Evaluation of mode identification techniques in two key white dwarf pulsators

dc.contributor.advisorWinget, Donald Earl, 1955-
dc.contributor.advisorOliveira Filho, Kepler de Souza
dc.creatorNitta, Atsukoen
dc.date.accessioned2015-05-13T18:42:13Zen
dc.date.available2015-05-13T18:42:13Zen
dc.date.issued2000-12en
dc.descriptiontexten
dc.description.abstractThe success of asteroseismology lies in the correct identification of the normal modes of oscillation. The Whole Earth Telescope (WET) identified the normal modes of a helium white dwarf pulsator, GD358, by analyzing the period distribution of the pulsation modes. Another way to identify modes is by comparing pulsation amplitudes in the UV to the optical. To cross-calibrate the two mode identification methods, we observed GD358 in August, 1996 with the Hubble Space Telescope (HST) to obtain the UV data while observing nearly simultaneously from the ground. During our observations, GD358 went through a very drastic amplitude modulation in a time scale of hours. These short time scale amplitude changes made the direct UV to optical amplitude determinations difficult. We successfully eliminate the possibility that the 423s mode, the dominant mode at the time of these observations, is an l=3 or 4 g-mode pulsation, but we cannot unambiguously decide if it is an l=1 or 2. Theoretical calculations indicate that the massive pulsating DA white dwarf BPM 37093 has a crystallized interior (Winget et al. 1997; Kanaan 1996; Montgomery 1998). Crystallization was predicted theoretically 40 years ago (Kirshnitz 1960; Abrisokov 1961; Salpeter 1961) although uncertainties in the nature and extent of crystallization, as well as its associated effects, are the largest sources of uncertainty in calculating the ages of the coolest white dwarf stars- important chronometers of the galactic disk. The WET observed BPM 37093 in April 1998 and again in April 1999, simultaneously with the HST, in hopes of using both the period distribution and the amplitude comparison method to identify the l value of the modes and measure the first crystallized mass-fraction of a stellar interior. Here we also rule out the possibility of the observed modes being l=3 and higher and demonstrate that not all the observed modes are l-1. If all the observed modes are l-2, then we conclude that the crystallization mass fraction is between 0-80%, depending mainly on the surface H layer. In the end, we evaluate the amplitude comparison method and address advantages and problems using this method compared to other mode identification methods.en
dc.description.departmentAstronomyen
dc.format.mediumelectronicen
dc.identifier.urihttp://hdl.handle.net/2152/29827en
dc.language.isoengen
dc.rightsCopyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works.en
dc.rights.restrictionRestricteden
dc.subjectMode identification techniquesen
dc.subjectWhite dwarf pulsatorsen
dc.subjectWhole Earth Telescopeen
dc.subjectAsteroseismologyen
dc.subjectGD358en
dc.titleEvaluation of mode identification techniques in two key white dwarf pulsatorsen
dc.typeThesisen
thesis.degree.departmentAstronomyen
thesis.degree.disciplineAstronomyen
thesis.degree.grantorThe University of Texas at Austinen
thesis.degree.levelDoctoralen
thesis.degree.nameDoctor of Philosophyen

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