Hubble Space Telescope Faint Object Research Articles

Photometric and Spectroscopic Studies of the Intermediate -Polar Cataclysmic System DQ Her

Abstract We report the results of spectral and photometric observations of DQ Her. Available spectra from International Ultraviolet Explorer (IUE), Hubble Space Telescope Faint Object Spectrograph (HST - FOS) and CCD photometry of one night were used. Some profiles revealing the variations of some spectral lines at different times are presented. There is variation with time for photometric observations and the brightness of DQ Her is changed from 14 mag. to 17.7 mag with clear display of burst. The ultraviolet luminosity for emitting region is in the range of (1.9x1031 erg s −1). The mass accretion rate is in the range of (3.5x10−12 M⊙yr −1). The line flux modulations at different times can be explained in terms of the accretion curtain model for intermediate polars, Kim & Beuermann (1996).

Ultraviolet to optical diffuse sky emission as seen by the Hubble Space Telescope Faint Object Spectrograph

Abstract We present an analysis of the blank-sky spectra observed with the Faint Object Spectrograph on board the Hubble Space Telescope. We study the diffuse sky emission from ultraviolet to optical wavelengths, which is composed of zodiacal light (ZL), diffuse Galactic light (DGL), and residual emission. The observations were performed towards 54 fields distributed widely over the sky, with spectral coverage from 0.2 to 0.7 μm. In order to avoid contaminating light from earthshine, we use the data collected only in orbital nighttime. The observed intensity is decomposed into the ZL, DGL, and residual emission, in eight photometric bands spanning our spectral coverage. We found that the derived ZL reflectance spectrum is flat in the optical, which indicates major contribution of C-type asteroids to the interplanetary dust (IPD). In addition, the ZL reflectance spectrum has an absorption feature at ∼0.3 μm. The shape of the DGL spectrum is consistent with those found in earlier measurements and model predictions. While the residual emission contains a contribution from the extragalactic background light, we found that the spectral shape of the residual looks similar to the ZL spectrum. Moreover, its optical intensity is much higher than that measured from beyond the IPD cloud by Pioneer 10/11, and also than that of the integrated galaxy light. These findings may indicate the presence of an isotropic ZL component, which is missed in the conventional ZL models.

ULTRAVIOLET SPECTROSCOPY OF PQ Gem AND V405 Aur FROM THEHSTANDIUESATELLITES

Ultraviolet spectra of two intermediate polars (IPs), PQ Gem and V405 Aur, observed with Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph and Faint Object Spectrograph and International Ultraviolet Explorer (IUE) satellites were analyzed during the period between 1994–2000. We estimated the reddening of the two systems from the 2200 A feature. Six spectra of the two systems revealing modulations of line fluxes at different times are presented. PQ Gem and V405 Aur are featured by spectral lines in different ionization states. This paper focuses on the third ionized carbon emission line at 1550 A and the first ionized helium emission line at 1640 A produced in the optically thin outer region of the accretion curtain for the two systems by calculating spectral line fluxes. From HST and IUE data, we deduced ultraviolet luminosities and ultraviolet accretion rates for the two binary stars. The average temperature of the accretion streams for PQ Gem and V405 Aur are ~4500 K and 4100 K, respectively. The results reveal that there are modulations in fluxes of spectral lines, ultraviolet luminosities, and ultraviolet accretion rates with time for both systems. These modulations are referred to the changes of both density and temperature as a result of the variations of mass transfer rate from the secondary star to the primary star. The current results are consistent with an accretion curtain model for IPs.

Ultraviolet spectroscopic study of EU UMa and ST LMi from HST and IUE observations

We present the first orbit-resolved ultraviolet spectroscopic observations of the two polar systems EU UMa and ST LMi obtained with the Hubble Space Telescope Faint Object Spectrograph (HST FOS) and International Ultraviolet Explorer (IUE) during their intermediate- and low-luminosity states in the period between 1982 and 2003. Different line profiles of the two systems showing variations of line fluxes at different orbital phases are presented. This paper focuses on the C IV emission line at 1550Å produced in the accretion stream, presenting calculations of spectral line fluxes, ultraviolet luminosities, and accretion rates for the two systems. Our analysis of the spectroscopic data reveals changes with orbital phase of the emission line profiles that correspond to the light curve variations of both EU UMa and ST LMi in the optical and infrared bands. The variations of line fluxes are attributed to variations of both density and temperature as a result of a changing rate of mass transfer from the secondary star to the white dwarf. The ultraviolet luminosity and accretion rate of EU UMa are smaller than the ultraviolet luminosity and accretion rate of ST LMi.

A SEARCH FOR MAGNESIUM IN EUROPA'S ATMOSPHERE

Europa's tenuous atmosphere results from sputtering of the surface. The trace element composition of its atmosphere is therefore related to the composition of Europa's surface. Magnesium salts are often invoked to explain Galileo Near Infrared Mapping Spectrometer spectra of Europa's surface, thus magnesium may be present in Europa's atmosphere. We have searched for magnesium emission in Hubble Space Telescope Faint Object Spectrograph archival spectra of Europa's atmosphere. Magnesium was not detected and we calculate an upper limit on the magnesium column abundance. This upper limit indicates that either Europa's surface is depleted in magnesium relative to sodium and potassium, or magnesium is not sputtered as efficiently resulting in a relative depletion in its atmosphere.

THE REDSHIFTED EXCESS IN QUASAR C IV BROAD EMISSION LINES

In this Letter, the Evans and Koratkar Atlas of Hubble Space Telescope Faint Object Spectrograph Spectra of Active Galactic Nuclei and Quasars is used to study the redward asymmetry in CIV broad emission lines (BELs). It is concluded that there is a highly significant correlation between the spectral index from 10 GHz to 1350 $\AA$ and the amount of excess luminosity in the red wing of the CIV BEL ($>99.9999%$ significance level for the full sample and the radio loud subsample independently, but no correlation is found for the radio quiet subsample). This is interpreted as a correlation between radio core dominance and the strength of the CIV redward asymmetry. The data implies that within the quasar environment there is BEL gas with moderately blueshifted emission associated with the purely radio quiet quasar phenomenon (the accretion disk) and the radio jet emission mechanism is associated with a redward BEL component that is most prominent for lines of sight along the jet axis. Thus, radio quiet quasars have CIV BELs that tend to show blueshifted excess and radio loud quasars show either a red or blue excess with the tendency for a dominant red excess increasing as the line of sight approaches the jet axis.

HIGH-DISPERSION SPECTRUM OF THE HALO PLANETARY NEBULA DdDm 1

Using the High Dispersion Spectrograph (HDS) at the Subaru Telescope, we secured the high-resolution line spectra in the 3600-7500 A wavelength range of the Galactic halo planetary nebula DdDm 1. We also analyzed the Hubble Space Telescope Faint Object Spectrograph data in the 1200-6730 A wavelength range. The diagnostic results indicate the electron temperatures of T {sub e}psilonapprox 11,000-14,000 K and the electron number densities of N {sub e}psilonapprox 2000-10,500 cm{sup -3}. In spite of high gaseous temperatures, we have not detected high excitation lines, e.g., He II. We derived abundance based on the ionic concentration of permitted and forbidden lines and the photoionization model. A comparison of the ionic concentrations from forbidden lines to recombination lines shows the abundance discrepancy between them. We tested various possibilities, e.g., temperature fluctuation and high-density blob components, to explain the discrepancy. The high-density components or density fluctuation might be partly responsible for the discrepancy. DdDm 1 shows a low carbon abundance that corresponds to metal-poor stars, [Fe/H] <=-1. Assuming a distance of 10 kpc to DdDm 1, theoretical models suggest that the central star has T {sub eff}approx = 39,000 K and Lapprox = 2000-3000 L {sub sun}. The relatively high gas temperaturesmore » appear to be caused by very low heavy elemental abundances or insufficient coolants in the shell gas. Its progenitor, born in an extremely carbon-poor environment as an initial mass of about 0.9 M {sub sun}, had probably experienced only the first dredge-up.« less

Resolving LB 11146 withHubble Space Telescope's Fine Guidance Sensor

On the basis of its observed spectra, LB 11146 was identified as a binary system composed of pair of massive (∼0.9 M⊙) white dwarf stars. However, observations with the Hubble Space Telescope (HST) Faint Object Camera did not resolve the system, and searches for radial velocity variations yielded null results. Subsequent observations of LB 11146 with the HST's Fine Guidance Sensor have resolved the system at three epochs spanning 295 days, thereby confirming the binary nature of this object. We find a nearly constant component separation of approximately 15 mas and a large change in position angle, consistent with a low-inclination, low-eccentricity orbit with a period of approximately 130 days. At a distance of 40 pc, this corresponds to a component separation of about 0.6 AU, which in turn also results in a Keplerian orbit with a period of about 130 days, assuming the spectroscopic masses of the white dwarfs. The size of the orbit is significantly smaller than the size of the progenitor stars as they evolved through the asymptotic giant branch, so one or more phases of binary star evolution is likely to have occurred. LB 11146 joins PG 1115+116, but in a more extreme sense, as an example of a close binary composed of massive white dwarfs that did not evolve to become a short-period system, thereby providing an additional key observational test for models of binary star evolution.

An Update on the X-Ray Transient Narrow-Line Seyfert 1 Galaxy WPVS 007:SwiftObservations of UV Variability and Persistence of X-Ray Faintness

We report on the detection of UV variability and the persistence of X-ray faintness of the X-ray transient narrow-line Seyfert 1 galaxy WPVS 007 based on the first year of monitoring with Swift between 2005 October and 2007 January. WPVS 007 has been an unusual source. Although it was X-ray-bright in the ROSAT All-Sky Survey, it has been extremely faint in all following X-ray observations. Swift also finds this NLS1 to be X-ray-faint, not detected at a 3 σ upper limit of 2.6 × 10-17 W m-2 in the 0.3-10.0 keV band, confirming that the AGN is still in a low state. During the 2006 July and December observations with the Swift UV-Optical Telescope (UVOT) the AGN became fainter by about 0.2 mag in the UV filters and by about 0.1 mag in V, B, and U compared with the 2005 October to 2006 January and 2006 September to October observations, followed by a rebrightening in the 2007 January observation. This variability can be caused either by a change in the absorption column density, and therefore the reddening in the UV, or by flux variations of the central engine. We also noticed that the flux in the UVOT filters agrees with earlier measurements by the International Ultraviolet Explorer taken between 1993 and 1995, but spectra taken by the Hubble Space Telescope Faint Object Spectrograph show that WPVS 007 was fainter in the UV by a factor of at least 2 in 1996. The flat optical/UV spectrum suggests that some UV extinction is present in the spectrum, but that alone cannot at all account for the dramatic fading in the X-ray flux. Most likely we see a partially covering absorber in the X-ray. Alternatively, the current X-ray emission seen from WPVS 007 may also be the emission from the host galaxy.

Quasar Spectrum Classification with Principal Component Analysis (PCA): Emission Lines in the Lyα Forest

We investigate the variety in quasar UV spectra (??1020-1600), with emphasis on the weak emission lines in the Ly? forest region, using principal component analysis (PCA). We use 50 smooth continuum-fitted quasar spectra (0.14 < z < 1.04) taken by the Hubble Space Telescope (HST) Faint Object Spectrograph. The first, second, and third principal component spectra (PCS) account for 63.4%, 14.5%, and 6.2% of the variance, respectively, and the first seven PCS account for 96.1% of the total variance. Three weak emission lines in the Ly? forest are identified as Fe II ?1070.95, Fe II + Fe III ?1123.17, and C III* ?1175.88. Using the first two standardized PCS coefficients, we introduce five classifications. By actively using PCS, we can generate artificial quasar spectra that are useful in testing the detection of quasars, DLAs, and the continuum calibration. We show that the power-law-extrapolated continuum is inadequate to perform precise measurements of the mean flux in the Ly? forest because of the weak emission lines and the extended tails of Ly? and Ly?/O VI emission lines. We show that we miss 5.3% of the flux on average, and there are cases where we would miss 14% of the flux. These corrections are essential in the study of the intergalactic medium at high redshift in order to achieve precise measurements of physical properties, cosmological parameters, and the reionization epoch.

Hubble Space Telescopeultraviolet spectroscopy of blazars: emission-line properties and black hole masses

The ultraviolet (UV) spectra of 16 blazars (<z) ≃ 1) from the archives of the Hubble Space Telescope Faint Object Spectrograph have been analysed in order to study in a systematic way the properties of their broad UV emission lines. We find that the luminosities of the most prominent and intense lines, Lyα and C IV λ1549, are similar to those of normal radio-loud quasars at comparable redshifts. However, the equivalent widths of blazar lines are significantly smaller than those of radio-loud quasars. Therefore, while the intrinsic broad-line region luminosity of blazars appears to be indistinguishable from that of radio-loud quasars, their continuum must be comparatively higher, most probably due to relativistic beaming. We have combined the UV luminosities of the debeamed continuum with the emitting gas velocity to derive estimates of the masses of the central supermassive black holes. The size of the broad-line region was computed in two ways: (1) via an empirical relationship between UV continuum luminosity and broad-line region size, and (2) through the external photon density required by blazar models to reproduce the inverse Compton components observed at γ-rays. The second method yields significantly different results from the first method, suggesting that it provides only a very rough estimate or a lower limit on the size of the broad-line region. We find that the average mass of the central black holes in blazars is ∼2.8 x 10 8 M ○. , with a large dispersion, comparable to those computed for other radio-loud active galactic nuclei.

Variability of Narrow, Associated Absorption Lines in Moderate‐ and Low‐Redshift Quasars

We present the results of a search for variability in the equivalent widths (EWs) of narrow, associated (|Δv| ≤ 5000 km s-1) absorption lines found in the UV spectra of z ≤ 1.5 quasars. The goal of this search was to use variability as a means of identifying absorption lines arising in gas that is intrinsic to the quasar central engine. We have compared archival Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) spectra of quasars with recent spectra obtained as part of our own snapshot survey of the same objects with the Space Telescope Imaging Spectrograph (STIS). The intervals between observations are 4-10 yr. We primarily focused on the C IV absorption lines, although we also studied other lines available in the same spectra (e.g., Lyα, N V, O VI). Our main result is that 4 out of 15 quasars, or 4 out of 19 associated absorption systems, contained variable narrow absorption lines, which are indicative of intrinsic absorption. We conclude that a minimum of 21% of the associated absorption-line systems are variable. Because not all systems will have necessarily varied, this is a lower limit on this fraction and is consistent with previous estimates based on variability, partial coverage analysis, or statistical arguments. If we interpret the upper limits on the variability timescale as upper limits on the recombination time of the absorber, we constrain the density of the absorber to be ne > 3000 cm-3 and its distance from the ionizing source to be R ≲ 100 pc. Moreover, we are now able to pick out specific intrinsic absorption-line systems to be followed up with high-dispersion spectroscopy in order to constrain the geometry, location, and physical conditions of the absorber. We briefly illustrate how follow-up studies can yield such constraints by means of a simulation.

A Complete Atlas of Recalibrated Hubble Space Telescope Faint Object Spectrograph Spectra of Active Galactic Nuclei and Quasars. I. Pre‐COSTAR Spectra

We have recalibrated all nonpolarimetric, pre-COSTAR, archival Hubble Space Telescope Faint Object Spectrograph UV and optical spectrophotometry of active galaxies and quasars in order to extract uniformly calibrated spectrophotometric data for further detailed scientific investigations. The raw archival spectra have been recalibrated using the latest algorithms and calibration data. Spectral data contaminated by intermittent noisy diodes and cosmic-ray events have been identified manually and eliminated. Wherever possible we have combined multiple observations of the same source to produce a single spectrum per object with the highest possible signal-to-noise ratio and covering the widest wavelength range, scaling the individual data sets to the same photometric scale where necessary. Detailed quality assurance has been performed to ensure that the merged object spectra are of the highest quality consistent with the limitations of the individual data sets and calibrations. The recalibrated, merged object spectra are available in electronic form. We use this data set to present statistics of the photometric accuracy in the wavelength overlap regions for observations spanning multiple gratings.

Observations of the Pulsating White Dwarf G 185–32

We observed the pulsating hydrogen atmosphere white dwarf G 185-32 with the Whole Earth Telescope in 1992. We report on a weighted Fourier transform of the data detecting 18 periodicities in its light curve. Using the Hubble Space Telescope Faint Object Spectrograph time resolved spectroscopy, and the wavelength dependence of the relative amplitudes, we identify the spherical harmonic degree (� ) for 14 pulsation signals. We also compare the determinations of effective temperature and surface gravity using the excited modes and atmospheric methods, obtaining Teff = 11 960 ± 80 K, log g = 8.02 ± 0.04 and M = 0.617 ± 0.024 M� .

An Archive of IUE Low‐Dispersion Spectra of the White Dwarf Stars

We have produced an archive of the ultraviolet low-dispersion spectra for the full set of white dwarf stars observed with the International Ultraviolet Explorer (IUE) satellite over the course of its 18 yr mission. This archive contains the spectra of 322 individual degenerate stars which have been processed to optimize the signal-to-noise for each star. In particular, all spectra have been corrected for residual temporal and thermal effects and placed on the Hubble Space Telescope Faint Object Spectrograph absolute flux scale using procedures described by Massa & Fitzpatrick. Wherever possible, multiple observations of individual stars have been co-added to further enhance signal-to-noise and have been combined into a single spectrum

Evidence of a Strong N [CSC]v[/CSC]/C [CSC]iv[/CSC] Correlation between Emission and Absorption Lines in Active Galactic Nuclei

The narrow absorption lines (NALs) that are seen in the rest-frame ultraviolet near the systemic redshift of active galactic nuclei (AGNs) are not always intrinsic to the near-nuclear region but may originate in the host galaxy or in neighboring galaxies intervening along the line of sight. A variety of criteria have been sought—and several identified—as evidence of an intrinsic origin. We have measured both emission and absorption lines in a Hubble Space Telescope Faint Object Spectrograph sample of objects with both C IV and N V NALs within ±5000 km s-1 of the systemic redshift. We find a strong (>99.5% confidence) linear correlation between the N V/C IV ratio in broad emission lines and that in NALs. A control sample of AGNs with NALs separated by larger velocities shows no such correlation. Our finding thus identifies an additional test for the intrinsic nature of NALs in any given object. The correlation shows that the chemical-enrichment histories and/or ionization parameters of the NAL clouds are closely related to those of clouds that produce the broad emission lines.

Extreme‐Ultraviolet Absorption Lines in Lyα Forest Absorbers and the Oxygen Abundance in the Intergalactic Medium

We create stacked composite absorption spectra from Hubble Space Telescope Faint Object Spectrograph data from four quasi-stellar objects to search for absorption lines in the extreme-ultraviolet wavelength region associated with Ly? forest absorbers in the redshift range 1.6 < z < 2.9. We successfully detect O V ?630 in Ly? absorbers throughout the 1013-1016.2 cm-2 column density range. For a sample of absorbers with 1013.2 cm-2 < N(H I) < 1014.2 cm-2, corresponding to gas densities ranging from around the universal mean to overdensities of a few, we measure an O V ?630 equivalent width of 10.9 ? 3.7 m?. We estimate that the detection is real with at least 99% confidence. We only detect O IV ?788, O IV ?554, O III ?833, and He I ?584 in absorbers with Ly? equivalent widths 0.6 ?, which are likely associated with traditional metal line systems. We find no evidence in any subsamples for absorption from N IV ?765, Ne V ?568, Ne VI ?559, Ne VIII ??770, 780, or Mg X ??610, 625. The measured equivalent widths of O V suggest values of O V/H I in the range -1.7 to -0.6 for 1013.2 cm-2 < N(H I) 1015 cm-2. The lack of detectable O IV absorption except in the strongest absorption systems suggests a hard ionizing background similar to the standard Haardt & Madau spectrum. Using photoionization models, we estimate that the oxygen abundance in the intergalactic medium with respect to the solar value is [O/H] ? -2.2 to -1.3. Comparing to studies of C IV, we estimate [O/C] ? 0.3-1.2. The overabundance of oxygen relative to carbon agrees with other low-metallicity abundance measurements and suggests enrichment of the intergalactic medium by Type II supernovae.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] and Ground-based Spectroscopy of K648 in M15

We derive some element abundances for the central star of the planetary nebula K648 from analysis of Hubble Space Telescope Faint Object Spectrograph spectra (1200–3300 A). Carbon is found to be approximately 4 times overabundant with respect to solar values, while oxygen is underabundant and silicon about solar, and He/H = 0.6. The stellar continuum indicates a reddening of E(B-V) = 0.08 and is well approximated by a synthetic model with Teff = 37,000 K and log g = 4.0, as found previously by Heber, Dreizler, & Werner. The scaling of the synthetic model with the measured UV flux indicates R = 1.3 R⊙, and thus log (L/L⊙) = 3.45, M = 0.62 M⊙, assuming a distance of D = 10.3 kpc. The C IV exhibits a wind profile, from which a terminal wind velocity of V∞ = 1600 ± 150 km s-1 is derived and a column density of Ni(C IV) = 6 ± 3 × 1015 cm-2. From ground-based spectra obtained at the William Hershel 4.2 m telescope we derive the first measurement of the expansion velocity for the nebula: Vexp([N II]) = 11.9 km s-1 and Vexp(Hα) = 16.7 km s-1. Combined with the nebular radius measured from HST imaging in our previous paper, the velocity yields an expansion age of the object Agekinematic ≥ 4270 yr. This value is slightly higher than predictions from post-AGB evolutionary models for a ≈0.6 M⊙ He-burning remnant. The photospheric overabundance of He and C, as well as the low luminosity of the central star, are consistent with a post-AGB object that experienced a late thermal pulse and consequent third dredge-up, and which is now in a luminosity-dip He-burning phase. The progenitor mass is fairly low.

The Cuspy Liner Nucleus of the S0/a Galaxy NGC 2681

The nucleus of the bulge-dominated, multiply barred S0/a galaxy NGC 2681 is studied in detail using the high-resolution Hubble Space Telescope Faint Object Camera (FOC), Near-Infrared Camera and Multiobject Spectrometer (NICMOS) imaging, and the Faint Object Spectrograph (FOS). The ionized gas central velocity dispersion is found to increase by a factor ≈2 when narrowing the aperture from R ≈ 15 (ground) to R ≈ 01 (FOS). Dynamical modeling of these velocity dispersions suggests that NGC 2681 does host a supermassive black hole (BH) for which one can estimate a firm mass upper limit MBH ≲ 6 × 107 M☉. This upper limit is consistent with the relation between the central BH mass and velocity dispersion MBH - σ known for other galaxies. The emission-line ratios place the nucleus of NGC 2681 among LINERs. It is likely that the emission-line region comes from a rather mild, but steady, feeding of gas to the central BH in this galaxy. The inner stellar population lacks any measurable color gradient (to a radius of 0.6 kpc) from the infrared to the ultraviolet, consistently with FOC, FOS, and IUE data, all indicating that this system underwent a starburst ≈1 Gyr ago that encompassed its whole interior, down to its very center. The most likely source of such a widely distributed starburst is the dumping of tidally extruded gas from a galaxy neighbor. If so, then NGC 2681 can be considered as the older brother of M82, seen face-on as opposed to the edge-on view we have for M82.

Observations of Lyα Absorption in a Triple Quasar System

We have obtained follow-up observations of the quasar pair LBQS 0107-025A,B and new observations of the nearby quasar LBQS 0107-0232 with the Hubble Space Telescope Faint Object Spectrograph. Extended wavelength coverage of LBQS 0107-025A and B using the G270H grating was also obtained. This triple system is unique in providing sensitivity to coherent Ly-alpha absorption on transverse scales of approximately 1 Mpc at z < 1. Monte Carlo simulations were used to establish the confidence level for matches between absorption features in different lines of sight as a function of velocity separation. Pairwise, there are 8, 9, and 12 lines that match between spectra. Three instances of matches between all three lines of sight were found with velocity separations of less than 550 kms^-1. Two of the pairings have coincident lines within |Delta v| < 200 kms^-1 that would occur with less than 10% probability by chance. Taking into account the equivalent widths of the lines, one of these triple coincidences is significant at the 99.99% confidence level, based on Monte Carlo simulations with random line placements. Matches with strong lines preferentially have small velocity separations. These same simulations are used to demonstrate that the distribution of matches for a population of absorbers randomly distributed in velocity space is peaked toward |Delta v| = 0, which has implications for the statistical significance of matches. One of the triple coincidences appears to be a strong absorber with a sheet-like, but inhomogeneous, geometry and a coherence length approaching or exceeding 1 Mpc.