Stars of all evolutionary phases have been found to have excess infrared emission due to the presence of circumstellar material. To identify such stars, we have positionally correlated the infrared MSX point source catalogue and the Tycho 2 optical catalogue. A near/mid infrared colour criteria has been developed to select infrared excess stars. The search yielded 1938 excess stars, over half (...

A Steiner star for a set P of n points in R connects an arbitrary center point to all points of P , while a star connects a point p ∈ P to the remaining n−1 points of P . All connections are realized by straight line segments. Fekete and Meijer showed that the minimum star is at most √ 2 times longer than the minimum Steiner star for any finite point configuration in R. The maximum ratio betwee...

Recent observations and models for horizontal branch stars are briefly described and compared to models for AmFm stars. The limitations of those models are emphasized by a comparison to observations and models for HgMn stars.

We report on XMM–Newton observations of the young open cluster NGC 2547 which allow us to characterize coronal activity in solar-type stars, and stars of lower mass, at an age of 30 Myr. X-ray emission is seen from stars at all spectral types, peaking among G stars at luminosities (0.3–3 keV) of L x 1030.5 erg s−1 and declining to L x 1029.0 erg s−1 among M stars with masses 0.2 M . Coronal spe...

Submillimeter and near-infrared images of cool dusty debris disks and rings suggest the existence of unseen planets. At dusty but non-imaged stars, semi-major axes of associated planets can be estimated from the dust temperature. For some young stars these semi-major axes are greater than an arc second as seen from Earth. Such stars are excellent targets for sensitive near-infrared imaging sear...

We have determined magnesium isotopic ratios (Mg/Mg) in metal-poor (−2.6 ≤ [Fe/H] ≤ −1.3) halo dwarfs employing high S/N (90-280) high spectral resolution (R = 10) Keck HIRES spectra. Unlike previous claims of an important contribution from intermediate-mass AGB stars at low metallicities, we find that the rise of the AGB contribution in the Galactic halo did not occur until intermediate metall...

A pedagogical overview of strange quark matter and strange stars is presented. After a historical notation of the research and an introduction to quark matter, a major part is devoted to the physics and astrophysics of strange stars, with attention being paid to the possible ways by which neutron stars and strange stars can be distinguished in astrophysics. Recent possible evidence for bare str...

Recent estimates of magnetic field strengths in T Tauri stars yield values B = 1–4 kG. In this paper, I present an upper limit to the photospheric values of B by computing the equipartition values for different surface gravities and effective temperatures. The values of B derived from the observations exceed this limit, and I examine the possible causes for this discrepancy. Subject headings: s...

We derive age-metallicity relations (AMRs) and orbital parameters for the 1658 solar neighbourhood stars to which accurate distances are measured by the HIPPARCOS satellite. The sample stars comprise 1382 thin disc stars, 229 thick disc stars, and 47 halo stars according to their orbital parameters. We find a considerable scatter for thin disc AMR along the one-zone Galactic chemical evolution ...