Astron. Astrophys. 331, 949-958 (1998)

3. Reduction

Hipparcos provides astrometric data, with errors, for positions ; proper motions (the factor is already included), ; and parallaxes , for the epoch 1991.25, in addition to photometric data, which will be the subject of a second paper (Marchenko et al. 1997). Only 4 O stars and 1 WR star have reliable parallaxes; two of them ( Vel and Pup) have been studied elsewhere (van der Hucht et al. 1997; Schaerer et al. 1997).

In addition to the random peculiar motions of stars in the Galaxy, the bulk of the systematic angular motion arises due to differential Galactic rotation. It thus makes most sense to transform from the equatorial to the Galactic coordinate system. We do this using the values recommended by the Hipparcos consortium:

where and .

Then we transform the proper motions (Trumpler & Weaver 1953; Scheffler & Elsässer 1987):

where is the parallactic angle satisfying:

The observed proper motions can then be expressed as sums of basic Solar motion, Galactic rotation, and peculiar motion:

where

for distance from the Sun, r, in kpc and µ in mas/yr, and km/s (Delhaye 1965). Galactic rotation is written as:

with

We also take for the distance perdendicular to the Galactic plane. Here we adopt a flat rotation curve with

in which Solar galactocentric distance and circular Galactic rotation velocity , adequate for galactocentric distances (Kerr & Lynden-Bell 1986).

For the O stars, distances were adopted based on photometric techniques by Cruz-Gonzalez et al. (1974), with updates/corrections using reliable Hipparcos parallaxes for 4 stars ( Cas, µ Col, Pup, Cep) and Hipparcos stellar magnitudes (transformed to the V-band). Furthermore, the distances were updated from the catalogues of Humphreys (1978), Savage et al. (1985) and Diplas & Savage (1994). For the MXRBs, we used the mean values from van Oijen (1989). In the case of the WR stars, distances are adopted from the catalogue of van der Hucht et al. (1988), with an update for Vel based on the new, reliable parallax from Hipparcos. The radial velocities and spectral types of the O stars are in accordance with Cruz-Gonzalez et al. (1974), with further corrections from Humphreys (1978), Gies & Bolton (1986), Gies (1987), Levato et al. (1988). The spectral types of the WR stars are adopted from van der Hucht et al. (1988) with some minor modifications.

A summary of the pertinent data for all 141 stars is presented in Table 1 ¹.

© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998
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