Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Hartmann”

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 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Afterglows from gamma-ray bursters - A way to estimate the burster distance scale?

We calculate the strength of secondary gamma-ray signals that could be produced by positron annihilation in a medium surrounding a gamma-ray burster. We predict that such signals should be observable for a fairly wide range of conditions of this medium for bursts at both Galactic and cosmological distances. Furthermore, the time behavior of such signals together with the redshift measurement could be used to determine the distance scale to bursters.

Fencl, H. S.↗
NASA NTRS1993

Diffuse gamma-ray emission from pulsars in the Large Magellanic Cloud

We investigate the contribution of pulsars to the diffuse gamma-ray emission from the LMC. The pulsar birth rate in the LMC is a factor of about 10 lower than that of the Galaxy and the distance to pulsars in the LMC is about 5-10 times larger than to Galactic pulsars. The resulting total integrated photon flux from LMC pulsars is thus reduced by a factor of about 100 to 1000. However, the surface brightness is not reduced by the same amount because of the much smaller angular extent of the LMC in comparison to the diffuse glow from the Galactic plane. We show that gamma-ray emission due to pulsars born in the LMC could produce gamma-ray fluxes that are larger than the inverse Compton component from relativistic cosmic-ray electrons and a significant fraction of the extragalactic isotropic background or the diffuse Galactic background in that direction. The diffuse pulsar glow above 100 MeV should therefore be included in models of high-energy emission from the LMC. For a gamma-ray beaming fraction of order unity the detected emissions from the LMC constrain the pulsar birth rate to less than one per 50 yr. This limit is about one order of magnitude above the supernova rate inferred from the historic record or from the star-formation rate.

Hartmann, Dieter H.↗
NASA NTRS1992

Cosmic gamma-ray bursts from BATSE - Another great debate

The BATSE detectors aboard Compton Observatory record about one cosmic gamma-ray burst (GRB) per day. Preliminary data analysis shows a highly isotropic sky map and a nonuniform brightness distribution. Anisotropies expected from a Galactic neutron star population, the most frequently considered source model, did not emerge from the data. Taken at face value, the data seem to suggest a heliocentric solution of the GRB puzzle. The observed isotropy can be achieved if sources are either very near or extragalactic. Pop I neutron stars in the disk do not simultaneously fit sky and brightness distributions. A possibility are sources in an extended Galactic halo with scale length large enough to avoid strong anisotropies due to the solar offset from the Galactic center. If GRBs are located in an extended halo we ask whether the neutron star paradigm can survive. We show that the recently discovered high velocity radio pulsars may provide a natural source population for GRBs. If these pulsars formed in the halo, as suggested by the radio data, the possibility arises that GRBs and high velocity pulsars are two related phenomena that provide observational evidence of the dark Galactic corona. We also discuss cosmological redshift constraints that follow from the observed brightness distribution.

Hartmann, Dieter H.↗
NASA NTRS1992

The ionization of gamma-ray burst environments

If a gamma-ray burst (GRB) occurs in a neutral medium, ionizing radiation associated with the burst will flash-photoionize a region surrounding the source. Detection of the line emission from this ionized region can constrain the flux of ionizing radiation accompanying the GRB and the density and ionization state of the environment surrounding the burst source. If the medium is sufficiently dense then the ionized region will recombine and fade on human timescales, aiding in the detection of the burst location and further constraining the density of the medium. However, dust within the dense medium may attenuate the line flux; the burst is unlikely to melt the dust along the line-of-sight. Astronomically interesting flux limits can be established by observations with reasonable integration times on a major telescope (e.g., 3 meter or larger).

Band, D. L.↗
NASA NTRS1993

The infrared continuum of pure water vapor - Calculations and high-temperature measurements

Results of experimental and theoretical studies of medium infrared absorption by pure water vapor are reported. The experiments were performed in the 1900-2600/cm and 3900-4600/cm regions for temperatures and pressures of 500-900 K and 0-70 atm, respectively. The results are consistent with data in the literature and enable the determination of continuous absorption parameters.

Hartmann, J. M.↗
NASA NTRS1993

Threshold effects on V/V(max) for gamma-ray bursts

The interpretation of the observed gamma-ray burst V/V(max) statistic in terms of spatial distributions is model-dependent. Detection of gamma-ray bursts requires the counting rate in one or more detectors to exceed a threshold C(lim) determined from a time-dependent background rate B(t). The sampling depth of the burst detector is thus time-dependent, and, if burst sources are nonuniform in space, the observed V/V(max) distribution will be affected by B(t). We demonstrate this effect with a simple geometric distribution of standard candles and argue that V/V(max) statistic without information on threshold variations is insufficient for rigorous data analysis. Peak count rates and threshold values must be given separately for all events in order to facilitate a meaningful comparison of observations with theoretical distribution models.

Hartmann, D. H.↗
NASA NTRS1993

Inhomogeneous chemical evolution of the Galactic disk

We present analytical models for inhomogeneous chemical evolution (ICE) of systems in which the star formation history resembles a series of bursts, localized in space and/or time, with intermittent periods of remixing. The additional parameter of this model is the metallicity increment of bursting subsystems, but this parameter is constrained by the spread in the age-metallicity relation. We apply this model to the solar annulus in the Galactic disk and show that ICE models yield an improved fit to the observed shape of the stellar abundance distribution function (ADF). The G-dwarf problem can be alleviated with ICE models, but infall of metal poor gas and/or some preenrichment of the disk during the epoch of protogalactic evolution is still required to explain the paucity of low-metallicity dwarfs. ICE models also suggest an explanation of the reduced frequency of metal-rich G-dwarfs relative to the predictions of the simple model. It does not seem likely that chemical evolution of the solar annulus proceeded in a medium that was well-mixed at all times.

Malinie, Guy↗
NASA NTRS1993

The seasonal cycle of low stratiform clouds

The seasonal cycle of low stratiform clouds is studied using data from surface-based cloud climatologies. The impact of low clouds on the radiation budget is illustrated by comparison of data from the Earth Radiation Budget Experiment with the cloud climatologies. Ten regions of active stratocumulus convection are identified. These regions fall into four categories: subtropical marine, midlatitude marine, Arctic stratus, and Chinese stratus. With the exception of the Chinese region, all the regions with high amounts of stratus clouds are over the oceans. In all regions except the Arctic, the season of maximum stratus corresponds to the season of greatest lower-troposphere static stability. Interannual variations in stratus cloud amount also are related to changes in static stability. A linear analysis indicates that a 6 percent increase in stratus fractional area coverage is associated with each 1 C increase in static stability. Over midlatitude oceans, sky-obscuring fog is a large component of the summertime stratus amount. The amount of fog appears to be related to warm advection across sharp gradients of SST.

Klein, Stephen A.↗
NASA NTRS1993

The embedded young stars in the Taurus-Auriga molecular cloud. I - Models for spectral energy distributions

We describe radiative transfer calculations of infalling, dusty envelopes surrounding pre-main-sequence stars and use these models to derive physical properties for a sample of 21 heavily reddened young stars in the Taurus-Auriga molecular cloud. The density distributions needed to match the FIR peaks in the spectral energy distributions of these embedded sources suggest mass infall rates similar to those predicted for simple thermally supported clouds with temperatures about 10 K. Unless the dust opacities are badly in error, our models require substantial departures from spherical symmetry in the envelopes of all sources. These flattened envelopes may be produced by a combination of rotation and cavities excavated by bipolar flows. The rotating infall models of Terebey et al. (1984) models indicate a centrifugal radius of about 70 AU for many objects if rotation is the only important physical effect, and this radius is reasonably consistent with typical estimates for the sizes of circumstellar disks around T Tauri stars.

Kenyon, Scott J.↗
NASA NTRS1993

The embedded young stars in the Taurus-Auriga molecular cloud. II - Models for scattered light images

We describe NIR imaging observations of embedded young stars in the Taurus-Auriga molecular cloud. We find a large range in J-K and H-K colors for these class I sources. The bluest objects have colors similar to the reddest T Tauri stars in the cloud; redder objects lie slightly above the reddening line for standard ISM dust and have apparent K extinctions of up to 5 mag. Most of these sources also show extended NIR emission on scales of 10-20 arcsec which corresponds to linear sizes of 1500-3000 AU. The NIR colors and nebular morphologies for this sample and the magnitude of linear polarization in several sources suggest scattered light produces most of the NIR emission in these objects. We present modeling results that suggest mass infall rates that agree with predictions for cold clouds and are generally consistent with rates estimated from radiative equilibrium models. For reasonable dust grain parameters, the range of colors and extinctions require flattened density distributions with polar cavities evacuated by bipolar outflows. These results support the idea that infall and outflow occur simultaneously in deeply embedded bipolar outflow sources. The data also indicate fairly large centrifugal radii and large inclinations to the rotational axis for a typical source.

Kenyon, Scott J.↗
NASA NTRS1993

On Al-26 and other short-lived interstellar radioactivity

Several authors have shown that massive stars exploding at a rate of about three per century can account for a large portion, if not all, of the observed interstellar Al-26. In a separate argument using models of Galactic chemical evolution, Clayton (1984) showed that the Al-26/Al-27 production ratio was not large enough to maintain enough Al-26 in the Galactic disk gas of about 10 exp 10 solar masses having solar composition. We present a resolution of those conflicting arguments. A past history of Galactic infall growing the Galactic disk so dilutes the stable Al-27 concentration that the two approaches can be brought into near agreement. If massive stars dominate the production of Al-26, we suggest that the apparent shortfall of their Al-26/Al-27 yield ratio is to be interpreted as evidence for significant growth of the Galactic disk. We also discuss the implications of these arguments for other extinct radioactivities in meteorites, using I-129 and Sm-146 as examples.

Clayton, Donald D.↗
NASA NTRS1931

Comparison of weights of 17ST and steel tubular structural members used in aircraft construction

Although the strong aluminum alloys have proved themselves to be very efficient in aircraft construction there is a growing competition from the high-strength steels for certain parts, especially for tubular members. This tendency is being reflected in research work carried on at the Bureau of Standards. This study will be based largely on data given in Technical Note No. 307 of the NACA.

Hartmann, E C↗
NASA NTRS1942

The direct-stress fatigue strength of 17S-T aluminum alloy throughout the range from 1/2 to 500,000,000 cycles of stress

Fatigue-test were conducted on six specimens made from 3/4-inch-diameter 17S-T rolled-and-drawn rod for the purpose of obtaining additional data on the fatigue life of the material at stresses up to the static strength. The specimens were tested in direct tension using a stress range from zero to a maximum in tension. A static testing machine was used to apply repeated loads in the case of the first three specimens; the other three specimens were tested in a direct tension-compression fatigue machine. The direct-stress fatigue curve obtained for the material indicates that, in the range of stresses above about two-thirds the tensile strength, the fatigue strength is higher than might be expected by simply extrapolating the ordinary curve of stress plotted against the number of cycles determined at lower stresses.

Hartmann, E C↗
NASA NTRS1942

Summary of Results of Tests Made by Aluminum Research Laboratories of Spot-welded Joints and Structural Elements

Available information concerning spot welding as a means of joining aluminum-alloy parts has been summarized and comparisons have been made of the relative merits of spot-welded and riveted aluminum-alloy structural elements. The results indicated that spot welding was as satisfactory as riveting insofar as resistance to static loads is concerned. Spot welds showed slightly lower resistance to impact loads but definitely lower resistance to repeated loads than rivets.

HARTMANN E C↗