Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Absence of a metallicity effect for ultra-short-period planets   Ultra-short-period (USP) planets are a newly recognized class of planets with
periods shorter than one day and radii smaller than about 2 Earth radii. It has
been proposed that USP planets are the solid cores of hot Jupiters that lost
their gaseous envelopes due to photo-evaporation or Roche lobe overflow. We
test this hypothesis by asking whether USP planets are associated with
metal-rich stars, as has long been observed for hot Jupiters. We find the
metallicity distributions of USP-planet and hot-Jupiter hosts to be
significantly different ($p = 3\times 10^{-4}$), based on Keck spectroscopy of
Kepler stars. Evidently, the sample of USP planets is not dominated by the
evaporated cores of hot Jupiters. The metallicity distribution of stars with
USP planets is indistinguishable from that of stars with short-period planets
with sizes between 2--4~$R_\oplus$. Thus it remains possible that the USP
planets are the solid cores of formerly gaseous planets smaller than Neptune.

23
The categories of this paper are: solv-int
27
The categories of this paper are: comp-gas
5
The categories of this paper are: gr-qc
29
The categories of this paper are: funct-an
34
The categories of this paper are: chem-ph
12
The categories of this paper are: nucl-th
18
The categories of this paper are: econ
37
The categories of this paper are: supr-con
20
The categories of this paper are: acc-phys
2
The categories of this paper are: math
30
The categories of this paper are: q-alg
15
The categories of this paper are: nucl-ex
11
The categories of this paper are: hep-lat
31
The categories of this paper are: ao-sci
0
The categories of this paper are: hep-ph
4
The categories of this paper are: cond-mat