Space Telescope Imaging Spectrograph Data Research Articles

3C 273 host galaxy with Hubble Space Telescope coronagraphy

The close-in regions of bright quasars’ host galaxies have been difficult to image due to the overwhelming light coming from quasars. With coronagraphic observations in visible light using the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope, we removed 3C 273 quasar light using color-matching reference stars. The observations revealed the host galaxy from 60″ to 0.​​″2 with nearly full angular coverage. Isophote modeling has revealed a new core jet, a core blob, and multiple smaller-scale blobs within 2.​​″5. The blobs could potentially be satellite galaxies or infalling materials towards the central quasar. Using archival STIS data, we constrained the apparent motion of its large scale jets over a 22 yr timeline. By resolving the 3C 273 host galaxy with STIS, our study validates the use of coronagraphs on extragalactic sources for obtaining new insights into the central (at ∼kpc scales) regions of quasar hosts.

Spectral analysis of cool white dwarfs accreting from planetary systems: from the ultraviolet to the optical

ABSTRACT The accretion of planetary debris into the atmospheres of white dwarfs leads to the presence of metal lines in their spectra. Cool metal-rich white dwarfs, which left the main sequence many Gyr ago, allow the study of the remnants of the oldest planetary systems. Despite their low effective temperatures (Teff), a non-negligible amount of their flux is emitted in the near-ultraviolet (NUV), where many overlapping metal lines can potentially be detected. We have observed three metal-rich cool white dwarfs with the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope, and compare the results determined from the NUV data with those previously derived from the analysis of optical spectroscopy. For two of the white dwarfs, SDSS J1038−0036 and SDSS J1535+1247, we find reasonable agreement with our previous analysis and the new combined fit of optical and NUV data. For the third object, SDSS J0956+5912, including the STIS data leads to a 10 per cent lower Teff, though we do not identify a convincing explanation for this discrepancy. The unusual abundances found for SDSS J0956+5912 suggest that the accreted parent body was composed largely of water ice and magnesium silicates, and with a mass of up to ≃ 2 × 1025 g. Furthermore, SDSS J0956+5912 shows likely traces of atomic carbon in the NUV. While molecular carbon is not observed in the optical, we demonstrate that the large quantity of metals accreted by SDSS J0956+5912 can suppress the C2 molecular bands, indicating that planetary accretion can convert DQ stars into DZs (and not DQZs/DZQs).

Transient obscuration event captured in NGC 3227

From Swift monitoring of a sample of active galactic nuclei (AGN) we found a transient X-ray obscuration event in Seyfert-1 galaxy NGC 3227, and thus triggered our joint XMM-Newton, NuSTAR, and Hubble Space Telescope (HST) observations to study this event. Here in the first paper of our series we present the broadband continuum modelling of the spectral energy distribution for NGC 3227, extending from near infrared (NIR) to hard X-rays. We use our new spectra taken with XMM-Newton, NuSTAR, and the HST Cosmic Origins Spectrograph in 2019, together with archival unobscured XMM-Newton, NuSTAR, and HST Space Telescope Imaging Spectrograph data, in order to disentangle various spectral components of NGC 3227 and recover the underlying continuum. We find the observed NIR-optical-UV continuum is explained well by an accretion disk blackbody component (Tmax = 10 eV), which is internally reddened by E(B − V) = 0.45 with a Small Magellanic Cloud extinction law. We derive the inner radius (12 Rg) and the accretion rate (0.1 M⊙ yr−1) of the disk by modelling the thermal disk emission. The internal reddening in NGC 3227 is most likely associated with outflows from the dusty AGN torus. In addition, an unreddened continuum component is also evident, which likely arises from scattered radiation, associated with the extended narrow-line region of NGC 3227. The extreme ultraviolet continuum, and the ‘soft X-ray excess’, can be explained with a ‘warm Comptonisation’ component. The hard X-rays are consistent with a power-law and a neutral reflection component. The intrinsic bolometric luminosity of the AGN in NGC 3227 is about 2.2 × 1043 erg s−1 in 2019, corresponding to 3% Eddington luminosity. Our continuum modelling of the new triggered data of NGC 3227 requires the presence of a new obscuring gas with column density NH = 5 × 1022 cm−2, partially covering the X-ray source (Cf = 0.6).

Detection of Na, K, and H2O in the hazy atmosphere of WASP-6b

ABSTRACT We present new observations of the transmission spectrum of the hot Jupiter WASP-6b both from the ground with the Very Large Telescope FOcal Reducer and Spectrograph (FORS2) from 0.45 to 0.83 μm, and space with the Transiting Exoplanet Survey Satellite from 0.6 to 1.0 μm and the Hubble Space Telescope (HST) Wide Field Camera 3 from 1.12 to 1.65 μm. Archival data from the HST Space Telescope Imaging Spectrograph (STIS) and Spitzer are also re-analysed on a common Gaussian process framework, of which the STIS data show a good overall agreement with the overlapping FORS2 data. We also explore the effects of stellar heterogeneity on our observations and its resulting implications towards determining the atmospheric characteristics of WASP-6b. Independent of our assumptions for the level of stellar heterogeneity we detect Na i, K i, and H2O absorption features and constrain the elemental oxygen abundance to a value of [O/H] ≃ −0.9 ± 0.3 relative to solar. In contrast, we find that the stellar heterogeneity correction can have significant effects on the retrieved distributions of the [Na/H] and [K/H] abundances, primarily through its degeneracy with the sloping optical opacity of scattering haze species within the atmosphere. Our results also show that despite this presence of haze, WASP-6b remains a favourable object for future atmospheric characterization with upcoming missions such as the James Webb Space Telescope.

SEARCHING FOR THE HR 8799 DEBRIS DISK WITH HST/STIS

ABSTRACT We present a new algorithm for space telescope high contrast imaging of close-to-face-on planetary disks called Optimized Spatially Filtered (OSFi) normalization. This algorithm is used on HR 8799 Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) coronagraphic archival data, showing an over-luminosity after reference star point-spread function (PSF) subtraction that may be from the inner disk and/or planetesimal belt components of this system. The PSF-subtracted radial profiles in two separate epochs from 2011 and 2012 are consistent with one another, and self-subtraction shows no residual in both epochs. We explore a number of possible false-positive scenarios that could explain this residual flux, including telescope breathing, spectral differences between HR 8799 and the reference star, imaging of the known warm inner disk component, OSFi algorithm throughput and consistency with the standard spider normalization HST PSF subtraction technique, and coronagraph misalignment from pointing accuracy. In comparison to another similar STIS data set, we find that the over-luminosity is likely a result of telescope breathing and spectral difference between HR 8799 and the reference star. Thus, assuming a non-detection, we derive upper limits on the HR 8799 dust belt mass in small grains. In this scenario, we find that the flux of these micron-sized dust grains leaving the system due to radiation pressure is small enough to be consistent with measurements of other debris disk halos.

Phase-resolved ultraviolet spectroscopy of the magnetic Of?p star HD 191612

We present for the first time phase-resolved UV spectroscopy of an Of?p star, namely, HD 191612. The observations were acquired with the Space Telescope Imaging Spectrograph (STIS) on-board the Hubble Space Telescope (HST). We report the variability observed in the main photospheric and wind features and compare the results with previous findings for the Of?p star HD 108. We show that UV line strengths, H(alpha), and longitudinal magnetic field, vary coherently according to the rotational period (P = 537.6d), providing additional support for the magnetic oblique rotator scenario. The stellar and wind parameters of HD 191612 are obtained based on NLTE expanding atmosphere models. The peculiar wind line profile variations revealed by the new STIS data - not reproduced by 1D atmosphere models - are addressed through non-spherical MHD simulations coupled with radiative transfer. The basic aspects of the UV variability observed are explained and the structure of the dynamical magnetosphere of HD 191612 is discussed.

UPDATED ANALYSIS OF THE UPWIND INTERPLANETARY HYDROGEN VELOCITY AS OBSERVED BY THEHUBBLE SPACE TELESCOPEDURING SOLAR CYCLE 23

The interplanetary hydrogen (IPH), a population of neutrals that fill the space between planets inside the heliosphere, carries the signature of the interstellar medium (ISM) and the heliospheric interface. As the incoming ISM-ionized component deflects at the heliopause, charge exchange reactions decelerate the bulk motion of the neutrals that penetrate the heliosphere. Inside the heliosphere, the IPH bulk velocity is further affected by solar gravity, radiation pressure, and ionization processes, with the latter two processes dependent on solar activity. Solar cycle 23 provided the first partial temporal map of the IPH velocity, including measurements from the Hubble Space Telescope (HST) spectrometers (Goddard High Resolution Spectrograph (GHRS) and Space Telescope Imaging Spectrograph (STIS)) and the Solar and Heliospheric Observatory/Solar Wind ANisotropies (SWAN) instrument. We present an updated analysis of IPH velocity measurements from GHRS and STIS and compare these results with those of SWAN and two different time-dependent models. Our reanalysis of STIS data reveals a significant change in IPH velocity relative to earlier reports, because of the contamination by geocoronal oxygen that was not accounted for. While current models of the heliospheric interface predict the observed IPH velocity for solar maximum, they are not consistent with data covering solar minimum.more » With updates to the HST data points, we now find that all data can be fit by the existing models to within 1{sigma}, with the exception of SWAN observations taken at solar minimum (1997/1998). We conclude that the current data lack the temporal coverage and/or precision necessary to determine the detailed characteristics of the solar cycle dependence. Hence, new observations are merited.« less

Hubble Space Telescope transmission spectroscopy of the exoplanet HD 189733b: high-altitude atmospheric haze in the optical and near-ultraviolet with STIS

We present Hubble Space Telescope optical and near-ultraviolet transmission spectra of the transiting hot-Jupiter HD189733b, taken with the repaired Space Telescope Imaging Spectrograph (STIS) instrument. The resulting spectra cover the range 2900-5700 Ang and reach per-exposure signal-to-noise levels greater than 11,000 within a 500 Ang bandwidth. We used time series spectra obtained during two transit events to determine the wavelength dependance of the planetary radius and measure the exoplanet's atmospheric transmission spectrum for the first time over this wavelength range. Our measurements, in conjunction with existing HST spectra, now provide a broadband transmission spectrum covering the full optical regime. The STIS data also shows unambiguous evidence of a large occulted stellar spot during one of our transit events, which we use to place constraints on the characteristics of the K dwarf's stellar spots, estimating spot temperatures around Teff~4250 K. With contemporaneous ground-based photometric monitoring of the stellar variability, we also measure the correlation between the stellar activity level and transit-measured planet-to-star radius contrast, which is in good agreement with predictions. We find a planetary transmission spectrum in good agreement with that of Rayleigh scattering from a high-altitude atmospheric haze as previously found from HST ACS camera. The high-altitude haze is now found to cover the entire optical regime and is well characterised by Rayleigh scattering. These findings suggest that haze may be a globally dominant atmospheric feature of the planet which would result in a high optical albedo at shorter optical wavelengths.

THE ABUNDANCE OF BORON IN DIFFUSE INTERSTELLAR CLOUDS

We present a comprehensive survey of boron abundances in diffuse interstellar clouds from observations made with the Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope. Our sample of 56 Galactic sight lines is the result of a complete search of archival STIS data for the B II 1362 resonance line, with each detection confirmed by the presence of absorption from O I 1355, Cu II 1358, and Ga II 1414 (when available) at the same velocity. Five previous measurements of interstellar B II from Goddard High Resolution Spectrograph observations are incorporated in our analysis, yielding a combined sample that more than quadruples the number of sight lines with significant boron detections. Our survey also constitutes the first extensive analysis of interstellar gallium from STIS spectra and expands on previously published results for oxygen and copper. The observations probe both high and low-density diffuse environments, allowing the density-dependent effects of interstellar depletion to be clearly identified in the gas-phase abundance data for each element. In the case of boron, the increase in relative depletion with line-of-sight density amounts to an abundance difference of 0.8 dex between the warm and cold phases of the diffuse interstellar medium. The abundance of boron in warm, low-density gas is found to be B/H = (2.4+/-0.6)*10^-10, which represents a depletion of 60% relative to the meteoritic boron abundance. Beyond the effects of depletion, our survey reveals sight lines with enhanced boron abundances that potentially trace the recent production of 11B, resulting from spallation reactions involving either cosmic rays or neutrinos. Future observations will help to disentangle the relative contributions from the two spallation channels for 11B synthesis.

Hubble Space TelescopeSurvey of Interstellar12CO/13CO in the Solar Neighborhood

We examine 20 diffuse and translucent Galactic sight lines and extract the column densities of the ^12CO and ^13CO isotopologues from their ultraviolet A--X absorption bands detected in archival Space Telescope Imaging Spectrograph data with lambda/Deltalambda geq 46,000. Five more targets with Goddard High-Resolution Spectrograph data are added to the sample that more than doubles the number of sight lines with published Hubble Space Telescope observations of ^13CO. Most sight lines have 12-to-13 isotopic ratios that are not significantly different from the local value of 70 for ^12C/^13C, which is based on mm-wave observations of rotational lines in emission from CO and H_2CO inside dense molecular clouds, as well as on results from optical measurements of CH^+. Five of the 25 sight lines are found to be fractionated toward lower 12-to-13 values, while three sight lines in the sample are fractionated toward higher ratios, signaling the predominance of either isotopic charge exchange or selective photodissociation, respectively. There are no obvious trends of the ^12CO-to-^13CO ratio with physical conditions such as gas temperature or density, yet ^12CO/^13CO does vary in a complicated manner with the column density of either CO isotopologue, owing to varying levels of competition between isotopic charge exchange and selective photodissociation in the fractionation of CO. Finally, rotational temperatures of H_2 show that all sight lines with detected amounts of ^13CO pass through gas that is on average colder by 20 K than the gas without ^13CO. This colder gas is also sampled by CN and C_2 molecules, the latter indicating gas kinetic temperatures of only 28 K, enough to facilitate an efficient charge exchange reaction that lowers the value of ^12CO/^13CO.

Sulfur volcanism on Io

In February 2003, March 2003 and January 2004 Pele plume transmission spectra were obtained during Jupiter transit with Hubble's Space Telescope Imaging Spectrograph (STIS), using the 0.1″ wide slit and the G230LB grating. The STIS spectra covered the 2100–3100 Å wavelength regions and extended spatially along Io's limb encompassing the region directly above and northward of the vent of the Pele volcano. The S 2 and SO 2 absorption signatures evident in these data indicate that the gas signature at Pele was temporally variable, and that an S 2 absorption signature was present ∼12° from the Pele vent near 6 ± 5 S and 264 ± 15 W , suggesting the presence of another S 2 bearing plume on Io. Contemporaneous with the spectral data, UV and visible-wavelength images of the plume were obtained in reflected sunlight with the Advanced Camera for Surveys (ACS) prior to Jupiter transit. The dust scattering recorded in these data provide an additional qualitative measure of plume activity on Io, indicating that the degree of dust scattering over Pele varied as a function of the date of observation, and that there were several other dust bearing plumes active during the observations. We present constraints on the composition and variability of the gas abundances of the Pele plume as well as the plumes detected by ACS and recorded within the STIS data, as a function of time.

Supermassive black holes in the Sbc spiral galaxies NGC 3310, NGC 4303 and NGC 4258

We have undertaken an HST Space Telescope Imaging Spectrograph survey of 54 late type spiral galaxies to study the scaling relations between black holes and their host spheroids at the low mass end. Our aim is to measure black hole masses or to set upper limits for a sizeable sample of spiral galaxies. In this paper we present new Space Telescope Imaging Spectrograph (STIS) observations of three spiral galaxies, NGC 4303, NGC 3310 and NGC 4258. The bright optical emission lines Hα λ , [NII] λλ and [SII] λλ were used to study the kinematics of the ionized gas in the nuclear region of each galaxy with a ~ spatial resolution. Our STIS data for NGC 4258 were analyzed in conjunction with archival ones to compare the gas kinematical estimate of the black hole mass with the accurate value from H2O-maser observations. In NGC 3310, the observed gas kinematics is well matched by a circularly rotating disk model but we are only able to set an upper limit to the BH mass which, taking into account the allowed disk inclinations, varies in the range – at the 95% confidence level. In NGC 4303 the kinematical data require the presence of a BH with mass (for a disk inclination deg) but the weak agreement between data and disk model does not allow us to consider this measurement completely reliable. If the allowed inclination values are taken into account, MBH varies in the range – at the 95% confidence level. In NGC 4258, the observed kinematics require the presence of a black hole with ( deg) and, taking into account reasonable limits for the inclination, MBH is in the range – at the 95% confidence level. This result is in good agreement with the published value , derived from H2O-maser observations. As in the case of NGC 4303, the agreement between observed and model kinematics is not strong but this does not affect the recovery of the correct MBH value. Our attempt at measuring BH masses in these 3 late type Sbc spiral galaxies has shown that these measurements are very challenging and at the limit of the highest spatial resolution currently available. Nonetheless our estimates are in good agreement with the scaling relations between black holes and their host spheroids suggesting that (i) they are reliable and (ii) black holes in spiral galaxies follow the same scaling relations as those in more massive early-type galaxies. A crucial test for the gas kinematical method, the correct recovery of the known BH mass in NGC 4258, has been successful.

The Low‐Redshift Lyα Forest toward PKS 0405−123

We present results for Lyα forest and metal absorbers from ~7 km s-1 resolution Space Telescope Imaging Spectrograph data for QSO PKS 0405-123 (z = 0.574). We analyze strong and weak samples of Lyα forest lines, containing 60 absorbers with column density log N ≥ 13.3 over 0.002 < z < 0.423 and 44 with log N ≥ 13.1 over 0.020 < z < 0.234, respectively. Seven absorbers contain metals, all with associated O VI and often offset in velocity from Lyα. The strong and weak Doppler parameter distributions have b = 47 ± 22 and 44 ± 21 km s-1, respectively. Simulated spectra suggest that line blending and signal-to-noise ratio effects inflate b. For absorbers with 13.1 < log N < 14.0, we find a redshift overdensity of ~0.2-0.3 dex at 0.127 < z < 0.234, which we attribute to cosmic variance. There is a void in the strong sample at 0.0320 < z < 0.0814 with probability of random occurrence P < 0.0004. We detect Lyα-Lyα clustering in our sample on a scale of Δv ≤ 250 km s-1 for log N ≥ 13.3, consistent with a numerical model of structure evolution. We detect velocity correlations of up to 250 km s-1 between Lyα absorbers and 39 galaxies at z < 0.43 out to a transverse distance of 1.6 h local frame Mpc. The Lyα-galaxy two-point correlation function is significant out to Δv < 250 km s-1 and grows with minimum H I column density. The strongest signal occurs for log N 13.5-14.0 absorbers and is similar to the galaxy-galaxy correlation, implying that such Lyα absorbers have masses log(M/M☉) = 11.3. We find a correlation between local galaxy counts and local summed H I column density, with peak significance on scales of 4000-6000 km s-1 and probability of random occurrence P = 0.0009. Finally, we present column densities for a number of Galactic species.

Resolving the Inner Cavity of the HD 100546 Disk: A Candidate Young Planetary System?

The inner 100 AU of HD 100546 is studied via far-ultraviolet long-slit spectroscopy with the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS). The star is surrounded by reflection nebulosity, which can be traced 100 AU in the continuum, and by emission from H I Lyα, N I, Si II, and fluorescent H2 transitions. The Lyα emission can be traced up to 200 AU along the system semimajor axis and 300 AU along the semiminor axis. The radial surface brightness profile and the presence of both reflection nebulosity and molecular gas suggest that we have detected the flared surface of the disk predicted from analysis of the IR spectral energy distribution. When corrected for the r-2 falloff in illumination from the Herbig Be star, the reflection nebulosity, neutral atomic gas, and H2 emission all reveal the presence of a central cavity extending 013 ± 0025 (13 AU) from the star, more than 20 times larger in radius than would be expected from dust sublimation alone. The reflection nebulosity within the cavity is centered on a location 005 (5 AU) to the southeast of the star along the system semimajor axis. The pericenter asymmetry in the cavity is inconsistent with cavity formation by the combined effects of ice sublimation, radiation pressure blowout on small grains, or other disk chemistry that should produce azimuthally symmetric features. The STIS data are also consistent with a current accretion rate onto the Herbig Be star no higher than a few times 10-9 M☉ yr-1, an order of magnitude lower than seen in 5-8 Myr old Herbig Ae stars. The low accretion rate, large cavity, pericenter asymmetry, and deficit of warm dust grain emission observed over 2-8 μm are all consistent with dynamical sculpting of the disk by one or more bodies within the disk. An upper limit to the flux from any chromospherically active, low-mass companion is a factor of 5-10 fainter than AU Mic (M1 Ve; t = 12 Myr) at the distance of HD 100546. This upper limit firmly excludes a low-mass stellar companion within the cavity but does not exclude a less active and/or luminous brown dwarf. The absence of similar central clearing in younger Herbig Ae stars suggests that any companion must become externally detectable late in the evolution of the disk, favoring a giant planet as the source of the structure in the HD 100546 disk.

Jet-Gas Interaction in Markarian 78. II. Ionization Mechanisms

We investigate the ionization mechanisms in the Seyfert galaxy Markarian 78, chosen because its narrow-line region (NLR) is dominated by a strong jet-gas interaction. Our principal aim is to ascertain whether or not shock-related ionization is important. We use Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) optical and UV spectra from four slits, as well as HST Faint Object Spectrograph optical spectra from 10 apertures. Ultimately, several lines of evidence argue against the importance of shocks but in favor of central source photoionization, even for gas that is clearly accelerated by the jet flow. Following earlier work, we compare emission-line ratios with four classes of ionization model: photoionization of optically thick gas (standard and dusty U sequences), photoionization of optically thick and thin gas (Am/i sequence), and fast shocks. Traditional excitation measures are well matched by all three models, with -3.0 < log U < -2.0, -0.5 < log Am/i < +1.0, and 300 km s-1 < Vshock < 500 km s-1. However, several other line ratios provide clear discrimination, with significantly poorer fits for standard and dusty U models and even poorer for shock models. Importantly, our data span a sufficient range of conditions that they show trends in the ratio-ratio plots, and these trends clearly favor central source photoionization models, particularly the Am/i models. To achieve essentially perfect agreement, we make two slight modifications to the Am/i models of Binette et al.: we lower the ionization of the optically thin component (Mrk 78 has somewhat lower than normal excitation), and we increase the nitrogen abundance toward the nucleus. We suspect that this latter effect has introduced ambiguities in previous work. The UV lines are less helpful than anticipated, partly because of their weakness and partly because high ionization parameter photoionization can mimic shocks. An exception is N V, which, in the absence of abundance effects, can provide unambiguous discrimination. We extend the above analysis to include comparison of line profiles. We first show that even slight differences in velocity between pre- and postshock gas (shock scenario) and optically thin and thick gas (Am/i scenario) predict significant profile differences between [O III] λ5007 and other emission lines. The marked absence of such differences argues strongly against the two-component shock models and also demands, for an Am/i model, that optically thin and thick gas have similar velocity distributions. The possible presence, however, of a weak broad low-ionization component may indicate a minor shock contribution. Further evidence against shock ionization is the absence of correlations between line ratios and kinematic parameters (e.g., FWHM or Vpeak); that is, the calmest and the most turbulent gas both have similar ionization. Conversely, a global decrease in excitation away from the nucleus qualitatively supports central source photoionization. Quantitatively, however, Mrk 78 exhibits the so-called Q problem seen in many other active galactic nuclei, although we speculate that a detailed extension of Am/i-type models may resolve the problem without requiring in situ sources of ionization. Finally, we use NLR excitation and far-infrared luminosity to provide two independent estimates of the nuclear ionizing luminosity. Both these estimates match the simple picture of central source photoionization, yielding the observed emission-line luminosity and a plausible (~20%–100%) NLR covering factor. We conclude that despite the NLR in Mrk 78 being strongly disturbed by radio-emitting ejecta, there is little or no evidence for shock-related ionization. There is, however, strong evidence for central source photoionization, including both optically thick and thin gaseous components. In Paper III we make further use of the STIS data to derive the dynamics of the emission-line region and the nature of the jet flow.

The Ultraviolet and Optical Spectra of Luminous B‐Type Stars in the Small Magellanic Cloud

ABSTRACTWe present ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) of 12 early B‐type stars in the Small Magellanic Cloud (SMC), composed of nine supergiants and three giants. A morphological comparison with Galactic analogs is made using archival data from the International Ultraviolet Explorer (IUE). In general, the intensity of the P Cygni emission in the UV resonance lines is greater and is seen to later spectral types in the Galactic spectra than in their metal‐deficient SMC counterparts. We attribute these effects as most likely arising from weaker stellar winds in the SMC targets, as predicted by radiatively driven wind theory. We also include unpublished STIS observations of two late O‐type stars in the SMC. In combination with published O‐type STIS data, we now have an extensive ultraviolet spectral library of metal‐deficient stars to use in the study of unresolved starbursts and high‐redshift star‐forming galaxies. In this context, we present empirical measurements for the B‐type spectra of the new “1978 index” suggested by Rix et al. as a probe of metallicity in such systems.

Ionized gas and stars in the central kiloparsec of the type 2 Seyfert galaxy NGC 2110 - I. The data

In this paper, we present new results from an extensive set of Hubble Space Telescope (HST) and ground-based observations of the Seyfert galaxy NGC 2110. The HST data sets include Wide Field Planetary Camera 2 (WFPC2) observations as well as the first high-spatial resolution spectroscopy ([O i], [N ii], Hα and [S ii] lines) of this galaxy obtained using the Space Telescope Imaging Spectrograph (STIS). The ground-based data are three-dimensional (x, y, λ) spectrographic observations obtained using the integral field spectrograph OASIS on the Canada–France–Hawaii Telescope, complemented by near-infrared long-slit spectroscopy obtained using NIRSPEC on the Keck–II telescope. The OASIS observations cover regions containing both stellar absorption lines and major optical emission lines. The NIRSPEC observations cover the H and K bands. Combined with archival HST observations, the WFPC2 data provide us with a high-spatial resolution extinction map. The OASIS data allowed bidimensional mapping of the stellar and gaseous kinematics, as well as of the spectral properties of the ionized gas. These results are compared to those obtained in the near-infrared with NIRSPEC/Keck. Last, we used the STIS data to probe the ionized gas kinematics and properties in the inner 4 arcsec along PA = 156° at unprecedented spatial resolution. Our two-dimensional (2D) map of the stellar velocity field and the near-infrared stellar velocity profile are symmetric about the nucleus, confirming the results of previous long-slit observations. The asymmetry of the velocity field of the ionized gas is present at the same level for visible and near-infrared lines, indicating this is not a reddening effect. Multiple Gaussian fitting of the emission-line profile allowed the contributions of the broad and a narrow components to be disentangled. The intensity peak of the [O iii] narrow component is located north of the nucleus, indicating that the bulk of the narrow [O iii] emission comes from the jet-like structure (Mulchaey et al.) and not from the nucleus itself. We suggest that the northern arm is the anomalous one, contrary to what has been claimed earlier. Last, we also show that the elongated region of high gas velocity dispersion located close to the nucleus and discovered by Gonzalez Delgado et al. is intrinsic to the narrow component.

Technique for Detecting Warm‐Hot Intergalactic Gas in Quasar Ultraviolet Spectra

The ionizing spectral energy distribution of quasars exhibits a steepening of the distribution shortward of ~1200 ?. The change of the power-law index from approximately -1 (near-UV) to -2 (far-UV) has so far been interpreted as being intrinsic to quasars. We consider the possibility that the steepening may result from a tenuous absorption component that is anticorrelated with large mass overdensities. UV-sensitive satellites, whose detectors can extend down to 1000 ?, can set a useful limit to such an absorption component through the search for a flux increase in the window 1050-1190 ? (observer frame) with respect to an extrapolation of the continuum above 1230 ?. Since the recent Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope Space Telescope Imaging Spectrograph data do not show any obvious discontinuity in this region, this effectively rules out the possibility that intergalactic H I absorption is very important, and we conclude that most if not all of the steepening is intrinsic to quasars. A smaller flux discontinuity of order 1% cannot, however, be ruled out yet and would still be consistent with the warm-hot intergalactic component if it amounts to 30% of the baryonic mass, as predicted by some models of large scale structure formation, provided its temperature lies around 105.3 K.

The Binary Central Star of the Planetary Nebula A35

Using new Far Ultraviolet Spectroscopic Explorer (FUSE) observations in conjunction with Space Telescope Imaging Spectrograph (STIS) and International Ultraviolet Explorer archive data, we have modeled both components of the binary central star of the planetary nebula A35. The white dwarf (the ionizing star) was modeled using the non-LTE, plane-parallel code TLUSTY. We find its parameters to be Teff = 80 ? 3 kK, log g = 7.70 cm s-2, and [He/H] = -4 ? 1 and C, N, O, Si, and Fe to be underabundant by 2 orders of magnitude with respect to their solar values. This confirms its classification as a DAO white dwarf, and using the Hipparcos distance D = 163 pc, we derive a radius of RWD 1.65 ? 10-2 R? and a mass of M 0.5 M?. The modeling of the far-ultraviolet spectra also constrains the extinction value; EB-V = 0.04 ? 0.01. Furthermore, the FUSE and STIS data allow us to measure the molecular hydrogen (H2) and neutral hydrogen (H I) column densities along the sight line, the majority of which we believe is associated with the circumstellar material. The FUSE spectrum is best fitted with a two-component model for H2, consisting of a cool component (T = 200 K) with log N(H2,cool) = 19.6 cm-2 and a hot component (T 1250 K) with log N(H2,hot) = 17.4 cm-2. The H I column density is log N(H ) = 20.9 ? 0.1 cm-2. Assuming a typical gas/dust ratio for the interstellar medium, our value of EB-V implies that log N(H ) = 20.8 cm-2 of this is circumstellar. Our low extinction value and the measured column densities imply that there is essentially no dust in the nebula. Assuming that the neutral and molecular hydrogen is contained in a sphere of comparable dimensions to the ionized shell, we derive the combined mass of the circumstellar H I and H2 to be ~2.7 M?. Other geometries, such as a shell surrounding the ionized region, can be excluded. The mass of the ionized hydrogen is 1% that of the neutral material. From comparison with evolutionary calculations, we estimate the progenitor mass to be ~3.2 M?.

The WR+OB Progenitor RY Scuti: Intensive Spectroscopy of Its Compact Double‐Ring Nebula

We present a detailed spectroscopic analysis of the peculiar nebula around RY Scuti using data from the Space Telescope Imaging Spectrograph (STIS) and various ground-based observatories. This massive contact binary may represent a rapid evolutionary phase in transition to a short-period WR+OB system, and it is surrounded by a young nebula with unusual geometry. The ionized nebula is only about 1'' across, so STIS has allowed us to spatially resolve the spectrum of the star from its circumstellar nebula for the first time. Combining STIS data with ground-based spectra at optical and infrared wavelengths solves some geometric ambiguities posed by previous images and reveals the kinematic relationship between the nebular geometry and RY Scuti's complex emission-line profiles. We offer a new determination of the systemic velocity of 20 ± 3 km s-1, and we estimate the distance to RY Scuti as 1.8 ± 0.1 kpc. Nebular lines show subtle phase-dependent variability in addition to expected changes in equivalent width due to eclipses of stellar continuum. These changes in equivalent width yield an updated ephemeris, but a previously suggested period change is still uncertain after considering these new data. The nebula contains roughly 0.003 M☉ of material that is He- and N-rich and probably O-deficient compared to solar abundances, indicating that CNO-processed material has reached the surface in at least one component of the binary system. Diagnostic line ratios suggest that the electron density and temperature in the nebula are 2 × 105 cm-3 and ~9500 K, respectively. There are concentrated regions of higher density as well; emission knots seen around the rings suggest that common envelope mass loss during massive contact binary evolution may be characterized by strong azimuthal asymmetry and sporadic mass ejection events.