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Dissertation Information for Styliani Kafka

NAME:
- Styliani Kafka

DEGREE:
- Ph.D.

DISCIPLINE:
- Astronomy

SCHOOL:
- Indiana University (USA) (2005)

ADVISORS:
- R. Kent Honeycutt

COMMITTEE MEMBERS:
- Constantine P. Deliyannis
- Catherine P. Pilachowski
- Michael Hamburger

MPACT Status: Incomplete - Not_Inspected

Title: Exploring stellar activity on the secondary star of cataclysmic variables

Abstract: The light curves of Cataclysmic Variables (CVs) exhibit a plethora of behaviors, from ephemeral changes (flickering) to large nova explosions; most are attributed to the accretion process. During the last couple of decades, it has been realized that activity (starspots) and activity cycles on the secondary star can be responsible for (quasi-periodic) modulations observed in long-term CV light curves, and may moderate, even halt accretion from the secondary star. The latter can lead to observed large drops in the system luminosity (VY Scl low states); the presence of low states in both magnetic and disk systems directs attention to the secondary star for their cause. Activity on the K-M dwarf mass-losing secondary is commonly invoked to explain sustained mass transfer caused by system angular momentum loss via a magnetized stellar wind, quasi-periodic changes in the mass transfer rate, the elusive CV period gap, and widely differing mass transfer rates among CVs at the same orbital period. On the other hand, there is a substantial lack of observational support for these scenarios, since conventional activity indicators (X-ray and Ha emission; flares) are usually mimicked by accretion.

This thesis aims to explore alternative methods to infer the presence stellar activity on CV secondary stars. For such a complicated problem, both photometric and spectroscopic indicators of activity are sought, focusing on systems showing large (2-5 mag) drops in their optical light curves (VY Scl systems), presumably due to disruptions of the mass transfer from starspots near the L1 point. One- and two-slope fits to the shapes of transitions from and to the high state have been successfully related to the presence of starspots, if the slopes are interpreted as being due to migration of the umbra and penumbra portion of the starspot in front of L1. The speeds and shapes of the transitions of both disk and magnetic systems above the CV period gap (which is defined by the lack of CVs with orbital periods between 2 and 3 hours) are consistent with the aforementioned scenario. (Abstract shortened by UMI.)

MPACT Scores for Styliani Kafka

A = 0
C = 0
A+C = 0
T = 0
G = 0
W = 0
TD = 0
TA = 0
calculated 2010-10-07 14:11:41

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