The internet is full of pictures of people with beautiful glowing gorgeous planetary nebulae painted on their bodies which attests to their public appeal. Their complex morphology has long been a fascination to many people, in particular astronomers. Using the early telescopes with poor optical quality, they were likened to the outer planets, Uranus and Neptune, hence their name, even though they have no relation at all to planets. As candidates for research, they make interesting laboratories with conditions that would be impossible to replicate on Earth. Over a star’s lifetime prior to evolving into a planetary nebulae (PNe), stars evolve through different stages known as the main sequence, ranging from red giants to horizontal and asymptomatic branches. The Hertzsprung-Russel diagram demonstrates the star’s evolution stages. Certainly, some stars evolve quickly while others jump some stages depending on their mass. Due to the chemical reaction occurring within, stars change physically. They expand in size then develop layers of material on the surface. Thereafter the temperature of the core of the star rises, heating the material that has been blown off making it glow. The glowing material is the nebula. Eventually, the core cools down and the size of the star diminishes resulting in a white dwarf, a low-mass star with a dense core.
During their evolutionary stage, PNe are compelling objects to study since a star’s life provides insights that could help with understanding subsequent stages. Likewise, the physics and chemical composition of PNe impart knowledge into other fields of astronomy like galactic, stellar, and interstellar astronomy. Planetary Nebulae are found within old and young galaxies by using mass and brightness measurement. These objects could show the size, structure as well as how far other galaxies are from the Milky Way. The material of the nebula found in new stars reveals similarities to the chemical elements (hydrogen, carbon, iron, etc) that dominate the gas and dust of stars that were formed from PNe.
The shape of Planetary Nebulae is mostly round even though more and more some are found to be intricate in their growth habits. The causes of their shapes have been studied extensively, but only in recent decades has evidence linked their aspherical shapes to the presence of binary stars. According to Kelebogile Gasealahwe whose master’s project was based on developing a quantitative time-series to determine whether these objects have binary central stars, “PNe and binary central stars (CSs) have been the favoured explanation for deviations from spherical symmetry. Finding and characterising the population of binary CSs is therefore important in understanding the physics behind their morphologies” she explained.
Kelebogile Gasealahwe and her supervisors developed a quantitative time-series analysis to determine whether these planetary nebulae have binary central stars then developed constraints for permissible orbital parameters. The focus of this study were Hen3-1333, Hen2-113, and Hen2-47, all with Wolf-Rayet (WR) central stars that often exhibit fast, dense stellar winds. Collectively they exhibit multi-polarity in their young nebulae, Hen3-1333 in particular has a disk and dual-dust chemistry, while the other two have central stars offset from the geometric centre of their nebulae. The objects were chosen because most of these features, mainly multipolar morphologies (growth habits), are not well represented amongst PNe with known binary central stars. Long observing periods help with determining whether there is any variability; as such the data-dense time-series was used to compare data that was collected with the Southern African Large Telescope (SALT) and The Exoplanet Survey Satellite (TESS).
The echelle spectroscopic data was collected inconsistently for ~3 years using the SALT High-Resolution Spectrograph (HRS). The Exoplanet Survey Satellite (TESS) obtained photometric data for the objects. Kelebogile says, “The spectra were taken with the medium resolution mode (R ~ 40000), obtaining between 35 – 60 spectra for each PN. The TESS data had continuous sampling with a 30-minute cadence recorded for an orbit period of 27.4 days. Using cross-correlation and Gaussian line fitting, radial velocity time-series were compared to light curves found from the TESS data. The quantitative variability analysis excluded short orbital period binary systems, indicating if their multiple features are due to binary interactions, the most likely cause is the long orbital period range. If the variability observed is due to a companion.”
The result is not conclusive yet, because of the minimal features seen in the objects. The assumption is that consistent and longer observation periods might yield conclusive results. The high cadence, short-term observations from TESS reveal that if there were any variability due to binaries, then the orbital period would not have been missed.
Now that her masters’ project is finished, Kelebogile has started working on her PhD on binary stars, though using different methods to understand these objects. She says her master’s project taught her spectral and time domain analysis, as well as scientific writing skills, which will be useful in her current project. What she enjoyed most about studying PNe was being able to connect the magnificent features seen in the images of these objects to the physics processes producing them.