Extreme Stars in Extreme Environments (Planetary Science)

Not all stars are the same. Not only because their fate varies according to the mass, but also because it is strongly influenced by the environment in which they are born or live. This is precisely the reason that prompted a group of astronomers to investigate in detail what happens to the stars found in the AGN accretion discs. The study traces its life, for the first time, and analyzes its consequences and the possibilities of observation

Most stars are born and spend their lives in a quiet and relatively empty environment, the interstellar medium . Even if two stars are found to coexist in a binary system , their routine is still very similar to that of isolated stars, except in a short – compared to their average life time – phase in which the more overbearing companion increases mass from the more resigned. The theories of stellar evolution are therefore based on these boundary conditions to follow the path of the stars and their contribution to the chemical enrichment of the cosmos. In a new theoretical study published in The Astrophysical JournalInstead, scientists have wondered what the fate of stars living in extreme environments such as the accretion disks of active galactic nuclei ( Agn ) is and what traces they can leave in the environment.

Agn are an extreme energetic phenomenon that occurs in the center of some galaxies whose black hole voraciously feeds on the material of a thin accretion disk that orbits around it. The energy released by the Agn is such that their light – emitted over the entire spectrum and coming from the accretion disk and the outgoing jets of material – almost completely obscures the surrounding galaxy, especially in the visible. Since the Agn condition is transient within a galaxy, some galaxies – like ours – are thought to have already ended and can be used as an example to search for observational evidence compatible with different theoretical scenarios.

It is unclear whether the stars dealing with these extreme environmental conditions were born in them or later migrated to the disk. What is clear, however, is that their condition strongly influences their evolutionary process. In this new study, the authors first had to modify the typical boundary conditions employed in stellar evolution codesto make them compatible with those of an Agn. In particular, they had to consider higher temperatures and much higher densities of the interstellar medium. The first consequence is that stars begin to gain mass very quickly and their internal composition is continually mixed by convective motions – on the contrary, low-mass stars have a well-defined stratification, with the lightest chemical elements in the center and the most heavy to the outside. The fusion process continues quickly and, thanks to the convective motions, the star continues to receive fresh hydrogen from the surrounding environment.

As they grow larger, the stars in the Agn disks continually live in the balance between the accretion of mass from the external material and the loss of it due to powerful stellar winds. Winds that enrich the surrounding environment with heavy elements such as helium, carbon, nitrogen and oxygen produced inside the star. There will come a time, as with any very massive star, when the energy resulting from the fusion will no longer be sufficient to counteract the gravitational collapse induced by its own weight: the star will explode as a supernova , enriching the surrounding medium with even heavier chemical elements. and leaving, in the center, a compact stellar remnant. Further accretion or merger processes could eventually result in a black hole.

Diagram showing stellar evolution in the accretion disk of an Agn. Low-mass stars can form or be welcomed by the disk, where they gain mass and eventually evolve to leave compact residues near the center of the disk. Credits: Cantiello et al. 2021

The road seems to be marked, destiny is written. But how can we identify, then, the presence of these stars in such an energetic and extreme environment? First of all, looking at its chemical composition. If most of the stars born – or happened – on the spot are to follow a sudden accretion and a fate similar to that described above, the gas in the accretion disk should fill with metals and heavy chemicals, produced by the stars themselves and by the their explosion as a supernova. In fact, astronomers have already noticed that Agn’s accretion discs are rich in heavy chemicals. Furthermore, given this explosive fate, the environment should be populated with far more compact stellar remnants than any other stellar environment. Finally,Ligo and Virgo .

The circle seems to be closing, but the work of astronomers is only at the beginning. Now, you have to roll your eyes.

Featured image: Centaurus A, a galaxy with an active core that emits fast-moving jets in its surroundings. Credits: Optical image, Eso / Wfi; Sub-millimeter image, MpIfr / Eso / Apex / A.Weiss et al .; X-ray image, NASA / Cxc / Cfa / R.Kraft et al.


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