From AAS NOVA : “Signs of Extreme Survivor Stars”

AASNOVA

From AAS NOVA

30 April 2021
Tarini Konchady

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Artist’s illustration of an active galactic nucleus shrouded by gas and dust. [NASA/JPL-Caltech (US)]

Active galactic nuclei are exactly what they sound like — central regions of galaxies that emit enormous amounts of energy. Typically they consist of a supermassive black hole surrounded by a hot disk of material being accreted onto the black hole. Hardly the most hospitable environment, but stars can still live in these surroundings!

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This now iconic composite image reveals Centaurus A, a galaxy with an active nucleus spewing fast-moving jets into its surroundings. [European Southern Observatory (EU)/Wide Field Imager(Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); National Aeronautics Space Agency (US)/Chandra X-ray Center (US)/Harvard Smithsonian Center for Astrophysics (US)/R.Kraft et al. (X-ray)]

Actively Hostile Environments

It would be hard to overstate how energetic active galactic nuclei (AGN) are.

Some can outshine the rest of their host galaxy at almost all detectable wavelengths! Spectra of material near the central black hole have shown that AGN environments contain a higher abundance of heavier elements than the environment of our Sun. So it’s possible that those heavier elements were produced in the accretion disk and then swept closer in towards the black hole.

But what produces heavier elements? Stars! Stars can be found near the central supermassive black holes of galaxies, like the Milky Way’s Sagittarius A*, but AGN have far more extreme environments than our placid central black hole.

So what sort of stars live in AGN environments? A recent study led by Matteo Cantiello (Flatiron Institute/ Princeton University (US) dives into this question.

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The mass and brightness of an AGN star over time. This star was modeled under specific AGN conditions. LEdd stands for Eddington luminosity, which is the maximum brightness a star can have when it has balanced its outward radiative pressure with its inward gravitational contraction. Mass loss starts roughly around when the star’s luminosity reaches the Eddington luminosity. [Cantiello et al. 2021]

What Massive Stars Make

Cantiello and collaborators were especially interested in how the evolution of stars in AGN environments differs from stellar evolution in calmer environments. To get where they are, AGN stars have to either form in accretion disks or get captured and pulled into the disks. Both models are viable and supported by looking at stellar populations around central black holes that were previously active, like Sagittarius A*.

Once in the disk, stars can rapidly accrete material and become hundreds of times more massive than the Sun. Massive stars experience more internal mixing than less massive stars, so the contents of a massive star are evenly distributed within the star’s interior. This is very different from stars like our Sun, where the outer layers of the star contain lighter elements like hydrogen and helium while inner layers are dominated by heavier elements.

However, massive stars are also unstable and can lose mass quickly as they teeter between expansion and collapse. Their sheer bulk also means that they will end their lives through core collapse — forming heavier and heavier elements through fusion until they run out of material to fuse and collapse onto themselves. The bottom line is that AGN stars are good at producing heavy elements and sending those elements out into the accretion disk.

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A schematic showing stellar evolution in the accretion disk of an AGN. Low mass stars can be formed in or accreted by the disk, where they gain mass and eventually evolve to leave behind compact remnants near the center of the disk. [Cantiello et al. 2021]

Signs of Stellar Life and Death

So Cantiello and collaborators identified two signatures of AGN stars: high abundances of heavy elements and compact stellar remnants left behind from core collapse. There are studies showing evidence for the first signature, and interestingly, this abundance of heavy elements doesn’t seem to depend on redshift.

The second signature is a bit trickier to tease out. Before gravitational-wave observatories, our best bet would be to search for the explosions associated with core collapse in the accretion disk of an AGN. Now, we can also look for the gravitational-wave signatures of the mergers of dense objects, with an expectation of how often these mergers would occur.

Sagittarius A* is a good proving ground for the findings of this study, since our galaxy’s nucleus may approximate the aftermath of an AGN. With predictions in hand, it’s now time to observe!

Citation

“Stellar Evolution in AGN Disks,” Matteo Cantiello et al 2021 ApJ 910 94.
https://iopscience.iop.org/article/10.3847/1538-4357/abdf4f

See the full article here .


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AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.