Conversation

What we do here is bring together two theoretical models in different domains: Bitsch & Battistini (2020), who consider how a star of a given metallicity creates planetesimals of a particular composition, and EAGLE, which makes many galaxies of varied star formation history.
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So a simulated galaxy makes lots of stars through time; the stars have disks that make planetesimals; the planetesimals become interstellar objects — tiny postcards to the future, outlasting their original home stars.
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We focus here on water, because there's lots of it, and it's a molecule that shows up with a clear variation depending on the star's metallicity. So we look at how water-rich and water-poor ISOs will be produced by different generations of stars.
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Galaxies make stars more at different times in their evolution. An early burst of star formation makes a lot of water-rich interstellar objects; a late burst makes water-poor interstellar objects.
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If you check this out for lots of simulated galaxies, it is pretty amazing that a property of interstellar objects (together with mean galaxy metallicity) can be useful in helping understand star formation history
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And so: a prediction! The interstellar object population should be dominated by twin populations of high and low water mass fraction, with relatively few ISOs appearing with an intermediate water fraction.
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Have we seen any of the ancient, water-rich ISOs so far? My thinking is probably no; we've seen two water-poor ones (for reasons we go into, though I am willing to be convinced otherwise!)
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Is oumuamua one of them? Is there any way to study these from interstellar distances? Like absorption spectra from clusters of them or something? I imagine there’s piles of rubble drifting around far from stars?
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yes, ʻOumuamua is one; from the analyses of the data that could be gathered I don't think there's a good argument for it being in the water-rich population. They're all rather too small to see unless they come calling. But a decent number will do that in the next ten years.