We present a homogeneous photometric and spectroscopic analysis of 18 stars along the evolutionary sequence of the metal-poor globular cluster NGC 6397 ([Fe/H]≈−2), from the main-sequence turnoff point to red giants below the bump. The spectroscopic stellar parameters, in particular stellarparameter differences between groups of stars, are in good agreement with broad-band and Strömgren photome...

In recent years several hypervelocity stars (HVSs) have been observed in the halo of our Galaxy. Such HVSs have possibly been ejected from the Galactic center and then propagated in the Galactic potential up to their current position. The recent survey for candidate HVSs show an asymmetry in the kinematics of candidate HVSs (position and velocity vectors), where more outgoing stars than ingoing...

The mass-transfer scenario links chemical peculiarities with stellar duplicity for an increasing number of stellar classes (classical and dwarf barium stars, subgiant and giant CH stars, S stars without technetium, yellow symbiotic stars, WIRRING stars, Abell-35-like nuclei of planetary nebulae...). Despite these successes, the mass-transfer scenario still faces several problems: What is the ma...

We show that the recently observed elemental abundance pattern of the carbon-rich metal-poor Damped Lyman α (DLA) system is in excellent agreement with the nucleosynthesis yields of faint core-collapse supernovae of primordial stars. The observed abundance pattern is not consistent with the nucleosynthesis yields of pair-instability supernovae. The DLA abundance pattern is very similar to that ...

Recent UV observations of the most massive Galactic globular clusters show a significant population of hot stars below the zero-age HB (“blue hook” stars), which cannot be explained by canonical stellar evolution. Stars which suffer unusually large mass loss on the red giant branch and thus experience the heliumcore flash while descending the white dwarf cooling curve could populate this region...

We present a comparison between the rotation period distributions of solartype single stars and primary stars in close binaries (0.1 AU . a . 5 AU) in the young (∼ 150 Myr) open cluster M35 (NGC2168). We find that the primary stars in the close binaries rotate faster than the single stars, on average. The differences in the means and medians between the period distributions are statistically si...

Radial velocity searches for substellar mass companions have focused primarily on stars older than 1 Gyr. Increased levels of stellar activity in young stars hinders the detection of solar system analogs and therefore there has been a prejudice against inclusion of young stars in radial velocity surveys until recently. Adaptive optics surveys of young stars have given us insight into the multip...

Neutral fluorine (F i) lines are identified in the optical spectra of cool EHe stars. These are the first identification of F i lines in a star’s spectrum, and provide the first measurement of fluorine abundances in EHe stars. The results show that fluorine is overabundant in EHe stars. The overabundance of fluorine provides evidence for the synthesis of fluorine in these stars, that is discuss...

Protostars are precursors to the nearly fully assembled T-Tauri and Herbig Ae/Be type stars undergoing quasi-static contraction towards the zero-age main sequence; they are in the process of acquiring the majority of their stellar mass. Although numerous young stars with spatially extended envelope-like structures appear to fit this description, their high extinction has inhibited observers fro...

We derive the effective temperatures and gravities of 461 OB stars in 19 young clusters by fitting the H profile in their spectra. We use synthetic model profiles for rotating stars to develop a method to estimate the polar gravity for these stars, which we argue is a useful indicator of their evolutionary status. We combine these results with projected rotational velocity measurements obtained...