Hubble Space Telescope Faint Object Research Articles

ITAL]Hubble Space Telescope[/ITAL] Observations of the Middle-aged Pulsar 0656+14

Multiwavelength spectral observations of middle-aged (τ~104-106 yr) isolated pulsars enable one in principle to separate thermal radiation emitted from the neutron star surface and nonthermal radiation from its magnetosphere. We have previously detected the middle-aged radio and soft X-ray pulsar 0656+14 with the Hubble Space Telescope Faint Object Camera (FOC) long-pass filter F130LP. In the present Letter we report new FOC observations of this pulsar with three broadband filters: F430W, F342W, and F195W. The optical-UV spectral flux can be fitted with a two-component spectral model that combines a power-law spectrum with a Rayleigh-Jeans spectrum. The nonthermal component with the power-law index α=1.4−0.7+ 0.6 dominates at λ3000 A. The thermal component is characterized by the Rayleigh-Jeans parameter, G ≡ T6(R10/d500)2, where T=106T6 K is the brightness temperature, R=10R10 km is the neutron star radius as seen by a distant observer, and d=500d500 pc is the distance. For a plausible extinction, E(B-V)=0.03, we obtained G=3.6+ 1.6−2.0. The observed shape of the optical-UV spectrum of PSR 0656+14 differs drastically from those of both younger pulsars (Crab, 0540-69, Vela) and the older pulsar Geminga.

ITAL]Hubble Space Telescope[/ITAL] Observations of the Dwarf Nova WZ Sagittae: A Very Rapidly Rotating White Dwarf

We present Hubble Space Telescope Goddard High-Resolution Spectrograph G140L spectra of the white dwarf in WZ Sge, exposed during quiescence. Our best-fitting synthetic spectra yield a rapidly rotating white dwarf with velocity Vrot sin i = 1200+ 300−400 km s-1, white dwarf effective temperature Twd = 14,800 K, the gravity log g = 8.0, and the chemical abundances relative to solar in number (with 3σ error bars) of C, 5.0+ 2.0−2.0; N, 3.0+ 1.0−1.0; and Si, <0.1. We also reexamined a previous Hubble Space Telescope Faint Object Spectrograph G130H spectrum reported by Sion and coworkers and obtained consistent values, but lower C and N abundances. We do not detect a measurable systematic shift in our observed spectra, presumably due to the extremely low mass of the companion.

Spin and beat phenomena in time-resolved Hubble Space Telescope UV spectroscopy of PQ Gem

Results of the first low-resolution (6 Å), time-resolved Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) observation of the prototypical strong-field intermediate polar system, PQ Gem, are presented. The λλ1150–2600 continuum light curve is dominated by the 13.9-min rotational signature of the white dwarf at all UV wavelengths covered, with a broadly constant fractional modulation depth. The rotational profile contains a dip which is deepest in the far-UVand which we believe, like its X-ray counterpart, is caused by stream occultation of the white dwarf. The continuum and emission-line fluxes are also modulated on the 14.5-min beat period but, remarkably, vary in antiphase. This complex behaviour facilitates the identification and partial isolation of two spin- modulated spectral components and a beat component. One spin component has a blue spectral distribution whose temperature is ≾ 50 000 K if no allowance is made for the absorbing effects of the stream, but may be much hotter (consistent with earlier X-ray estimates) if, as seems likely, the absorber is not completely optically thick. The other spin-modulated component has a red spectral distribution whose temperature (≾ 10 000 K) and luminosity probably associate it with the magneto spheric accretion flow itself. The beat continuum component has a temperature in the region of 17 000 K and appears to be radiated by a region whose size is comparable to that of the white dwarf. The beat pulsation in the emission lines may also originate from a region of similar dimensions. We consider one- and two-site hypotheses to explain the antiphased line and continuum beat modulations, but are unable to arrive at a convincing solution.

Final Astro‐1 Calibration of the Hopkins Ultraviolet Telescope: A Test of White Dwarf Model Atmospheres as Far‐Ultraviolet Calibration Standards

We present the final calibration of the Hopkins Ultraviolet Telescope (HUT) for its flight during the Astro-1 space shuttle mission in 1990 December. The calibration is defined by a comparison between an observation obtained in flight of the hot DA white dwarf G191-B2B and the model atmosphere for this star calculated by P. Bergeron. We also present the postflight laboratory calibration of the HUT sensitivity, which we find to be in excellent agreement with the white dwarf calibration. The ratio of the white dwarf to the laboratory effective area calibration has a mean value of 1.0044 over the wavelength range 912-1840 Å, a slope of 7.61 × 10-6 Å-1 (which corresponds to a relative difference of only 0.35% at either end of the spectrum), and fluctuations about the mean value with an rms amplitude of 3%. The uncertainties in the laboratory calibration are 4% for the overall normalization, 5% relative to the mean on wavelength scales of 50 to several hundred Å, and an uncertainty in the overall slope that corresponds to 1% at 1840 Å and that increases smoothly to 3% at 912 Å. These results validate the use of white dwarf models as absolute flux standards in the far-ultraviolet, especially for wavelengths shortward of Lyα, where previous experiments often disagreed by large factors. In particular, the normalization and wavelength dependence of the sensitivity calibration adopted by the Hubble Space Telescope Faint Object Spectrograph is confirmed to within the uncertainties of our laboratory calibration, while the normalization of the IUE final calibration is found to be low by an amount just exceeding our 1 σ uncertainty. Fits to the spectra of both G191-B2B and the hot DA white dwarf HZ 43 yield effective temperatures and surface gravities in good agreement with values derived from Balmer line profiles. We also present a comparison with Voyager spectra for the stars G191-B2B, HZ 43, BD +28°4211, π Aqr, and HD 25443. When averaged over broad bands (912-1050 Å, 1050-1180 Å), fluxes from the two instruments typically agree to within 10%.

Hubble Space Telescope Faint Object Camera calculated point-spread functions

A set of observed noisy Hubble Space Telescope Faint Object Camera point-spread functions is used to recover the combined Hubble and Faint Object Camera wave-front error. The low-spatial-frequency wave-front error is parameterized in terms of a set of 32 annular Zernike polynomials. The midlevel and higher spatial frequencies are parameterized in terms of set of 891 polar-Fourier polynomials. The parameterized wave-front error is used to generate accurate calculated point-spread functions, both pre- and post-COSTAR (corrective optics space telescope axial replacement), suitable for image restoration at arbitrary wavelengths. We describe the phase-retrieval-based recovery process and the phase parameterization. Resultant calculated precorrection and postcorrection point-spread functions are shown along with an estimate of both pre- and post-COSTAR spherical aberration.

Hubble Space TelescopeFaint Object Spectrograph Spectroscopy of Localized Chemical Enrichment from Massive Stars in NGC 5253

Optical and ultraviolet spectroscopy obtained with the Hubble Space Telescope Faint Object Spectrograph at three locations in the central H II complex of the amorphous starburst galaxy NGC 5253 confirms an apparent area of enhanced nitrogen abundance seen in ground-based studies. At two positions, N appears enhanced by a factor of 3 [log (N/O) = -0.85] compared to a third location 50 pc away where the measured N abundance is typical of metal-poor galaxies [log (N/O) = -1.30]. No other elemental species shows spatial abundance fluctuations, including C as measured from the C III] ??1907, 1909 ? emission lines. Extinction to the emission-line regions is quite low (AV = 0.25), and we rule out reddening uncertainties as the cause of the apparent N enhancement. Comparison of the observed ionic fractions to predictions of photoionization models shows that ionization uncertainties cannot be responsible for the anomalously high N/O ratio. These findings require the existence of a N production mechanism that is decoupled from C and O production. Although the high N/He enrichment ratio is consistent with that expected from non-Type I planetary nebulae (PNs), the localized nature of the enrichment, the large number of PNs (150-500), and the long timescales (109 yr) required make this explanation highly unlikely. Given the proximity of the N overabundance to a very young, heavily obscured star cluster, we attribute the N excess to recent pollution from massive stars. Plausible N enrichment scenarios involve O star winds, He-deficient W-R star winds, and/or ejection events from luminous blue variables. If the N enrichment is due to localized pollution from the winds of W-R stars observed in the burst, then an accompanying He enrichment of 40% ought to be observed, but we are able to rule out He enhancements at the 2 ? level at both locations showing N enhancement. We propose that if the N-rich wind or ejecta is incorporated into self-gravitating clumps of molecular gas, and if the 106 yr old clusters of massive stars in starburst galaxies are precursors of globular clusters, then N-overabundant halo and globular cluster stars in the Galaxy may owe their chemical peculiarity to similar N-enrichment episodes early in the history of the Milky Way.

Motion of the Ultraviolet R Aquarii Jet

We present evidence for subarcsecond changes in the ultraviolet (~2550 A) morphology of the inner 5 arcseconds of the R Aqr jet over a 2 yr period. These data were taken with the Hubble Space Telescope (HST) Faint Object Camera (FOC) when the primary mirror flow was still affecting observations. Images of the R Aqr stellar jet were successfully restored to the original design resolution by completely characterizing the telescope-camera point spread function (PSF) with the aid of phase-retrieval techniques. Thus, a noise-free PSF was employed in the final restorations which utilized the maximum entropy method (MEM). We also present recent imagery obtained with the HST/FOC system after the COSTAR correction mission that provides confirmation of the validity of our restoration methodology. The restored results clearly show that the jet is flowing along the northeast (NE)-southwest (SW) axis with a prominent helical-like structure evident on the stronger NE side of the jet. Transverse velocities increase with increasing distance from the central source, providing a velocity range of 36-235 km s-1. From an analysis of proper motions of the two major ultraviolet jet components, we detect an ~40.2 yr event separation of this apparent enhanced material ejection occurring probably at periastron which is consistent with the suspected ~44 yr binary period; this same analysis shows that the jet is undergoing nonlinear acceleration which suggests that the propulsive force probably results from magnetohydromagnetic effects. The restoration computations and the algorithms employed demonstrate that mining of flawed HST data can be scientifically worthwhile.

Lyα Absorption‐Line Systems in the Gravitational Lens Q0957+561

Far-ultraviolet spectra of the gravitational lens components Q0957+561A and B were obtained with the Hubble Space Telescope Faint Object Spectrograph (HST FOS). Two previously known absorption-line systems were detected at redshifts zdamped = 1.3911 and zLyα = 1.1249. Their prominent absorption features are superposed on intense QSO continuum emission between λλ900-1400 in the quasar rest frame. Strong O VI λ1033, Lyα λ1216, and N V λ1240 line emission found at the QSO redshift (zQSO = 1.41) accompany the absorption-line systems. Lyα through Ly associated with the damped absorption system were found in both lensed components, together with other ionic species of N I, N III, C II, C III, Si II, Si III, and O I. We tentatively identify O VI λλ1033, 1037 absorption at the damped Lyα redshift, which, if confirmed, would be the highest ionization species yet detected in such systems. The equivalent widths of the Lyman series in Q0957+561A are measurably greater compared with absorption in 0957+561B, consistent with the narrower and shallower depth of the Lyman series line profiles in image B. The differences of the damped Lyman series absorption in the lensed components are the only significant spectral characteristic that distinguishes the far-ultraviolet spectra of 0957+561A and B. These results indicate that the damped Lyα absorber is inhomogeneous over scale lengths of ~200 pc, which corresponds to the beam separation at the damped Lyα redshift. However, the equivalent widths of neutral and ionized metals in lens components A and B are correlated, which suggests these spectral features arise in an extended region. The metal line-absorption strength is consistent with lower column densities compared with the hydrogen line-forming region. Thus, the small coherence length scale indicated by the difference in hydrogen line absorption between the lensed components suggests the geometric ray paths intercept different regions of a galactic disk that is viewed pole-on, while the metal absorption occurs in the halo.

ITAL]Hubble Space Telescope[/ITAL] Discovers Optical Emission from the Radio Pulsar PSR 1055−52

Optical/UV emission from isolated neutron stars, if detected, represents a valuable indicator of the pulsar's emission mechanisms, and more so for those objects of "intermediate" age, where thermal and nonthermal processes are known to coexist. PSR 1055-52 is a middle-aged pulsar and a strong γ-ray emitter. As such, it represents a logical target, but the presence of a nearby star renders optical ground-based searches totally hopeless. The Hubble Space Telescope Faint Object Camera (FOC), however, was successful in detecting the pulsar's emission through its U (F342) filter. It is interesting to compare such emission with an extrapolation of the soft, thermal part of the ROSAT X-ray spectrum. Within the uncertainties, it appears that the FOC flux is compatible with the Rayleigh-Jeans side of such a spectrum, as was the case for Geminga, in many ways a similar object. PSR 1055-52 becomes one of three pulsars (besides the Crab and Vela) visible at all wavelengths.

An Atlas of Hubble Space Telescope Ultraviolet Images of Nearby Galaxies

We present an atlas of UV ($\sim 2300$ \AA) images, obtained with the Hubble Space Telescope (HST) Faint Object Camera, of the central $22''\times 22''$ of 110 galaxies. The observed galaxies are an unbiased selection constituting about one half of a complete sample of all large ($D>6'$) and nearby ($V< 2000$ km s$^{-1}$) galaxies. This is the first extensive UV imaging survey of normal galaxies. The data are useful for studying star formation, low-level nuclear activity, and UV emission by evolved stellar populations in galaxies. At the HST resolution ($\sim 0.05''$), the images display an assortment of morphologies and UV brightnesses. These include bright nuclear point sources, compact young star clusters scattered in the field or arranged in circumnuclear rings, centrally-peaked diffuse light distributions, and galaxies with weak or undetected UV emission. We measure the integrated $\sim 2300$ \AA flux in each image, classify the UV morphology, and examine trends between these parameters and the optical properties of the galaxies.

Ultraviolet Extinction by Interstellar Dust in External Galaxies: M31

Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) spectra of stars in OB associations of M31 are used to derive the UV extinction by interstellar dust in M31 by three different methods: (1) comparing spectra of M31 star pairs, (2) comparing spectra of M31 stars to those of Galactic standard stars, and (3) comparing M31 star spectra to atmosphere models. The derived intrinsic M31 extinction curve has an overall wavelength dependence very similar to that of the average Galactic extinction curve but possibly has a weaker 2175 Å bump, however, with a significance of only 1 σ. This result is different from the LMC (30 Dor)-like curves published earlier, which contained both intrinsic M31 extinction and "foreground" extinction, and were based either on low-signal IUE spectra, or on FOS data affected by inaccuracy in the preliminary flux calibration, and were not computed with the pair method used in this work.

The [ITAL]Hubble Space Telescope[/ITAL] Quasar Absorption Line Key Project: The Unusual Absorption-Line System in the Spectrum of PG 2302+029—Ejected or Intervening?

We report the discovery of a high-ionization broad absorption line system at a redshift of zabs = 0.695 in the spectrum of the zem = 1.052 radio-quiet quasar PG 2302+029. Broad absorption with FWHM from 3000 to 5000 km s-1 is detected from C IV, N V, and O VI in Hubble Space Telescope (HST) Faint Object Spectrograph spectra of the quasar. A narrow-line system (FWHM ~ 250 km s-1) at zabs = 0.7016 is resolved from the broad blend and includes absorption by Lyα and the C IV, N V, and O VI doublets. No absorption by low-ionization metal species (e.g., Si II and Mg II) is detected in the HST or ground-based spectra for either the broad or the narrow system. The centroids of the broad system lines are displaced by ~56,000 km s-1 to the blue of the quasar's broad emission lines. The reddest extent of the broad-line absorption is more than 50,000 km s-1 from the quasar. The properties of this system are unprecedented, whether it is an intervening or an ejected system.

Far-Ultraviolet Spectra of Broad Absorption Line QSOs and Constraints on Models for the Ionization Structure and Metallicity of the BAL-Region Gas

view Abstract Citations (54) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Far-Ultraviolet Spectra of Broad Absorption Line QSOs and Constraints on Models for the Ionization Structure and Metallicity of the BAL-Region Gas Turnshek, David A. ; Kopko, Michael, Jr. ; Monier, Eric ; Noll, Donette ; Espey, Brian R. ; Weymann, Ray J. Abstract Hubble Space Telescope Faint Object Spectrograph G270H spectra of four moderate-to high-redshift broad absorption line (BAL) QSOs are presented. In addition, evidence is discussed that indicates that the gas that gives rise to the BALs has nonuniform ionization and enhanced abundances. In the context of a photoionization model, ionization parameter fluctuations of at least ~32-16 are needed to explain the observed column densities of different ions of the same element. In such a model, the gas must have very enhanced metal-to-hydrogen abundances relative to solar composition. However, the actual metal-to-hydrogen abundance enhancements are difficult to constrain because they are so model-dependent. For example, in a photoionization model the shape of the photoionizing continuum has a significant influence on the derived metal-to-hydrogen enhancement, but for normally adopted shapes the enhancement is very large, i.e., an enhancement of ~120-230 times solar for nitrogen. If collisional ionization is important, the need for metal-to-hydrogen abundance enhancements would be severely reduced. The derived relative metal abundances are somewhat more robust because the relevant metal-line transitions correspond to similar ionization potentials. In a photoionization model, nitrogen enhancements relative to oxygen and carbon of ~4-3 and 9:-10: times solar values, respectively, are indicated. If normal stellar nucleosynthesis is important, the results may be indicative of star formation with a relatively flat initial mass function, with the nitrogen overabundance produced by secondary processing in massive stars. However, large enhancements of elements like phosphorus in some objects suggest that the enrichment scenario may be more complex. In addition, the results discussed here place constraints on the details of a photoionization model for the BAL-region gas. These constraints are unphysical enough that they may indicate that central- source photoionization is not dominant. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177227 Bibcode: 1996ApJ...463..110T Keywords: GALAXIES: QUASARS: ABSORPTION LINES; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: H0059-735; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: Q0226-024; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: H0903+1734; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: Q0932+5006; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: Q1309-0536 full text sources ADS | data products SIMBAD (7) NED (4) MAST (1) ESA (1)

The Ultraviolet Spectrum of DQ Herculis: Detection of Line and Continuum Pulsation

view Abstract Citations (20) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Ultraviolet Spectrum of DQ Herculis: Detection of Line and Continuum Pulsation Silber, Andrew D. ; Anderson, Scott F. ; Margon, Bruce ; Downes, Ronald A. Abstract The old nova DQ Herculis was observed with the Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) in the 1150-2500 Å range. Spectra were taken with a time resolution of 4.08 s, allowing a search for pulsations of the UV light at the 71 s white dwarf spin period, analogous to those long known to be present in visible light. The integrated UV spectrum shows strong line emission in Lyα, N V, Si IV/O IV], C IV, and He II. The flux of the UV continuum, Si IV/O IV], C IV, and He II drop during the eclipse by the late-type star, indicating that most of this emission is generated near the white dwarf. There is a hump in the UV light curve prior to eclipse, due to a hot spot at the edge of the accretion disk with T ∼ 25,000 K and area ∼4 x 1018 cm-2. Our FOS data yield the first report of coherent 71 s pulsed UV emission from DQ Her. Pulsations in the UV continuum are detected with half-amplitudes of 4.5% during the first HST orbit, 2.5% during the second HST orbit prior to eclipse, and 1.2% during the eclipse ingress. The UV amplitude is clearly time-variable, also known to be the case in visible light, as no continuum pulsations are seen during the third HST orbit (upper limit 2%). Most interesting, however, is the behavior of the UV emission lines. Prior to the eclipse ingress, Lyα is seen to pulse with at least a 16% amplitude, and displaced 0.27 in phase later than that of the continuum pulse. During the ingress, C IV pulses with an amplitude of 6%, displaced by 0.30 earlier in phase than the continuum. These phase shifts may indicate that the true period of the line emission is slightly different from the continuum pulsation. The UV pulsations clearly hold great promise as an aid in unraveling the complexities of this well-studied but still poorly understood system. ROSAT has detected faint X-ray emission from DQ Her with LX(0.1-2.O keV) 1030 ergs s-1. While the observed X-ray emission is consistent with an origin from the late-type secondary star, the strength of He II λ1640 suggests there is an additional hidden X-ray component, of strength (LX 1034 ergs s-1) comparable to that seen in other DQ Her stars. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177162 Bibcode: 1996ApJ...462..428S Keywords: STARS: NOVAE; CATACLYSMIC VARIABLES; STARS: INDIVIDUAL CONSTELLATION NAME: DQ HERCULIS; STARS: OSCILLATIONS; ULTRAVIOLET: STARS full text sources ADS | data products SIMBAD (7) MAST (1) HEASARC (1) ESA (1)

Mars ozone measurements near the 1995 aphelion: Hubble space telescope ultraviolet spectroscopy with the faint object spectrograph

Ultraviolet (225–330 nm) spectral scans of Mars were obtained with the Hubble space telescope (HST) faint object spectrograph (FOS) in February of 1995. These spectra yield ozone column abundances, cloud opacities (0.2–0.4 at low latitudes), and polar seasonal ice albedos from southern midlatitudes to northern high latitudes on Mars. At the time of these measurements, Mars was at a solar longitude (Ls) of 63.5°, corresponding to the late northern spring season on Mars, and very near to Mars aphelion. The most important result of these observations is the measurement of low‐latitude ozone abundances (3.1−0.5+2.1), which are significantly (≥100%) elevated relative to the northern fall (Ls = 208°, pre‐perihelion) IR ozone measurements of Espenak et al. [1991] in 1988. The implied perihelion‐to‐aphelion increase in the global Mars ozone column (from 1.5−1.0+0.4 to 3.1−0.5+2.1 μm atm) is quantitatively consistent with photochemical modeling analysis of Clancy and Nair [this issue], which predicts large annual variations in Mars photochemistry due to orbital forcing of the altitude of global water vapor saturation on Mars [Clancy et al., 1996]. However, the HST FOS observations are not diagnostic of the altitudes at which Mars ozone densities vary with Ls, which is a key aspect of the Clancy and Nair model prediction. Furthermore, it is the uncertain ozone density profile which leads to the large asymmetric uncertainties in the derived FOS and IR ozone columns. An ozone column of 7.3 ± 2.5 μm atm is retrieved for a high northern latitude region (71–75°N). The derived ultraviolet albedo of the north polar seasonal CO2 cap is 0.18 ± 0.07, which is roughly 10 times the ultraviolet albedo of the silicate surface of Mars, but only one quarter the visible albedo of the seasonal CO2 ice cap.

Fluorescent hydroxyl emissions from Saturn's ring atmosphere.

Just before earth passed through Saturn's ring plane on 10 August 1995, the Hubble Space Telescope Faint Object Spectrograph detected ultraviolet fluorescent emissions from a tenuous atmosphere of OH molecules enveloping the rings. Brightnesses decrease with increasing distance above the rings, implying a scale height of about 0.45 Saturn radii (Rs). A spatial scan 0.28Rs above the A and B rings indicates OH column densities of about 10(13) cm(-2) and number densities of up to 700 cm(-3). Saturn's rings must produce roughly 10(25) to 10(29) OH molecules per second to maintain the observed OH distribution.

Correction: Rapid energy dissipation and variability of the lo-Jupiter electrodynamic circuit

Nature 379, 323-325 (1995) IN Fig. 1 of this Letter, a dashed line was omitted that was referred to in the legend to Fig. 30. The corrected figure is shown here. Images of the FUV Jovian aurorae and of the far-ultraviolet signature of the lo magnetic flux tube with the Hubble Space Telescope Faint Object Camera (11 96).

On the Nature of the Outer Rings around SN 1987A

We present recent Hubble Space Telescope (HST) Faint Object Spectrograph and ground-based spectroscopic observations of SN 1987A and its surroundings and discuss them in conjunction with HST Faint Object Camera and Wide Field Planetary Camera 2 narrowband imaging. We have determined the following properties of the outer rings around SN 1987A: (1) the ionization and the temperature in the northern outer ring of SN 1987A are similar to those measured in the inner ring; (2) the [O II] electron density in the outer rings, both the northern and the southern, is ~800 cm-3, less than a third of that in the inner ring; (3) the N/O and N/C abundance ratios measured in the northern ring are ~3 times lower than in the inner ring, whereas the N/H abundance ratio is about half the value measured in the inner ring. Moreover, narrowband imaging indicates that the prevailing conditions in both the northern and the southern outer rings are quite similar to each other and to those measured in the portion of the northern outer ring that projects on the ejecta. These findings indicate that the gas in the outer rings consists of material that was ejected by the supernova progenitor at an earlier stage of evolution than the inner ring, when CNO processing was less advanced. Comparison with evolutionary models suggests that the material that constitutes the outer rings was ejected from the supernova progenitor ~104 yr before the bulk of the inner-ring material was ejected.

The albedo spectrum of Europa from 2200 Å to 3300 Å

We obtained a spectrum of Europa from 2224 to 3302 Å with high signal to noise and a resolution of 6.3 Å with the Hubble space telescope faint object spectrograph. We detect a broad absorption feature centered at 2800 Å, seen previously at lower signal to noise in International Ultraviolet Explorer spectra of Europa. No additional absorption features are detected. We compare the observed spectrum to laboratory spectra of sulfur ions in water ice and sulfur dioxide ice on water ice. The absorption in Europa is a closer match to sulfur dioxide ice. The poor fit of ice irradiated by sulfur ions may indicate that a direct source of SO2 is required to explain the observed absorption.

The Hubble Space Telescope Sample of Radio-loud Quasars: The LY alpha /H beta Ratio

view Abstract Citations (64) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Hubble Space Telescope Sample of Radio-loud Quasars: The LY alpha /H beta Ratio Netzer, Hagai ; Brotherton, M. S. ; Wills, Beverley J. ; Han, Mingsheng ; Wills, D. ; Baldwin, J. A. ; Ferland, G. J. ; Browne, I. W. A. Abstract We have used the first Hubble Space Telescope Faint Object Spectrograph spectra of our sample of radio- loud quasars, and quasi-simultaneous ground-based spectrophotometry, to investigate the intensity ratio Lyα/Hβ, whose small observed values are one of the outstanding problems of active galactic nuclei research. The present sample of 20 quasars with complete flux and profile data shows the first significant correlations of this ratio with other observed properties. The strongest correlations are with various continuum slope indicators: we find smaller Lyα/Hβ ratios in quasars whose continua rise more steeply into the red. The long-wavelength continuum slope (1909-4861 Å) is strongly correlated with Lyα/Hβ, but the short-wavelength continuum slope (1215ff1909 Å) is not. A separation into line components shows that the above correlations arise mostly from the red wings of the lines. The core-to-wing flux ratio is also correlated with the slope. The correlation of Lyα/Hβ with continuum slope is consistent with line and continuum reddening by an external dust screen with Galactic-type extinction of up to EB-V = 0.3. In this case the intrinsic Lyα/Hβ ratio is ∼20. However, other trends expected if dust were the sole factor are not seen. There are indications that core-dominated and lobe-dominated sources differ in their Lyα/Hβ and continuum slope dependence. We calculate a grid of theoretical hydrogen line ratios and use it to investigate reddening and alternative explanations, such as dependence upon ionizing flux. We suggest that several different mechanisms are operating. Publication: The Astrophysical Journal Pub Date: July 1995 DOI: 10.1086/175939 Bibcode: 1995ApJ...448...27N Keywords: GALAXIES: ACTIVE; GALAXIES: QUASARS: EMISSION LINES; GALAXIES: QUASARS: GENERAL; LINE: PROFILES; RADIO CONTINUUM: GALAXIES; ULTRAVIOLET: GALAXIES full text sources ADS | data products SIMBAD (20) NED (20) MAST (1) ESA (1)