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At least 1,261 records · Page 70
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DOE OSTI2022

A Rapid Ionization Change in the Nebular-phase Spectra of the Type Ia SN 2011fe

Abstract We present three new spectra of the nearby Type Ia supernova (SN Ia) 2011fe covering ≈480–850 days after maximum light and show that the ejecta undergoes a rapid ionization shift at ∼500 days after explosion. The prominent Fe iii emission lines at ≈4600 Å are replaced with Fe i +Fe ii blends at ∼4400 Å and ∼5400 Å. The ≈7300 Å feature, which is produced by [Fe ii ]+[Ni ii ] at ≲400 days after explosion, is replaced by broad (≈±15,000 km s −1 ) symmetric [Ca ii ] emission. Models predict this ionization transition occurring ∼100 days later than what is observed, which we attribute to clumping in the ejecta. Finally, we use the nebular-phase spectra to test several proposed progenitor scenarios for SN 2011fe. Nondetections of H and He exclude nearby nondegenerate companions, [O i ] nondetections disfavor the violent merger of two white dwarfs, and the symmetric emission-line profiles favor a symmetric explosion.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1975

On the effect of timing errors in run length codes

Many redundancy removal algorithms employ some sort of run length code. Blocks of timing words are coded with synchronization words inserted between blocks. The probability of incorrectly reconstructing a sample because of a channel error in the timing data is a monotonically nondecreasing function of time since the last synchronization word. In this paper we compute the 'probability that the accumulated magnitude of timing errors equal zero' as a function of time since the last synchronization word for a zero-order predictor (ZOP). The result is valid for any data source that can be modeled by a first-order Markov chain and any digital channel that can be modeled by a channel transition matrix. An example is presented.

Wilkins, L. C.↗
NASA NTRS1979

Cryostatic influence of Formvar coatings on sizing of the stabilizer in superconductive NbTi-Cu - Conductor bundle effects

In continuation of single-conductor studies, quench onset data of quasi-steady operation are reported for a Formvar-coated composite (Nb48Ti/Cu) in bundle geometry (conductor thickness about 0.2 cm, approximately square cross section). Overall thermal conductances of the heater simulation technique applied are of the order 0.1 W/(sq cm-K), comparable to single-specimen results, around 4 K and below the lambda transition in agreement with model predictions. Considerable deterioration, however, occurs below 3 K and down to the lambda temperature during operation in near-saturated He I. Consequences for magnets of intermediate energy density are discussed with emphasis on the modifications necessitated by coatings.

Caspi, S.↗
NASA NTRS1980

Spatially resolved methane band photometry of Jupiter. III - Cloud vertical structures for several axisymmetric bands and the Great Red Spot

The paper presents cloud structure models for Jupiter's Great Red Spot, Equatorial and North Tropical Zones, North and South Temperate Zones, and North and South Polar Regions. The models are based on images of Jupiter in three methane bands and nearby continuum; radiative transfer calculations include multiple scattering and absorption from three aerosol layers. The model results include the transition in the upper-cloud altitude to 3 km lower altitude from the tropical zones to temperate zones and polar regions, a N/S asymmetry in cloud thickness in the tropical and temperature zones, and the presence of aerosols up to about 0.3 bar in the Great Red Spot and Equatorial Zone. It is concluded that polarization data are sensitive to aerosols in and above the upper cloud layer but insensitive to deeper cloud structure.

West, R. A.↗
NASA NTRS1982

Model chromospheres of RS CVn stars - Balmer line profiles in Gamma Andromedae

Two models have been constructed for the chromosphere of the RS CVn star Gamma And, one with a low-pressure transition zone, and one with a high pressure transition zone. The high pressure model predicts an H alpha line profile which agrees fairly well with high resolution observations of Gamma And, without the need to include nonthermal line broadening. The low pressure model predicts an H alpha profile which agrees very well with the observations, after application of macroturbulent broadening with a gaussian velocity parameter of about 30 km/sec. Methods which could distinguish between the two alternatives are discussed, and it is suggested that if the low-pressure model is valid, the large macroturbulence may be associated with unstable magnetic flux loops in the atmosphere.

Mullan, D. J.↗
NASA NTRS1983

S II and S III branching ratios in the 600- to 1200-A interval

Branching ratios are presented of singly and doubly ionized sulfur EUV emissions. They are determined by measuring the relative photon intensities of each of the branching components. For several transitions in S II for which mean lifetimes have been measured with fast-beam spectroscopy, the data presented here are used to determine transition probabilities. The S II transitions originate from the 2P, 4s-prime 2D, and 4s 2P terms and terminate on the metastable states of the ion. The S III transitions originate from the 3d 3D0, 4s 3P0, 3p3 3S0, 4s 1P0, and 3s3p3 1P0 terms and terminate on the metastable and ground ionic states. The results for S III include branching ratios involving intercombination transitions that affect ongoing modeling of the energy budget of the Io plasma torus.

Morrison, M. D.↗
NASA NTRS1985

Stellar evolution at high mass with convective core overshooting

The transition from stellar evolution models with no convective core overshooting (CCO) at all to models in which homogeneous mixing due to CCO reaches far beyond the formal convective core boundary is systematically explored. Overshooting is parameterized in terms of the ratio d/H(p), where d is the distance of convective overshoot beyond the formal convective core boundary and H(p) is the local pressure scale height. It is concluded that CCO in very massive main sequence stars produces a great expansion of the stellar envelope if d/H(p) is large but not excessively large. CCO does not entirely suppress convective instability above the overshoot zone in the envelopes of main sequence stars more massive than about 15 solar masses. A general comparison of theoretically constructed isochrones for young stars with observed main sequence turnups indicates that the observed turnups are longer, brighter, and cooler at the tip than those expected on thfe basis of standard evolutionary theory.

Stothers, R. B.↗
NASA NTRS1985

A review of some approximate methods used in aerodynamic heating analyses

It is pointed out that preliminary design and optimization studies for new aerospace vehicles require techniques which can calculate aerodynamic heating rates accurately and efficiently. The method employed to calculate the flow field depends to a large extent on the shape of the vehicle, Mach number, Reynolds number, and Knudsen number. In the case of the aero-assisted orbital transfer vehicle (AOTV), a substantial portion of the flight will be in the transitional regime between continuum and free molecule flow. The present paper discusses some approximate methods which have been used to calculate heating rates on high-speed vehicles. Attention is given to the stagnation point and leading edges, the downstream region, the axisymmetric analog, laminar and turbulent heating rates, transition heating rates, gas models, and three-dimensional applications.

Dejarnette, F. R.↗
NASA NTRS1986

The Orbital Acceleration Research Experiment

The hardware and software of NASA's proposed Orbital Acceleration Research Experiment (OARE) are described. The OARE is to provide aerodynamic acceleration measurements along the Orbiter's principal axis in the free-molecular flow-flight regime at orbital attitude and in the transition regime during reentry. Models considering the effects of electromagnetic effects, solar radiation pressure, orbiter mass attraction, gravity gradient, orbital centripetal acceleration, out-of-orbital-plane effects, orbiter angular velocity, structural noise, mass expulsion signal sources, crew motion, and bias on acceleration are examined. The experiment contains an electrostatically balanced cylindrical proofmass accelerometer sensor with three orthogonal sensing axis outputs. The components and functions of the experimental calibration system and signal processor and control subsystem are analyzed. The development of the OARE software is discussed. The experimental equipment will be enclosed in a cover assembly that will be mounted in the Orbiter close to the center of gravity.

Blanchard, R. C.↗
NASA NTRS1988

Extreme ultraviolet quantum efficiency of opaque alkali halide photocathodes on microchannel plates

Comprehensive measurements are presented for the quantum detection efficiency (QDE) of the microchannel plate materials CsI, KBr, KCl, and MgF2, over the 44-1800 A wavelength range. QDEs in excess of 40 percent are achieved by several materials in specific wavelength regions of the EUV. Structure is noted in the wavelength dependence of the QDE that is directly related to the valence-band/conduction-band gap energy and the onset of atomic-like resonant transitions. A simple photocathode model allows interpretation of these features, together with the QDE efficiency variation, as a function of illumination angle.

Siegmund, O. H. W.↗
NASA NTRS1989

Investigation of CAST-10-2/DOA 2 airfoil in NAE high Reynolds number two-dimensional test facility

A common airfoil model with the CAST 10-2/DOA-2 profile and 228 mm (9 inches) chord length was tested. The tests performed in NAE covered the Mach numbers from 0.3 to 0.8 and Reynolds numbers from 10 to 30 million. The model was tested with transition free and with transition fixed at 5 percent chord for both the upper and the lower surfaces. The data obtained were analyzed for the effects of Reynolds number, transition fixing and Mach number. The role of the boundary layer on the displacement effect, the interaction with the shock wave and the trailing edge separation are examined. The results are summarized as follows: (1) the airfoil performance depends strongly on Reynolds number and transition fixing; (2) with transition fixed, the aerodynamic quantities such as lift, pitching moment and drag show a monotonic variation with Reynolds number; (3) with transition free, the aerodynamic quantities vary less regularly with Reynolds number and a slight parametric dependency is shown. The weak dependency is due to the compensatory effect of the forward shift of the transition position and the thinning of the turbulent boundary layer as Reynolds number increases; (4) the shock Mach number and the shock position are weakly dependent on Reynolds number; and (5) the long extent of the laminar boundary layer at transonic speeds reduces the drag appreciably at low Reynolds numbers. The drag bucket around the design Mach number can be observed below Reynolds number 15 million.

Chan, Y. Y.↗
NASA NTRS1990

Self-organized criticality in sandpiles - Nature of the critical phenomenon

The scaling behavior of sandpile models is investigated analytically. First, it is shown that sandpile models contain a set of domain walls, referred to as troughs, which bound regions that can experience avalanches. It is further shown that the dynamics of the troughs is governed by a simple set of rules involving birth, death, and coalescence events. A simple trough model is then introduced, and it is proved that the model has a phase transition with the density of the troughs as an order parameter and that, in the thermodynamic limit, the trough density goes to zero at the transition point. Finally, it is shown that the observed scaling behavior is a consequence of finite-size effects.

Carlson, J. M.↗
NASA NTRS1991

A novel boundary layer sensor utilizing domain switching in ferroelectric liquid crystals

This paper describes the design and the principles of operation of a novel sensor for the optical detection of a shear stress field induced by air or gas flow on a rigid surface. The detection relies on the effects of shear-induced optical switching in ferroelectric liquid crystals. It is shown that the method overcomes many of the limitations of similar measuring techniques including those using cholesteric liquid crystals. The present method offers a preferred alternative for flow visualization and skin friction measurements in wind-tunnel experiments on laminar boundary layer transition investigations. A theoretical model for the optical response to shear stress is presented together with a schematic diagram of the experimental setup.

Parmar, D. S.↗