Mind Your Metals (especially when modeling the collapse of super massive stars into black holes)

Title: The formation of black holes from rapidly accreting supermassive stars is not trivial: Simulations of thermonuclear pulsations and explosions

Authors: Chris Nagele & Hideyuki Umeda

First Author’s Institution: Department of Astronomy, School of Science at the University of Tokyo & William H. Miller Department of Physics and Astronomy, Johns Hopkins University Baltimore

Status: Published in The Physical Review D. [closed access]

Astronomers have observed massive black holes in the early universe for some time (check out UHZ-1), but we still don’t fully understand how black holes can grow to be so massive in the early universe. You may be asking, “Why is it a problem that black holes are so big in the early universe?” and the answer is basically this: black holes form in the early universe through a variety of hypothesized pathways (starting as either light or heavy “seeds”) and then grow by accreting material over time, but there is a limit to how quickly the black holes can accrete material–called the Eddington limit. Light black hole seeds cannot accrete mass fast enough, if they follow the Eddington limit, to reach the size of the massive black holes we observe in the early universe. Instead light seeds have to grow at a rate faster than the Eddington limit, called the super- or hyper-Eddington limit. While this type of accretion is technically allowed (the physicists checked and you can read more about in this white paper), it ~feels~ illegal.

Heavy black hole seeds start off much more massive and can accrete matter following the Eddington limit to reach the masses we observe in the early universe. But how do we form the heavy black hole seeds? One method is called the direct collapse scenario and describes when a super massive star (SMS; M > 10,000 M_☉) made from primordial gas (gas made up of hydrogen and helium) rapidly accretes gas until it becomes so massive (and unstable) that it collapses to form a black hole. Today’s paper uses general relativistic (GR) hydrodynamical models to explore if SMSs that are rapidly accreting gas and encounter a GR instability will explode or collapse into black holes.

Modeling the life and death of an SMS is not an easy job, since SMSs can live for thousands of years but the collapse of an SMS happens very quickly (on timescales of a few days to a few weeks). So the authors use two modeling codes: one called the HOngo Stellar Hydrodynamics Investigator (HOSHI) code to model the SMS and the accretion on to the SMS until a GR instability is detected, and a second code, a 1D GR Lagrangian hydrodynamics code that reads in the SMS model with the GR instability and models the SMS until it collapses into a black hole, pulsates, or explodes (ejects a large amount of mass, allowing the SMS to become stable again). They run these models assuming three different metallicities (a measure of elements other than hydrogen and helium, and is represented with the symbol Z–0 Z_☉ metallicity means the only elements present are H and He!) for the accreting gas and several different accretion rates.

Their main finding is that it’s actually easier and faster (by an order of magnitude) to create a black hole from an SMS that is not accreting, which is the opposite of what was previously thought!This is because SMSs that are accreting have smaller cores that collapse slowly due to the nuclear energy generated, which works to stabilize the star and prevent collapse for a longer amount of time. Additionally, they found that higher metallicity (Z = 0.1-1 Z_☉) accreting SMSs never collapsed and instead were equally likely to pulsate or explode. Finally, they found that a higher accretion rate did not result in a higher final mass for all the metallicities tested, which is a behavior unseen in other models that requires further investigation (see in Figure 1).

In summary, this paper finds it is difficult to model the collapse of an accreting super massive star into a massive black hole and it’s even harder if the accreting gas is metal rich! If you liked learning about super massive stars (and how they could’ve formed the universe’s first massive black holes) you’re in luck because you can check out these other Astrobites articles (“Upbringing of Supermassive Stars”, “It’s over 4000! This supermassive black hole has gone Super-Eddington!”, and “A New Way to Die: What Happens to Supermassive Stars?”) that have summarized related works to this paper!

Astrobite edited by Dee Dunne

Featured image credit: bratgenerator

• About the Author

About Erica Sawczynec

I am a fourth year graduate student at the University of Texas at Austin working on NIR spectroscopy instrumentation. When I'm not in the lab I manage the archive for IGRINS (RRISA) and use the data products to study molecular hydrogen emission in circumstellar disks. Outside of work you can find me reading sci-fi and fantasy novels, baking bread, hanging out with my cat, or over on Twitter @EricaSawczynec.

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