Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Runts of the Litter: Why planets formed through gravitational
  instability can only be failed binary stars   Recent direct imaging discoveries suggest a new class of massive, distant
planets around A stars. These widely separated giants have been interpreted as
signs of planet formation driven by gravitational instability, but the
viability of this mechanism is not clear cut. In this paper, we first discuss
the local requirements for fragmentation and the initial fragment mass scales.
We then consider whether the fragment's subsequent growth can be terminated
within the planetary mass regime. Finally, we place disks in the larger context
of star formation and disk evolution models. We find that in order for
gravitational instability to produce planets, disks must be atypically cold in
order to reduce the initial fragment mass. In addition, fragmentation must
occur during a narrow window of disk evolution, after infall has mostly ceased,
but while the disk is still sufficiently massive to undergo gravitational
instability. Under more typical conditions, disk-born objects will likely grow
well above the deuterium burning planetary mass limit. We conclude that if
planets are formed by gravitational instability, they must be the low mass tail
of the distribution of disk-born companions. To validate this theory, on-going
direct imaging surveys must find a greater abundance of brown dwarf and M-star
companions to A-stars. Their absence would suggest planet formation by a
different mechanism such as core accretion, which is consistent with the debris
disks detected in these systems.

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The categories of this paper are: patt-sol
15
The categories of this paper are: nucl-ex
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The categories of this paper are: math-ph
22
The categories of this paper are: chao-dyn
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The categories of this paper are: q-bio
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The categories of this paper are: nlin
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The categories of this paper are: quant-ph
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The categories of this paper are: nucl-th
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th
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The categories of this paper are: atom-ph
0
The categories of this paper are: hep-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: solv-int
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The categories of this paper are: acc-phys
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The categories of this paper are: stat