No abstract

Aims. We exploit nine years of photometry in the ASAS V-band catalogue to develop a method to select in an objective way the best quality data and detect low-amplitude variables. We concentrate on bright stars (V < 10) and avoid open cluster stellar densities. Methods. We use the (probable) members of the Sco-Cen associations as a test sample of bright stars that includes periodic early-type photometric variables as well as young late-type stars with cyclic or irregular variability patterns. We select grade-A observations with consistent photometry in all apertures, amounting to 88 ± 4 per cent of all grade-A data in most cases, and use the aperture with lowest dispersion of magnitudes (excluding one percent of data to avoid sparse surviving outliers) to search for periodicities, cycles and long-term trends. The well-known alias problems, due to the once-per-night observing cadence in the ASAS project, can often be alleviated for the bright stars by use of the Hipparcos satellite photometry. Results. Micro-variables with a range of variability as low as 0.01-0.02 mag for periodic variables (V magnitudes 7-10) and 0.04 mag for irregular long-term variables are detected. In total 37 light variables are identified: 14 were previously unknown, and another 12 were listed as suspected variables. The light variability in the young associations in Sco-Cen is briefly discussed. Conclusions. The ASAS database contains much more information on photometric variables than retrieved commonly.

ABSTRACT. This is the first paper in a series in which we study the kinematical structure and dynamical evolution of OB associations and young stellar clusters. By means of high-resolution high-signal-to-noise (S/N) spectroscopic observations, our goal is to obtain accurate stellar radial and rotational velocities and information on binarity in a selected number of such systems. In the present paper, we discuss the observations and data reduction for a sample of 156 early-type, established or probable, members of the Sco OB2 association. These stars form a subset of a larger sample that was observed by the HIPPARCOS satellite. The observations presented here were performed at the ESO 1.5-m telescope using the ECHELEC echelle CCD spectrograph. The formal S/N obtained is typically 70 at 3800 Â and 300 at 4050 Â. A new data-reduction package was developed. For the sake of quality control over the final spectra, we critically evaluated the performance of each procedure in the reduction. Emphasis is on techniques for detecting systematic errors in an empirical way and for maximally eliminating them, at least on a differential level. The accuracy of our reduced spectra is limited by intrinsic instrumental imperfections. Local random errors are given and the amplitude of systematic residuals that (may) occur in certain spectral regions is estimated in considerable detail. In anticipation of subsequent papers in this series, we finally show that the accuracy of differential radial velocities (for spectra of the same star) is essentially limited by centering and zero-point uncertainties amounting to ^ 1 km s -1 (rms).

No abstract

No abstract