## 5. Space motion and Galactic orbit of Pal 5Knowing the cluster's absolute proper motion and also its distance ( kpc, Harris 1996) and radial velocity ( km/s, Smith 1985) with respect to the Sun, we finally proceed to its motion in three-dimensional Galactic space. On the assumption of kpc for the distance Sun - Galactic center and km/s for the rotation of the local standard of rest (IAU 1986) and with the 'basic solar motion' of Delhaye (Mihalas & Binney 1981) as a reasonable estimate of the Sun's peculiar motion in the LSR , the cartesian components of the Galactocentric position and velocity of Pal 5 come out as shown in Table 2. As viewed from a point near the Sun the present location of the cluster lies on the opposite side of the Galaxy. It has a height of 16 kpc above the Galactic plane and a projected distance of 7 kpc from the Galactic center. While the direction of the velocity vector depends on the choice between the two different proper motion results given above (see the two cases in Table 2), the absolute value of the velocity is in both cases close to 130 km/s (see Table 3 b). This confirms that Pal 5 is indeed among the clusters with the lowest space velocity in the globular cluster system of the Galaxy.
Schweitzer, Cudworth & Majewski (1993) have pointed out that using their proper motion for Pal 5, the velocity vector is perpendicular to the position vector and hence the cluster is very close to apogalacticon. Our proper motion results do not confirm this particular orientation of the velocity vector. Nevertheless, the conclusion that Pal 5 is near apogalacticon remains true also with regard to the proper motions obtained in this paper. This can be anticipated from the fact that the total space velocity is much below the circular velocity of the Galaxy, and it is confirmed in a more precise manner by the results of a numerical calculation of the cluster's Galactic orbit.
In analogy to our previous studies on other globular clusters we
integrated the orbit of Pal 5 for the past 10 Gyrs, using the
model by Allen & Santillan (1991) as a representation of the
Galactic gravitational potential. The resulting orbital parameters are
summarised in Table 3 and a plot of the orbital path is shown in
Fig. 8. While with the proper motion from Schweitzer, Cudworth
& Majewski (1993) the distance of the cluster from the Galactic
center is limited by the current distance of 17 kpc, with our
proper motion the orbit yields a maximum distance of 18.9 kpc and
the most recent apogalacticon lies 0.05 Gyr (i.e. 1/8 of a mean
orbital period) back from present. Perigalactic distances are found to
be not smaller than 5.3 kpc. The eccentricity of the orbit (as
measured by ) returns a value of 0.56.
Naturally, the distribution of values of Galactocentric distance
In order to find out how much the orbital characteristics change
within the estimated range of error in our proper motions, we modified
the cluster's initial velocity accordingly and calculated a number of
alternative orbits. The result of this test was, that the distance
parameters do not undergo extreme changes, i.e.
and fell within 18 and 20 kpc and
was between 3 and 8 kpc. The direction of
rotation was in all cases prograde with average rotational velocities
between 30 and 70 km/s. Although the © European Southern Observatory (ESO) 1998 Online publication: April 20, 1998 |