Engineering topics

Bohlin, R. C.

Publications and source records attributed to Bohlin, R. C..

At least 55 records · Page 3

NASA NTRS1981

Effects of temperature fluctuations of IUE data quality

Analysis of IUE calibration lamp images shows that variation in the temperature of the scientific instrument causes shifts in the location of the spectral format with respect to the reseau grid on the detector and in the location of the reseaux themselves. In high dispersion, a camera head amplifier temperature difference of 6C corresponds to a shift of 4 pixels in the spectral format for LWR and 2 pixels for SWP along the dispersion direction. Shifts perpendicular to the disperson (for the same temperature difference) are less than one pixel for both cameras. In low dispersion spectra, the shifts are similar but orthogonal to those described above with the larger motion lying in the direction perpendicular to the dispersion. In both dispersion modes, the observed shifts are apparently independent of wavelength. In high dispersion, the constant pixel shift mimics a constant velocity error.

Thompson, R. W.↗

NASA NTRS1980

Long-slit spectroscopy in the rocket ultraviolet of the Orion Nebula

The study deals with the extensive nebulosity in the Trapezium region of Orion. An entrance slit was placed at the focal plane of rocketborne telescope, creating a long-slit spectrograph with a wavelength resolution of 22 A FWHM and a spatial resolution of 14 in. along the slit. Data reduction is outlined, the structure of the Orion Nebula is discussed, and radiative transfer considerations are presented.

Bohlin, R. C.↗

NASA NTRS1980

Photometric calibration of the International Ultraviolet Explorer /IUE/ - Low dispersion

Absolute sensitivity curves for IUE low-resolution spectra are obtained by comparing IUE measurements of hot stars with the absolute energy distributions established for these objects by previous satellite and rocket experiments. The differences between these earlier experiments are discussed quantitatively, and a common absolute flux scale is proposed as the basis for the absolute calibration of IUE.

Bohlin, R. C.↗

NASA NTRS1979

Absolute calibration in the 1750 A-3350 A region

The absolute flux measurements in the rocket ultraviolet made by Bohlin, Frimout, and Lillie (BFL) are revised using a more correct treatment of the air extinction that enters the air calibration of their instrument. The absorption by molecular oxygen and ozone, Rayleigh scattering, and extinction by aerosols is tabulated for general use in ultraviolet calibrations performed in air. The revised absolute flux of Eta UMa and final fluxes for Alpha Lyr and Zeta Oph are presented in the 1750 A-3350 A region. The absolute flux of the star Eta UMa (B3 V) is compared to four other independent determinations in the 1200 A-3400 A region and a maximum difference of 35% is found near 1500 A between the OAO-2 and Apollo 17 fluxes. Longward of 1700 A the typical scatter in the different determinations is only plus or minus 5%. The rocket measurements of BFL, the ANS and TD-1 satellite data, and the Apollo 17 data are compared to the ultraviolet fluxes from the OAO-2, demonstrating a photometric reproducibility of about plus or minus 3%. Therefore, all four sets of spectrophotometry can be reduced to a common absolute scale.

Strongylis, G. J.↗

NASA NTRS1979

The depletion of interstellar gaseous iron

The Copernicus UV telescope was used to measure equivalent widths of interstellar Fe II resonance lines toward 55 early-type stars; the measurements permit the determination of Fe II column densities. The depletion of interstellar gaseous iron was obtained by combining these measurements with the results from a previous atomic and molecular hydrogen survey program; the derived depletions refer mostly to matter in H I regions. As an example, the nearly normal gaseous iron abundance in the distant high-latitude intermediate-velocity cloud toward HD 93521 is consistent with the idea that these clouds are produced by galactic supernova explosions.

Savage, B. D.↗

NASA NTRS1978

In-flight performance of the IUE

A report on the initial performance of the IUE satellite, launched in January, 1978, is given. Attention is given to optical performance, camera performance, background corrections, photometric performance, and data reduction. Samples of high-dispersion and low-dispersion spectra are shown. A remaining problem of proper separation of high dispersion orders in data reduction is discussed.

Boggess, A.↗

NASA NTRS1978

IUE observations of the interstellar medium

Results from a preliminary study of interstellar absorption lines in the spectra of the three stars HD149757, HD93521 and HD153919, observed by the IUE during its commissioning phase are presented. They demonstrate the capabilities of the IUE for interstellar research. Results from a preliminary study of the emission line spectrum of the planetary nebula NGC 7027 are outlined and used to study its interstellar reddening.

Grewing, M.↗

NASA NTRS1978

A survey of interstellar H I from L-alpha absorption measurements. II

This paper presents interstellar H I column densities, mean H I space densities, total neutral hydrogen column densities, and total mean hydrogen space densities toward 100 stars surveyed in the L-alpha region with the U2 detector of the Copernicus satellite. The data for 47 stars are compared with OAO 2 results, and ratios of gas to E(B-V) color excess are computed for 30 'intercloud' stars, 45 'cloud' stars, and the combined sample of 75 stars. A definitive value of 5.8 x 10 to the 21st power atoms/sq cm per mag is obtained for the mean ratio of total neutral hydrogen to color excess, indicating that this ratio seems to be a constant, independent of location in the sky, except possibly toward stars with anomalous extinction. The anomalous ratio determined for Rho Oph is examined, and the galactic distribution of neutral hydrogen is evaluated. The following values are estimated for matter in the galactic plane within 500 pc of the sun: a total mean hydrogen space density of 1.15 atoms/cu cm, a mean H2 space density of 0.143 molecule/cu cm (possibly a lower limit), and a mean H I space density of 0.86 atom/cu cm (a possible overestimate).

Bohlin, R. C.↗

NASA NTRS1978

The rocket-ultraviolet spectrum and models of the planetary nebula NGC 7662

Ultraviolet spectral analysis of NGC 7662 has revealed a Balmer line distribution indicating a C/O ratio of unity. This abundance of carbon is characteristic of mixing between the central star and the outer envelope of the planetary nebula. A continuum spectrum is observed from the star and the nebula, radiating its energy via the two photon process through neutral hydrogen. Spectral lines were fitted to apply to the predictions of three models of the nebular structure. UV observations were compared with visible, IR, and radio observations.

Bohlin, R. C.↗

NASA NTRS1977

A survey of interstellar HI from L alpha absorption measurements 2

The Copernicus satellite surveyed the spectral region near L alpha to obtain column densities of interstellar HI toward 100 stars. The distance to 10 stars exceeds 2 kpc and 34 stars lie beyond 1 kpc. Stars with color excess E(B-V) up to 0.5 mag are observed. The value of the mean ratio of total neutral hydrogen to color excess was found to equal 5.8 x 10 to the 21st power atoms per (sq cm x mag). For stars with accurate E(B-V), the deviations from this mean are generally less than a factor of 1.5. A notable exception is the dark cloud star, rho Oph. A reduction in visual reddening efficiency for the grains that are larger than normal in the rho Oph dark cloud probably explains this result. The conversion of atomic hydrogen into molecular form in dense clouds was observed in the gas to E(B-V) correlation plots. The best estimate for the mean total gas density for clouds and the intercloud medium, as a whole, in the solar neighborhood and in the plane of the galaxy is 1.15 atoms per cu. cm; those for the atomic gas and molecular gas alone are 0.86 atoms per cu cm and 0.143 molecules per cu cm respectively. For the intercloud medium, where molecular hydrogen is a negligible fraction of the total gas, atomic gas density was found to equal 0.16 atoms per cu cm with a Gaussian scale height perpendicular to the plane of about 350 pc, as derived from high latitude stars.

Bohlin, R. C.↗

NASA NTRS1977

Absolute calibration in the 1750 - 3350 A region

The absolute flux measurements in the rocket ultraviolet made by Bohlin, Frimout, and Lillie (BFL) are revised using a more correct treatment of the air extinction that enters the air calibration of their instrument. The absorption by molecular oxygen and ozone, Rayleigh scattering, and extinction by aerosols is tabulated for general use in ultraviolet calibrations performed in air. The revised absolute flux of eta UMa and final fluxes for alpha Lyr and zeta Oph are presented in the 1750-3350 A region. The absolute flux of the star eta UMa is compared to four other independent determinations in the 1200-3400 A region and a maximum difference of 35% is found near 1500 A between the OAO-2 and Apollo 17 fluxes. The rocket measurements of BFL, the ANS and TD-1 satellite data, and the Apollo 17 data are compared to the ultraviolet fluxes from the OAO-2, demonstrating a photometric reproducibility of about + or - 3 percent. Therefore, all four sets of spectrophotometry can be reduced to a common absolute scale.

Strongylis, G. J.↗

NASA NTRS1977

A survey of interstellar molecular hydrogen. I

Data from the Copernicus satellite's ultraviolet telescope were used to survey column densities of atomic and molecular hydrogen from a large sample of early-type stars; these data have bearing on an eventual understanding of diffuse and dense interstellar clouds. Column densities are derived by fitting damping profiles to the observed spectra, most of which exhibit strong damping lines in the lower rotational levels surveyed. Plots of dust column density, fractional abundance of molecular hydrogen, and the logarithm of fractional abundance versus total gas column density are given for many of the stars; stars with abnormally large or small hydrogen column densities, as well as some distant stars at high galactic latitudes, are considered. Equilibrium and nonequilibrium theories accounting for the abundance of interstellar hydrogen are compared, and support is found in the data for an account which balances hydrogen formation on interstellar grains with destruction through photodissociation. Overall averages for atomic and molecular hydrogen levels in the galactic plane are also calculated.

Savage, B. D.↗

NASA NTRS1975

The rocket-ultraviolet spectrum of the planetary nebula NGC 7027

An ultraviolet spectrum of NGC 7027, obtained with a rocket-borne telescope, is analyzed. Absolute values are presented for the observed fluxes, and upper limits are given for the strongest predicted lines that were not observed. The results are corrected for interstellar extinction using the observed and calculated line ratios between H-beta and the hydrogenic recombination line of He II at 1640 A. The corrected C IV resonance line at 1549 A is found to be in good agreement with the intensity calculated from models, but the intercombination line of C III at 1909 A is found to be too bright by a factor of 10. This discrepancy is reduced to a factor of 4 by taking dielectric recombination into account and is eliminated by using the solar carbon abundance, which implies attenuation in the C IV line. It is shown that no appreciable number of absorbing grains can exist in the C IV-producing region of the nebula since the optical depth is of the order of 10,000 at the line center.

Bohlin, R. C.↗

NASA NTRS1975

Mariner 9 observations of beta Cephei stars

Ultraviolet spectra of three beta Cep variables and a nonvariable star of similar spectral type were observed by the ultraviolet spectrometer on Mariner 9. A significant light variation at 1400 A may have been detected for beta Cep. The variable stars have mean ultraviolet spectra similar to that of the nonvariable star and do not show any significant short-period spectral variations.

Lesh, J. R.↗

NASA NTRS1975

The rocket ultraviolet spectrum of the planetary nebula NGC 7027

An ultraviolet spectrum of NGC 7027 was obtained with a rocket-borne telescope. The observed fluxes are given on an absolute basis and upper limits are given for the strongest predicted lines which were not observed. The extinction correction was made on the basis of the observed and calculated line ratios for the hydrogenic recombination line of He 2 at 1640A to H beta. The extinction is in agreement with ground based determinations. When corrected for extinction the C 4 resonance line at 1549A is in good agreement with the intensity calculated from models, but the C 3 intercombination line at 1909A is a factor of ten too bright. The addition of dielectronic recombination to the models sufficiently changes the C 3 concentration to reduce the discrepancy to a factor of four. The abundance of carbon is assumed to be 2 x 0.0001 that of hydrogen. Using carbon abundances for the sun, this discrepancy disappears and there must be attenuation in the C 4 line. Since the optical depth is approximately 10,000 at the line center, no appreciable number of absorbing grains can exist in the C 4 producing region of the nebula.

Bohlin, R. C.↗

NASA NTRS1975

Copernicus observations of interstellar absorption at Lyman alpha

Column densities NH of atomic hydrogen have been derived for 40 OB stars from spectral scans at Lyman alpha obtained by the Copernicus (OAO-3) satellite. The stars are all between 60 and 1100 pc away with a range of mean densities n sub H of 0.01 to 2.5 atoms cm-3. The gas to color-excess ratio in clouds varies from 1 to 3 times the mean outside of clouds. The presence of molecular hydrogen correlates with E(B-V), but the best tracer for H2 is atomic hydrogen. The mean density of the gas for all 40 stars is much smaller than the mean of 0.7 atoms cm-3 obtained from 21-cm observations, because the brightest stars with less than average amounts of matter in the line of sight were selected for observation.

Bohlin, R. C.↗

NASA NTRS1975

Small Astronomy Payloads for Spacelab

The workshop to define feasible concepts in the UV-optical 1R area for Astronomy Spacelab Payloads is reported. Payloads proposed include: high resolution spectrograph, Schmidt camera spectrograph, UV telescope, and small infrared cryogenic telescope.

Bohlin, R. C.↗

NASA NTRS1974

Absolute flux measurements in the rocket ultraviolet

A two-channel spectrometer was calibrated in the wavelength region 1200-3400 A and flown on an Aerobee rocket to observe the stars alpha Lyr, eta UMa, and zeta Oph. Standard tungsten lamps provided the absolute calibration down to 2250 A, and a photodiode calibrated by the National Bureau of Standards was the reference at shorter wavelengths. The molecular branching-ratio technique of relative calibration using the gases CO, NO, and N2 was a check on the absolute calibration. The flux from eta UMa agrees with the prediction of a hydrogen line blanketed model atmosphere within 10% between 1700 and 3400 A and within 4% over most of this wavelength region.

Bohlin, R. C.↗