6. The orbits of M 3 and M 22
Scholz, Odenkirchen & Irwin (1993) measured the proper motion of M 3 relative to the extragalactic reference frame, which enables a true orbit to be calculated. For M 22, Cudworth & Hanson (1993) converted the relative proper motion of Cudworth (1986) to an absolute proper motion, using assumptions concerning the motion of the field stars and of the Sun, derived from the Lick absolute proper motion programme.
Using a galactic potential based on the mass model of Allen & Santillán (1991), orbits have been calculated for M 3, and M 22 and the resultant relevant orbit details are given in Tables 4 and 5. As the values for M 22 are derived using assumptions of the motions of field stars and the Sun, the orbit will be less certain than that calculated for M 3.
Table 4. Orbit of M 3. This table covers the period between the present and 115 Myr ago, when the cluster last passed through the Galactic plane. The x, y, z are space coordinates in a right-handed Galactocentric Cartesian coordinate system with z pointing to the North Galactic Pole and x opposite to the Sun. R is the distance from the Galactic centre
Table 5. Orbit of M 22. This table covers the period between the present and 3.56 Myr ago, when the cluster last passed through the Galactic plane. The columns are the same as in Table 4
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998