Scientists have studied how polycyclic aromatic hydrocarbons (PAHs) are created in an aging population of stars in one of the Milky Way’s satellite galaxy, the Large Magellanic Cloud. They have found that the types of PAHs found in the atmospheres of these stars are much more varied than the PAHs observed in our galaxy.
“We were surprised because previous measurements of PAHs in interstellar dust in the Large Magellanic Cloud were very similar to those in the Milky Way,” said Mikako Matsuura from University College London. “Our studies suggest these organic molecules are processed and change their composition soon after they are ejected by dying stars to fill the matter within the galaxy. Dying stars in the neighboring galaxy are richer in carbon than the Milky Way’s stars, so are more likely to trigger these wide varieties of organic compounds.”
Matsuura and an international team observed 24 carefully selected stars with the Spitzer Space Telescope and analyzed the light to find features linked to PAHs. Stars are powered by nuclear fusion in their cores, converting hydrogen to helium. Toward the end of their lives, low-mass stars like the Sun consume all their hydrogen in their stellar cores and start to convert helium to oxygen and carbon. Eventually, they run out of fuel and expel their outer layers, creating nebulae around central white dwarfs. The gas and molecules from these nebulae are mixed over time into the interstellar medium. The team focused their study on stars that appeared to be in the process of fusing carbon.
“We think that as the central star evolves, the increasingly energetic radiation effects the compositions of the PAHs, leading to more varieties. However, once the PAHs are mixed into the interstellar medium, this variability decreases,” said Matsuura. “One possibility for this contrast is that ultraviolet radiation might change the profiles of the PAHs. Another is that PAHs are reprocessed further in the interstellar medium, changing their composition.”