Engineering topics
Publications and source records attributed to Landsman, W. B..
Small-aperture, high-dispersion IUE spectra have been obtained of seven late-type stars that, in general, confirm previous Copernicus results concerning the distribution of hydrogen and deuterium in the local interstellar medium. In addition, the IUE Ly Alpha spectra of Altair, and of the Alpha Cen components, suggest that multiple velocity components exist in these two directions.
Theoretical line profiles are presented that demonstrate why 100,000 spectral resolution and high S/N are needed to derive accurate column densities from spectral lines that lie close to the flat part of the curve of growth and may contain multiple velocity components. The aim of the Space Telescope high-resolution-spectrograph observing program is to obtain column densities in the hydrogen and deuterium Lyman alpha lines along a variety of lines of sight within and extending beyond the local cloudlet, in which the sun is located near an edge. The broad chromospheric Lyman alpha emission lines of late-type stars are used as background sources against which to measure the interstellar absorption features. Profiles of interstellar absorption features in Mg II and Fe II lines will be obtained to derive the broadening parameter and/or identify possible multiple velocity components in the lines of sight.
High-disperson Copernicus and IUE observations of chromospheric Ly alpha emission are used to study the distribution of HI in the local interstellar medium. Interstellar parameters are derived toward 3 stars within 5 pc of the Sun, and upper limits are given for the Ly alpha flux from 9 other stars within 10 pc.
A composite profile is presented of the Ly-alpha emission line of Alpha Cen A, obtained from 10 individual spectra with the high-resolution spectrograph aboard the International Ultraviolet Explorer (IUE) satellite. There is excellent overall agreement with two previous Copernicus observations. Interstellar deuterium is detected, and a lower limit is set on the deuterium to hydrogen ratio of nDI/nHI greater than 8 x 10 to the -6th. In addition, the deuterium bulk velocity appears blueshifted by 8 + or - 2 km/s with respect to interstellar hydrogen, suggesting a nonuniform medium along the line of sight.
Ultraviolet photometry on the International Ultraviolet Explorer (IUE) was used to study four stars located in one target of an earlier diffuse background sounding-rocket experiment. The resulting stellar correction is much smaller than that previously estimated, giving a higher diffuse background at this target. Visible photometry appears to be a better indicator of ultraviolet flux than spectral type. The discrepancy between previous predictions and the present observations is explained in terms of: (1) misclassification of two stars; (2) use of a spectral type/effective temperature calibration hotter than more recent determinations; and (3) inadequacy of the Kurucz models, in the far ultraviolet, for A-type stars.