SIMBAD references

An important goal in astronomy is to understand the star formation process. Studies thereof have led to fairly detailed theoretical knowledge of the formation of individual stars. However, there is no detailed understanding of the origin of global properties of the stellar population, such as the initial mass function. To constrain possible theories, studies of young stellar groups are required. OB associations near the Sun are excellent candidates. However, reliable membership determinations (based on proper motions) for OB associations are limited to spectral types earlier than B5.

To remedy this problem the observation by HIPPARCOS of over 10,000 candidate members of OB associations within 1∼kpc from the Sun was proposed. The results will be available by early 1996. To complement the HIPPARCOS data ground based studies have been carried out. These include the collection of photometry and the gathering of accurate radial velocities for the early type stars in Sco OB2. The combined data will lead to a much improved knowledge of the stellar content and internal kinematics of nearby OB associations.

The aim of this thesis is to further investigate nearby OB associations by ground-based observations, to carry out a first analysis of the data from the radial velocity programme on Sco OB2, and to prepare for the release of HIPPARCOS data.

The first part concerns the Orion OB1 association. Walraven photometry is presented of established and probable members of Orion OB1. The physical parameters of the stars are derived. These are used to determine the distance to the association, the ages of the subgroups, the depth of the molecular clouds along the line of sight, and the IMF. Using the derived IMF and the ages, the energy (in the form of stellar winds and supernovae) released into the interstellar medium by the stars in Orion OB1 is calculated.

Next, the Orion-Eridanus bubble surrounding the association is investigated. Data on neutral hydrogen from the Leiden/Dwingeloo survey are combined with infrared (IRAS), CO (literature), and X-ray data (HEAO1). The neutral-hydrogen maps allow identification of the H I filaments and arcs delineating the Bubble and a derivation of the expansion velocity of the surrounding shell. The X-ray data are shown to anti-correlate in a detailed way with kinematically narrow features in H I. Comparison of IRAS 100 micron data with the H I data shows that the H I shell emission is optically thin, which justifies a derivation of its mass by direct conversion of the H I emission to column densities. Using a model that takes the density stratification of the Galactic H I layer into account, it is shown that the stellar winds and supernovae from Orion OB1 can account for the size as well as for the expansion velocity of the H I shell.

The second part of the thesis concerns the radial velocity programme in Sco OB2. Observations for this programme were carried out with the CASPEC and ECHELEC echelle spectrographs at ESO, La Silla. The data reduction for ECHELEC spectra is described in detail. The differential wavelength calibration of the spectra is better than 0.25 km s^-1. From these spectra precise rotational velocities are derived for a sample of 156 stars that are established or probable members of Sco OB2. This sample is extended with literature data for 47 established members of Sco OB2. Analysis of the v sin i distributions shows that there are no significant differences between the subgroups of Sco OB2. It is shown that members of the binary population of Sco OB2 on the whole rotate more slowly than the single stars. Analysis of Walraven photometric colours shows that positions of B7-B9 single dwarfs above the main sequence of Sco OB2 are a consequence of rotation.

The last part focuses on the preparation for the release of HIPPARCOS data. N-body models of OB associations are employed to investigate the reliability of so-called kinematic ages. These are the ages derived from proper motions, based on the model of linear expansion. It is shown that the tracing back of proper motions in time always leads to underestimated ages. If the proper motion in a certain direction is plotted vs. the corresponding coordinate to derive an expansion coefficient, the ages can be overestimated as well as underestimated, depending on the chosen coordinate direction and the magnitude of the effects of virtual expansion caused by radial motion. The conclusion is that the longstanding discrepancy between the kinematic and nuclear ages for OB associations can be attributed to underestimates of the kinematic age.