Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Rocky Extrasolar Planetary Compositions Derived from Externally-Polluted
  White Dwarfs   We report Keck High Resolution Echelle Spectrometer data and model atmosphere
analysis of two helium-dominated white dwarfs, PG1225-079 and HS2253+8023,
whose heavy pollutions most likely derive from the accretion of
terrestrial-type planet(esimal)s. For each system, the minimum accreted mass is
~10^22 g, that of a large asteroid. In PG1225-079, Mg, Cr, Mn, Fe and Ni have
abundance ratios similar to bulk Earth values, while we measure four refractory
elements, Ca, Sc, Ti and V, all at a factor of ~2-3 higher abundance than in
the bulk Earth. For HS2253+8023 the swallowed material was compositionally
similar to bulk Earth in being more than 85% by mass in the major element
species, O, Mg, Si, and Fe, and with abundances in the distinctive proportions
of mineral oxides - compelling evidence for an origin in a rocky parent body.
Including previous studies we now know of four heavily polluted white dwarfs
where the measured oxygen and hydrogen are consistent with the view that the
parents bodies formed with little ice, interior to any snow-line in their
nebular environments. The growing handful of polluted white dwarf systems with
comprehensive abundance measurements form a baseline for characterizing rocky
exoplanet compositions that can be compared with bulk Earth.

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The categories of this paper are: q-bio
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The categories of this paper are: ao-sci
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The categories of this paper are: hep-lat
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The categories of this paper are: mtrl-th
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The categories of this paper are: funct-an
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The categories of this paper are: gr-qc
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The categories of this paper are: hep-ex
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The categories of this paper are: patt-sol
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The categories of this paper are: nucl-ex
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The categories of this paper are: chao-dyn
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The categories of this paper are: dg-ga
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The categories of this paper are: stat
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The categories of this paper are: eess
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The categories of this paper are: comp-gas
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The categories of this paper are: hep-ph
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The categories of this paper are: cond-mat