Authors: Claudia Aguilera-Gómez, Laura K. Rogers, Amy Bonsor, Paula Jofré, Simon Blouin, Oliver Shorttle, Andrew M. Buchan, Yuqi Li, Siyi Xu
First Author’s Institution: Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
Status: Posted to arXiv [open access]
When studying stars, we use a combination of photometry and spectroscopy to determine their key parameters such as temperature, surface gravity, and chemical composition. These parameters are very important since they allow us to study the internal structure and composition of stars in detail. And because the chemistry of a star remains largely unchanged throughout its life, its present-day chemistry provides valuable insights about the gas and dust that formed it.
Attempting a similar kind of analysis on exoplanets to learn about their composition is distinctly more difficult for various reasons, ranging from generally not being hot enough to exhibit absorption lines, to being orders of magnitude less luminous than their host stars. And yet, understanding the chemical composition of exoplanets, especially terrestrial ones, is incredibly important. So, how do we square this issue?
Well, we use white dwarfs. White dwarfs, the remnants of stars that were not massive enough to end their lives as supernovae, have atmospheres composed almost entirely of hydrogen and helium. Due to their extreme gravity, denser elements quickly sink to their interiors and are undetectable. Because of this, astronomers can use the presence of elemental lines in the spectra of white dwarfs as proof of pollution events, or events that alter the chemistry of a star that aren’t tied to its birth or evolution. One common pollution event is, you guessed it, the accretion of material from an orbiting exoplanet!
However, studying the spectra of white dwarfs is also tricky (this is, at best, an understatement). Due to their extreme surface gravity, their spectral lines are incredibly broad and hard to measure, making the detection of potential accretion from exoplanets difficult. Great. Now, how do we square this issue?
We use binary stars! Binary stars form from the same gas cloud, meaning each member in the system shares the same chemical composition. Stellar spectra are relatively easier to analyze. As a result, if you have a binary system of a star and a white dwarf, you can use the chemistry of the star to infer the intrinsic chemistry of the white dwarf. With that resolved, you can more easily determine the composition of orbiting exoplanets using the chemistry of the now-identifiable accreted material!
Figure 1: From top to bottom, the path of a star in a binary undergoing stellar evolution, and eventually becoming a white dwarf. As it undergoes this process, planetary material can be scattered or accreted, depending on the evolutionary state of the star. Finally, as a white dwarf, accreted material is initially visible, but over time sinks beneath the surface. As a result, when analyzing the chemistry of a white dwarf, astronomers must account for how long ago that material was accreted to accurately determine the chemistry of the accreted material. Figure 1 in the paper.
That brings us neatly to today’s paper. A group of astronomers set out to measure the chemistry of an exoplanet orbiting a main sequence (HD69962)-white dwarf (WD J0820) binary system. Specifically, they aimed to compare the chemistry of the accreted material on the white dwarf with the host star’s chemistry to assess their similarities. This is important because while exoplanets form from the same gas and dust as their host stars, their compositions can vary due to the amounts of refractory (high condensation temperature) and volatile (low condensation temperature) elements they contain.
Using high-resolution spectra from the Magellan Inamori Kyocera Echelle spectrograph on the 6.5 meter Magellan Clay Telescope, located at Las Campanas Observatory, the authors analyzed the chemical abundances of Fe, Mg, Si, Ca, and Ti in both members of the binary. Because these accreted elements sink into the white dwarf at different rates over time, the authors measured the ratios of these elements to determine what phases of accretion the white dwarf could be in. Finally, they used this information, to infer the origins of the accreted planetary material.
Figure 2: A plot comparing the chemistry of the main-sequence star (black), to the chemistry of the white dwarf in various stages of accretion (blue, orange, green). These are also compared to different potential sources of accreted exoplanet material (gray). Figure 12 in the paper.
By comparing the chemistry of the white dwarf and the main-sequence star, the authors found that the accreted material was depleted in moderately refractory elements (Mg, Si, and Fe) compared to other refractory elements such as (Ca, Ti, and Al). This element grouping is linked to their condensation temperatures, or the temperature at which that element can form as a solid. This metric provides a valuable connection between the compositions of exoplanets and their host stars. While the authors couldn’t pinpoint a definitive root cause for the observed difference in moderately refractory to refractory elements, they explored several plausible explanations. These ranged from incomplete condensation during planetary formation to heating of the exoplanet during the white dwarf’s evolution. Given the inherent challenges of measuring exoplanet chemical compositions, these findings provide a strong starting point and foundation for future studies. More importantly, they bring us one step closer to a deeper understanding how planets beyond our solar system are formed.
Astrobite edited by Janette Suherli
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