Modeling Circumstellar Gas Emission around a White Dwarf Using Cloudy

Abstract: The chemical composition of an extrasolar planet is fundamental to its formation, evolution and habitability. In this study, we explore a new way to measure the chemical composition of the building blocks of extrasolar planets, by measuring the gas composition of the disrupted planetesimals around white dwarf stars. As a first attempt, we used the photo-ionization code Cloudy to model the circumstellar gas emission around a white dwarf Gaia J0611$-$6931 under some simplified assumptions. We found most of the emission lines are saturated and the line ratios approaching the ratios of thermal emission; therefore only lower limits to the number density can be derived. Silicon is the best constrained element in the circumstellar gas and we derived a lower limit of 10$^{10.3}$ cm$^{-3}$. In addition, we placed a lower limit on the total amount of gas to be 1.8 $\times$ 10$^{19}$ g. Further study is needed to better constrain the parameters of the gas disk and connect it to other white dwarfs with circumstellar gas absorption.