Engineering topics

Snijders, M. A. J.

Publications and source records attributed to Snijders, M. A. J..

14 records

NASA NTRS1990

The missing UV absorption lines of NGC 4151

Near simultaneous high dispersion long and short wavelength International Ultraviolet Explorer (IUE) observations of the Seyfert galaxy NGC 4151 are discussed. Previous observations revealed a narrow absorption system in Mg II not present in Ly alpha or C IV. The new observations confirm the presence of this system in Mg II and its absence in the other lines. Possible reasons for this are discussed. Future Hubble Space Telescope studies of NGC 4151 are discussed.

Leech, K. J.↗

NASA NTRS1987

The Mg II line profile in the Seyfert galaxy NGC 4151 - A new outflowing component

This paper examines the Mg II 2795-2802 A doublet in the Seyfert galaxy NGC 4151 at a higher resolution than has previously been used, searching for velocity systems in absorption and emission. Evidence is presented for a new, narrow, outflowing absorption system in Mg II having a velocity of 825 km/s relative to the sun, and -165 km/s relative to the systemic velocity of NGC 4151. This feature is not present in Ly-alpha or C IV and possible explanations for this are considered. For the Mg II and C IV lines, a model decomposition of the line profile is presented.

Leech, Kieron J.↗

NASA NTRS1987

Of Galactic novae

Observations of novae in outburst and old quiescent novae in the Galaxy, obtained with the IUE satellite since its launch in 1978, are surveyed. The advantages of UV observations of novae are summarized; the astrophysics of a nova outburst are reviewed; and data from IUE observations of 17 novae are presented in tables and graphs and characterized in detail. Consideration is given to the power-law distribution of the continuum flux in old novae, the problems involved in determining temperatures and mass-accretion rates, evidence for mass loss in some old novae, the nonsolar abundances observed in novae in outburst, and the fact that some outbursts occur on ONeMg (rather than CO) white dwarfs.

Starrfield, Sumner↗

NASA NTRS1986

The Mg II line profile in the Seyfert galaxy NGC 4151: A new outflowing component

The Mg II 2795, 2802A doublet in the Seyfert galaxy NGC 4151 was examined to search for velocity systems in absorption and emission. Evidence for a narrow, outflowing absorption system in Mg II having a velocity of +825 km/sec relative to the Sun, -165 km/sec relative to the systemic velocity of NGC 4151 is presented. This feature is not present in Ly alpha or C IV and possible explanations for this are considered. For the Mg II and C IV lines a model decomposition of the line profile is shown.

Leech, Kieron J.↗

NASA NTRS1982

Ultraviolet observations of nova Aquila, 1982

A bright nova in Aquila was discovered with a visual magnitude of 6 to 7. A number LWR and SWP spectra of the object were obtained with the IUE. The nova has a heavily reddened spectrum, E(B-V)= 0.55 + or - 0.15, and a complex shot wavelength spectrum with both narrow emission features and broad absorption troughs, indicating terminal velocities as high as 10,000 km/s. Both the continuum flux and the absorption line profiles changed between two observations. Short period variations were also observed.

Snijders, M. A. J.↗

NASA NTRS1981

IUE observations of Fe 2 galaxies

Repeated observations of the Seyfert 1 galaxies I Zw 1 and II Zw 136, which have very strong Fe II emission lines in the optical region, were made at low resolution with the IUE Satellite. The ultraviolet spectra are very similar: both are variable and show broad emission features of Fe II (especially the UV multiplets 1, 33, 60, 62, and 63) as well as the emission lines usually strong in Seyferts and quasars. The data strongly support the hypothesis that the optical Fe II emission lines are primarily due to collisional excitation and that resonance fluorescence makes only a minor contribution to the excitation of these lines.

Penston, M. V.↗

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

The ultraviolet flux distribution of Alpha-2 Canum Venaticorum

Intermediate- and narrow-band UV spectrophotometry from Copernicus, OAO 2, the S2-68 experiment on TD 1, and a sounding-rocket experiment are combined with ground-based observations to define the absolute flux distribution of the bright magnetic Ap star Alpha-2 CVn over the wavelength range from 1030 to 7580 A. Two flux distributions are presented which coincide more or less with the rare-earth maximum and minimum in the star's cycle. The results are compared with those for two normal stars, the UV variability of Alpha-2 CVn is characterized as a function of wavelength, and non-LTE effects on the UV continua of C I and Si I are analyzed. Some physical properties of Alpha-2 CVn are estimated, and the evolutionary age of the star is estimated to be between 220 million and 300 million years. It is concluded that Alpha-2 CVn is old enough to have been magnetically braked to its current rotational velocity by either the centrifugal-wind or the accretion mechanism and to have developed its chemical peculiarities by either mechanism.

Leckrone, D. S.↗

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

IUE observations of extragalactic objects

During the commissioning phase of IUE several extragalactic objects were observed spectrally at low dispersion in the UV range lambda lambda 1150-3200: the Seyfert galaxies NGC4151 and NGC1068, the QSO 3C273, the BL Lacertae object B2 1101+38, the giant elliptical galaxy M87 and the spiral galaxy M81. The results obtained are presented and a preliminary analysis given for all six objects, discussing the continuous spectrum, extinction, emission line spectrum and absorption line spectrum, where possible for each case. Several new or confirmatory astrophysical results are obtained.

Boksenberg, A.↗

NASA NTRS1977

The continuous UV flux of Alpha Lyrae - Non-LTE results

Non-LTE calculations for the ultraviolet C I and Si I continuous opacity show that LTE results overestimate the importance of these sources of opacity and underestimate the emergent flux in Alpha Lyr. The largest errors occur between 1100 and 1160 A, where the predicted flux in non-LTE is as much as 50 times larger than in LTE, in reasonable accord with Copernicus observations. The discrepancy between LTE models and observations has been interpreted to result from the existence of a chromosphere. Until a self-consistent non-LTE model atmosphere becomes available, such an interpretation is premature.

Snijders, M. A. J.↗

NASA NTRS1975

The He II lines in the spectrum of zeta Puppis

The observed He II line spectrum in zeta Pup is compared with that predicted from model atmospheres in an attempt to establish the parameters (effective temperature, log g) which characterize the model that best represents this star's atmosphere. The study is conducted by compiling equivalent widths of the He II lines in the series (n = 2,3,4,5) and comparing them with predictions from plane-parallel static model atmospheres using a non-LTE theory of line formation. Good agreement between observation and prediction for a model atmosphere with an effective temperature of 50,000 K and log of 4.0 is found for the upper members of the n = 3 and n = 5 series, but discrepancies are found for the two lines of the n = 2 series and the upper members of the n = 4 series. It is shown that the n = 4 level appears to be more overpopulated at moderate atmospheric depths than the non-LTE calculations with plane-parallel layers indicate. It is suggested that this may be due to an overlap of the H and He II lines in the even-even series caused by macroturbulent velocities of the hydrogen and helium atoms.

Snijders, M. A. J.↗

NASA NTRS1975

Observations and theory of Mg II lines in early type stars. II - Theory and predicted profiles. III - The observations and a comparison with the predictions

Profiles of the UV Mg II lines in the spectra of early type stars are computed in a grid of model atmospheres with effective temperatures between 8000 and 35,000 K and log g values of 2.5 to 4.0 using the LTE and non-LTE theories of line formation. The theoretical results indicate that the line cores are strengthened by non-LTE effects over the entire temperature range, resonance-line wings are weaker in the cooler models than in the LTE case, and very large deviations from LTE occur in the hot low-gravity models. These predictions are compared with the equivalent widths of the UV Mg II lines in 106 stars and of the visual lines in 48 stars (spectral types O4 to A3). The observed equivalent widths of normal stars in luminosity classes II through V are found to agree with the predictions over the entire range from O8 to A2 if a certain Mg/H abundance is adopted. The line intensities observed in supergiants, Be, Bp, and Ap stars are discussed.

Snijders, M. A. J.↗

NASA NTRS1974

A bibliography of atomic line identification lists

A bibliography of atomic-line-identification lists is presented to supplement the material contained in the Ultraviolet and Revised Multiplet Tables and in the finding list by Kelly and Palumbo (1973). The list covers the wavelength range from 911 A to 8205 A.

Adelman, S. J.↗