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Thread: Implications of pebble accretion models

  1. #1
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    Implications of pebble accretion models

    While researching Hubble tension (and black-body radiation) I came across some old lecture notes on pebble accretion.
    Basically if pebble accretion happens then interstellar space would be full of pebbles and pebble-piles.
    At the time no interstellar object had been seen.

    So, New Horizons and data from other probes going to comets has put pebble accretion back as the front runner.

    What does this mean for the size distribution of interstellar objects?
    For every object big enough to be seen how many pebbles are out there?
    Twitter https://twitter.com/CarlNGraham -- Carl N Graham -- Sci-fi blog: The Alien Reporter

  2. #2
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    Pebble accretion, to my understanding, occurs only within the gravity well of a young star or giant planets/brown dwarf. It does not happen in otherwise empty interstellar space.
    Do good work. —Virgil Ivan "Gus" Grissom

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    I was assuming that most interstellar objects are formed in star systems & then ejected.
    But thinking about it 486958 Arrokoth is 40 AU out so could pebbles be forming under dense molecular cloud conditions?

    So, on a sample size of 2 we have an object of a few hundred metres an comet of a few kilometres.
    Compared to other populations it does not seem to matter if these two are larger or smaller than average for interstellar objects for estimating size distribution.

    Would it be reasonable to assume for every object like 2i/borisov there would be a few tens of million of pebbles in smaller objects?
    Twitter https://twitter.com/CarlNGraham -- Carl N Graham -- Sci-fi blog: The Alien Reporter

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    Good point. Have to wait and see.
    Do good work. —Virgil Ivan "Gus" Grissom

  5. #5
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    How roughly how big is our detection volume for finding objects of comparable size to 2i/borisov?
    I was guessing at about 35 cubic AU?
    Twitter https://twitter.com/CarlNGraham -- Carl N Graham -- Sci-fi blog: The Alien Reporter

  6. #6
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    Quote Originally Posted by PetTastic View Post
    How roughly how big is our detection volume for finding objects of comparable size to 2i/borisov?
    I was guessing at about 35 cubic AU?
    Going back to post #2 in this thread, pebble accretion occurs in planet-forming disks (rings) around young stars using matter which has been concentrated. The pebbles are not an immediate result, but probably come from an accumulation of stringy dust, and may initially look like tiny cotton candy blobs until enough of them collide together to start being boulders with beards. This process does not happen in interstellar clouds. 2i/Borosov was created around a young star and ejected. As to observing volume, 35 cubic AU is a good enough value to use for any back-of-the-envelope calculations. New equipment will extend that volume. I imagine that the LSST (Vera Rubin Observatory, which should be in full operation in 2022) will probably take it to between 150 to 500 cubic AU.
    Forming opinions as we speak

  7. #7
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    This is not my field but some of my late night reading suggested things might be the other way around.

    It was pointed out that Saturn's rings are formed of pebbles and might hint at ring formation around stars.
    Collisions involving water ice are highly inelastic. "Ice does not bounce" causing very high energy loss (cooling) in particle systems.

    The chances are that without pebbles protoplanetary disk would be dominated by powerful rapidly rotating storms (large scale turbulence) caused by the differential rotation in the disk.

    Such storms would hinder pebble and planet formation.
    So, in this line of thinking the pebbles had to come first.
    Twitter https://twitter.com/CarlNGraham -- Carl N Graham -- Sci-fi blog: The Alien Reporter

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