Evolution of Sun-like Stars With Age and Its Impact on Planets

Das, Srijan Bharati (2018) Evolution of Sun-like Stars With Age and Its Impact on Planets. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

From the time humanity has looked up to the stars, it has wondered about the possibility of life in other stellar systems. With the advancement of human civilizations, a multitude of concerns — natural as well as man-made, have ushered into the scientific community. To a near-sighted analyser, the Earth and its habitation is "here-to-stay". But, the far-sighted questions would be : "How long would the earth be habitable? Why is it habitable? What is the role of the Sun in forming the Earth’s atmosphere? Is there another planet that we might migrate to incase the Earth is rendered inhabitable?" The host star of our stellar system, the Sun, is approximately 4.6 billion years old. During the period of evolution, the solar properties, especially those related to magnetic activity have changed. The Sun has a spectral type G2V and therefore, other stars of comparable spectral types are expected to demonstrate similar evolutionary trends. There have been previous studies along similar lines but with a large relaxation on the span of spectral type, F, G, K, of "solar-like" stars. Contrary to expectations, many of these scientific works have found that the Sun is not "solar-like", which means, that the Sun tends to fall outside the cluster of solar-like stars in a scatter plot of various parameters. Confining the study to a much smaller section of spectral class - G2, we find that the Sun lines up along the "evolutionary track of the magnetic properties" along with the other stars in our sample. In doing so, we propose a set of empirical relations between various observational parameters with stellar age. Moreover, the chosen observational parameters are proxies of the magnetic peculiarities of the star. Thus, we devise a tool to not only predict the magnetic properties of a solar-like star, given the knowledge of any of the observational parameters, but more importantly, to predict such properties for solar-like stars of different ages. In a way, this study shall be able to predict what the magnetic field parameters for a(n) young (old) Sun looked (will look) like, therefore helping us understand the influence of the evolving Sun on planetary evolution. As a second part of the study, we formulate an algorithm to harness this knowledge of the magnetic evolution of the Sun-like star to simulate its impact on a magnetized planet that it might host. For this, we have taken a hypothetical planet with a dipolar magnetic field configuration. To study the modifications of the local environment of the planet as impacted by different types of stellar winds, we perform 3D magnetohydrodynamic (MHD) simulations using the PLUTO code. Configuring this code to suit our system, we have been able to analyse and observe different aspects of the star-planet interaction. In doing so, we started off with various quantitative and qualitative sanity checks to validate the robustness of our initial configuration. Thereafter, we explore the phenomena of injection of stellar winds into the inner magnetosphere as well as the loss of atmospheric plasma contained in the planet. Our simulations say that both these phenomena does occur, also in accordance with other simulations and observations. In the third and final part of our study, we introduce extremely high tilt angles to our hypothetical planet to study how the stellar wind injection and loss of atmosphere compares with the present Earth-like cases. The added motivation behind this study is in the fact that this highly tilted magnetic axis is one of the hypothetical magnetic field configurations that a planet might have during a geomagnetic polarity reversal. The last polarity reversal of the Earth had occurred around 780,000 years ago. Bearing in mind that the earth is known to flip its magnetic poles every 200,000-300,000 years and that we are long over-due for another reversal, the task of getting a qualitative idea of the magnetospheric dynamics during such reversals, is not only an exotic scientific problem, but rather an immediate necessity. Based on our simulations we discuss the expected influence of the solar wind on the Earth’s magnetosphere during such a reversal and the resulting impacts such a phenomena may have on our atmosphere.

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