The Milky Way's 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.

Remarkable inconsistencies between elemental abundances in the main sequence stars and their progeny F-G supergiants are discussed. Comparative abundance analysis of the hot main sequence stars, the cepheid U Sgr and two cool supergiants belonging to young open cluster M 25 is performed. The detected disaccord in the abundances of carbon, oxygen and other elements between these stars having a common origin but occupying at present different evolutionary stages may be due to the fact that the chemical anomalies observed in B stars are caused by the mechanism of the radiative diffusion in the upper atmosphere layers. The chemical composition of B stars determined spectroscopically may not reflect correctly their true chemical composition, nor the chemical composition of the in-terstellar medium. On the other hand such abundance anomalies are not expected for F-G supergiants which have suffered the large scale mixing in the red giant phase. The observed abundances for these objects are much more reliable as a reference point in the study of galactic chemical evolution. Three new Be stars are discovered in M 25. Our study has doubled the number of Be stars known in this cluster.

The World Space Observatory UltraViolet (WSO-UV) is an international space mission devoted to UV spectroscopy and imaging. The observatory includes a 170 cm 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 ultraviolet domain where most of astrophysical processes can be efficiently studied with unprecedented capability.

We present the first intermediate-resolution (l/Dl ϭ 3000) spectrum of the bright quasi-stellar object 3C 273 at wavelengths between 900 and 1200 Å . Observations were performed with the Berkeley spectrograph aboard the ORFEUS II mission. We detect Lyb counterparts to intergalactic Lya features identified by Morris and coworkers at cz ϭ 19,900, 1600, and 1000 km s ; counterparts to other putative Lya clouds along the sight Ϫ1 line are below our detection limit. The strengths of the two very low redshift Lyb features, which are believed to arise in Virgo intracluster gas, exceed preflight expectations (Weymann and coworkers), suggesting that the previous determination of the cloud parameters may underestimate the true column densities. A curve-of-growth analysis sets a minimum H i column density of 4 # 10 14 cm for the 1600 km s cloud. We find marginally Ϫ2 Ϫ1 significant evidence for Galactic H along the sight line, with a total column density of about 10 15 cm . We Ϫ2 2 detect the stronger interstellar O vi doublet member unambiguously; the weaker member is blended with other features. If the Doppler b-value for O vi is comparable to that determined for N v by Sembach and collaborators, then the O vi column density is (7 ‫ע‬ 2) # 10 14 cm , significantly above the only previous estimate, by Davidsen. Ϫ2The O vi/N v ratio is about 10, consistent with the low end of the range observed in the disk, as shown in the compilation by Hurwitz & Bowyer. Additional interstellar species detected for the first time toward 3C 273 (at modest statistical significance) include P ii, Fe iii, Ar i, and S iii.

Abstract. 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 inflight 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/∆λ = 10 000, where ∆λ is measured as FWHM of the instrumental profile. The effective area peaks at 1.3 cm 2 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.

Continuous access to the UV domain has been considered of importance to astrophysicists and planetary scientists since the mid-sixties. However, the future of UV missions for the post-HST era is believed by a significant part of astronomical community to be less encouraging. We argue that key science problems of the coming years will require further development of UV observational technologies. Among these hot astrophysical issues are: the search for missing baryons, revealing the nature of astronomical engines, properties of atmospheres of exoplanets as well as of the planets of the Solar System etc. We give a brief review of UV-missions both in the past and in the future. We conclude that UV astronomy has a great future but the epoch of very large and efficient space UV facilities seems to be a prospect for the next decades. As to the current state of the UV instrumentation we think that this decade will be dominated by the HST and coming World Space ObservatoryUltraviolet (WSO-UV) with a 1.7 m UV-telescope onboard. The international WSO-UV mission is briefly described. It will allow high resolution/high sensitivity imaging and B. Shustov · M. Sachkov ( )

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The World Space Observatory Ultraviolet (WSO-UV) is a multi-national project lead by the Russian Federal Space Agency (Roscosmos) with the objective of high performance observations in the ultraviolet range. The 1.7 m WSO-UV telescope is equipped with UV spectrographs (responsibility of Russia and Germany) and UV imagers (responsibility of Spain). The UV spectroscopic instrumentation comprises two high resolution echelle spectrographs operating in wavelength ranges of 102-176 nm and 174-310 nm respectively, and a Long Slit Spectrograph designed to operate in the range of 102-310 nm. All three spectrographs represent individual instruments. In order to save mass while maintaining high stiffness, the instruments are combined to a monoblock, World Space Observatory Ultraviolet Spectrographs (WUVS). Due to strict technical requirements stated by the customer the material CeSiC (provided by the company ECM) has been selected for the design of the spectrograph structure. In contrast to aluminium, the stable structure of CeSiC is significantly less sensitive to thermal gradients. No further mechanism for focus correction with high functional, technical and operational complexity and corresponding additional System costs are necessary. Using CeSiC also relaxes the thermal control requirements of ±5°C, which represents a considerable cost driver for the S/C design. The phase B2 study of the WUVS instrument finished in December 2010 in collaboration with Russia and with industrial support of the Kayser-Threde company. It included construction of a Structural Thermal Model (STM) for verification of thermal and mechanical loads, stability with respect to thermal distortions and CeSiC manufacturing feasibility.