Time series analysis of IM Pegasi photometry : comparing two method

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http://urn.fi/URN:NBN:fi:hulib-202012074709
Title: Time series analysis of IM Pegasi photometry : comparing two method
Author: Rantakylä, Julia
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2020
Language: eng
URI: http://urn.fi/URN:NBN:fi:hulib-202012074709
http://hdl.handle.net/10138/322421
Thesis level: master's thesis
Discipline: Tähtitiede
Abstract: Active longitudes are areas, where star spot activity is centered in and reappears on a periodic manner. Star spots are cooler areas on the star surface, caused by rising magnetic field lines inhibiting the flow of the convective region. The ways to observe active longitudes is limited, but in some stars the phenomenon has clearly been present, as Lehtinen et al. (2016) has showed. One of the observation methods is to analyse the primary and secondary minima epochs of the star’s light curve relative to its orbital period. Time series analyses are tools to gather these phases from light curves. Here two different methods were used to analyse a RS CVn binary member IM Pegasi. Continuous Period Search (CPS) (Lehtinen et al. 2011) defines an adaptive,single periodic model to a moving window of observations, allowing the light curve to contain sudden changes. Discrete Chi-square Method (DCM) (Jetsu 2020)) applies a multiperiod, polynomial-trended model to fit to the data with constant parameters, assuming all changes in the light curve are part of periodic changes. Using these two methods the light curve of IM Pegasi is studied in order to determine if there could be active longitudes present. Four data segments were chosen to be further analysed with DCM based on the CPS results. One of the segments showed a flip-flop effect in the CPS phase results, which was showed to be apparent based on the successful DCM performance. Two segments, which had rather steady phase trend in the CPS results, performed well with the DCM analysis. The fourth segment, which showed strong migrating of the secondary minima phase in CPS analysis, had problems performing with DCM as a whole segment. The primary periodicity is detected in both CPS and DCM withing good limits of agreement. The DCM dual-periodic model results in all four segments indicate of an additional, more fragile irregular structure in the star, like separated dynamo waves.
Subject: IM Pegasi
RS CVn binaries
active longitudes
star spots
light curves
time series analysis


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