The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usuallyhidden intershell region. Thus, the photospheric elemental abundances of these stars allow to draw conclusions about details of nuclear burning and mixing processes in the pre...

We present a final summary of all ROSAT X-ray observations of nearby stars. All available ROSAT observations with the ROSATPSPC, HRI and WFC have been matched with the CNS4 catalog of nearby stars and the results gathered in the Nearby X-ray and XUV-emitting Stars data base, available via www from the Home Page of the Hamburger Sternwarte at the URL http://www.hs.uni-hamburg.de/DE/For/Gal/Xgrou...

Aims. The statistical equilibrium of neutral and ionized silicon in the atmospheres of metal-poor stars is discussed. Non-local thermodynamic equilibrium effects are investigated and the silicon abundances in metal-poor stars determined. Methods. We have used high resolution, high signal to noise ratio spectra from the UVES spectragraph at the ESO VLT telescope. Line formation calculations of S...

Pulsars have been recognized as normal neutron stars or quark stars. Submillisecond pulsars, if detected, would play an essential and important role in distinguishing quark stars from neutron stars. A key question is how sub-millisecond pulsars could be formed. Both sub-Keplerian (for neutron and quark stars) and super-Keplerian cases (only for quark stars, which are bound additionaly by strong...

We report the discovery of two new R Coronae Borealis (RCB) stars in the Large Magellanic Cloud (LMC) using the MACHO project photometry database. The identiication of both stars has been connrmed spectroscopically. One is a cool RCB star (T eff 5000 K) characterized by very strong Swan bands of C 2 and violet bands of CN, and weak or absent Balmer lines, G-band and 12 C 13 C bands. The second ...

Cosmological simulations of Population III star formation suggest an initial mass function (IMF) biased toward very massive stars (M & 100 M⊙) formed in minihalos at redshift z & 20, when the cooling is driven by molecular hydrogen. However, this result conflicts with observations of extremely-metal poor (EMP) stars in the Milky Way halo, whose r-process elemental abundances appear to be incomp...

We present Hα spectropolarimetry observations of a sample of 23 Herbig Ae/Be stars. A change in the linear polarisation across Hα is detected in a large fraction of the objects, which indicates that the regions around Herbig stars are flattened (disclike) on small scales. A second outcome of our study is that the spectropolarimetric signatures for the Ae stars differ from those of the Herbig Be...

Using near-infrared imaging and spectroscopy, we discuss the discovery of six emission-line sources within D20 pc of Sgr A West but outside of the central parsec. The objects are coincident with stars seen in the J, H, and K@ images ; all have Brc (2.166 km) in emission, several also have He I (2.058 and 2.112/3 km), and some have He II (2.189 km) emission lines. Comparison of the Hand K-band s...

The standard model for the operation of the s-process in asymptotic giant branch (AGB) stars predicts that low-metallicity ([Fe/H] < ∼ − 1) AGB stars should exhibit large overabundances of Pb and Bi as compared to other s-elements. The discovery of the first three such 'lead stars' (defined as stars enriched in s-elements with [Pb/hs] > ∼ 1, hs being any of Ba, La or Ce) among CH stars has been...

The apparent absence of stars in the Milky Way halo with −5 . [Fe/H] . −4 suggests that the gas out of which the halo stars were born experienced a period of low or delayed star formation after the local universe was lit up by the first, metalfree generation of stars (Pop III). Negative feedback owed to the Pop III stars could initially have prevented the pre-Galactic halo from cooling, which t...