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Title: The Cepheids of M33 : distance and metallicity effects through Sloan filters
Author: Tsang, K. G.-J.
ISNI:       0000 0004 8507 0525
Awarding Body: Liverpool John Moores University
Current Institution: Liverpool John Moores University
Date of Award: 2019
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Cepheids are a class of variable stars that undergo regular pulsations in size and brightness. There exists a strong correlation between the period of pulsation and the average intrinsic brightness of a Cepheid; this correlation is known as the period-luminosity relation, or Leavitt Law. It is this Law that bestows Cepheids their importance as distance indicators on both interstellar and intergalactic scales, enabling accurate distance estimates and studies into galactic structure. Cepheids are found in many different environments and are therefore imparted with varying levels of metallicity. The difference in Cepheid metallicities may give rise to variations in their observable characteristics, leading to changes in the Leavitt Law. These changes may manifest themselves as a metallicity dependence on the slope or zero-point of the Leavitt Law which, if not accounted for, introduce uncertainties into the resulting distance estimates. Observational studies of Cepheids have typically used Johnson-Cousin filters. With the advent of large-scale sky surveys using Sloan-band filters, Cepheid studies in Sloan filters will become more prevalent. It is important that there exist baseline Leavitt Laws against which future work can be compared. This thesis aims to provide Leavitt Laws in Sloan filters and to quantify the effects of metallicity on Leavitt Laws in said filters. Sloan-band magnitudes were obtained with PSF photometry for over 1.3 million stars in the nearby spiral galaxy M33. Over 1500 Cepheids were identified and Leavitt Laws were empirically derived. The slopes were found to be consistent with semi-empirical determinations. By adopting the semi-empirical slopes, a mean distance modulus of 24.57 ± 0.06 mag to the system and a reddening of E(B-V) = 0.144 ± 0.020 mag were obtained. Cepheids were found throughout the disk of M33 and so sample the galaxy's metallicity gradient. Cepheids were binned into galactocentric annuli and a series of Leavitt Laws were constructed to assess the effects of metallicity on the slopes and zero-points. For the slopes, no significant dependence on metallicity was found. For the zero-points, a metallicity effect is seen: metal-rich Cepheids are brighter than metal-poor Cepheids of the same pulsation period. Metallicity corrections of -0.089 ± 0.107 mag/dex in g', -0.188 ± 0.071 mag/dex in r', and -0.250 ± 0.080 mag/dex in i' were measured. In Wesenheit indices, the values all cluster around -0.468 mag/dex. These metallicity parameter values are predicated upon the adoption of a metallicity gradient determined from beat Cepheids. Shallower metallicity gradients derived from HII regions produce unreasonably large corrections, suggesting that the use of chemical abundances derived from such tracers as a proxy for Cepheid metallicity is unreliable. An independent distance determination to M33 was obtained with the tip of the red giant branch method. An average distance modulus of 24.53 \pm 0.11 mag was found, consistent with the value derived from Cepheids.
Supervisor: Bersier, D. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: QB Astronomy ; QC Physics