Vande Putte, D.W.; (2011) Riding the galactic potential. Doctoral thesis , UCL (University College London).

Abstract

At the outset of this study, we implemented a series of axisymmetric, time-invariant Galactic potential models, by coding these numerically. With these potentials, we carried out a series of investigations in Galactic dynamics, related to Globular Cluster disk impacts, Open Cluster birth scenarios, and Planetary Nebula trajectories. The first of these studies relates to the crossing of the Galactic disk by a Globular Cluster to examine whether these could produce star formation due to gravitational focussing or compression of disk material. We report on simulations of the effect on disk material which reveal that the crossing can sometimes cause local gravitational focussing of disk material. This adds to the strong compression that can result from the shock wave generated by a GC disk crossing examined by Levy (2000). The main thrust of this first study is a search for any remnants of disk crossings by Globular Clusters. Using the gravitational potential of the Galaxy to locate the position of the most recent crossings of a subset of fifty-four Globular Clusters reveals that systematic errors and uncertainties in initial conditions limit the scope for unequivocal identification. From the subset of fifty-four, six possible search sites with the best constraints are retained for further scrutiny. Three of the six potentially promising search areas in the disk relate to Globular Clusters NGC 3201, 6397 and NGC 6838, for which we cannot rule out some observed star associations observed nearby as being remnants. The three other of the six areas are too large to provide meaningful identification of remnants. Also, a possible remnant (Open Cluster NGC6231) is shown not to be due to Globular Cluster impact, contrary to a previous report. In a more wide-ranging screening of one hundred and fifty-five Globular Clusters we identify which Globular Clusters are compatible with being responsible for the formation of any of the Galaxy’s five most prominent Star Super Clusters. For the second study, we considered six mechanisms so far proposed to account for the Galactic disk heating. Of these, the most important appear to be a combination of scattering of stars by molecular clouds and by spiral arms. We study a further mechanism, namely the repeated disk impact of the original Galactic Globular Cluster population up to the present. We find that Globular Clusters could have contributed a small fraction of the current vertical energy of the disk, as they could heat the whole disk to σz = 5.5kms−1 (c.f. the observed 18 and 39 kms−1 for the thin and thick disks respectively). We find that the rate of rise of disk heat (α=0.22 in σz ∼ tα with t being time) from GC heating, is close to that found for scattering by molecular clouds. Our third investigation relates to the Galactic population of Open Clusters which provides an insight into star formation in the Galaxy. The Open Cluster catalogue by Dias et al.(2002b) is a rich source of data, including kinematic information. Using this large sample we carry out a systematic analysis of 481 Open Cluster orbits, using parameters based on orbit eccentricity and separation from the Galactic plane. These two parameters may be indicative of cluster origin, and we find them to be correlated. We also find them to be correlated with metallicity, another parameter suggested elsewhere to be a marker for origin, in that high values of any of these two parameters generally indicate a low metallicity ([Fe/H] Solar< −0.2 dex). The resulting analysis points to four Open Clusters in the catalogue potentially being of extra-Galactic origin by impact of high velocity clouds on the disk: Berkeley21, 32, 99, and Melotte66, with a possible further four due to this origin (NGC2158, 2420, 7789, IC1311). A further three may be due to Galactic Globular Cluster impact on the disk i.e. of internal Galactic origin (NGC6791, 1817, and 7044). We show that clusters with highly eccentric orbits and thus low metallicity move in and out radially in the disk, thus transporting low metallicity radially across the disk. This led us to undertake observations at the WHT (November 2010), to assess the metallicity of the top six candidates with the most eccentric orbits. In a fourth study, we use the orbit sizes to constrain parameters related to the core star of Planetary Nebulae. Firstly, we map the orbit of the core of Sh-2-216 and observe from its vertical extent, that this is a thin disk object. Secondly, we plot the orbits of the core of Sh-2-68 for various values of its current distance, this being a parameter affected by a large uncertainty, and we fix the maximum current distance to at most 600pc; any larger value would lead to an unbound orbit. In a final study we examine cases where the Galactic potential is not axisymmetric (due to spirals), and cases where the axisymmetric potential varies in time, due to Galactic evolution (due to mass increase). The non-axisymmetric features allow us to examine the extent of radial stellar mixing in the disk, and we find that stars can be ejected from their Keplerian disk orbit up to an altitude of several kpc, and this may contribute to disk heating discussed above. This underlines the role of non-axisymmetric features in Galactic evolution. Lastly, we investigate the impact on stellar orbits of a Galaxy whose mass increases with time, and find that large effects can be expected, but not for orbits occurring within the last ∼10 Myr or ∼1 Gyr. We conclude the work by suggesting further model improvements and their applications.

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