Barry Welsh

We present an abundance analysis of the oxygen detected in the FUSE spectra of three hot white dwarfs with effective temperatures lower than 50,000 K: GD984, REJ1032+532, and REJ2156-543. The FUSE data show unexpectedly strong O VI λλ1032 and 1038 lines. We demonstrate that stellar atmosphere models with homogeneous oxygen abundances neither reproduce the O VI doublet nor the Extreme Ultraviolet Explorer data. Our models show that some O VI ions are formed by photoionization in the upper layers of the atmosphere and are sensitive to the EUV opacity of O III and O IV at lower depths. Based on these findings we compute models that include an oxygen abundance that varies as a function of depth. We show that the oxygen abundance should be lower at depths where the O III and O IV ions predominate in order to reduce the opacity in the EUV range, and it should be higher in the upper layers to allow the production of strong O VI lines. Our results provide one of the best evidence that the abundance of an element heavier than hydrogen is stratified in the atmospheres of white dwarfs.

The total ultraviolet (UV) flux (from 1412 to 2718 Å) of M 101 is compared on a pixel-to-pixel basis with the total far-infrared (FIR) flux (from 60 to 170 {μ }m), using the maps of the galaxy taken by GALEX in the near-UV and far-UV and by ISOPHOT at 60, 100 and 170 {μ }m. The main result of this investigation is the discovery of a tight dependence of the FIR/UV ratio on radius, with values monotonically decreasing from ˜ 4 in the nuclear region to nearly zero towards the edge of the optical disk. Although the tightness of this dependence is in part attributable to resolution effects, the result is consistent with the presence of a large-scale distribution of diffuse dust having a face-on optical depth which decreases with radius and which dominates over the more localized variations in opacity between the arm and interarm regions. We also find a trend for the FIR/UV ratio to take on higher values in the regions of diffuse interarm emission than in the spiral-arm regions, at a given radius. This is interpreted quantitatively in terms of the escape probability of UV photons from spiral arms and their subsequent scattering in the interarm regions, and in terms of the larger relative contribution of optical photons to the heating of the dust in the interarm regions.

The Carina star HD 93206 has been observed at high resolution using the IUE satellite to study the neutral atomic inter-stellar absorption lines in this direction. A line profile fitting analysis has been carried out on these resonance lines to determine cloud component column densities. The state of ionization of these clouds is discussed in detail and, where possible, the respective electron and hydrogen atom densities and electron temperatures are determined.

We present details of the design of a micro-channel based photon counting image-tube developed for ground-based visible astronomical observations with a timing precision <1 micro-sec. The image-tube, developed at UC Berkeley, was subsequently attached to the prime focus of the 40 inch Nickel telescope at the Lick Observatory to obtain high time resolution photometric data on a variety of objects

High-velocity interstellar gas has been detected in IUE spectra of the stars HD 192639, 193514 and 193443 which are members of the OB association Cyg OB1. In all cases where blending between different absorption components is not severe, the authors have derived ion column densities appropriate to the high-velocity gas by fitting theoretical profiles to the observed absorption lines. Previous observations of interstellar gas in the direction of Cyg OB1 are reviewed in an attempt to determine the origin of the high-velocity ultraviolet features. The possibility is discussed that the present observations sample different sections of a large-scale expanding shell around Cyg OB1. Alternatively, the high-velocity gas may be local to the individual stars observed, resulting from the interaction of their energetic stellar winds with the ambient interstellar medium.

We describe the Lyman Imaging Telescope Experiment (LITE) which is a NASA Ultraviolet Astrophysics Branch supported Advanced Mission Concept mission. The prime scientific aim of the LITE mission will be to carry out the first set of very high spatial resolution (0.2 arc sec), wide field of view (10 arc minute), pointed observations in several narrow wavelength bands in the far ultraviolet region of the spectrum (900 - 1600 angstroms). LITE will possess excellent detection sensitivity, such that limiting magnitudes for typical images are expected to be close to that of the HST WFPC II instrument. The proposed far ultraviolet astrophysical studies will encompass the emission of diffuse gas with temperatures in the range 80,000 - 1,000,000 K.

The Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR) payload, launched on the STSAT-1 Mission 27 September 2003, is scheduled to begin (December 2003) the first imaging (10' resolution) spectral survey of diffuse far ultraviolet (900-1750 Angstrom, lambda /delta lambda 600) emission from interstellar and intergalactic matter. The primary science objectives of the mission are to determine the flow of mass and energy through the interstellar medium (ISM), mapping the global distribution of the multiphase ISM, and testing competing models for the origin of the hot interstellar plasma. We report the status and preliminary science results from the beginning of the mission and describe opportunities for investigators. The SPEAR payload is supported in part by NASA grant NAG5-5335. The Korea Ministry of Science and Technology supports the STSAT Mission.

The large field of view (1.2 degrees), high sensitivity at ultraviolet wavelengths of 1350-2800 Å (AB magnitude about 23 in 1500 s exposure), and repeated sky coverage of the Galaxy Evolution Explorer (GALEX) satellite have resulted in many serendipitous detections of variable and transient ultraviolet sources. 84 of these sources, detected during the first 12 months of the mission, have been reported in version 1.0 of the GALEX Ultraviolet Variability (GUVV) catalog by Welsh et al. (2005). Since then, new software detection tools based on aperture curves of growth, relative photometry and statistical tests have been developed that are now able to reveal many new and lower amplitude time-variable UV sources contained within the ever-expanding GALEX data archives. Here, we report on the preliminary results of implementing these variability detection tools on new data obtained with extended GALEX observations of two regions of the Hyades cluster. The cluster is known to contain many late-type M-dwarf stars that can produce highly energetic UV flares, typically of 200-1000s duration. We also report on the detection of a wide variety of other variable sources which include Seyfert Galaxies, QSOs, field flare stars and many new RR Lyrae stars, which can vary by up to 6 magnitudes in the far ultraviolet (1350-1800 Å) during their pulsation cycle. We anticipate that the RR Lyrae variables will be the predominant type of UV variable source contained in version 2.0 of the GUVV catalog.

Until recently, RR Lyrae stars have been sparsely observed at wavelengths shorter than 2000Å. The NASA Galaxy Evolution Explorer (GALEX) satellite has serendipitously obtained simultaneous near and far ultraviolet light curves of several RR Lyrae stars, which vary during their pulsation cycle by up to 6.3 magnitudes in the GALEX FUV channel (1350Å-1800Å). This variation is far greater than the <1.5 mag seen at visible wavelengths. We show a comparison of several GALEX UV light curves with those calculated from Kurucz stellar atmosphere models, and also describe methods for identifying new RR Lyrae stars using GALEX in conjunction with other survey photometric data.

M32, the compact elliptical galaxy companion to the Andromeda spiral galaxy, has been imaged by the Galaxy Evolution Explorer (GALEX) in two UV bands, centered at ~1500 (far-ultraviolet [FUV]) and 2300 Å (near-ultraviolet). The imaging data have been carefully decomposed so as to properly account for the complicated background contamination from the disk of M31. We have derived the surface brightness and color profiles finding a slightly positive color gradient of Δ(FUV-B)/Δlogr=+0.15+/-0.03 within one effective radius. Earlier data from the Ultraviolet Imaging Telescope suggested that M32 had an extremely large (negative) FUV-optical color gradient [Δ(FUV-B)/Δlogr<-2], inverted with respect to the majority of gradients seen in giant elliptical galaxies. Our new results show that despite its very low UV upturn, M32 has similar UV properties to those observed in luminous elliptical galaxies.

Contamination control program results from three years of ground operations on the Extreme Ultraviolet Explorer instruments. [Proceedings of SPIE 1329, 24 (1990)]. David C. Ray, Sharon R. Jelinsky, Barry Y. Welsh, Roger F. Malina. Abstract. ...

We present laboratory measurements of the reduction in UV-to-visible/IR reflectivity (1200Å - 10,000Å) of optical surfaces due to 50Å - 500Å deposited layers of out-gassed molecular contaminants from 9 commonly used spacecraft/space instrument materials that include Apiezon-L hi-vacuum grease, Braycote-601EF lubricant, DC 704 silicon oil, EPOM rubber clad wire, PVC clad wire, Scotchweld 2216 epoxy, Uralane 5753 staking compound, Tefzel cable tie and Aeroglaze Z306 black paint on Kapton. Our results are compared with predictions from theoretical models currently being widely used throughout the space industry. Good agreement is found at UV wavelengths, but large differences occur at visible wavelengths. This latter effect is due to the application of Beer's Law, which ignores the non-negligible effect of reflectance scattering from the thin film of the contaminant.

We present medium resolution spectra (R ~ 7 km s-1) of the interstellar NaI D1 & D2 absorption lines observed towards 7 early-type stars with distances ranging from 221 to ~ 1.5 kpc in the line-of-sight to the RCW 114 nebula. Our observed pattern of absorption is in conflict with the findings of Bedford et al. (1984) who placed an upper limit to the distance to this nebula of <200 pc. Instead, our spectral data are consistent with a marked increase in NaI absorption complexity occuring only for sight-lines with distances >1 kpc. A similar pattern of absorption behavior is seen for the SiII lambda 1304 Å interstellar line observed in the ultraviolet towards four of the aforementioned stars. We propose a scenario in which the absorption components with velocities <-10 km s-1 seen towards RCW 114 are caused by an expanding stellar wind-blown bubble associated with the Wolf-Rayet (WR) star, HD 156385 (d ~ 1.5 kpc), and that the Hα and [SII] nebular emission probably arises in shocked filamentary gas associated with a pre-existing evolved supernova remnant cavity produced by the high-mass progenitor of the present WR star.

We present several high time-resolution (0.1 s) lightcurves of bright, short-duration (< 1000 s) flare events recorded at ultraviolet wavelengths (1350 - 2750 A) during the first 18 months of observations by the NASA Galaxy Evolution Explorer (GALEX). These outbursts originate on nearby M-dwarf (dMe) flare stars, and can result in a brightening of over 5 UV magnitudes in a period of less than 200 seconds. We compare and contrast pre and post flare behavior in order to derive meaningful parameters that can be used as a general diagnostic tool of the complex emission structure often exhibited by these outbursts. Finally, we show plots of SDSS (g - r) and GALEX UV magnitudes that can be used to identify previously unknown active dM stars. GALEX is a NASA Small Explorer. We gratefully acknowledge NASA's support for construction, operation, and science analysis for the GALEX mission.

The GALEX (Galaxy Evolution Explorer) Nearby Galaxies Survey is providing deep far-UV and near-UV imaging for a representative sample of galaxies in the local universe. We present early results for M51 and M101, from GALEX UV imaging and SDSS optical data in five bands. The multi-band photometry of compact stellar complexes in M101 is compared to population synthesis models, to derive ages, reddening, reddening-corrected luminosities and current/initial masses. The GALEX UV photometry provides a complete census of young compact complexes on a approximately 160pc scale. A galactocentric gradient of the far-UV - near-UV color indicates younger stellar populations towards the outer parts of the galaxy disks, the effect being more pronounced in M101 than in M51.

We present an analysis of the far-ultraviolet spectrum of the post-AGB star Barnard 29 in the globular cluster M13 (NGC 6205) obtained with COS and FUSE. The data extend from the Lyman limit to about 1780 A with a spectral resolution R 20,000. Previous observers have derived the star's effective temperature (Teff = 20,000 K), surface gravity (log g = 3.0), and abundances of He, C, N, O, Mg, Si, Al, S, and Fe. In addition to these species, we detect absorption from the light elements P and Cl, the iron-peak elements Ti, Cr, and Ni, and the heavy elements Ge and Zr. Using LTE and NLTE stellar atmosphere models, we derive the abundances of these elements and set upper limits for several more. Our Ge and Zr abundances will provide useful constraints on models of nucleosynthesis in AGB stars. This work is supported by NASA grant NNX08AC14G.

We shall implement newly developed software tools to search the GALEX archival all-sky survey data for both transient and variable UV sources. Our aim is to detect and model the UV emission from stellar flares, dwarf novae, soft X-ray transients and intrinsically variable stars such as RR Lyraes and Cepheids. Preliminary inspection of 37 observations of 1 of the 80 proposed Deep Imaging Survey fields has revealed a Mv = 13.5 RR Lyrae stars that exhibits over 5 magnitudes of variability in the far UV. Such large amplitude UV variations provide a more stringent limit to the stellar metallicity and/or temperature as derived from stellar atmosphere models for this class of star. We also hope to detect the associated UV emission from known gamma-ray bursters, which if detected would help constrain their luminosity function. Most of these serendipitous detections will require follow-up ground-based observations to determine their optical identification.

We shall probe the ionization state and kinematics of the lower halo-Local Bubble interaction region using hot white dwarf stars of known, increasing distance away from the galactic plane up to z sim 0.3 kpc that of neutral gas. Our main goal is to obtain better constraints on the location of the hot gas of the Local Bubble and the associated conductive interfaces. We shall also search for signs of the infall or outflow of neutral andor ionized gas in this particular region (which could be related to a galactic fountain), and investigate the way the kinetic temperature, electron density, element abundancesdepletions and gross velocity structure of gas in the Local Bubble varies as a function of increasing height above the galactic disk as it merges into the lower galactic halo.

We review some of the mission concepts currently being considered by NASA's Astrophysics Division to carry out future observations in the 100-3000 Angstrom region. Examples of possible future missions include UV and visible interferometric experiments, a next generation Space Telescope and lunar-based UV instrumentation. In order to match the science objectives of these future missions with new observational techniques, critical technology needs in the ultraviolet regime have been identified. Here we describe how NASA's Astrophysics Division Advanced Programs Branch is attempting to formulate an integrated technology plan called the 'Astrotech 21' program in order to provide the technology base for these astrophysics missions of the 21st century.

The various concepts in ultraviolet detector technology currently being funded by NASA's Astrophysics Division to carry out observations in the 100 to 3000 A region are reviewed. In order to match the science objectives of future space missions with new observational techniques, critical detector technology needs in the ultraviolet regime have been identified. The attempt by NASA's Astrophysics Division Advanced Programs Branch to formulate an integrated detector technology plan as part of the ongoing 'Astrotech 21' program in order to provide the technology base for these astrophysics missions of the 21st century is described.

We present Galaxy Evolution Explorer (GALEX) far-UV and near-UV mosaic observations covering the entirety of M31 and M33. For both targets, we measure the decline of surface brightness (in FUV and NUV) and changes in FUV--NUV color as a function of galactocentric radius. These UV radial profiles are compared to the distribution of ionized gas traced by H-alpha emission. We find that the extent of the UV emission, in both targets, is greater than the extent of the observed HII regions and diffuse ionized gas. We determine the ultraviolet diffuse fraction in M33 using our FUV observations and compare it to the H-alpha diffuse fraction obtained from wide-field narrow-band imaging. The FUV diffuse fraction appears to be remarkably constant near 0.65 over a large range in galactocentric radius, with departures to higher values in circumnuclear regions and, most notably, at the limit of the H-alpha disk. We suggest that the increase in FUV diffuse fraction at large galactocentric radii could indicate that a substantial portion of the diffuse emission beyond this point is not generated in situ but rather scattered from dust, after originating in the vicinity of the disk's outermost HII regions. Radial variation of the H-alpha diffuse fraction was also measured. We found the H-alpha diffuse fraction generally near 0.4 but rising toward the galaxy center, up to 0.6. We made no attempt to correct our diffuse fraction measurements for position-dependent extinction, so the quoted values are best interpreted as upper limits given the plausibly higher extinction for stellar clusters relative to their surroundings.

We present a short overview of high resolution (R = 100,000) spectroscopic observations of interstellar absorption in the Local Bubble, which is an asymmetric region of very low density gas surrounding the Sun extending to almost 200pc in some galactic directions. Absorption in the local interstellar cloud, the local fluff, the Local Bubble neutral interface wall and highly ionized local regions are presented.

We shall perform extended pointed observations of two regions in the Pleiades open cluster to observe UV flux variability due to flare activity on M-type stars. Using newly developed software tools we shall be able to probe a more comprehensive range of level of magnetic activity on a wider range of spectral types than that presently achieved at X-ray wavelengths. Our observations will determine flare energies and activity levels as a function of spectral type for the 110 Myr old cluster that can be compared with similar data obtained in Cycle 2 from the older 600Myr old Hyades cluster. These data are of particular relevance to the determination of habitability zones around M-stars, which account for 80 percent of the stellar population in the Galaxy.

Number counts of galaxies in two Galaxy Evolution Explorer (GALEX) bands [far-UV (FUV: 1530 Å) and near-UV (NUV: 2310 Å) both in AB magnitudes] are reported. They provide for the first time in the literature homogeneously calibrated number counts of UV galaxies continuously covering a very wide range in UV magnitude (14-23.8). Both the FUV and NUV counts are inconsistent with a nonevolution model, whereas they are in good agreement with evolution models (essentially luminosity evolution) derived from the high-z UV luminosity functions of Arnouts et al. We find that the contribution from GALEX-detected galaxies to the UV background is 0.68+/-0.10 nW m-2 sr-1 at 1530 Å and 0.99+/-0.15 nW m-2 sr-1 at 2310 Å. These are 66%+/-9% and 44%+/-6% of the total contributions of galaxies to the UV background at 1530 Å (1.03+/-0.15 nW m-2 sr-1) and at 2310 Å (2.25+/-0.32 nW m-2 sr-1), respectively, as estimated using the evolution models. Galaxy counts and star counts in seven regions, each containing a few square degrees of GALEX coverage in an area of ~4-13 deg2, are compared with each other to study the region-by-region variance. This shows that for the galaxy counts, the cosmic variance is comparable to the net error due to other uncertainties. The star counts increase with decreasing absolute Galactic latitude |b|.

We describe the Galaxy Evolution Explorer (GALEX) satellite that was launched in April 2003 specifically to accomplish far ultraviolet (FUV) and near ultraviolet (NUV) imaging and spectroscopic sky-surveys. GALEX is currently providing new and significant information on how galaxies form and evolve over a period that encompasses 80% of the history of the Universe. This is being accomplished by the precise measurement of the UV brightness of galaxies which is a direct measurement of their rate of star formation. We briefly describe the design of the GALEX mission followed by an overview of the instrumentation that comprises the science payload. We then focus on a description of the development of the UV sealed tube micro-channel plate detectors and provide data that describe their on-orbit performance. Finally, we provide a short overview of some of the science highlights obtained with GALEX.

We report on the detection of over 90 variable and transient sources serendipitously detected at ultraviolet wavelengths (1350 - 2750 Å) during the first year's observations by the NASA Galaxy Evolution Explorer (GALEX). Many of of the brightest time-variable objects are nearby M-dwarf flare stars, one of which exhibited a brightening of 10 UV magnitudes in a period of only 600 seconds. This explosive event was over 20 times more energetic than any previously recorded flare in the ultraviolet. We present our data in a preliminary catalogue of time-variable objects that includes their position, UV magnitude variation and possible source identity. Finally, we derive the approximate energy input into the interstellar medium from these UV outbursts to determine if this is a significant source of ionization for the local interstellar gas.

We present the spectrum the brightest known extreme ultraviolet source, epsilon Canis Majoris, between 600 and 920 Angstroms. epsilon Canis Majoris (B2 II) was discovered to be the brightest EUV source in the sky during the Extreme Ultraviolet Explorer (EUVE) all-sky survey. Subsequent spectroscopic observations found an anomolously low neutral hydrogen column density, allowing for the first time the observation of a B star between 500 and 730 Angstroms. Unexpectedly, the spectrum is still increasing at 730 Angstroms. By observing epsilon CMa between 600 and 920 Angstroms, the spectrum of epsilon CMa is now complete from the x-ray thru to the infrared. This provides a unique opportunity for studying the atmospheres of B stars and refining the current atmospheric models, which cannot currently model the stellar flux from epsilon CMa. epsilon CMa was observed on March 4, 1996 from White Sands Missile Range using a sounding rocket borne spectrograph. The spectrum has a resolution ranging between 1000 at 735 Angstroms to 450 at 900 Angstroms . Two data sets were acquired during the flight; one with a tin filter, to block out potential scattered light longward of 920 Angstroms, and one without the filter. We will present the spectrum epsilon CMa and discuss and the current state of our analysis.

We present a study of the morphological nature of redshift z~0.7 star-forming galaxies using a combination of HST ACS, GALEX, and ground-based images of the COSMOS field. Our sample consists of 8146 galaxies, 5777 of which are detected in the GALEX near-ultraviolet band (2310 Å or ~1360 Å rest frame) down to a limiting magnitude of 25.5 (AB), and all of which have a brightness of F814W(HST)<23 mag and photometric redshifts in the range 0.55<z<0.8. We make use of the UV to estimate star formation rates, correcting for the effect of dust using the UV slope, and of the ground-based multiband data to calculate masses. For all galaxies in our sample, we compute, from the ACS F814W images, their concentration (C), asymmetry (A), and clumpiness (S), as well as their Gini coefficient (G) and the second moment of the brightest 20% of their light (M20). We observe a bimodality in the galaxy population in asymmetry and in clumpiness, although the separation is most evident when either of those parameters is combined with a concentration-like parameter (C, G, or M20). We further show that this morphological bimodality has a strong correspondence with the FUV-g color bimodality and conclude that UV-optical color predominantly evolves concurrently with morphology. We observe many of the most star-forming galaxies to have morphologies approaching that of early-type galaxies, and we interpret this as evidence that strong starburst events are linked to bulge growth and constitute a process through which galaxies can be brought from the blue to the red sequence while simultaneously modifying their morphology accordingly. We conclude that the red sequence has continued growing at z<~0.7. We also observe z~0.7 galaxies to have physical properties similar to that of local galaxies, except for higher star formation rates. Whence we infer that the dimming of star-forming galaxies is responsible for most of the evolution in star formation rate density since that redshift, although our data are also consistent with a mild number evolution. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by Association of Universities for Research in Astronomy, Inc. (AURA), under NASA contract NAS 5-26555 and with the NASA Galaxy Evolution Explorer (GALEX); also based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation; the Canada-France-Hawaii Telescope with MegaPrime/MegaCam operated as a joint project by the CFHT Corporation, CEA/DAPNIA, the National Research Council of Canada, the Canadian Astronomy Data Centre, the Centre National de la Recherche Scientifique de France, TERAPIX, and the University of Hawaii.

Data from contaminant-induced UV optics degradation studies and particulate models are used here to develop end-of-service-life instrument contamination requirements which are very stringent but achievable. The budget is divided into allocations for each phase of hardware processing. Optical and nonoptical hardware are monitored for particulate and molecular contamination during initial cleaning and baking, assembly, test, and calibration phases. The measured contamination levels are compared to the requirements developed for each phase to provide confidence that the required end-of-life levels will be met.

We present high resolution (R = 60,000) measurements of the NaI D1 and D2 (5890 A) and CaII K (3933 A) interstellar absorption line profiles recorded towards several post-AGB stars located within the M13 and M15 globular clusters, supplemented with a lower resolution spectrum of the CaII K-line observed in absorption towards an Ofpe/WN9 star in the central region of the M33 galaxy. The normalized interstellar absorption profiles have been fit with cloud component velocities, doppler widths and column densities in order to investigate the kinematics and physical conditions of the neutral and partially ionized gas observed along each sight-line. Our CaII observations towards M13 have revealed 4 absorption components that can be identified with galactic Intermediate Velocity Clouds (IVCs) spanning the -50 > Vlsr > -80 km/s range. The NaI/CaII ratio for these IVC's is<0.3, which characterizes the gas as being warm (T=1000 K) and partially ionized. Similar observations towards two stars within M15 have revealed absorption due to a galactic IVC at Vlsr=+65 km/s. This IVC is revealed to have considerable velocity structure, requiring at least 3 cloud components to fit the observed NaI and CaII profiles. CaII K-line observations of a sight-line towards the center of the M33 galaxy have revealed at least 10 cloud components. A cloud at Vlsr=-130 km/s is either an IVC associated with the M33 galaxy occurring at +45 km/s with respect to the M33 local standard of rest, or it is a newly discovered HVC associated with our own Galaxy. In addition, 4 clouds have been discovered in the -165 > Vlsr > -205 km/s range. Three of these clouds are identified with the disk gas of M33, whereas a component at - 203 km/s could be IVC gas in the surrounding halo of M33.

Detectors with cross strip anodes have been used in open face and sealed tube detectors for detection of particles and light. In combination with high resolution position encoding electronics these have the capability of being able to independently record the accurate time and position of each detected photon event. Sealed tube (18 mm) microchannel plate cross strip anode detectors with supergen-II photocathodes, and open face cross strip anode detectors up to 40mm have been commissioned. These have high spatial resolution (≤ 30 μm), are capable of high counting rates (input rates > 5 MHz), demonstrate time tagging of events with ~1.5 ns accuracy, and provide extended lifetimes. Related developments of high efficiency GaN photocathodes and novel MCP structures capable of providing 20 x 20 cm devices potentially enhance the applicability of these devices.

High-resolution absorption measurements of the interstellar Na I D lines at 5890 A observed toward 27 early-type stars in the local interstellar medium (LISM) are presented. These results are combined with other high-resolution sodium measurements to map the space distribution of neutral sodium column density for some 118 stars out to less than 200 pc. These measurements indicate an upper limit to the neutral sodium column density of log N(Na I) less than 10.0/sq cm can be inferred out to a distance of 50 pc in most directions in the LISM. Also, the rarefield region of the Local Bubble may extend beyond 60 pc in at least 35 percent of the directions sampled thus far. Evidence is shown for a ubiquitous, comoving vector for neutral NaI gas clouds in the LISM which is in a different direction to LISM vectors previously reported for more ionized local gas clouds. A comparison of the measured sodium columns with those of interstellar Ca II for a sample of 12 stars within 95 pc results in a ratio of Na I/Ca II less than 0.5 for most stars. This value implies that there could be warm, neutral gas with T of about 12,500 K beyond 50 pc in the LISM.

We will repeatedly observe 4 early-type Herbig Be stars which are thought to possess gaseous circumstellar disks. Our investigation will focus on the strong stellar FUV line profiles to determine the extent of time variability due to mass loss and accretion processes. We will also analyze the physical state of the molecular absorption lines sampled in the interstellar gas serendipitously observed in the line-of-sight to these objects.

The microchannel plates for the detectors in the SUMER and UVCS instruments aboard the Solar Orbiting Heliospheric Observatory (SOHO) mission to be launched in late 1995 are described. A low resistance Z stack of microchannel plates (MCPs) is employed in a detector format of 27 mm multiplied by 10 mm using a multilayer cross delay line anode (XDL) with 1024 by 360 digitized pixels. The MCP stacks provide gains of greater than 2 multiplied by 10(superscript 7) with good pulse height distributions (as low as 25% FWHM) under uniform flood illumination. Background rates of approximately equals 0.6 event cm(superscript -2) sec(superscript -1) are obtained for this configuration. Local counting rates up to approximately equals 800 events/pixel/sec have been achieved with little drop of the MCP gain. MCP preconditioning results are discussed, showing that some MCP stacks fail to have gain decreases when subjected to a high flux UV scrub. Also, although the bare MCP quantum efficiencies are close to those expected (approximately equals 10%), we found that the long wavelength response of KBr photocathodes could be substantially enhanced by the MCP scrubbing process. Flat field images are characterized by a low level of MCP fixed pattern noise and are stable. Preliminary calibration results for the instruments are shown.

High-resolution observations of the sodium D1 and D2 interstellar absorption lines have been made toward 19 stars in the local ISM at distances ranging from 20 to 220 pc. A least-squares absorption line-fitting analysis is carried out on 10 of the stars that show detectable sodium. The velocity structure, velocity dispersions, and column densities of the various interstellar cloud components are derived. For the majority of stars, the best-fit dispersion parameter is about 2 km/s, corresponding to an upper limit for the kinetic temperature of the interstellar gas of about 5500 K. No sodium is detected toward stars with distances less than 42 pc, placing an upper limit 2.5 x 10 to the 18th/sq cm to the hydrogen column density in these lines of sight.

We shall implement our specialized software tools to serendipitously search the GR2 data archive for transient and variable UV stellar sources. Our aim is to detect and assign probable identifications and positions for these sources for inclusion in version 2.0 of the GALEX UV Variability (GUVV) catalog. Such sources will include dMe flare stars, cataclysmic variables, soft X-ray transients and intrinsically variable stars such as RR Lyraes and delta Scutis. This work supports the NASA strategic objective of understanding stellar structure and evolution.

We present high spectral resolution UV absorption measurements of the sight-lines towards two B-type stars in the local interstellar medium recorded with the newly refurbished HST-STIS instrument. We comment on the physical condition of the local interstellar gas within 80pc of the Sun, especially with regard to the possibility that the Local Bubble cavity may not be filled with a high temperature, highly ionized plasma.

We present HST-COS observations of the interstellar sight-line towards the M13 globular cluster. High velocity gas at V -120 km/s and intermediate velocity gas at V -65 km/s has been detected in many UV absorption lines. The HVC gas is probably associated with the Complex K IVC with a distance of 1.0 < z < 4.7 kpc. This would place the infalling HVC gas far closer than previous estimates of HV clouds surrounding our Galaxy.