AGB Stars in the Large Magellanic Cloud are Shrouded in Iron Dust

Through a combination of theoretical models and Infrared observations, Researchers found a specific group of Asymptotic Giant Branch (AGB) stars that shrouded in a high fraction of iron dust.  

This latest research, published in the journal ‘The Astrophysical Journal Letters’, compared the Infrared observations of the Spitzer Space Telescope with the theoretical models of the formation of dust in circumstellar envelopes. The researching team discovered that these massive AGB stars were formed about 100 million years ago and are about 5 times heavier than our Sun. Against all expectations, it was found that the Infrared Spectral Energy Distributions can only be retraced if iron dust is the most abundant component of the circumstellar envelopes (which was not the case). What made this discovery even more surprising is that such an environment is a rarity when it comes to metal-poor AGB stars.    

AGB Stars

Stars that come under the category of AGB are generally 1-8 times heavier than the Sun of our Solar System and end their lives as White Dwarfs. During the phase of evolving along the asymptotic giant branch, these stars expand rigorously while they cool down. Consequently, they lose a huge fraction of their mass due to the powerful stellar winds. These winds provide ideal conditions (high density and low temperature) for the condensation of dust grains in their circumstellar envelopes. This dust is extremely important for the survival of galaxies because it is a vital commodity for the formation of new stars and planets. This is the reason why the scientific community is so keen to determine the quality (the type of dust) and quantity of the dust produced by these giant stars.

Unique Spectral Properties of these AGB Stars

Ester Marini, a Doctoral Student at the Roma Tre University who is the First Author of the study, mentioned that they have successfully characterized (for the first time) this class of AGB stars with unique spectral properties. She explained that the poor metal content of these massive stars offers an ideal set of conditions for producing large quantities of iron dust. In these circumstances, the process of nucleosynthesis is so extreme that it removes the elements that could produce other types of dust. She elaborated that in the following words:

“In fact, in metal-poor environments, the complex nucleosynthesis within massive AGB stars is so advanced that it burns up almost all the magnesium and oxygen, necessary to form other types of dust, such as the silicates.”

Aníbal García Hernández, a Co-author of the research who is working as a Researcher in the Instituto de Astrofísica de Canarias (IAC), strengthened the argument even more by referring to some past observational evidence. She commented that iron dust is the main component of the stardust formed in poor metal environments and we have sufficient independent studies to show that. She said,

“This result is an important confirmation of the theory of iron dust in metal-poor environments, already hinted at in independent observational evidence.”

James Webb Space Telescope (JWST)

This research also included some predictions for the future James Webb Space Telescope of NASA. The researching team hopes that this mission will give a whole new dimension to this discovery as we will be able to study more of these AGB stars in other galaxies. The MIRI instrument on the JWST is being considered the ideal instrument for making these critical observations. Flavia Dell’Agli, the Second Author of the study who a Postdoctoral Researcher at the IAC, referred to that by saying,

“The arrival of the James Webb Space Telescope (JWST) will open up new possibilities for investigating this case in depth. That future telescope will greatly enhance the number of resolved extragalactic AGB stars and that the MIRI instrument on the JWST will be ideal for identifying this class of stars in other galaxies of the Local Group.”

JWST, a successor to the Hubble Space Telescope, is expected to launch in March 2021. According to an estimate, a whooping amount of 10 billion USD will be spent on this mission. It will not only improve the sensitivity and resolution of the images but will also cover a larger part of the sky. It will enable the scientists to carry out a wider range of investigations in the fields of cosmology and astronomy. You can know more about JWST at the following link: https://www.jwst.nasa.gov/