Local galactic kinematics from Hipparcos proper motions

The stellar velocity field within two kiloparsecs of the solar system is in vestigated from the Hipparcos proper motions and parallaxes of about 20 000 single stars farther than 100 pc. The motion of the sun relative to the Lo cal Standard of Rest and the relative systematic velocity field due to the galactic rotation are studied simultaneously for several categories of star s ranging from early type dwarfs to K and M giants observed by Hipparcos. T he relative Velocity field is modeled in the framework of the Milne-Ogorodn ikov hydrodynamic representation and led to the determination of eight para meters characterizing the flow, including the usual Oort Constants. The pec uliar motion of these stars is also studied from the residuals and led to a new determination of the components of the velocity dispersion for every c lass of stars considered. The components of the solar motion are ascertained to within +/- 0.2 - 0.3 km s(-1) in each direction and are found to vary significantly with the sta r sample, except for the component normal to the galactic plane always clos e to 7 km s(-1). Regarding the Oort constants, one finds A = 11.0 +/- 1.0 k m s(-1)kpc(-1) for early type dwarfs and A = 14.5 +/- 1.0 km s(-1)kpc(-1) f or the distant giants. The constant B shows a less obvious difference with respectively -13.2 +/- 0.5 km s(-1)kpc(-1) and -11.5 +/- 1.0 km s(-1)kpc(-1 ). The galactic center is found to lag by 6 degrees from the normal to the direction of the galactic motion of the LSR in the solar neighborhood. Fina lly the three components of the velocity dispersion are determined and disc ussed for all the stellar groups considered in this paper with an accuracy of +/- 0.2 - 0.4 km s(-1).