A search for transiting extrasolar planets in the open cluster NGC 4755

Jayawardene, Bandupriya S. (2015) A search for transiting extrasolar planets in the open cluster NGC 4755. Professional Doctorate (Research) thesis, James Cook University.

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The search for ESP (extra-solar planets) has become a very popular astronomical research activity since the first discovery of ESP in 1995. Although, there are many ways of finding these exotic bodies, the transit method has become a widely used method; even amateurs have their opportunity to become planet hunters. This requires high precision time-series photometry and light curve analysis of large numbers of stars. When a planet transits, its radii ratio with the primary star can be determined accurately. Thus, combining this ratio with radial velocity data, the mass and radius of the planet can be realized, assuming the primary star's radius is known. The information gained from the transiting planets makes it possible to unravel the structure and composition of ESPs, understand the formation and the evolution process, and find the physical properties of the planet.

The detection of a weak, short, periodic transit signal in noisy light curves is a challenging task. As large numbers of light curves are to be analyzed, automation and an optimization of the search and analysis process is a necessity. The fluxes of stars on CCD (Charge Coupled Device) images are measured and the de-trended flux is used to draw the light curve. Normally, the search is done by a ground based detection system; hence the light curve is contaminated with noise components coming from atmospheric variations and systematic errors. To obtain high precision data without atmospheric noise, space based CCD cameras are already active.

Based on the above transit theory, this search was first done by using REST (Really Embarrassing Small Telescope) at JCU for field stars in the solar neighbourhood, GL 581, HD 13445 and HD 27894. The CCD images of the stars were subjected to CCD data reduction, pre-processing, differential photometry and analyzing by transit identification algorithms. Differential photometry, the ratio of the target star flux and reference star flux was used to nearly nullify the atmospheric variations.

The target open cluster for the main search is NGC 4755, which is widely known as the Jewel Box, in the constellation of the Southern Cross. The Perth Automatic Telescope, which can be remotely controlled, was used to obtain the data for the open cluster. 176 cluster stars brighter than 14th magnitude with published 'B' and 'V' magnitudes and another 994 faint stars in the cluster frame have been analyzed. Several analytical signal-processing methods have been used to process the light curve to get the best light signal, having a SD (Standard Deviation) less than 5 milli-magnitudes. Later, fast wavelet transform was used to remove high frequency noise components and to produce an approximate signal which shows the long-term trend of the light curve and contain a possible transit. While no planetary transits have been identified in this cluster before, the ability to get light curves with standard deviation less than 5 milli-magnitudes is a significant achievement. The approximated light curves (using wavelets) are almost flat indicating that there are no signals with cycle time of 90 minutes or more.

The PSD (Power Spectral Density) of the light curve gives the frequency components associated with the curve. As there is a limitation of using this FFT based method, the Lomb-Scargle method was used to generate PSD.

This data was compared with 2MASS data to find closer brown dwarfs and a dozen possible candidates were found.

Variable stars in the cluster can also be studied with the light curve of stars. As there are at least 19 known variable stars in the NGC 4755; this is an opportunity to study the known variable stars in the cluster as well as to discover additional ones.

Item ID: 41511
Item Type: Thesis (Professional Doctorate (Research))
Keywords: astronomy; ESP; exoplanets; extra-solar planets; Jewel Box Cluster; Kappa Crucis Cluster; NGC 4755; solar systems; stellar bodies; transit method; transit photometry
Date Deposited: 03 Dec 2015 00:26
FoR Codes: 02 PHYSICAL SCIENCES > 0201 Astronomical and Space Sciences > 020199 Astronomical and Space Sciences not elsewhere classified @ 100%
SEO Codes: 96 ENVIRONMENT > 9699 Other Environment > 969999 Environment not elsewhere classified @ 100%
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