2 The working samples

2.1 Sample of O and B stars

Our initial sample contains 6922 Hipparcos O- and B-type stars. The astrometric data for these stars come from the Hipparcos Catalogue (ESA 1997), radial velocities from the compilation of Grenier (1997) and Strömgren photometry from Hauck & Mermilliod's (1998) catalogue. The procedure followed to elaborate the sample considered here is fully described in Torra et al. (; hereafter referred to as Paper I) along with a study of the possible biases in the trigonometric distances and the availability of radial velocities. Our sample contains 3915 stars with known distance and proper motions (2272 stars with known radial velocity). In Paper I we characterized the structure and kinematics of the Gould Belt system using this sample of O and B stars, establishing its boundary to a distance of about 0.6 kpc from the Sun. Taking into account the kinematic peculiarities of the Gould Belt, in this paper we do not consider those stars with R < 0.6 kpc. In Fig. 1 we show the position of those stars with 0.6 < R < 2 kpc projected on the galactic plane (Xpositive towards the galactic center and Y positive towards the galactic rotation direction), which is our working sample (448 stars; 307 of them with distance, radial velocity and proper motions and 141 with only distance and proper motions).

Figure 1: Star distribution in the X-Y galactic plane for the sample of O and B stars with 0.6 < R < 2 kpc.

2.2 Sample of Cepheid stars

The initial sample contains all the Hipparcos classical Cepheids. Astrometric data were taken from the Hipparcos Catalogue (ESA 1997), whereas radial velocities come from Pont et al. (, 1997).

Individual distances were computed following two period-luminosity (PL) relations. In both, periods come from the Hipparcos Catalogue (ESA 1997) and individual reddenings from Fernie et al.'s () compilation (continuous updating). A classification between fundamental and overtone Cepheids from light curves and Fourier analysis was adopted (Beaulieu 1999), using only the former in the kinematic analysis.

The first PL relation (Luri 2000) adopts a slope from EROS (Beaulieu 1999) and corresponds to the short cosmic distance scale:

$$M_{\rm v}^{{\rm Short}} = -1.08 - 2.72 \log P.$$ (1)

On the other hand, the second PL relation (Feast & Catchpole 1997) corresponds to the large distance scale:

$$M_{\rm v}^{{\rm Large}} = -1.41 - 3.46 \log P.$$ (2)

The sample contains 186 stars with known distance and proper motions (165 stars with radial velocity). Their distribution in the galactic plane ( 0.6 < R < 4 kpc) is shown in Fig. 2 (164 stars; 145 of them with distance, radial velocity and proper motions and 19 with only distance and proper motions).

Figure 2: Star distribution in the X-Y galactic plane for the sample of Cepheid stars with 0.6 < R < 4 kpc. Distances computed from a short cosmic distance scale (Luri 2000).