Samuel T. Durrance

The Hopkins Ultraviolet Telescope - Performance and calibration during the Astro-1 mission

Results are reported of spectrophotometric observations, made with the Hopkins Ultraviolet Telescope (HUT), of 77 astronomical sources throughout the far-UV (912-1850 A) at a resolution of about 3 A, and, for a small number of sources, in the extreme UV (415-912 A) beyond the Lyman limit at a resolution of about 1.5 A. The HUT instrument and its performance in orbit are described. A HUT observation of the DA white dwarf G191-B2B is presented, and the photometric calibration curve for the instrument is derived from a comparison of the observation with a model stellar atmosphere. The sensitivity reaches a maximum at 1050 A, where 1 photon/sq cm s A yields 9.5 counts/s A, and remains within a factor of 2 of this value from 912 to 1600 A. The instrumental dark count measured on orbit was less than 0.001 counts/s A.

Performance and Preliminary Calibration of the Hopkins Ultraviolet Telescope on the Astro-2 Mission

An improved version of the Hopkins Ultraviolet Telescope (HUT) made its second flight on the Astro-2 mission aboard the Space Shuttle Endeavour from 1995 March 2-18. The longer mission duration and greatly improved pointing stability relative to Astro-1 made possible 385 observations of 265 celestial targets at far-ultraviolet wavelengths. Observing efficiency exceeding 60% over 14 days of science operations yielded 205 hr of on-source integration time, a factor of 5.1 increase over Astro-1. We describe changes to the instrument following Astro-1 and the in-flight photometric calibration, which is based on a comparison of our observations of the hot DA white dwarf HZ 43 with a model atmosphere whose parameters were derived from optical observations. The peak effective area is 24.1 cm2 at 1160 A, where the inverse sensitivity is 7.09 × 10-13 ergs cm-2 count-1. This is an improvement by a factor of 2.3 over Astro-1, largely attributable the installation of new optics coated with ion-sputtered silicon carbide. Observations of several other white dwarfs indicate that the calibration is accurate to about 5%, after correction for modest, but significant, time-dependent degradation during the mission. The spectral resolution varied from 2 to 4 A over the first-order wavelength range of 820-1840 A. The wavelength scale is established to better than 1 A. As on Astro-1, dark counts and scattered light were extremely low. Airglow line intensities were much lower because of the lower level of solar activity. When all factors are considered together, HUT performance on Astro-2 was a full order of magnitude better than that achieved on the highly successful Astro-1 mission.

Hopkins ultraviolet telescope observations of the far-ultraviolet spectrum of NGC 4151

Observations of the FUV spectrum of the Seyfert galaxy NGC 4151 from 912 to 1860 A during the flight of Astro-1 aboard the space shuttle Columbia in December 1990 are reported. Broad emission lines with full-width at half-maximum of about 8500 km/s dominate the spectrum. Numerous absorption features modify the continuum shape, particularly at wavelengths shortward of Ly-alpha. The continuum turns over sharply below 1000 A and disappears by 924 A, well above the redshifted Lyman edge of NGC 4151 at 915 A. The absorption lines have intrinsic widths of about 1000 km/s and are blueshifted relative to the system velocity of NGC 4151 by 200-1300 km/s. Absorption of the continuum by the converging higher-order Lyman lines explains the sharp turnover of the continuum below 1000 A. The blueshifts of the absorption lines, their large intrinsic widths, and the inferred high densities are all consistent with outflowing material originating in the broad-line region.

Far‐Ultraviolet Spectroscopy of Venus and Mars at 4 A Resolution with the Hopkins Ultraviolet Telescope on Astro‐2

Far-ultraviolet spectra of Venus and Mars in the range 820-1840 A at ~4 A resolution were obtained on 1995 March 13 and 12, respectively, by the Hopkins Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the space shuttle Endeavour. Longward of 1250 A, the spectra of both planets are dominated by emission of the CO fourth positive (A1Π-X1Σ+) band system and strong O I and C I multiplets. In addition, CO Hopfield-Birge bands, B1Σ+-X1Σ+ (0, 0) at 1151 A and C1Σ+-X1Σ+ (0, 0) at 1088 A, are detected for the first time, and there is a weak indication of the E1Π-X1Σ+ (0, 0) band at 1076 A in the spectrum of Venus. The B-X band is blended with emission from O I λ1152. Modeling the relative intensities of these bands suggests that resonance fluorescence of CO is the dominant source of the emission, as it is for the fourth positive system. Shortward of Lyα, other emission features detected include O II λ834, O I λ989, H I Lyβ, and N I λλ1134 and 1200. For Venus, the derived disk brightnesses of the O I, O II, and H I features are about one-half of those reported by Hord et al. from Galileo EUV measurements made in 1990 February. This result is consistent with the expected variation from solar maximum to solar minimum. The Ar I λλ1048, 1066 doublet is detected only in the spectrum of Mars, and the derived mixing ratio of Ar is of the order of 2%, consistent with previous determinations.

Constraints on the origin of the ultraviolet upturn in elliptical galaxies from Hopkins Ultraviolet Telescope observations of NGC 1399

Results are presented of observations of the giant elliptical galaxy NGC 1399 in December 1990, which provide measurements of the spectral energy distribution of the elliptical galaxy and allow detection of absorption features in the sub-2000-A UV. The spectrum shows no emission features other than well-known geocoronal lines. Strong Lyman series absorption is evident at the galaxy redshift. Other absorption features attributable to the hot stellar population are detected, but there is no evidence of C IV 1548, 1551 absorption. Thse data are used to set constraints on the origin of the UV upturn in elliptical galaxies. The lack of detectable C IV absorption and the shape of the continuum spectrum exclude star formation with a standard IMF as the sole contributor to the 1550-A flux. The observed continuum flux decreases from 1050 A to the Lyman limit, indicating that the light is dominated by stars with temperatures less than 25,000 K. For star-formation models, this temperature puts the main-sequence turnoff near B0, implying either that star formation ended about 2 x 10 exp 7 yr ago, or that stars are currently forming with a truncated IMF.

Determination of ionic abundances in the IO torus using the Hopkins ultraviolet telescope

Attention is given to a 415-1864-A spectrum of the emissions from the Io torus with about 3-A resolution in first order (830-1864 A) obtained with HUT in December 1990. A list of the 31 strongest features shows that, apart from five features that are totally or partially terrestrial, all are due to O(+), S(+), S(2+), and S(3+). There is no evidence for elements other than oxygen and sulfur. The spectrum shows many additional weaker emission features so that the existence of minor species cannot be ruled out at this time. Because the HUT spectrum contains well-resolved lines for all four of the major species, the mixing ratio (Ni/ne) for each one can be determined without using models of the spectral emissions, or a detailed geometric model of the torus plasma. The relative concentration of O(+) determined from the HUT observation is 38 percent less than that obtained from the Voyager UVS data.

The spectrum of the Jovian dayglow observed at 3 A resolution with the Hopkins ultraviolet telescope

Ultraviolet spectra of the Jovian equatorial dayglow in the spectral range 830-1850 A were obtained at about 3 A resolution. The observed spectrum is dominated by electron impact excitation of the H2 Lyman and Werner band systems. Solar Lyman-beta induced fluorescence in the (6, nu-double prime) Lyman band progression is clearly identified in five distinct P(1) lines, and the contribution of solar fluorescence to the total 2.3 kR slit-averaged H2 emission rate is estimated to be 17-22 percent. The electron excitation spectrum is characterized by a relative weakness of the Werner band system and the absence of cascade contributions to the Lyman system and is very similar to that of the south polar aurora. The integrated H2 emission rate in the 900-1100 A band is a factor of two lower than that measured by the Voyager UVS. Based on model calculations, photoelectron excitation does not appear able to account for the amount of observed electron-excited H2 emission.

Coronagraphic imaging of the Beta Pictoris circumstellar disk - Evidence of changing disk structure within 100 AU

New R-band images of the β Pictoris circumstellar disk obtained with the Adaptive Optics Coronagraph expose the disk inward to 40 AU from the star. From these images, the first reliable optical photometry of the disk within 100 AU of β Pic is reported. Across a radius of 100 AU, the radial power-law dependence of the disk-midplane surface brightness undergoes an abrupt transition, with power-law indices changing within 100 AU from -3.5 to -2.4 in the NE extension and from -4.2 to -1.9 in the SW extension. This result confirms the previously noted asymmetry in the brightness gradient beyond 250 AU, and suggests an inverted asymmetry within 100 AU. The geometrical thickness of the disk appears nearly constant within ∼115 AU and increases proportionally with radius beyond ∼115 AU

The spectrum of the tropical oxygen nightglow observed at 3 A resolution with the Hopkins Ultraviolet Telescope

Ultraviolet spectra of the tropical oxygen night-glow in the range of 830 to 1850 A (in first order) at 3 A resolution were obtained with the Hopkins Ultraviolet Telescope during the Astro-1 space shuttle mission in December 1990. We present here the data obtained on a setting celestial target as the zenith angle of the line-of-sight varied from 77 to 95°. The dominant features in the spectrum (other than geocoronal hydrogen) are OIλλ1304 and 1356 and the radiative recombination continuum near 911 A. The continuum is resolved and found to be consistent with an electron temperature in the range 1000 – 1250 K. The observed ratio of the brightness of OIλ1356 to the continuum suggests that O+ - O− mutual neutralization contributes about 40% to the 1356 A emission. The dependence of the optically thin emissions on zenith angle is consistent with a simple ionospheric model. Weak OIλ989 emission is also detected, but there is no evidence for any similarly produced atomic nitrogen emissions.

Discovery of a fast radiative shock wave in the Cygnus Loop using the Hopkins Ultraviolet Telescope

Observations of the far-ultraviolet spectrum of a bright radiative filament on the eastern edge of the Cygnus Loop supernova remnant are described which were made through the Hopkins Ultraviolet Telescope aboard the Astro-1 Space Shuttle mission in December 1990. Observations include strong emission lines of C III 977, N III 991, and O VI 032, 1038 and many fainter lines. A comparison of this spectrum with shock model calculations indicates that the emission at this position in the Cygnus Loop is dominated by 170 km/s shock wave.