Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The GAPS Programme with HARPS-N at TNG. III: The retrograde orbit of
  HAT-P-18b   The measurement of the Rossiter-McLaughlin effect for transiting exoplanets
places constraints on the orientation of the orbital axis with respect to the
stellar spin axis, which can shed light on the mechanisms shaping the orbital
configuration of planetary systems. Here we present the interesting case of the
Saturn-mass planet HAT-P-18b, which orbits one of the coolest stars for which
the Rossiter-McLaughlin effect has been measured so far. We acquired a
spectroscopic time-series, spanning a full transit, with the HARPS-N
spectrograph mounted at the TNG telescope. The very precise radial velocity
measurements delivered by the HARPS-N pipeline were used to measure the
Rossiter-McLaughlin effect. Complementary new photometric observations of
another full transit were also analysed to obtain an independent determination
of the star and planet parameters. We find that HAT-P-18b lies on a
counter-rotating orbit, the sky-projected angle between the stellar spin axis
and the planet orbital axis being lambda=132 +/- 15 deg. By joint modelling of
the radial velocity and photometric data we obtain new determinations of the
star (M_star = 0.770 +/- 0.027 M_Sun; R_star= 0.717 +/- 0.026 R_Sun;
Vsin(I_star) = 1.58 +/- 0.18 km/s) and planet (M_pl = 0.196 +/- 0.008 M_J; R_pl
= 0.947 +/- 0.044 R_J) parameters. Our spectra provide for the host star an
effective temperature T_eff = 4870 +/- 50 K, a surface gravity of log(g_star) =
4.57 +/- 0.07 cm/s, and an iron abundance of [Fe/H] = 0.10 +/- 0.06. HAT-P-18b
is one of the few planets known to transit a star with T_eff < 6250 K on a
retrograde orbit. Objects such as HAT-P-18b (low planet mass and/or relatively
long orbital period) most likely have a weak tidal coupling with their parent
stars, therefore their orbits preserve any original misalignment. As such, they
are ideal targets to study the causes of orbital evolution in cool
main-sequence stars.

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The categories of this paper are: quant-ph
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The categories of this paper are: cs
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The categories of this paper are: adap-org
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The categories of this paper are: chao-dyn
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The categories of this paper are: patt-sol
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The categories of this paper are: ao-sci
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The categories of this paper are: funct-an
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The categories of this paper are: q-alg
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The categories of this paper are: comp-gas
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The categories of this paper are: gr-qc
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The categories of this paper are: mtrl-th
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The categories of this paper are: solv-int
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The categories of this paper are: hep-th
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The categories of this paper are: acc-phys
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The categories of this paper are: econ
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The categories of this paper are: cond-mat