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Several recent studies derived the existence of plumes on Jupiter's moon Europa. The only technique that provided multiple detections is the far-ultraviolet imaging observations of Europa in transit of Jupiter taken by the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST). In this study, we reanalyze the three HST/STIS transit images in which Sparks et al. identified limb anomalies as evidence for Europa's plume activity. After reproducing the results of Sparks et al., we find that positive outliers are similarly present in the images as the negative outliers that were attributed to plume absorption. A physical explanation for the positive outliers is missing. We then investigate the systematic uncertainties and statistics in the images and identify two factors that are crucial when searching for anomalies around the limb. One factor is the alignment between the actual and assumed locations of Europa on the detector. A misalignment introduces distorted statistics, most strongly affecting the limb above the darker trailing hemisphere where the plumes were detected. The second factor is a discrepancy between the observation and the model used for comparison, adding uncertainty in the statistics. When accounting for these two factors, the limb minima (and maxima) are consistent with random statistical occurrence in a sample size given by the number of pixels in the analyzed limb region. The plume candidate features in the three analyzed images can be explained by purely statistical fluctuations and do not provide evidence for absorption by plumes.

We report results from a new technique for mapping Io's SO2 vapor distribution. The Space Telescope Imaging Spectrograph (STIS) instrument on the Hubble Space Telescope observed Io during four Jupi ...

In late 2006, ground-based photometry of $\eta$ Car plus the Homunculus showed an unexpected decrease in its integrated apparent brightness, an apparent reversal of its long-term brightening. Subsequent HST/WFPC2 photometry of the central star in the near-UV showed that this was not a simple reversal. This multi-wavelength photometry did not support increased extinction by dust as the explanation for the decrease in brightness. A spectrum obtained with GMOS on the Gemini-South telescope, revealed subtle changes mid-way in $\eta$ Car's 5.5 yr spectroscopic cycle 0when compared with HST/STIS spectra at the same phase in the cycle. At mid-cycle the secondary star is 20--30 AU from the primary. We suggest that the spectroscopic changes are consistent with fluctuations in the density and velocity of the primary star's wind, unrelated to the 5.5 yr cycle but possibly related to its latitude-dependent morphology. We also discuss subtle effects that must be taken into account when comparing ground-based and HST/STIS spectra., Comment: 34 pages, 9 Figures

Two-dimensional processing of Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) echelle spectra of the eclipsing M2 Iab + B binary system, VV Cep, has revealed extended emission in all 21 epochs observed, ranging from midtotality through the first quadrature. In the ultraviolet accessible to STIS (1150- 3150 ?), essentially all observed photons originate from the hot component. In total eclipse, a pure emission spectrum is seen, with strong lines from lower levels up to ~2 eV cut by blue-shifted absorption in the M supergiant wind. Out of eclipse, a continuum is observed upon which a rich absorption spectrum of mostly neutral and singly-ionized iron group elements is superimposed. Out of eclipse, the strongest absorption features sit atop the emission profiles seen during eclipse. Far from eclipse, the neutral lines disappear, and the strongest of the remaining absorption lines develop complex inverse P-Cygni profiles. The continuum and spectrally extended blue wings of the inverse P-Cygni spectral lines show a spatial extension characteristic of the HST/STIS response to an unresolved source. However, the emission components of spectral lines lying longward of the blue-shifted wind absorption typically show flux extending beyond the unresolved hot component, implying formation over an extended region much larger than the size of the binary orbit. This extended line emission is believed to be formed by scattering of photons from the hot component's continuum by ions in ground and low-lying metastable levels in the extended wind of the M supergiant. Extension is observed out of eclipse for lines with lower levels up to approximately 4 eV, but the greatest degree of extension is observed for lines with lower levels less than 2 eV. Radial velocity differences across the spatially extended emission correlate with the aperture position angle, consistent with model predictions for wind flow in a binary system in which the wind outflow is comparable to the orbital velocity, and indicate an orbital plane oriented from +11? E of N (receding) through ?169? (approaching).

We have obtained Space Telescope Imaging Spectrograph long-slit spectra of the central region of the advanced merger NGC 4194. The spectra cover the wavelength ranges 1150–1750 A in the UV and 2900–10270 A in the visible. Results from the study of the properties of 14 star-forming regions (knots) are presented. If the [N II] contribution is 40% of the combined Hα + [N II] flux, then the average E(B - V) = 0.7 mag. The metal abundances are approximately solar, with individual knot abundances ranging from log(O/H) + 12 = 8.1 ± 0.5 to 8.9 ± 0.4. The Hα luminosities of the 14 observed knots yield a total star formation rate (SFR) of ~46 M⊙ yr-1. The sizes of the H II regions associated with the knots were determined from L(Hβ) and range from ~28 to ~119 pc when a filling factor of 0.1 is assumed. The sizes are a factor of ~2.15 smaller for a filling factor of 1. Using Starburst99, the EW(Hα + [N II]), and EW(Hβ), we estimate the ages of the star-forming regions to be 5.5–10.5 Myr. From ground-based spectra the effective temperatures of the H II regions are found to be ~11,000 K, and the electron densities are determined to be ~530 cm-3. We find that eight of the knots probably formed with a Salpeter initial mass function truncated at an upper mass of 30 M⊙, and one of the knots likely formed with a standard Salpeter initial mass function. We suggest that the knots in our sample are the precursors of globular clusters. Two of the knots are in a region of flowing gas and are among the most massive, are the largest in radius, have the highest SFR, and are among the youngest of the knots.

The gallium abundance in HgMn stars has been reported to be different for analyses conducted from spectral lines in the optical versus those from the ultraviolet. We pursue this ultraviolet-optical discrepancy from a line-blending perspective by investigating the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) spectrum of the sharp-lined HgMn binary star chi Lup. Its gallium abundance is derived to be log N-Ga = 4.5 (log N-H = 12) based on nonresonance Ga II lines, and we are able to demonstrate the line-blending problems associated with the abundances determined from the resonance lines of Ga II lambda 1414 and Ga III lambda 1495. The HST STIS spectrum for chi Lup is also compared with a co-added International Ultraviolet Explorer (IUE) spectrum to further illustrate the importance of accounting for the line blending in quantitative abundance analyses. We have compared an IUE co-added spectrum of mu Lep with an LTE-based synthetic spectrum and managed to fit both Ga II lambda 1414 and Ga III lambda 1495 with an abundance of log N-Ga = 7.3. However, the fitting of these lines requires a dramatic change in the radiative damping constants, which may indicate a breakdown in our LTE modeling or the need to include other atmospheric effects. (Less)

Long-slit spectra of several dozen young star clusters have been obtained at three positions in the Antennae galaxies with the Space Telescope Imaging Spectrograph (STIS) and its 52"x0.2" slit. Based on H-alpha emission-line measurements, the average cluster-to-cluster velocity dispersion in 7 different cluster aggregates ("knots") is

To measure the column densities of interstellar and intergalactic gas clouds using absorption line spectroscopy, the apparent optical depth technique (AOD) of Savage & Sembach (1991) can be used instead of a curve-of-growth analysis or profile fit. We show that the AOD technique, whilst an excellent tool when applied to data with good S/N, will likely overestimate the true column densities when applied to data with low S/N. This overestimation results from the non-linear relationship between the flux falling on a given detector pixel and the apparent optical depth in that pixel. We use Monte Carlo techniques to investigate the amplitude of this overestimation when working with data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Space Telescope Imaging Spectrograph (STIS), for a range of values of S/N, line depth, line width, and rebinning. AOD measurements of optimally sampled, resolved lines are accurate to within 10% for FUSE/LiF and STIS/E140M data with S/N>7 per resolution element., 7 pages, 3 figures (2 in color), accepted for publication in AJ

Using The Hubble Space Telescope's Space Telescope Imaging Spectrograph HST's STIS, observations of the OIII emission from the narrow-line region (NLR) of NGC 4151 were obtained and radial velocities determined. Five orbits of HST time were used to obtain spectra at five parallel slit configurations, at a position angle of 58 degrees, with spatial resolution 0.2 arcseconds across and 0.1 arcseconds along each slit. A spectral resolving power of ~ 9,000 with the G430M grating gave velocity measurements accurate to ~ 34 km/s. A kinematic model was generated to match the radial velocities, for comparison to previous kinematic models of biconical radial outflow developed for low-dispersion spectra at two slit positions. The new high-resolution spectra permit the measurement of accurate velocity dispersions for each radial-velocity component. The full-width at half-maximum (FWHM) reaches a maximum of 1000 km/s near the nucleus, and generally decreases with increasing distance to about 100 km/s in the extended narrow-line region (ENLR), starting at about 6 arcseconds from the nucleus. In addition to the bright emission knots, which generally fit our model, there are faint high velocity clouds which do not fit the biconical outflow pattern of our kinematic model. These faint clouds occur at the turnover points of the outflowing bright clouds. We suggest possible scenarios that could explain these rogue clouds: (1) backflow resulting from shocks and (2) outflow outside of the bicones, although the latter does not explain how the knots are ionized and accelerated. A comparison of our observations with a high-resolution radio map shows that there is no evidence that the kinematics of the NLR clouds are affected by the radio lobes that comprise the inner jet., 30 pages, 15 figures (some color), accepted for publication in the Astronomical Journal. Downloadable versions of the paper with high resolution figures/images are available here: http://www.chara.gsu.edu/~crenshaw/NGC4151_kinematics.pdf

We obtained ultraviolet and optical images of star-forming regions in the interacting galaxy system NGC 3994/3995 using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We describe the characteristics of 56 detected knots, a majority of which are younger than 20 Myr old. There is no discernible pattern of knot age with respect to position within the galaxies. The knots have masses and radii ranging from 7.2 × 103 to 4.4 × 106 M⊙ and from 4 to 23 pc, respectively. Using a conservative criterion, we find that ~15% of these knots may be proto-globular clusters; the percentage of proto-globular clusters may be as high as ~70%. The UV flux distribution of the knots in NGC 3995 can be fitted with a power law with α = -0.72 ± 0.11, with no turnover detected brightward of the completeness limit.

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.

Secondary flux standards are established by measuring their brightness relative to primary standard stars. The Hubble Space Telescope (HST) primary standards are the three pure-hydrogen white dwarf (WD) flux standards that determine the sensitivity calibration for the Space Telescope Imaging Spectrograph. STIS observations have defined the flux of the Sloan Digital Sky Survey (SDSS) standard BD +17°4708 from 0.17 to 1.0 μm with an uncertainty of less than 0.5% relative to the HST primary standards, as verified by two independent sets of photometry. The two CCD cameras, HRC and WFC, in the ACS instrument package on HST confirm the average ratio of BD +17°4708 to the three fainter WD primary standards to better than 0.5% if a few of the ACS filter bandpasses have small wavelength shifts. With an uncertainty of less than 0.5% in the transfer of the flux calibration from the primary standards, the accuracy of the BD +17°4708 relative flux distribution is essentially the same as that of the primary WDs, that is, ~2% in the relative fluxes over the 0.17–1 μm range. Our flux distribution agrees with the SDSS fluxes to within their quoted 3% uncertainty.

We have obtained an ultraviolet spectrum covering the range 917–1182 A of the high-field magnetic cataclysmic variable AR Ursae Majoris when the binary was in its typical low-accretion state. The data reveal an array of deep Zeeman absorption features due to hydrogen Lyα–Lyγ, including several transitions that are normally forbidden but become enabled in the strong electric fields that exist in the atmospheres of highly magnetic white dwarfs. Spectral models indicate a magnetic field strength and structure consistent with that determined from longer wavelength Hubble Space Telescope Space Telescope Imaging Spectrograph UV observations; that is, a dipole of strength ~235 MG offset along its axis by ~0.15 of the stellar radius.

The Space Telescope Imaging Spectrograph (STIS) has measured the absolute flux for Vega from 0.17–1.01 μm on the Hubble Space Telescope white dwarf (WD) flux scale. These data are saturated by up to a factor of 80 overexposure but retain linearity to a precision of 0.2% because the charge bleeds along the columns and is recovered during readout of the CCD. The signal-to-noise ratio per pixel exceeds 1000, and the resolution R is about 500. A V magnitude of 0.026 ± 0.008 is established for Vega, and the absolute flux level is 3.46 × 10-9 ergs cm-2 s-1 at 5556 A. In the regions of Balmer and Paschen lines, the STIS equivalent widths differ from the pioneering work of Hayes in 1985 but do agree with predictions of a Kurucz model atmosphere, so that the STIS flux distribution is preferred to that of Hayes. Over the full wavelength range, the model atmosphere calculation shows excellent agreement with the STIS flux distribution and is used to extrapolate predicted fluxes into the IR region. However, the IR fluxes are 2% lower than the standard Vega model of Cohen. IUE data provide the extension of the measured STIS flux distribution from 0.17 down to 0.12 μm. The STIS relative flux calibration is based on model atmosphere calculations of pure hydrogen WDs, while the Hayes flux calibration is based on the physics of laboratory lamps and blackbody ovens. The agreement to 1% of these two independent methods for determining the relative stellar flux distributions suggests that both methods may be correct from 0.5–0.8 μm and adds confidence to claims that the fluxes relative to 5500 A are determined to better than 4% by the pure hydrogen WD models from 0.12 to 3 μm.

We have measured the brightness variations in Eta Carinae for the past six years using the Hubble Space Telescope Space Telescope Imaging Spectrograph and Advanced Camera for Surveys. Unlike ground-based data, observations by the HST allow direct measurement of the brightness of the central star by resolving it from the surrounding bright ejecta. We find interesting behavior during 2003 in the continuum and H$\alpha$ emission. The data show that the established long term brightening trend of Eta Car continues, including regular events which coincide with the 5.5 year spectroscopic cycle and other more rapid and unexpected variations. In addition to the HST data, we also present ground-based data obtained from the AAVSO which show many of the same features. The dip in the apparent brightness of the central star at the time of the 2003.5 event is wavelength dependent with no decrease in the continuum. These observations cast doubt on asimple eclipse or occultation as the explanation for the dip and place constraints on the models for the event.

We present a detailed study of the Seyfert 2 galaxy Markarian 78, using continuum and emission-line images and multiaperture spectra from the Hubble Space Telescope (HST) and a deep 3.6 cm VLA image. Our overall aim is to study the interaction between the radio source and the emission-line gas, since ground-based data already indicate the presence of a strong bipolar jet-driven flow. First, in the wider context, Mrk 78 is probably a post-merger system, with a nuclear dust lane, approximate r1/4 continuum profile, and highly extended asymmetric gas distribution. The [O III] and radio images both show complex structures with many similarities but also important differences. A careful comparison shows convincing morphological evidence for jet-gas interaction: (1) the western inner radio jet terminates and flares at the position of a bright [O III] knot; (2) the weaker eastern radio jet changes direction as it encounters a large [O III] knot; (3) most [O III] features appear limb brightened on the upstream side, and flared on the downstream side; and (4) in the outer regions the radio components tend to lie between or adjacent to [O III] knots, indicating the radio and line emitting phases do not easily interpenetrate. In addition to evidence of jet-gas interaction, two features indicate the importance of a central ionizing radiation field: an inner fanlike structure to the east, with a straight kinematically quiet northern edge; and an approximately fanlike extended narrow-line region to the west, lying well outside the radio source. The [O III] line profiles from 10 Faint Object Spectrograph apertures provide further kinematic insight into the jet-gas interaction. (1) On the eastern side, where the radio source is deflected, the [O III] profile contains a highly redshifted component (~700 km s-1) that is also narrow (FWHM ≤ 200 km s-1), indicating significant coherent gas acceleration with almost no induced turbulence. (2) At this location, the [O III] profile is also double, suggesting lateral expansion away from a jet axis, as it burrows into the cloud complex. (3) In contrast, the bright western inner knot, which seems to disrupt the inner radio jet, has essentially undisturbed kinematics. (4) Across the outer complex western region, gas velocities are higher in the region center, decrease at the leading edge and may be highest where radio flows "blow out" of the region. Overall, morphology and kinematics suggest the western side is best described as an initially disrupted jet that then fills, accelerates, and leaks out of a complex incomplete "bubble" of ionized gas. The eastern side is best described as a large centrally illuminated fanlike gas structure that is penetrated, accelerated, and ultimately deflects the radio source. We use these different regions to construct a plausible evolutionary sequence: Initially, a dense (molecular?) cloud enters the jet flow and disrupts it (inner western knot); as time passes, the jet begins to penetrate, accelerate, and ablate the cloud (eastern knot complex); continued jet influence further disperses the cloud fragments, sweeping out channels and gaps (western lobe region). This process may repeat itself many times over the lifetime of the jet. In companion papers, we study the ionization mechanisms and jet properties in more detail using recently acquired Space Telescope Imaging Spectrograph spectra, taking a more complete account of the energetics of the radio source and the ionized gas.

The Space Telescope Imaging Spectrograph (STIS) was used to obtain the ultraviolet spectrum of AL Comae 5.5 yr past its 1995 April superoutburst. The spectrum can be fitted by a model of a log g = 8 ± 0.5 white dwarf with a temperature of 16,300 ± 1000 K. Surprisingly, the flux distribution is exactly the same as the Faint Object Spectrograph data obtained 4.5 yr earlier, but the UV fluxes are a factor of 2 lower than the previous measurement, while the optical fluxes remained the same. The implication could be that the temperature of the UV component has remained very similar, but the size of the emitting area has decreased by ~30%. Alternatively, there could be multiple components contributing in the UV, with corresponding changes in temperature and size that produce a similar flux distribution at the two epochs.

We present the second gravitationally lensed quasar discovered during the course of a Hubble Space Telescope Space Telescope Imaging Spectrograph snapshot survey for small-separation gravitational lenses. CTQ 327 is a double quasar with an image separation of 122 and a g-band flux ratio of roughly 5 to 1. Spectra reveal both components to be z = 1.37 quasars, and the lensing galaxy is clearly visible after point-spread function subtraction of the two quasar components. The light profile of the lensing galaxy is well modeled by an r1/4 law, indicative of an early-type elliptical galaxy. An estimate of the lens galaxy redshift is z ~ 0.4–0.6, based on the Faber-Jackson relationship and photometric considerations, although values outside this range are still consistent with the present data. Resolved spectra of the two quasars show similar, but not identical, continuum and emission-line features: component A exhibits weaker emission lines with respect to the continuum than does component B, and there is evidence of intrinsic differences in the emission-line profiles between the two components. Optical monitoring of the quasar pair also shows a change in the g-band flux ratio of 0.14 mag over a 3 month period. These spectral and photometric differences may be due to microlensing fluctuations from stars in the lensing galaxy, intrinsic quasar variability coupled with the system's differential time delay, or some combination of the two. The observed variability makes CTQ 327 an attractive target for future flux monitoring, aimed at time-delay or microlensing studies.

We report long-slit spectroscopic mapping of the η Carinae nebula obtained using the Space Telescope Imaging Spectrograph. The observations reveal the presence of a previously unknown bipolar emission nebula (roughly ±2'' along its major axis) embedded within the well-known and larger Homunculus Nebula. A preliminary analysis suggests that this embedded nebula may have originated from a minor eruption event circa 1890, 50 years after the formation of the larger Homunculus.

We have obtained ultraviolet and visible wavelength images for the central regions of the interacting galaxies NGC 3395 and NGC 3396, using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. The images show many isolated sources of ultraviolet emission produced by young star-forming regions (knots). The FWHM of most of the knots in both galaxies is ≤20 pc. Far-UV flux distributions for the complete sample of knots can be fitted with a power law with exponent α = -1.20 ± 0.35 for NGC 3395 and a power law with exponent α = -0.69 ± 0.21 for NGC 3396. Comparison with models from Leitherer et al. indicates that the ages of most of the knots are ≤80 Myr. Reddening of the knots ranges from E(B-V) = 0.0 to E(B-V) = 0.3 mag, indicating variable amounts of dust in these regions. Almost all the knots have masses less than 106 M⊙. Many of the knots are probably bound and at least six knots are good proto–globular cluster candidates. There are no significant differences in the fluxes, sizes, and ages of the knots in the two galaxies. The average mass of the knots in NGC 3395 is an order of magnitude less than the average mass of the knots in NGC 3396. There is no obvious correlation between the age of a knot and its position the galaxy.

We have carried out a synthetic spectral analysis of a Hubble Space Telescope Space Telescope Imaging Spectrograph observation of the old nova DI Lacertae (Nova Lacerta 1910). The spectrum, obtained with the E140M disperser, reveals a rising continuum shortward of 1560 A, a C IV P Cygni profile indicative of wind outflow associated with disk accretion, a deep Lyα profile, and strong N V (1238 and 1242 A) and O V (1371 A) wind/coronal absorption lines. Numerous sharp interstellar resonance lines are also present. A grid of accretion disk models, spanning a wide range of inclinations, accretion rates, and white dwarf masses, was compared with three sets of dereddened data. From the three best fits, we conclude that the most likely parameters characterizing the far-UV spectrum of DI Lac are an inclination below 18°, a white dwarf mass between 0.6 and 0.8 M⊙, and an accretion rate between 10-9.0 and 10-9.5 M⊙ yr-1. The scale factors for the three best-fit disk models indicate distances between 2 and 2.5 kpc. The extreme weakness or absence of silicon features in the observed spectrum corresponds to a silicon underabundance of 10-4 times solar. The best-fitting high-gravity photosphere model has log g = 8, Teff = 27,000 K, V sin i = 200 km s-1, and a scale factor distance (for a 0.7 M⊙ white dwarf) of only 76 pc. Thus, a white dwarf photosphere cannot be contributing appreciably to the far-UV spectrum.

We present Hubble Space Telescope observations of the northern part of NGC 7635, a circular shell around the O6.5 IIIf star BD +60°2522. The nebula, which lies within the large emission-line region S162, is notable not only for its symmetric shell, but also for a complex of "cometary" knots close to the central star. Our observations include spectra taken with the Space Telescope Imaging Spectrograph and narrowband images taken with the Wide Field Planetary Camera 2. The high spatial resolution of these data reveals the knots to be the ionized edges of a much larger mass of neutral material, with strong photoevaporative flows toward the central star. The cometary appearance of the knots is produced by the intersection of two ridges, one in the plane of the sky and the other 65° relative to it. Stratification in the emission from the shell can also be seen, the result of shock heating as material is swept into the expanding shell. We also see for the first time a small loop of emission between the central star and the cometary knot complex. We propose that this was formed by the collision between the strong stellar wind and the photoevaporative flow from the closest and brightest of these knots.

We have obtained Hubble Space Telescope/Space Telescope Imaging Spectrograph low-resolution ultraviolet spectra of the X-ray pulsar 4U 1626-67 (=KZ TrA); 4U 1626-67 is unusual even among X-ray pulsars because of its ultrashort binary period (P = 41.4 minutes) and remarkably low mass function (≤1.3 × 10-6 M⊙). The far-UV spectrum was exposed for a total of 32 ks and has sufficient signal-to-noise ratio to reveal numerous broad emission and prominent narrower absorption lines. Most of the absorption lines are consistent in strength with a purely interstellar origin. However, there is evidence that both C I and C IV require additional absorbing gas local to the system. In emission, the usual prominent lines of N V and He II are absent, while both O IV and O V are relatively strong. We further identify a rarely seen feature at ~1660 A as the O III] multiplet. Our ultraviolet spectra therefore provide independent support for the recent suggestion that the mass donor is the chemically fractionated core of either a CONe or ONeMg white dwarf; this was put forward to explain the results of Chandra high-resolution X-ray spectroscopy. The velocity profiles of the ultraviolet lines are in all cases broad and/or flat topped or perhaps even double peaked for the highest ionization cases of O; in either case, the ultraviolet line profiles are in broad agreement with the Doppler pairs found in the X-ray spectra. Both the X-ray and far-UV lines are plausibly formed in (or in an corona just above) a Keplerian accretion disk; the combination of ultraviolet and X-ray spectral data may provide a rich data set for follow-on detailed models of the disk dynamics and ionization structure in this highly unusual low-mass X-ray pulsar system.

We analyze HST/STIS spectra (see Paper I) of the central region of the dense globular cluster M15. We infer the velocities of 64 individual stars, two-thirds of which have their velocity measured for the first time. This triples the number of stars with measured velocities in the central 1 arcsec of M15 and doubles the number in the central 2 arcsec. Combined with existing ground-based data we obtain the radial profiles of the projected kinematical quantities. The RMS velocity sigma_RMS rises to 14 km/s in the central few arcsec, somewhat higher than the values of 10-12 km/s inferred previously from ground-based data. To interpret the results we construct dynamical models based on the Jeans equation, which imply that M15 must have a central concentration of non-luminous material. If this is due to a single black hole, then its mass is M_BH = (3.9 +/- 2.2) x 10^3 solar masses. This is consistent with the relation between M_BH and sigma_RMS that has been established for galaxies. Also, the existence of intermediate-mass black holes in globular clusters is consistent with several scenarios for globular cluster evolution proposed in the literature. Therefore, these results may have important implications for our understanding of the evolution of globular clusters, the growth of black holes, the connection between globular cluster and galaxy formation, and the nature of the recently discovered `ultra-luminous' X-ray sources in nearby galaxies. Instead of a single black hole, M15 could have a central concentration of dark remnants (e.g., neutron stars) due to mass segregation. However, the best-fitting Fokker-Planck models that have previously been constructed for M15 do not predict a central mass concentration that is sufficient to explain the observed kinematics.[ABRIDGED], 43 pages, LaTeX, with 14 PostScript figures. Astronomical Journal, in press (Dec 2002). Please note that the results reported here are modified by the Addendum available at astro-ph/0210158 (Astronomical Journal, in press, Jan 2003). This second version submitted to astro-ph is identical to first, with the exception of the preceeding remark

This article presents and discusses a method for measuring the proper motions of the Galactic dwarf spheroidal galaxies using images taken with the Hubble Space Telescope. The method involves fitting an effective point spread function to the image of a star or quasi-stellar object to determine its centroid with an accuracy of about 0.005 pixel (0.25 milliarcseconds) -- an accuracy sufficient to measure the proper motion of a dwarf spheroidal galaxy using images separated by just a few years. The data consist of images, dithered to reduce the effects of undersampling, taken at multiple epochs with the Space Telescope Imaging Spectrograph or the Wide Field Planetary Camera. The science fields are in the directions of the Carina, Fornax, Sculptor, and Ursa Minor dwarf spheroidal galaxies and each has at least one quasi-stellar object whose identity has been established by other studies. The rate of change with time of the centroids of the stars of the dwarf spheroidal with respect to the centroid of the quasi-stellar object is the proper motion. Four independent preliminary measurements of the proper motion of Fornax for three fields agree within their uncertainties. The weighted average of these measurements is mu_alpha = 49 +- 13 milliarcseconds/century and mu_delta = -59 +- 13 milliarcseconds/century. The Galactocentric velocity derived from the proper motion implies that Fornax is near perigalacticon, may not be bound to the Milky Way, and is not a member of any of the proposed streams of galaxies and globular clusters in the Galactic halo. If Fornax is bound, the Milky Way must have a mass of at least (1.6 +- 0.8) x 10^{12} solar masses., Comment: 4 tables, 16 figures (22 postscript files), 30+ pages. Accepted for publication in AJ. v2: Revised to remove a typo in the abstract and improve the formatting

Spectra taken with the Space Telescope Imaging Spectrograph allow accurate location and extraction of the nuclear spectrum of NGC 4151, with minimal contamination by extended line emission and circumnuclear starlight. Spectra since 1997 show that the P Cygni Balmer and He I absorption seen previously in low nuclear states is present in higher states, with outflow velocity that changes with the nuclear flux. The phenomenon is discussed in terms of some of the absorbers seen in the UV resonance lines, and in terms of outflows from the central source and surrounding torus.

New observations with the Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2), combined with prior archival data, provide nearly complete coverage of the inner 30 Doradus Nebula in both nebular lines and continuum at 0″.1 resolution. The developing windblown cavity surrounding the massive first-generation, central cluster and its interface with the remanent molecular clouds, the site of second-generation, triggered star formation, are imaged in their entirety. Dithered Planetary Camera observations with 0″.03 resolution of Knots 1-3 reveal further structural details of the large dust pillars oriented toward the central cluster and the newborn, massive multiple systems of the second generation in those fields. The new data also provide the first WFPC2 coverage of the fields of two interesting, luminous infrared sources observed in our previous Near Infrared Camera and Multi-Object Spectrometer program; comparisons of the high-resolution optical and IR images are illuminating. In addition, we have obtained Space Telescope Imaging Spectrograph optical, long-slit observations of seven O-type multiple systems in the region, including those in Knots 1-3 and four representatives of the older generation. The spatially resolved spectrograms and classifications of the close pairs, with separations ranging from 0″.09 to 1″.18, are presented, as well as WFPC2 photometry of the individual components and the compact clusters of fainter stars associated with them. These new observations, and planned further analysis of them, offer significant new information about the intricate structure and evolution of the two-stage starburst in 30 Dor., Facultad de Ciencias Astronómicas y Geofísicas

We have obtained Hubble Space Telescope Space Telescope Imaging Spectrograph long-slit spectroscopy of the aligned emission-line nebulae in three compact steep-spectrum radio sources: 3C 67, 277.1, and 303.1. We find systematic offsets (~300–500 km s-1) of the emission-line velocities on one or both sides of the radio sources. We also see evidence for broad lines (FWHM ~ 500 km s-1) and complex emission-line profiles. In 3C 303.1 the data are consistent with multiple components and possibly split lines. The amplitude of the velocity variations is not so large as to exclude gravitationally induced motions. However, the complex kinematics, the lack of a signature of Keplerian rotation, and the association of the velocity variations with the radio lobes are consistent with the motions being driven by the expansion of the radio source. These kinematic signatures are consistent with an interaction between the expanding radio lobe and the emission-line clouds in which the clouds have been accelerated to velocities ~300–500 km s-1. Acceleration of the clouds by the bow shock is plausible given the estimated densities in the clouds and the velocities observed in the much smaller compact symmetric objects and with expansion velocities estimated from spectral aging.

We report the discovery of an ultraviolet filament detected in a new Space Telescope Imaging Spectrograph (STIS) NUV-MAMA image of the cD galaxy UGC 9799, located in the cooling-flow cluster Abell 2052 and host to the radio source 3C 317. The filament is ~2 kpc in length and is located at a distance of ~4 kpc from the nucleus along a north-south axis. It consists of three knots embedded along the edges of a diffuse filamentary halo. The northern half of the filament is narrow (~100 pc) and straight while the southern half is bent and more diffuse. The blue color (NUV-V ≈ -2.4) and morphology of the filament are most consistent with a recent episode of star formation (T ≈ 5 Myr). Only a few × 104 M⊙ of young stars or a star formation rate of ≈10-3 M⊙ yr-1 is required to produce the feature. A steep ultraviolet halo is detected around the unresolved nucleus, and it may be associated with an old stellar component. No ultraviolet features are identified at the location of the extended emission-line nebulae observed from the ground, indicating that OB stars are not the primary source of ionization in these regions. We consider cooling flows and a merger with a satellite galaxy the trigger for the starburst regions and conclude that the latter is the more consistent with the chaotic dust lanes spread throughout the host galaxy. The star formation observed is orders of magnitude less than the inferred cooling rate in the cooling flow scenario.

Coronagraphic imaging of the nearest Herbig Be star with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope, Ks (2.15 ?m) imaging with ADONIS at the 3.6 m telescope at La Silla, and mid-infrared imaging with OSCIR using the 4 m Blanco Telescope at Cerro Tololo Inter-American Observatory confirm the detection of the disk reported by Pantin et al. and map the disk out to 5'' (~515 AU) in the optical and at Ks. While the source is unresolved at 10 and 18 ?m, it can be traced to 15 at 11.7 ?m. We confirm the change in the radial dependence of the disk surface brightness near 27 seen at 1.6 ?m by Augereau et al. at Ks. No such break in the power law is seen in the optical. The STIS data reveal spiral dark lane structure, making HD 100546 the third near?zero-age main-sequence Herbig Ae/Be star with structure more than 100 AU from the star. We also optically detect a low surface brightness envelope extending 10'' (1000 AU) from the star, in addition to nebulosity, which is probably associated with DC 292.6-7.9. The survival of the envelope through essentially the entire pre?main-sequence lifetime of the star, coupled with the absence of physical companions within 1500 AU of the star, suggests that envelope lifetimes owe more to the star-forming environment than to mass-loss activity from the Herbig Ae/Be star.

With photometric repeatability in the 0.2%–0.4% range and wavelength coverage from 1150 to 10,200 A, the Space Telescope Imaging Spectrograph (STIS) low-dispersion spectrophotometry provides excellent absolute flux standards. STIS observations of 19 stars are used to update the database of Hubble Space Telescope standard stars with absolute fluxes that are based on the white dwarf pure hydrogen scale. Uncertainties in the STIS absolute flux calibration range from 4% in the far-UV to 2% at longer wavelengths. The relative and absolute fluxes of our composite standards in the IR are verified by the Near Infrared Camera and Multi-Object Spectrograph (NICMOS) observations and by ground-based IR photometry.

We observed Triton using the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) on 1999 August 28, September 11, and September 13 UT. On all three occasions, we obtained images at both 271 and 374 nm, and spectra from 215 to 318 nm. We find that Triton's geometric albedo is high through the near- and mid-UV, decreasing from 0.64 ± 0.02 at 374 nm to 0.53 ± 0.05 at 223 nm. We compare these data with a previous UV data set obtained by the HST Faint Object Spectrograph (FOS), taken primarily during 1993 September 1–4. Between 250 and 320 nm, Triton was roughly 20% brighter in the STIS (1999) data set than in the FOS (1993) data set, but between 220 and 250 nm the albedos are essentially unchanged. As expected from the HST FOS and other data sets, we find that Triton had higher spatial contrast at shorter wavelengths in 1999, but it exhibited a light-curve amplitude that is as much as 33% lower than derived from Voyager 2 photometry taken in 1989. We discuss some possible causes for these observed changes.

Doppler velocities give information about the three-dimensional shape and orientation of the bipolar ejecta of η Car. We report slit spectroscopy obtained using the Space Telescope Imaging Spectrograph (HST/STIS), with spatial resolution adequate for good models. These data allow an independent estimate of the distance to this object. Equatorial velocities, not clearly recognized in earlier work, provide the first definite measure of the bipolar inclination angle relative to our line of sight. Equatorial structure is arguably the most significant new spectroscopic result reported here, and some of the equatorial gas has higher and lower velocities which probably represent later and earlier ejection events. A range of bipolar lobe shapes fit the data reasonably well; the worst ambiguities result from the nature of the object more than from instrumental limitations.

Observations of the interstellar and circumstellar absorption components obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) along the line of sight toward the Wolf-Rayet–luminous blue variable (LBV) system HD 5980 in the Small Magellanic Cloud are analyzed. Velocity components from C I, C I*, C II, C II*, C IV, N I, N V, O I, Mg II, Al II, Si II, Si II*, Si III, Si IV, S II, S III, Fe II, Ni II, Be I, Cl I, and CO are identified, and column densities estimated. The principal velocity systems in our data are (1) interstellar medium (ISM) components in the Galactic disk and halo (Vhel = 1.1 ± 3, 9 ± 2 km s-1); (2) ISM components in the SMC (Vhel = +87 ± 6, +110 ± 6, +132 ± 6, +158 ± 8, +203 ± 15 km s-1); (3) SMC supernova remnant SNR 0057-7226 components (Vhel = +312 ± 3, +343 ± 3, +33, +64 km s-1); (4) circumstellar (CS) velocity systems (Vhel = -1020, -840, -630, -530, -300 km s-1); and (5) a possible system at -53 ± 5 km s-1 (seen only in some of the Si II lines and marginally in Fe II) of uncertain origin. The supernova remnant SNR 0057-7226 has a systemic velocity of +188 km s-1, suggesting that its progenitor was a member of the NGC 346 cluster. Our data allow estimates to be made of Te ~ 40,000 K, ne ~ 100 cm-3, N(H) ~ (4–12) × 1018 cm-2 and a total mass between 400 and 1000 M⊙ for the supernova remnant (SNR) shell. We detect C I absorption lines primarily in the +132 and +158 km s-1 SMC velocity systems. As a result of the LBV-type eruptions in HD 5980, a fast-wind/slow-wind circumstellar interaction region has appeared, constituting the earliest formation stages of a windblown H II bubble surrounding this system. Variations over a timescale of 1 year in this circumstellar structure are detected.

We present a simple kinematic model for the narrow-line region (NLR) of the Seyfert 1 galaxy NGC 4151, based on our previous observations of extended [O III] emission with the Space Telescope Imaging Spectrograph (STIS). The model is similar to a biconical radial outflow model developed for the Seyfert 2 galaxy NGC 1068, except that the bicone axis is tilted much more into our line of sight (40 degrees out of the plane of the sky instead of 5 degrees), and the maximum space velocities are lower (750 km/s instead of 1300 km/s. We find evidence for radial acceleration of the emission-line knots to a distance of 160 pc, followed by deceleration that approaches the systemic velocity at a distance of 290 pc (for a distance to NGC 4151 of 13.3 Mpc). Other similarities to the kinematics of NGC 1068 are: 1) there are a number of high-velocity clouds that are not decelerated, suggesting that the medium responsible for the deceleration is patchy, and 2) the bicone in NGC 4151 is at least partially evacuated along its axis. Together, these two Seyfert galaxies provide strong evidence for radial outflow (e.g., due to radiation and/or wind pressure) and against gravitational motion or expansion away from the radio jets as the principal kinematic component in the NLR., 31 pages, Latex, includes 11 figures in postscript, Figures 5a,5b,6a,6b in color, to appear in the Astronomical Journal

We present a spatially-resolved ultraviolet spectrum of a non-radiative shock front in the Cygnus Loop, obtained with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The spectrum covers the wavelength range 1118 - 1716 angstroms, with an effective spectral resolution of ~12 angstroms. The 0.1" spatial resolution of these data provides a huge improvement over earlier ultraviolet spectra, allowing us to study the spatial distribution of high ionization line emission directly behind the shock front. We are able to isolate individual shock features in our spectrum by comparing the STIS spectrum with a WFPC2 H-alpha image of the region. Isolating the brightest shock tangency, we identify lines of NV, CIV, HeII, OV, OIV] and SiIV, and NIV] as well as the hydrogen 2-photon continuum. The NV line peaks about 0.3" behind the CIV and HeII emission and is spatially broader. Also, the observed line ratios of CIV and HeII to NV are higher in our bright shock spectrum than in previous observations of the same filament obtained through much larger apertures, indicating that there must be a more widely distributed component of the NV emission. We calculate shock models and show that the observed separation between the CIV and NV emission zones and observed line intensities constrain the combinations of shock velocity and pre-shock density that are allowed.

Computed flux distributions for four white dwarf (WD) stars with nearly pure hydrogen atmospheres are tied to the Vega flux scale with V-band Landolt photometry. With broadband photometric precision in the range 0.2%–0.4%, Space Telescope Imaging Spectrograph (STIS) low-dispersion spectrophotometry tightly constrains the relative continuum fluxes of the various model atmosphere computations. The set of non-LTE (NLTE) model continua has residuals that are slightly less than the residuals with respect to LTE models or with respect to a NLTE set that includes a metal line–blanketed model for G191B2B. The LTE models have Balmer line profiles that are matched to ground-based observations, while a consistent NLTE Balmer line analysis has not been published for all four stars. Residuals with respect to the LTE model continua are less than ~1% from 2000 to 9000 A. Total uncertainties in the adopted absolute flux distributions for the four WD standard stars decrease from 4% at 1300 A to 2% at 5000–10000 A and include estimates for all systematic effects. The STIS sensitivity in the broad hydrogen lines is uncertain with respect to the adjacent continuum by 2%–3% in the wings and by up to 5% in the line cores, because the flux in the wings differs among the various models and because the observed cores are contaminated by out-of-band light in the wide 52'' × 2'' calibration slit.

A search was conducted using the Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) in 1994 August for Iogenic H Lyα emission predicted from electron capture by precipitating trapped magnetospheric protons. The Doppler-shifted diffusely reflected solar Lyα component was detected in the combined high-resolution spectra, partly resolved from the geocoronal emission. This component had a disk-averaged intensity of 2.5 ± 0.7 kR. Io's corresponding geometric albedo in this line, which must include the effects of atmospheric SO2 absorption, was 0.055 ± 0.015. Iogenic emission from ambient H atoms was probably also detected, with a disk-averaged intensity greater than 280 R, most likely ≈485 R, which suggests a significant column of atmospheric or coronal H. The dominant brightness of the reflected solar line in our GHRS data supports the proposed hypothesis that the primary source of Io's H Lyα emission observed more recently (1997–1998) by the HST Space Telescope Imaging Spectrograph at low spectral resolution is diffusely reflected sunlight, rather than Iogenic H. The consequence is a significant increase in the absorbing SO2 column of Io's trailing atmosphere as Jupiter moved 0.37–0.44 AU closer to the Sun; this could result from the higher SO2 equilibrium vapor pressure over the 4–5 K warmer ice. Averaging low-resolution HST GHRS archival far-ultraviolet spectra of Io obtained during 1994–1996 provided a 2 σ upper limit of 5.2 R A-1 for the disk-averaged intensity of Lyα emission from energetic (27–223 keV) precipitating protons. This corresponds to an estimated upper limit of 47 R, integrated over the red Doppler wing of the thermal distribution for protons of mean energy 30–60 keV. This is close to estimates for a proton aurora generated in the SO2 column of Io's trailing atmosphere, indicating that SO2 may be Io's dominant atmospheric gas. A 2 σ upper limit of 123 R per line is derived for nearby fluorescence of extreme-UV radiation by ambient atmospheric sulfur.

We compare the morphology of the cometary knots in the Helix Nebula in different emission lines using Hubble Space Telescope Wide Field Planetary Camera 2 and Space Telescope Imaging Spectrograph observations. We find that the [N II] 6658 A line emission from the cometary heads is displaced with respect to Hα, peaking at a position that is closer to the central ionizing star. This result seems at first sight to be in conflict with simple photoionization models, which predict that the [N II] emission is closer to the ionization front (IF) because it is confined to a thin H+-He0 layer, a prediction confirmed by calculations with both our own and the CLOUDY programs. However, the ratio of [N II] to Hα is very temperature sensitive, and the observations can be explained if the knots are modeled as photoevaporating globules. In this case, there is a strong temperature gradient across the IF, resulting in the [N II] emission being depressed in the partially neutral zones. We also find a strong correlation between the strength of the [N II] and [O III] emission in individual knots, with both being higher in the knots that are closer to the central star. On the current evidence, it is unclear whether this is due to interknot variations in metal abundances or in gas temperature.

Narrowband imaging of the nuclear region of NGC 4151 with the Hubble Space Telescope is presented. The filter bandpasses isolate line emission in various high-velocity ranges in several ions. Slitless and long-slit spectra of the region with the Space Telescope Imaging Spectrograph also indicate the locations of high-velocity gas. These emission regions are faint and interspersed among the bright emission clouds seen in direct images. They have radial velocities up to 1400 km s-1 relative to the nucleus and are found in both approach and recession on both sides of the nucleus. This contrasts strongly with the bright emission-line clouds, which have been discussed previously as showing bidirectional outflow with velocities within 400 km s-1 of the nucleus. We discuss the possible connections of the high-velocity material with the radio jet and the nuclear radiation.

HST/Space Telescope Imaging Spectrograph data show that the apparent near-UV, visual-wavelength, and near-IR brightness of η Car increased by a factor of two during 1998. Meanwhile its Homunculus ejecta nebula brightened by about 30%, the largest fluctuation of this type in the past 40 years. These developments were quite unexpected and are not easy to explain. Some dust has probably been destroyed, while the star's luminosity may have increased even though it was already close to the Eddington limit. Such a rapid luminosity change would be a truly remarkable phenomenon, not predicted by existing models.

In this extension of our previous ultraviolet classification study of Small Magellanic Cloud stars, we construct an ultraviolet classification system for stars of the Large Magellanic Cloud, a galaxy with a metallicity intermediate to those of the Milky Way and the SMC. Employing the same method as was used for the SMC, we compile a set of LMC spectra drawn from the International Ultraviolet Explorer archives and use these spectra to construct a framework of reliable UV spectral classification criteria. We classify 143 LMC IUE spectra and identify the LMC standard stars that define our UV classification system. Representative spectra are presented to illustrate the patterns that the UV spectral classification features exhibit. The LMC and SMC UV classification systems are intended to be applicable to UV spectra of stars in external galaxies for which no MK standards have been defined. Such UV spectra are expected to be collected from spaceborne instruments such as the Space Telescope Imaging Spectrograph, aboard the Hubble Space Telescope.

The combination of Space Telescope Imaging Spectrograph (STIS) and optical spectroscopy with STIS imaging of the field surrounding J2233-606 affords a unique opportunity to study the physical nature of the quasar absorption-line systems. We present an analysis of the ionization state, chemical abundances, and kinematic characteristics of two metal line systems at z = 1.92 and 1.94 toward J2233-606. We focus on these two systems because (1) the observations provide full coverage of the Lyman series, and hence an accurate determination of their H I column densities, and (2) they exhibit many metal line transitions that allow for a measurement of their ionization state and chemical abundances. Line profile fits of the Lyman series for the two systems indicate log N(H I) = 17.15 ± 0.02 cm-2 evenly distributed between two components for the z = 1.92 system and log N(H I) = 16.33 ± 0.04 cm-2 for the z = 1.94 system. By comparing observed ionic ratios of C and Si against calculations performed with the CLOUDY software package, we find the ionization state is high and well constrained in both systems. Applying ionization corrections to the measured ionic column densities, we determine that the systems exhibit significantly different metallicities: ≈1/50 solar and less than 1/200 solar for the two components of z = 1.92 and ≈40% solar at z = 1.94. The most significant uncertainties regarding these measurements are associated with the assumptions inherent to the CLOUDY simulations. The properties of the z = 1.92 system are consistent with a low-metallicity galaxy (e.g., a dwarf galaxy) as well as absorption by large-scale structure of the intergalactic medium. On the other hand, the high metallicity of the z = 1.94 system suggests significant star formation and is therefore assumed associated with a galactic system. Furthermore, we predict it is the more likely to be observed with the STIS imaging and follow-up observations. If a galaxy is identified, our results provide direct measurements on the properties of the interstellar medium for a z ≈ 2 galaxy.

We report Hubble Space Telescope observations that provide evidence for a companion to Proxima Centauri (Gl 551), the closest star to the Sun. Data acquired with the Faint Object Spectrograph (FOS), used as a coronagraphic camera, show excess light that can be interpreted as being from a substellar object within 05 of Proxima Cen. Two observations of Proxima Cen separated by 103 days indicate a point source (or a feature) superposed on the wing of the point-spread function in the FOS images. This feature moves relative to the aperture, and on the plane of the sky. Comparisons with other FOS images of stars acquired using the coronagraphic mode reveal no comparable features or evidence that this feature can be explained by any instrumental anomaly. If this feature denotes a companion to Proxima Cen, it has an apparent separation corresponding to ~0.5 AU at Proxima Cen and is ≈7 mag fainter than Proxima Cen in the bandpass of the FOS red detector. The small apparent separation could result from a highly eccentric orbit, which could project a close companion. Alternately, the small separation could imply a short (~1 yr) period. Further coronagraphic observations, using the Space Telescope Imaging Spectrograph, the Near-Infrared Camera and Multiobject Spectrometer, or some other instrumentation, are needed to verify this tentative result.

We report far-ultraviolet observations of Europa in transit of Jupiter obtained with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope on six occasions between 2014 December and 2015 March. Absorption of Jupiter's bright hydrogen Ly alpha dayglow is detected in a region several moon radii above the limb in all observations. The observed extended absorption provides the first detection of an atomic hydrogen corona around Europa. Molecular constituents in Europa's global sputtered atmosphere are shown to be optically thin to Lya. The observations are consistent with a radially escaping H corona with maximum densities at the surface in the range of (1.5-2.2) x 10(3) cm(-3), confirming the abundances predicted by Monte Carlo simulations. In addition, we search for anomalies around the limb of Europa from absorption by localized high H2O abundances from active plumes. No significant local absorption features are detected. We find that an H2O plume with line-of-sight column density in the order of 10(16) cm(-2), as inferred by Roth et al. would not be detectable based on the statistical fluctuations of the transit measurements, and hence is not excluded or further constrained. The presence of plumes with line-of-sight column densities of >2 x 10(17) cm(-2) can be excluded at a 3-sigma level during five of our six observations. QC 20170329

The Space Telescope Imaging Spectrograph (STIS) has measured the flux for Sirius from 0.17 to 1.01 μm on the Hubble Space Telescope (HST) White Dwarf scale. Because of the cool debris disk around Vega, Sirius is commonly recommended as the primary IR flux standard. The measured STIS flux agrees well with predictions of a special Kurucz model atmosphere, adding confidence to the modeled IR flux predictions. The IR flux agrees to 2%-3% with respect to the standard template of Cohen and to 2% with the Midcourse Space Experiment absolute flux measurements in the mid-IR. A weighted average of the independent visible and mid-IR absolute flux measures implies that the monochromatic flux at 5557.5 A (5556 A in air) for Sirius and Vega, respectively, is 1.35 × 10–8 and 3.44 × 10–9 erg cm–2 s–1 A–1 with formal uncertainties of 0.5%. Contrary to previously published conclusions, the Hipparcos photometry offers no support for the variability of Vega. Pulse pileup severely affects the Hp photometry for the brightest stars.

We have imaged the ejecta of GK Persei (Nova Persei 1901 A.D.) with the Hubble Space Telescope (HST), whose 0.1 arcsec resolution reveals hundreds of cometary-like structures with long axes aligned toward GK Per. One or both ends of the structures often show a brightness enhancement relative to the structures' middle sections, but there is no simple regularity to their morphologies (in contrast with, for example, the Helix nebula). Some of structures' morphologies suggest the presence of slow-moving or stationary material with which the ejecta is colliding, while others suggest shaping from a wind emanating from GK Per itself. The most detailed expansion map of any classical nova's ejecta was created by comparing HST images taken in successive years. Wide Field and Planetary Camera 2 narrowband images and Space Telescope Imaging Spectrograph spectra demonstrate that the physical conditions in this nova's ejecta vary strongly on spatial scales much smaller than those of the ejecta. Directly measuring accurate densities and compositions, and hence masses of this and other nova shells, will demand data at least as resolved spatially as those presented here. The filling factor of the ejecta is 1% or less, and the nova ejecta mass must be less than 10–4 M ☉. A modest fraction of the emission nebulosities vary in brightness by up to a factor of two on timescales of one year. Finally, we present the deepest images yet obtained of a jet-like feature outside the main body of GK Per nebulosity, and the first spectrum of that feature. Dominated by strong, narrow emission lines of [N II], [O II], [O III], and [S II], this feature is probably a shock due to ejected material running into stationary interstellar matter, slowly moving ejecta from a previous nova episode, or circumbinary matter present before 1901. An upper limit to the mass of the jet is of order a few times 10–6 M ☉. If the jet mass is close to this limit then the jet might be an important, or even dominant mass sink from the binary system. The jet's faintness suggests that similar features could easily have been missed in other cataclysmic binaries.