Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Envelope”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Personnel occupied woven envelope robot power

The Human Occupied Space Teleoperator (HOST) system currently under development utilizes a flexible tunnel/Stewart table structure to provide crew access to a pressurized manned work station or POD on the space station without extravehicular activity (EVA). The HOST structure facilitates moving a work station to multiple space station locations. The system has applications to orbiter docking, space station assembly, satellite servicing, space station maintenance, and logistics support. The conceptual systems design behind HOST is described in detail.

Source record↗
NASA NTRS1988

Theoretical studies of mass loss and shock phenomena in cool star envelopes

To show the difficulty of producing the blue-shifted emission of (O I) and (S II) from T Tauri stars directly in the stellar wind, an element of gas in a radially expanding stellar wind was followed as it cooled and recombined. Results indicate that T Tauri winds must be heated at large distances from the star to produce the (S II) emission. A shock between the wind and the disk is an attractive mechanism to produce this heating. When the theory is applied to a simple disk model, a number of predictions and implications are evident, for example, that some T Tauri stars eject mass near the equatorial plane. In a second study, spectral energy distributions of T Tauri stars were analyzed to place limits on the amount of accretion which might occur during the early phase of stellar evolution. The best match to H-alpha line profiles is for models in which the turbulent velocity dominates close to the star, while expansion dominates farther out. Such a model predicts, for instance, that a mass loss rate of 1/10,000,000 solar masses per year is required to account for the blue-shifted Na I absorption of some objects.

Hartmann, Lee↗
NASA NTRS1988

Operating envelopes of particle sizing instrumentation used for icing research

The Forward Scattering Spectrometer Probe and the Optical Array Probe are analyzed in terms of their ability to make accurate determinations of water droplet size distributions. Sources of counting and sizing errors are explained. The paper describes ways of identifying these errors and how they can affect measurement.

Hovenac, Edward A.↗
NASA NTRS1988

The equation of state for stellar envelopes. II - Algorithm and selected results

A free-energy-minimization method for computing the dissociation and ionization equilibrium of a multicomponent gas is discussed. The adopted free energy includes terms representing the translational free energy of atoms, ions, and molecules; the internal free energy of particles with excited states; the free energy of a partially degenerate electron gas; and the configurational free energy from shielded Coulomb interactions among charged particles. Internal partition functions are truncated using an occupation probability formalism that accounts for perturbations of bound states by both neutral and charged perturbers. The entire theory is analytical and differentiable to all orders, so it is possible to write explicit analytical formulas for all derivatives required in a Newton-Raphson iteration; these are presented to facilitate future work. Some representative results for both Saha and free-energy-minimization equilibria are presented for a hydrogen-helium plasma with N(He)/N(H) = 0.10. These illustrate nicely the phenomena of pressure dissociation and ionization, and also demonstrate vividly the importance of choosing a reliable cutoff procedure for internal partition functions.

Mihalas, Dimitri↗
NASA NTRS1988

The equation of state for stellar envelopes. III - Thermodynamic quantities

A method is described for deriving general expressions for all thermodynamic quantities of interest of a partially ionized multicomponent gas in terms of derivatives of the free energy. Explicit analytical formulas for all derivatives required in the evaluation of these quantities are given. Representative results for a hydrogen-helium mixture are shown.

Daeppen, Werner↗
NASA NTRS1988

Direct UV observations of the circumstellar envelope of alpha Orionis

Observations were made in the IUE LWP camera, low dispersion mode, with alpha Ori being offset various distances from the center of the Long Wavelength Large Aperture along its major axis. Signal was acquired at all offset positions and is comprised of unequal components of background/dark counts, telescope-scattered light, and scattered light emanating from the extended circumstellar shell. The star is known from optical and infrared observations to possess an extended, arc-minute sized, shell of cool material. Attempts to observe this shell with the IUE are described, although the deconvolution of the stellar signal from the telescope scattered light requires further calibration effort.

Stencel, R. E.↗
NASA NTRS1989

Slow expansion of the shell of the recurrent nova T Pyxidis and detection of a faint extended envelope

New H-alpha narrow-based CCD imaging of the recurrent nova T Pyxidis and the detection of a very faint, extended H-alpha halo surrounding the already known shell are reported. A forbidden O III image containing an emitting shell with a morphology different from that of the H-alpha shell is presented, and measurements of the H-alpha shell expansion are reported which rule out the 1966 eruption date for the shell origin, assuming uniform expansion. It is proposed that the observed shell consists of slowly moving, solar abundance ejecta which are photoionized.

Shara, Michael M.↗
NASA NTRS1989

Asymmetric envelope expansion of supernova 1987A

Analytic models for the spherically symmetric expansion of SN 1987 A are discussed. The SN model includes a bend in the expanded density profiles at a velocity of about 4000 km/s and employs an asymmetric initial density distribution or energy deposition on the assumption of radial flow. The calculations show that there is a tendency for the flow to become more spherical during the expansion phase, and that the sense of density asymmetry changes across the bend in the density profile.

Chevalier, Roger A.↗
NASA NTRS1990

The equation of state for stellar envelopes. IV - Thermodynamic quantities and selected ionization fractions for six elemental mixes

The free-energy minimization technique in the form developed in the preceding papers in this series is employed to evaluate thermodynamic quantities and ionization fractions on a fine temperature and density grid for six astrophysical mixtures of 15 elements. The mixtures range from that appropriate to super-metal-rich stars, through solar abundance, to that for extreme Population II objects. In this paper, the results for solar abundances are summarized in a form that is illustrative and which facilitates comparison with the results from other equation of state calculations.

Mihalas, Dimitri↗
NASA NTRS1990

The envelopes of RV Tauri stars. I - Linear polarimetric and spectroscopic observations

Simultaneous spectroscopic and polarimetric observations are presented for five RV Tauri stars and one RV Tauri-like semiregular variable. Time-dependent variations are found in the polarizations for four of the objects, as are wavelength-dependent variations in two and possibly three stars. In general, the largest polarizations and strongest wavelength dependences in the polarization occur immediately following primary light minima.

Nook, Mark A.↗
NASA NTRS1991

The shock process and light-element production in supernova envelopes

Detailed hydrodynamic modeling of the passage of supernova shocks through the hydrogen enevlopes of blue and red progenitor stars was carried out to explore the sensitivity to model conditions of light element production (specifically Li7 and B-11) which was noted by Dearborn, Schramm, Steigman and Truran (1989) (DSST). It is found that, for stellar models with M is less than or approximately 100 M solar mass, current state of the art supernova shocks do not produce significant light element yields by hydrodynamic processes alone. The dependence of this conclusion on stellar models and on shock strengths is explored. Preliminary implications for Galactic evolution of lithium are discussed, and it is suspected that intermediate mass red giant stars may be the most consistent production site for lithium.

Brown, Lawrence E.↗
NASA NTRS1991

The light curve and changes in the circumstellar envelope around IRC + 10216

This paper presents results of NIR photometric observations of IRC + 10216 star during the time interval 1965-1990, which were combined with measurements of the circumstellar shell surrounding IRC + 10216 by IR speckle interferometry. Using these data together with published observations, a very precise light curve was constructed, demonstrating the occurrence of long-term changes in the circumstellar shell surrounding IRC + 10216. The observations are discussed in the framework of two alternatives: (1) the occurrence of dynamical changes in the shell and (2) blocking of the direct light from the star to a portion of the shell by intervening material.

Dyck, H. M.↗
NASA NTRS1990

IRAS observations of extended dust envelopes around evolved stars

Deconvolved IRAS profiles, with resolution 2-3 time better than detector sizes 1.5 and 3 arcmin at 60 and 100 microns, are presented for a number of evolved stars with extended emission. These include VY UMa, Mu Cep, S Sct, U Hya, Y CVn, U Ant, alpha Ori, Y Pav, UU aur, IRC + 10216, RZ Sgr, and R Lyr. Simple models suggest that extended IRAS emission results from stars which greater mass loss rates in the past, rather than from stars with large current mass loss rates.

Hawkins, George↗
NASA NTRS1993

PG 0308 + 096 and PG 1026 + 002 - Two new short period binary stars resulting from common-envelope evolution

Ultraviolet spectroscopy, optical spectroscopy, and spectrophotometry have been used to study the excess UV stars PG 0308 + 096 and PG 1026 + 002. Both objects are short-period binary systems, each containing a DA white dwarf star and a dM star. Orbital periods of approximately 0.284 day for PG 0308 + 096, and aproximately 0.597 day for PG 1026, have been found by spectroscopic analysis of the H-alpha emission line. Ly-alpha and Balmer line profile fitting were used to estimate the mass of white dwarf stars; mass estimates for the dM stars are based on their spectral types. The orbital inclinations are derived from these masses, the periods, and amplitudes of the H-alpha radial velocity curves. The equivalent width of the H-alpha emission line, in each binary system, varies with the orbital phase in such a manner as to imply that it arises, in large part at least, from the hemisphere of the M star that faces the white dwarf star.

Saffer, Rex A.↗
NASA NTRS1993

The contribution of disks and envelopes to the millimeter continuum emission from very young low-mass stars

We investigate the question of disk formation during the protostar phase. We model the dust continuum emission from the dense cloud core using the cloud-collapse models of Terebey et al. (1984) and show that dust emission from the dense core is important when measured with large single-dish telescopes at 1.3 mm, but nearly negligible with interferometers at 2.7 mm. From published and new data, we conclude that massive disks are also seen toward a number of other sources including L1448 IRS 3, whose disk mass is estimated to be 0.5 solar mass. However, 1.3 mm data show that massive disks are relatively rare, occurring around perhaps 5 percent of young embedded stars. This implies that either massive disks occur briefly during the embedded phase or that relatively few young stars form massive disks. The median 1.3 mm flux density of IRAS-Dense cores in our sample is nearly the same as T Tauri stars in the sample of Beckwith et al. (1990). We conclude that the typical disk mass is not significantly higher during the embedded phase than during the later T Tauri phase.

Terebey, S.↗