Engineering topics

Jura, M.

Publications and source records attributed to Jura, M..

At least 73 records · Page 4

NASA NTRS1983

The dust around the carbon star IRC +10216

Mass loss from late-type stars is important both in the evolution of these objects and for replenishment of the interstellar medium with processed material. Because the outflows are cool and dense, grains form. The present investigation is concerned with the nature of the extinction of the grains in the carbon star IRC+10216. The basic method employed involves the study of the extinction as a function of wavelength around IRC+10216. By a comparison with different models, it is then possible to infer which grain model is more likely to be correct. It is concluded that the dust around IRC+10216 is composed of relatively small grains. The grains are either amorphous carbon or graphite similar to that described by Mezger et al. (1982), but not like the graphite described by Mitchell and Robinson (1980). In particular, the grains around IRC + 10216 are relatively efficient emitters in the infrared. If this result is true of carbon stars in general, it means that the carbon grains present in the interstellar medium are much cooler than often believed.

Jura, M.↗

NASA NTRS1983

The nature of NML Cygnus

Since the discovery of NML Cyg by Neugebauer et al. (1965), its nature has been uncertain because its distance and luminosity are not known. NML Cyg is partially surrounded by an H II region which is heavily obscured by intervening interstellar matter. This H II region has been mapped in the continuum at 21 cm, and it seems clearly associated with the star. In the present investigation it is proposed that the H II region has been created by ionizing radiation from the Cyg OB2 association. The close agreement between the observed structure of the H II region and that predicted by the considered model provides strong evidence that NML Cyg is indeed at the distance of the association, 2 kpc. At this distance it is among the most luminous red stars shown. Because of its high luminosity, NML Cygnus is probably a massive star of perhaps 50 solar masses.

Morris, M.↗

NASA NTRS1983

Molecular self-shielding in the outflows from late-type stars

It is proposed that molecules, especially CO and H2, in the circumstellar outflows from late-type stars can be self-shielded from ambient interstellar UV radiation. The theory of self-shielding in spherical, expanding envelopes is developed for the case in which the photodestruction is dominated by absorption in the Doppler cores of the relevant UV lines, and it is shown that this theory can account for the observations of IRC plus 10216. Also considered is the case in which photodestruction takes place in the damping wings of the UV lines as is appropriate for H2. It is found that most of the hydrogen remains molecular, although the amount of atomic hydrogen is not completely negligible.

Morris, M.↗

NASA NTRS1982

Large-scale magnetic fields in spiral galaxies

The optical polarization vectors which measurements have shown to be parallel to the pronounced dark lanes of such galaxies as NGC 5128 and 4590 are interpreted as the consequence of a large scale, systematic field parallel to the dark lane that leads to the alignment of the grains. It is suggested that such polarization may also be produced by scattering off grains concentrated in the dark lane. It is recommended that there be further observational tests of the hypothesis that the observed polarization indicates the magnetic field in the galaxy.

Jura, M.↗

NASA NTRS1982

Current NASA studies for a Far Ultraviolent Spectrographic Explorer (FUSE)

The NASA plans for FUSE, a satellite which obtains spectra with resolutions between 100,000 and 100 in the spectral regions from 912 to 1216A and 100 to 912A, are outlined. Scientific problems which can be tackled by FUSE, but not by IUE or the Space Telescope, are discussed. A grazing incidence echelle and a hybrid echelle design are presented. They have high throughput, large simultaneous spectral range, and low background photon counting statistics. The satellite operational organization is similar to that of IUE.

Linsky, J.↗

NASA NTRS1982

Search for interstellar superheavy hydrogen

Models for fundamental physical interactions allow for the existence of stable or nearly stable elementary particles much heavier than the proton. Stellar spectra were searched for a positively charged superheavy particle, X(+), which, with a bound electron, should appear as apparently superheavy neutral hydrogen in the interstellar medium. An upper limit for the abundance of X relative to normal hydrogen in the line of sight toward the bright star gamma Cassiopeiae is 2 x 10 to the -8th.

Jura, M.↗

NASA NTRS1982

Models for far-infrared emission from normal galaxies

It is shown that the presence of grains distributed throughout either disk or spherical galaxies can lead to detectable far-infrared sources. In particular, an elliptical such as NGC 4278 with an appreciable amount of neutral gas should be easily detectable with the Infrared Astronomical Satellite (IRAS) if it has a dust-to-gas ratio similar to the value in the solar neighborhood. A disk should be detectable at 100 Mpc with IRAS. Also, disks may emit most of their energy longward of 100 microns; this could be important for estimating the mass-to-light ratios of these galaxies.

Jura, M.↗

NASA NTRS1982

Observations of interstellar zinc

IUE observations toward 10 stars have shown that zinc is not depleted in the interstellar medium by more than a factor of two, suggesting that its abundance may serve as a tracer of the true metallicity in the gas. A result pertinent to the history of nucleosynthesis in the solar neighborhood is that the local interstellar medium has abundances that appear to be homogeneous to within a factor of two, when integrated over paths of about 500 pc.

York, D. G.↗

NASA NTRS1982

Abundance fluctuations in the interstellar medium

The determination of abundances within the interstellar medium is reviewed. It appears that interstellar abundances within 1 kpc of the Sun are uniform to within a factor of two or three, but it is not yet possible to determine whether there are real fluctuations at this level except for deuterium for which the factor of two variations appear to be real. Establishing the level of local fluctuations in the abundances is of considerable importance for understanding the history of nucleosynthesis in the solar neighborhood, the evolution of the interstellar medium and the formation of stars.

Jura, M.↗

NASA NTRS1982

The High Resolution Spectrograph for the Space Telescope

The High Resoltuion Spectrograph (HRS) in conjunction with the Space Telescope (ST), extends ultraviolet astronomical spectroscopy to higher spectra, spatial, and time resolutions than previously achieved, as well as to fainter and more distant celestial objects. Other significant advances inherent in the instrument are high photometric accuracy and efficient operation via exposure meter control and real time rejection of bad data. These capabilities are provided to accomplish the scientific programs of the HRS investigation definition team, which concern the interstellar medium, stellar winds, and evolutionary aspects of stellar atmosphere studies; the determination of chemical abundances relevant to stellar evolution; the investigation of quasars and Seyfert galaxy nuclei; and the analysis of the atmospheres of solar system objects, including comets.

Brandt, J. C.↗

NASA NTRS1981

Model for the circumstellar gas around alpha Orionis

Models are presented to explain the physical conditions in the circumstellar envelope surrounding the red supergiant Alpha Orionis. The models of the CO density and neutral atomic potassium density are calculated for the outer shell to reproduce simultaneously both the radio detection of CO and optical measurements of the amount of scattering by neutral potassium around Alpha Ori. CO and potassium loss rates of 7 x 10 to the 39th/sec and 8 x 10 to the 37th/sec are obtained, which imply a potassium to carbon ratio greater than that of the sun by about a factor of 25. The low carbon abundance may be attributed to extensive CNO processing, and, together with the potassium abundance, indicates a total mass loss rate of 0.000015 solar masses/year. The mass loss, mass and abundance properties of Alpha Ori suggest it to be the progenitor of a Type II supernova such as Cas A.

Jura, M.↗

NASA NTRS1981

Discovery of interstellar rubidium

Interstellar rubidium is detected through observations of the resonance line of Rb I at 7800 A towards zeta Oph. The abundance ratio of rubidium to potassium is estimated to be approximately solar, and if rubidium is generally found to have an abundance similar to potassium, it is indicated that the local interstellar medium is well mixed with a wide variety of the products of nucleosynthesis.

Jura, M.↗

NASA NTRS1981

Observations of interstellar zinc

The International Ultraviolet Explorer observations of interstellar zinc toward 10 stars are examined. It is found that zinc is at most only slightly depleted in the interstellar medium; its abundance may serve as a tracer of the true metallicity in the gas. The local interstellar medium has abundances that apparently are homogeneous to within a factor of two, when integrated over paths of about 500 pc, and this result is important for understanding the history of nucleosynthesis in the solar neighborhood. The intrinsic errors in detecting weak interstellar lines are analyzed and suggestions are made as to how this error limit may be lowered to 5 mA per target observation.

Jura, M.↗

NASA NTRS1981

High-resolution profiles for the diffuse interstellar band at 6196 A

The 6196.0 A diffuse feature is observed in various directions towards moderately and slightly reddened stars at high spectral resolution and signal to noise ratio. The feature is found to be symmetric with no fine structure with an equivalent-width upper unit paralleling that of H2. The feature is as narrow as 0.4 A, although much broader in other stars due to cloud blending in the line of sight, and is absent in all observed stars with E(B-V) less than 0.1, and upper limits more than an order of magnitude below values extrapolated from more reddened stars. Upper limits are determined by multiplying the square root of the number of channels in which the line may lie, and another narrow feature, although highly asymmetric, is observed at 6194.6 A. It is concluded that the results increase the possibility of at least one of the diffuse band carriers being a molecule.

Smith, W. H.↗

NASA NTRS1981

Very dusty galaxies

The existence of a significant population of dust shrouded galaxies that do not emit much optical light but instead are primarily infrared sources is considered. It is noted that if very dusty galaxies are assumed to have a luminosity function similar to that of normal galaxies, then current observations seem to show that there are not more dusty galaxies than normal objects. Until more extensive infrared observations are obtained, however, it seems at least possible, though perhaps unlikely, that there are in fact a significant number of dusty galaxies.

Jura, M.↗

NASA NTRS1980

Comparisons of interstellar CH/+/ and H2

Copernicus observations of H2 toward stars in the Pleiades and 23 Orionis have been obtained because of their importance in understanding the formation of interstellar CH(+). No model of CH(+) equilibrium seems to agree very well with these observations; in particular, the suggestions that CH(+) is formed either from collisions between vibrationally excited H2 and C(+) or from radiative association of C(+) and H2 can apparently be excluded. These data do suggest that there is a correlation between the amounts of rotationally excited H2 and CH(+) which are present. Observations of the Pleiades also help interpretations of the reflection nebulae near these stars.

Frisch, P. C.↗

NASA NTRS1980

Copernicus observations of C I and CO in diffuse interstellar clouds

Copernicus was used to observe absorption lines of C I in its ground state and excited fine structure levels and CO toward 29 stars. We use the C I data to infer densities and pressures within the observed clouds, and because our results are of higher precision than previous work, much more precise estimates of the physical conditions in clouds are obtained. In agreement with previous work, the interstellar thermal pressure appears to be variable, with most clouds having values of p/k between 1000/cu cm K and 10,000/cu cm K, but there are some clouds with p/k as high as 100,000/cu cm K. Our results are consistent with the view that the interstellar thermal pressure is so variable that the gas undergoes continuous dynamic evolution. Our observations provide useful constraints on the physical processes on the surfaces of grains. In particular, we find that grains are efficient catalysts of interstellar H2 in the sense that at least half of the hydrogen atoms that strike grains come off as part of H2. Results place strong constraints on models for the formation and destruction of interstellar CO. In many clouds, an order of magnitude less CO than predicted in some models was found.

Jenkins, E. B.↗

NASA NTRS1980

Circular polarization from scattering by circumstellar grains

It is suggested that observations of circular polarization can be a valuable tool for the study of circumstellar nebulae. A simple model is presented to predict the percentage of circular polarization produced by scattering from the dust contained within such nebulae; it was found that a significant component of circular polarization can be produced in nebulae which have a net linear polarization. Possible observations of the red supergiants mu Cep and alpha Ori are given as examples.

Shafter, A.↗