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The formation and evolution of the large magellanic cloud from selected clusters and star fields

Show simple item record Olsen, Knut A. G en_US 2009-10-05T23:22:48Z 2009-10-05T23:22:48Z 1998 en_US
dc.identifier.other b42245928 en_US
dc.identifier.other 40857392 en_US
dc.identifier.other Thesis 47311 en_US
dc.description Thesis (Ph. D.)--University of Washington, 1998 en_US
dc.description.abstract We have obtained deep Hubble Space Telescope color-magnitude diagrams of fields centered on the six old LMC globular clusters NGC 1754, NGC 1835, WGC 1898, NGC 1916, NGC 2005, and NGC 2019. The data have been carefully calibrated and the effects of crowding on the photometric accuracy have been thoroughly investigated. The observations have been used to produce $V-I,V$ color-magnitude diagrams of the clusters and of the background field stars, which we have separated from each other through a statistical cleaning technique.The cluster color-magnitude diagrams show that the clusters are old, with main sequence turnoffs at $V\sim 22.5$ and well-developed horizontal branches. We used the slopes of the red giant branches to measure the abundances, which we find to be 0.3 dex higher, on average, than previously measured spectroscopic abundances. In two cases there is significant variable reddening across at least part of the image, but only for NGC 1916 does differential reddening preclude accurate measurements of the CMD characteristics. The mean reddenings of the clusters, measured both from the color of the red giant branch and through comparison with Milky Way clusters, are $\le$0.10 magnitudes in $E(B-V)$ in all cases.By matching tbe color-magnitude diagrams of the clusters to fiducial sequences of the Milky Way globular clusters M3, M5, and M55, we find that the mean difference of the LMC and Milky Way cluster ages is 1.0 $\pm$ 1.2 Gyr, calculated such that a positive difference indicates that the LMC clusters are older. Through Monte Carlo simulations, errors in the individual measurements of the ages relative to Milky Way clusters are found to be $\sbsp{\sim}{<}$1.0 Gyr. We find a similar chronology by comparing the horizontal branch morphologies and abundances with HB evolutionary tracks, assuming that age is the "second parameter". These results imply that the LMC formed at the same time as the Milky Way Galaxy.The evolution of the LMC following its formation has been studied through an analysis of the field star CMDs. We used an automated technique to disentangle the evolutionary tracks of varying age and composition that are represented in the CMDs. We computed star formation rates as a function of age for a number of models having different initial mass function slopes, distances, and uniform reddenings, assuming that the chemical evolution follows that implied by LMC clusters.Our results show that the LMC has been actively forming stars over the last 4 Gyr, with evidence for a decline in the last 0.5-1 Gyr. While the NGC 1754 field, which lies in the disk, has had only a low level of star formation after the globular cluster formation epoch until 4 Gyr ago, we find that the bar has been actively forming stars for the past 6-8 Gyr. We find that these qualitative results are robust against errors in the model parameters. (Abstract shortened by UMI.)* ftn*Originally published in DAI Vol. 59, No. 6. Reprinted here with corrected author name. en_US
dc.format.extent ix, 235 p. en_US
dc.language.iso en_US en_US
dc.rights Copyright is held by the individual authors. en_US
dc.rights.uri en_US
dc.subject.other Theses--Astronomy en_US
dc.title The formation and evolution of the large magellanic cloud from selected clusters and star fields en_US
dc.type Thesis en_US

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