_{URL: https://export.arxiv.org/pdf/1901.00007.pdf}

Gravitational stability and fragmentation condition for discs around accreting supermassive stars

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QUESTION 1:

Can you give me a very clear explanation of the core assertions, implications, and mechanics elucidated in this paper?

ANSWER 1:

Based on my analysis, the key points and implications of this paper are:

1. The authors investigate the gravitational stability of accretion disks around rapidly accreting protostars that may become supermassive stars (SMSs) and eventually seed black holes for supermassive black holes in the early universe.

2. They construct steady, one-dimensional disk models with detailed chemical and thermal processes for a range of central star masses (10-10^5 solar masses) and accretion rates (10^-3 to 1 solar mass per year).

3. Two types of accretion flows are considered: atomic flows (relevant for SMS formation via atomic cooling) and molecular flows (relevant for SMS formation in regions with large baryon-dark matter streaming velocities).

4. For atomic flows, the authors find:

   • Disks become gravitationally unstable for accretion rates above ~0.1 solar masses per year, regardless of central star mass.
   • At lower accretion rates, H2 formation can occur in the disk, leading to rapid cooling.

5. For molecular flows, they find:

   • There is a critical radius beyond which the disk becomes unstable.
   • This radius depends on accretion rate and central star mass.

6. The results suggest disk fragmentation may be common during SMS formation, potentially leading to:

   • Episodic accretion onto the central protostar
   • Formation of supermassive binary stars

7. The authors use the α-viscosity prescription and consider disks unstable when α > 1, based on previous numerical simulations.

8. The detailed chemical/thermal modeling allows the authors to identify transitions between atomic and molecular gas in the disks that were not captured in previous simplified models.

The key implication is that disk fragmentation could be an important process during SMS formation, potentially affecting the growth of SMSs and leading to the formation of binary SMSs. This has implications for early supermassive black hole formation and potential future gravitational wave detections.

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QUESTION 2:

Can you explain the value of this in basic terms? Like you're talking to a CEO. So what? What's the bottom line here?

ANSWER 2:

Certainly. Here's a high-level explanation for a CEO:

Bottom line: This research gives us crucial insights into how the universe's first supermassive black holes might have formed, which is a major unsolved mystery in astrophysics.

Key points:

1. Big Picture: We're trying to understand how supermassive black holes, which power the brightest objects in the universe (quasars), could have formed so quickly after the Big Bang.

2. Potential Solution: One leading theory suggests these black holes came from "supermassive stars" - stars about 100,000 times more massive than our Sun.

3. The Problem: We need to understand if these supermassive stars can actually form and grow rapidly enough.

4. This Study's Contribution: The researchers investigated whether the disks of gas feeding these growing supermassive stars would remain stable or break apart.

5. Key Findings:

   • Under certain conditions, these disks are likely to fragment.
   • This fragmentation could interrupt the growth of supermassive stars or even lead to the formation of binary supermassive stars.

6. Implications:

   • This could make it harder for individual supermassive stars to grow, potentially challenging our current theories.
   • However, it might also open up new possibilities, like the formation of supermassive black hole binaries.

7. Future Impact: These results could guide future observations and potentially help us interpret future gravitational wave detections from the early universe.

In essence, this work is helping us refine our understanding of how the first giant black holes in the universe may have formed, which is crucial for our overall picture of how the universe evolved from its earliest stages to what we see today.

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QUESTION 3:

Can you give me an analogy or metaphor that will help explain this to a broad audience?

ANSWER 3:

Certainly! Here's an analogy that might help explain this concept to a broad audience:

Imagine you're building the world's largest snowman in a very short time. This snowman represents a supermassive star, and your goal is to make it as big as possible as quickly as possible.

Now, you're not building this snowman by hand. Instead, you've set up a giant conveyor belt (the accretion disk) that's bringing snow from all around and dumping it onto your snowman.

The key question is: Can you keep piling on snow fast enough to build your massive snowman before it melts (or in the case of stars, before it collapses into a black hole)?

What this study is looking at is whether that conveyor belt of snow (the accretion disk) stays intact or breaks apart under different conditions.

The researchers found that:

1. If you try to pour snow too quickly (high accretion rate), the conveyor belt might break apart, spilling snow everywhere instead of neatly piling it onto your snowman.

2. Even at slower rates, if your conveyor belt gets too long (large disk radius), it might start to break at the edges.

3. Sometimes, the snow might clump together on the conveyor belt, creating separate smaller snowballs instead of contributing to your main snowman.

These findings suggest that building a supermassive snowman (or star) might be more complicated than we thought. It might explain why these gigantic snowmen are so rare in the universe, and it could even suggest that sometimes we end up with two large snowmen (binary supermassive stars) instead of one.

Understanding this process better could help us know where to look for these rare, gigantic "snowmen" in the early universe, and might even help us detect the "rumble" of two such snowmen colliding (gravitational waves from binary supermassive black holes) in the future.

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QUESTION 4:

Can you provide a specific example that best illustrates the key points elucidated in this paper?

ANSWER 4:

Certainly! Let's consider a specific example that illustrates the key points of the paper:

Imagine we're looking at a protostar that's on its way to becoming a supermassive star. Let's say it currently has a mass of 1,000 times that of our Sun (10^3 solar masses) and is accreting material at a rate of 0.1 solar masses per year. This scenario is directly examined in the paper and illustrates several key findings:

1. Atomic Flow Scenario: If the gas flowing into the disk is primarily atomic (which might happen in regions with strong UV radiation that dissociates molecules):

   • The disk temperature would start at about 4,000 K at the outer edge (1,000 AU from the star) and gradually decrease inward to about 3,000 K.
   • At this accretion rate, the disk would be on the verge of instability. The paper shows that for atomic flows, disks become unstable when the accretion rate exceeds about 0.1 solar masses per year.
   • This means our disk is just at the point where it might start to fragment, potentially disrupting the smooth accretion onto the central star.

2. Molecular Flow Scenario: If the inflowing gas is primarily molecular (which might occur in regions with high dark matter-baryon streaming velocities):

   • The disk would start cooler, around 2,000 K at the outer edge.
   • As we move inward, we'd see a sudden jump in temperature to about 3,000 K where the H2 dissociates into atomic hydrogen.
   • In this case, the disk would likely be unstable in its outer parts. The paper shows a critical radius beyond which molecular disks become unstable, and for this stellar mass and accretion rate, that radius is about 100 AU.
   • This means the outer parts of the disk (beyond 100 AU) would likely fragment into clumps.

3. Implications:

   • In both scenarios, we're seeing conditions that could lead to disk fragmentation, which could disrupt the steady growth of the star.
   • This fragmentation might lead to episodic accretion as clumps fall onto the star, or even the formation of a companion star if large clumps survive.
   • The detailed temperature and chemical profiles calculated in the paper (like the H2 dissociation front in the molecular case) show the importance of accurate thermal and chemical modeling in understanding these disks.

This example illustrates how the paper's findings apply to a specific case of a growing supermassive star, showing how the disk's stability depends on the nature of the accreting material and how fragmentation might occur under these realistic conditions.