Hipparcos astrometric data were used to derive the spiral structure of the Galaxy in the solar neighbourhood. We considered two different samples of stars as tracers of this structure. In the sample of O and B stars the astrometric data were complemented with a careful compilation of radial velocities and Strömgren photometry, providing reliable distances and spatial velocities for these stars. In the sample of Cepheid stars we used two period-luminosity relations, one considering a short cosmic scale (Luri 2000) and another one standing for a large cosmic scale (Feast & Catchpole 1997). A kinematic model of our galaxy that takes into account solar motion, differential galactic rotation, spiral arm kinematics and a K-term was adopted. The model parameters were derived via a classical weighted least squares fit. The robustness of our results was checked by means of careful insimulations.
A galactic rotation curve with a value of A Oort constant of 13.7-13.8 km s-1 kpc-1 was found for the sample of O and B stars. In the case of Cepheid stars, we found A = 14.9-16.9 km s-1 kpc-1, depending on the case and the cosmic scale chosen. We confirmed the discrepancies appearing in A when samples with a different distance horizon were used (Olling & Merrifield 1998). For both cosmic scales we found an acceptable coherence between radial velocity, proper motion and combined solutions. Concerning the second-order term of the rotation curve, we always found a low value, compatible with zero.
The study of the residuals for radial velocity data made it evident that a K-term was needed, which was found to be K = -(1.4-3.2) km s-1 kpc-1 for O and B stars and K = -(0.8-1.2) km s-1 kpc-1for Cepheids. Although small, an apparent compression motion seems to exist in the galactic local neighbourhood for heliocentric distances up to 4 kpc, though its physical mechanism is still unknown.
The phase of the spiral structure at the Sun's position obtained
(
-20
)
places it between the center and the outer
edge of an arm. This is in good agreement with the spatial distribution of
Cepheid stars. The angular rotation velocity of the spiral structure was
found to be
30 km s-1 kpc-1, which
places the Sun near the corotation circle.
Acknowledgements
This work has been supported by the CICYT under contracts ESP 97-1803 and AYA 2000-0937. DF acknowledges the FRD grant from the Universitat de Barcelona (Spain).
Copyright ESO 2001