Title: Oxygen Opacity Measurements at High-Energy-Density Conditions

Authors: J. E. Bailey, D. C. Mayes, G. P. Loisel, T. Nagayama, D. Aberg, C. Blancard, J. Colgan, Ph. Cossé, G. S. Dunham, G. Faussurier, C. J. Fontes, F. Gilleron, I. Golovkin, T. A. Gomez, M. F. Gu, S. B. Hansen, H. Huang, C. A. Iglesias, C. Monton, J.-C. Pain, R. Santana, and B. W. Wilson

First Author’s Institution: Sandia National Laboratories, Albuquerque, New Mexico 87185, USA

Status: Published in the Physical Review Letters [closed access]

A student once asked me, “How can we measure the temperature of the Sun if it’s so hot, wouldn’t our thermometer melt?” They were onto something bigger that is really fundamental to astrophysics in general; the truth is, we rarely physically measure anything we study, the thermometer would melt! (Not to mention, most things are too far away to send a thermometer anyway.) Instead, we must infer measurements from our observations using physics models.

For measurements of the Sun, we use the standard solar model. This is the framework you’ll learn in any introductory astronomy class – the Sun is a giant sphere of gas with a core of hydrogen undergoing nuclear fusion and releasing energy. That energy is transported outwards via radiation, then convection, and eventually reaches the surface, causing the Sun to shine. See a schematic of this model in Figure 1. Combining this model with observations like luminosity and surface abundances from spectroscopy allows us to infer properties like temperature, density, and interior structure.

Another way the Sun’s interior structure is probed is through helioseismology, or observing oscillations on the Sun’s surface. This works just like seismology on Earth: oscillations measured on the surface allow us to understand how waves propagate from below, and what the structure down there must be like. This is particularly useful for directly measuring the depth of the Sun’s convection zone without relying on models.

The Solar Problem

A little over two decades ago, tension began to rise between observation methods. Specifically, feeding new solar abundance observations into solar models predicted a shallower convection zone than was measured from helioseismological observations. So are our models wrong? And how could we test that? Today’s authors tackled these questions by creating Sun-like conditions at Sandia National Laboratory’s Z facility so they could compare the models to actual measurements.

A Likely Culprit

Something researchers suspect the solar models could get wrong is opacity, which describes how often photons get absorbed as they try to push through plasma to the Sun’s surface. Because photon transport is so important in the Sun, opacity has a big effect on the interior structure models predict. Essentially, if the solar opacity in models was higher, they would predict an interior structure more like that measured from helioseismology. And it could currently be wrong – the opacity used in models is an approximation, because we don’t fully understand opacity in plasmas as hot and dense as those in the Sun.

To better understand opacity in Sun-like plasma, today’s author created an extremely hot and dense oxygen plasma in a lab. They focused on oxygen because it is one of the elements that affects opacity the most at solar temperatures. Recreating solar conditions is quite difficult so the plasma wasn’t as hot or dense as the Sun, with a temperature and density lower by a factor of 10 and 1.3, respectively. Still, this was the hottest and densest oxygen plasma opacity measurements have ever been taken from. Then, they measured the radiation that could be transmitted through the plasma using a handful of spectrometers to limit experimental uncertainties. Finally, they compared these opacity measurements to seven state-of-the-art opacity models commonly used to model the Sun.

As shown in Figure 2, the opacity measurements match the models surprisingly well, especially considering this is the first time such a plasma has been studied in a lab. The slight differences between the measurements and models will be used to make model improvements. But, these differences aren’t great enough to solve the solar problem – the oxygen opacity isn’t high enough to make models match helioseismology measurements.

Other possible solutions exist though. For example, a similar study done with an iron plasma did result in higher opacities, which partially resolves the problem. As our laboratory technologies improve, we will be able to make even hotter and denser plasmas to replicate the Sun better. This will ultimately improve the standard solar model, and therefore, our understanding of all other stars in the universe – without any melted thermometers.

Astrobite edited by Viviana Cáceres

Featured image credit: NASA/SDO (AIA), Porapak Apichodilok CC BY-SA 4.0