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We report the discovery of a 16th magnitude star, HE0437-5439, with a heliocentric radial velocity of +723+-3km/s. A quantitative spectral analysis of high-resolution optical spectra obtained with the VLT and the UVES spectrograph shows that HE0437-5439 is a main sequence B-type star with Teff=20350K, log g=3.77, solar within a factor of a few helium abundance and metal content, rotating at v sin i=54km/s. Using appropriate evolutionary tracks we derive a mass of 8 Msun and a corresponding distance of 61 kpc. Its galactic rest frame velocity is at least 563km/s, almost twice the local Galactic escape velocity, indicating that the star is unbound to the Galaxy. Numerical kinematical experiments are carried out to constrain its place of birth. It has been suggested that such hyper-velocity stars can be formed by the tidal disruption of a binary through interaction with the super-massive black hole at the Galactic center (GC). HE0437-5439 needs about 100Myrs to travel from the GC to its presentposition, much longer than its main sequence lifetime of 25Myrs. This can only be reconciled if HE0437-5439 is a blue straggler star. In this case, the predicted proper motion is so small that it can only be measured by future space missions. Since the star is much closer to the Large Magellanic Cloud (LMC, 18kpc) than to the GC, it can reach its position from the center of the LMC. The proper motion predicted in this case is about 2mas/y (relative to the LMC), large enough to be measurable with conventional techniques from the ground. The LMC origin could also be tested by a high-precision abundance analysis., Comment: 13 pages, 4 figures. Astrophysical Journal Letters, accepted

We provide an in-depth analysis of the He II and H I absorption in the intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Ultraviolet-Visual Echelle Spectrograph (UVES) on the VLT telescope. Following up on our earlier study (Kriss et al. 2001, Science, 293, 1112), we focus here on two major topics: (1) small-scale variability (Delta z = 10^-3) in the ratio eta = N(He II)/N(H I); and (2) an observed correlation of high-eta absorbers (soft radiation fields) with voids in the (H I) Ly-alpha distribution. These effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc, together with radiative transfer through a filamentary IGM whose opacity variations control the penetration of 1-5 ryd radiation over 30-40 Mpc distances. Owing to photon statistics and backgrounds, we can measure optical depths over the ranges 0.1 < tau(HeII) < 2.3 and 0.02 < tau(HI) < 3.9, and reliably determine values of eta = 4 tau(HeII)/tau(HI) over the range 0.1 to 460. Values of eta = 20-200 are consistent with models of photoionization by quasars with observed spectral indices alpha_s = 0-3. Values of eta > 200 may require additional contributions from starburst galaxies, heavily filtered quasar radiation, or density variations. Regions with eta < 30 may indicate the presence of local hard sources. We find that eta is higher in "void" regions, where H I is weak or undetected and 80% of the path length has eta > 100. These voids may be ionized by soft sources (dwarf starbursts) or by QSO radiation softened by escape from the AGN cores or transfer through the "cosmic web". The apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag between the reionization epochs, z(HI) = 6.2 +/-0.2 and z(HeII) = 2.8 +/-0.2., 27 pages, 7 figures, to appear in ApJ

Using new HST/FOS, NTT/EMMI, and Keck/HIRES spectra of the gravitationally-lensed double QSO HE 1104-1805 AB (z=2.31, \theta=3.2), and assuming UV photoionization by a metagalactic radiation field, we derive physical conditions (ionization levels, metal abundances and cloud sizes along the lines of sight) in five CIV+MgII absorption systems clustered around z=1.66 along the two lines of sight. Three of these systems are associated with a damped Ly-alpha (DLA) system with logN(HI)=20.85, which is observed in A. The other two systems are associated with a Lyman-limit system with logN(HI)=17.57 (B). The CIV and MgII line profiles in A resemble those in the B spectra, and span \Delta v=360 km/s. Assuming that the relative metal abundances in these absorption systems are the same as observed in the DLA system, we find that the observed N(CIV)/N(MgII) ratios imply ionization parameters of log\Gamma=-2.95 to -2.35. Consequently, these clouds should be small (0.5-1.6 kpc with a hydrogen density n_H= 100 kpc) highly ionized low-density clouds (n_H\sim e-4 cm-3), in which OVI, but only weak CIV absorption occurs. In this state, log\Gamma>=-1.2 reproduces the observed ratio of N(OVI)/N(NV)>60. These results are discussed in view of the disk/halo and hierarchical structure formation models., Comment: LaTeX, 49 pages including 10 figures, minor changes introduced. Now in press at ApJ (1999 March 10 issue)

We report on the detection of strongly varying intergalactic HeII absorption in HST/COS spectra of two z~3 quasars. From our homogeneous analysis of the HeII absorption in these and three archival sightlines, we find a marked increase in the mean HeII effective optical depth from tau~1 at z~2.3 to tau>5 at z~3.2, but with a large scatter of 2< tau 2.7, probably indicating HeII reionization was incomplete at z>2.7. Likewise, recent three-dimensional numerical simulations of HeII reionization qualitatively agree with the observed trend only if HeII reionization completes at z=2.7 or even below, as suggested by a large tau>3 in two of our five sightlines at z, 6 pages, 4 color figures; ApJL in press; One clarification, typo corrections

X-ray observations of four hybrid stars, with the position-sensitive proportional counter on the ROSAT satellite are reported. Three of our program stars, β Ind, μ UMa, and y Aql were detected as X-ray sources for the first time. For one star, y Aql, the detection is of low significance, while for the other two stars, β Ind and μ UMa, we have collected enough counts to accumulate-for the first time for a hybrid star-a pulse height spectrum and perform a spectral analysis of the pulse height distribution. The dominant emission feature component for both stars is located at temperatures of log T ∼6.4