively low z the data toward some stars suggest a low effective scale height and fairly high average foreground density, while toward others the effective scale height is large and the average density is low. This can be understood as the result of irregularities in the interstellar medium. A model with half of the HI mass in clouds having radii of a few pc and a Gaussian vertical distribution with sigma sub 2 = 135 pc, and half of the mass in an exponential component with a scale height of 500 pc, gives a satisfactory fit to the data. The technique of comparing Ly alpha and 21 cm column densities is also used to discuss the problem of estimating the distance to several possibly subluminous stars
Geometry-Dependent Oxygen Diffusion Flux and Limiting Current Density of the Cathode in a Cathode-Supported Solid Oxide Fuel Cell
An analysis is performed on the diffusion of oxygen through tubular porous cathode substrates having several different geometries. It is shown that the flux of oxygen as it diffuses through these different substrate geometries can be explicitly expressed by a general analytical form with a unique geometric factor for each type of substrate geometry. Experimental measurements of the geometry-independent term, oxygen diffusivity, were conducted for two representative geometries: cylindrical and triangular tubes. These measurements show good agreement between samples with similar porosities and thus favorably support the oxygen flux equations presented. Formulations for the limiting current density were also derived directly from the oxygen flux equations. These are similarly expressed by a general analytical form with the same unique geometric factor depending on the particular substrate. Finally, three representative geometries, cylindrical, triangular, and square tubes, are used to illustrate the influence of the various dimensional parameters on the limiting current density
Highly Ionized High-Velocity Clouds toward PKS 2155-304 and Markarian 509
To gain insight into four highly ionized high-velocity clouds (HVCs)
discovered by Sembach et al. (1999), we have analyzed data from the Hubble
Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer (FUSE) for the
PKS 2155-304 and Mrk 509 sight lines. We measure strong absorption in OVI and
column densities of multiple ionization stages of silicon (SiII/III/IV) and
carbon (CII/III/IV). We interpret this ionization pattern as a multiphase
medium that contains both collisionally ionized and photoionized gas. Toward
PKS 2155-304, for HVCs at -140 and -270 km/s, respectively, we measure
logN(OVI)=13.80+/-0.03 and log N(OVI)=13.56+/-0.06; from Lyman series
absorption, we find log N(HI)=16.37^(+0.22)_(-0.14) and 15.23^(+0.38)_(-0.22).
The presence of high-velocity OVI spread over a broad (100 km/s) profile,
together with large amounts of low-ionization species, is difficult to
reconcile with the low densities, n=5x10^(-6) cm^(-3), in the
collisional/photoionization models of Nicastro et al. (2002), although the HVCs
show a similar relation in N(SiIV)/N(CIV) versus N(CII)/N(CIV) as high-z
intergalactic clouds. Our results suggest that the high-velocity OVI in these
absorbers do not necessarily trace the WHIM, but instead may trace HVCs with
low total hydrogen column density. We propose that the broad high-velocity OVI
absorption arises from shock ionization, at bowshock interfaces produced from
infalling clumps of gas with velocity shear. The similar ratios of high ions
for HVC Complex C and these highly ionized HVCs suggest a common production
mechanism in the Galactic halo.Comment: 38 pages, including 10 figures. ApJ, 10 April, 2004. Replaced with
accepted versio
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