N. Kappelmann

Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo

The Milky Ways halo contains clouds of neutral hydrogen with high radial velocities which do not follow the general rotational motion of the Galaxy. Few distances to these high-velocity clouds are known, so even gross properties such as total mass are hard to determine. As a consequence, there is no generally accepted theory regarding their origin. One idea is that they result from gas that has cooled after being ejected from the Galaxy through fountain-like flows powered by supernovae; another is that they are composed of gas, poor in heavy elements, which is falling onto the disk of the Milky Way from intergalactic space. The presence of molecular hydrogen, whose formation generally requires the presence of dust (and therefore gas, enriched in heavy elements), could help to distinguish between these possibilities. Here we report the discovery of molecular hydrogen absorption in a high-velocity cloud along the line of sight to the Large Magellanic Cloud. We also derive for the same cloud an iron abundance which is half of the solar value. From these data, we conclude that gas in this cloud originated in the disk of the Milky Way.

The ORFEUS II Echelle Spectrometer: Instrument description, performance and data reduction

During the second flight of the ORFEUS-SPAS mission in November/December 1996, the Echelle spectrometer was used extensively by the Principal and Guest Investigator teams as one of the two focal plane instruments of the ORFEUS telescope. We present the in-flight performance and the principles of the data reduction for this instrument. The wavelength range is 90 nm to 140 nm, the spectral resolution is significantly better than λ /Δ λ  = 10 000, where Δ λ is measured as FWHM of the instrumental profile. The effective area peaks at 1.3 cm2 near 110 nm. The background is dominated by straylight from the Echelle grating and is about 15% in an extracted spectrum for spectra with a rather flat continuum. The internal accuracy of the wavelength calibration is better than ±  0.005 nm.

HIRDES UV spectrographs

The World Space Observatory Ultraviolet (WSO/UV) is a multi-national project grown out of the needs of the astronomical community to have future access to the ultraviolet range of the spectrum. The development of the WSO/UV S/C and the telescope is headed by the Russian Federal Space Agency (Roscosmos). The mission is scheduled to be launched in 2010 into the L2 orbit. The WSO/UV consists of a single Ultraviolet Telescope, incorporating a primary mirror of 1.7 m diameter feeding UV spectrometer and UV imagers. The UV spectrometer comprises three different single spectrographs, two high resolution echelle spectrographs - the High Resolution Double Echelle Spectrograph (HIRDES) - and a low dispersion long slit instrument. Within the HIRDES the spectral band (102 - 310 nm) is separated into two echelle spectrographs covering the UV range between 174- and 310 nm (UVES) and VacuumUV range between 102 and 176 nm (VUVES) with a very high spectral resolution of > 50000. Each spectrograph encompass a stand alone optical bench structure with a fully redundant high speed MCP detector system, the optomechanics and a network of mechanisms with different functionalities. The fundamental technical concept is based on the heritage of the two previous ORFEUS SPAS missions. The phase B1 development activities are described in this paper under consideration of performance aspects, design drivers, the related trade offs (e.g. mechanical concepts, material selection etc.) and the critical functional and environmental test verification approach. Furthermore the actual state of the other scientific instruments of the WSO/UV (e.g. UV imagers) project is described.

ORFEUS II Far-Ultraviolet Observations of 3C 273: The Intrinsic Spectrum

Using the Berkeley spectrometer of the ORFEUS-SPAS II mission, we observed the spectrum of the bright, low-redshift QSO 3C 273 in the wavelength range 900-1200 A (780-1040 A in the QSOs rest frame). The QSOs spectrum is dominated by broad emission blends of the resonance lines of O VI, C III, N III, and S VI. Only relatively weak emission is detected at the wavelengths of the higher Lyman lines, and no significant Lyman discontinuity is present at the QSOs redshift. The reddening-corrected underlying smooth continuum shows (for the epoch of the ORFEUS observations) a turnover of the νFν spectrum at about 2.5×1015 Hz. While this turnover frequency is in the general range expected for active galactic nucleus accretion disks, it is lower than predicted for QSOs as luminous as 3C 273 by standard accretion disk models.

The ORFEUS II Echelle spectrum of HD 93521: A reference for interstellar molecular hydrogen

During the second flight of the ORFEUS-SPAS mission in November/December 1996, the Echelle spec- trometer was used extensively by the Principal and Guest Investigator teams as one of the two focal plane in- struments of the ORFEUS telescope. The spectrum of HD 93521 was obtained during this mission with a total integration time of 1740 s. This spectrum shows numer- ous sharp interstellar absorption lines. We identied 198 lines of molecular hydrogen including at least 7 lines with a high velocity component. Also most of the 67 identi- ed interstellar metal lines are visible with a high velocity component. We present plots of the complete ORFEUS II Echelle spectrum together with tables of all identied interstel- lar absorption lines including all 14 detectable H I lines. In addition several identied stellar lines, partially with narrow absorption components, and stellar wind lines are given in a separate table.

The WSO: a world-class observatory for the ultraviolet

The World Space Observatory is an unconventional space project proceeding via distributed studies. The present design, verified for feasibility, consists of a 1.7-meter telescope operating at the second Largangian point of the Earth-Sun system. The focal plane instruments consist of three UV spectrometers covering the spectral band from Lyman alpha to the atmospheric cutoff with R~55,000 and offering long-slit capability over the same band with R~1,000. In addition, a number of UV and optical imagers view adjacent fields to that sampled by the spectrometers. Their performance compares well with that of HST/ACS and the spectral capabilities of WSO rival those of HST/COS. The WSO, as presently conceived, will be constructed and operated with the same distributed philosophy. This will allow as many groups and countries to participate, each contributing as much as feasible but allowing multi-national participation. Although designed originally with a conservative approach, the WSO embodies some innovative ideas and will allow a world-class mission to be realized with a moderate budget.

WSO‐UV—Ultraviolet Mission for the Next Decade

The World Space Observatory Ultraviolet (WSO‐UV) is an international space mission devoted to UV spectroscopy and imaging. The observatory includes a 1.7 m aperture telescope capable of high‐resolution and long‐slit low‐resolution spectroscopy, and deep UV and optical imaging. The observatory is designed for observations in the UV domain where many astrophysical processes can be studied efficiently with unprecedented capability.

Medium-resolution far-ultraviolet spectroscopy of PKS 2155-304

Using the Berkeley spectrometer of the Orbiting Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) we observed the 87-117 nm UV spectrum of the BL Lac object PKS 2155-304 with about 0.5 A resolution. In addition to the expected interstellar lines we detected higher quantum number counterparts of the intergalactic Lyman alpha lines discovered earlier with IUE and the Hubble Space Telescope (HST) in the direction of PKS 2155-304. The Lyman discontinuities indicate for three of the redshifted clouds a combined H I column density of 2-5 x 10(exp 16)/sq cm, while the column density for another cloud appears to be well below 5 x 10(exp 15)/sq cm. No siginificant O VI absorption in the galactic halo toward PKS 2155-304 could be detected from our data. Assuming that saturation effects are negligible for these weak features, we obtain for the O VI column density toward PKS 2155-304 a 3 sigma upper limit of 2.7 x 10(exp 14)/sq cm.

UV MCP Detectors for WSO-UV: Cross Strip Anode and Readout Electronics

The main instrument of the WSO-UV satellite covers the wavelength range of 102-176 nm and 174-310 nm with two high resolution echelle spectrographs. The essential requirements for the associated detectors are high quantum efficiency, solar blindness, and single photon detection. To achieve this, we are developing microchannel plate (MCP) detectors in sealed tubes. It is planned to use cesium activated gallium nitride as semitransparent photocathode, a stack of two microchannel plates in chevron configuration, and a 33 mm × 44 mm cross strip anode with 64 horizontal and 64 vertical electrodes. This type of anode requires a lower gain of the MCPs ( ≈ 106) compared to other types of anodes. Therefore, it extends the expected lifetime of the detectors to about five to ten years. The challenge for the use of a cross strip anode onboard the WSO-UV satellite is the combination of contradictory constraints on the readout electronics: On the one hand it should be able to handle a maximum count rate of 3·105 s-1 with a spatial resolution better than 15 μm . On the other hand the power consumption is limited to about 8 W. One feasible solution is the so-called Beetle chip. This chip provides 128 input channels with charge-sensitive preamplifiers and shapers. It stores the sampled data temporarily in a ring buffer and multiplexes it to four analogue readout channels. The output is then digitized by four ADCs and processed in a radiation hard FPGA, which also contains the space-wire interface to the satellite bus.

MCP detector development for WSO-UV

The spectrographs of WSO-UV cover the wavelength range of 102 - 310 nm. The essential requirements for the associated detectors are high quantum effciency, solar blindness, and single photon detection. To achieve this, we develop a microchannel plate detector in a sealed tube. We plan to use cesium activated gallium nitride as semitransparent photocathode, a stack of two microchannel plates and a cross strip anode with advanced readout electronics. Challenges are the degradation of the photocathode under atmospheric conditions and the sealing process. We present the detector concept, details of the transfer and sealing processes under UHV, and the current status.