Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: High resolution spectroscopy for Cepheids distance determination. II. A
  period- projection factor relation   The projection factor is a key quantity for the interferometric
Baade-Wesselink (hereafter IBW) and surface-brightness (hereafter SB) methods
of determining the distance of Cepheids. Indeed, it allows a consistent
combination of angular and linear diameters of the star. We aim to determine
consistent projection factors that include the dynamical structure of the
Cepheids' atmosphere. Methods. Hydrodynamical models of delta Cep and l Car
have been used to validate a spectroscopic method of determining the projection
factor. This method, based on the amplitude of the radial velocity curve, is
applied to eight stars observed with the HARPS spectrometer. The projection
factor is divided into three sub-concepts : (1) a geometrical effect, (2) the
velocity gradient within the atmosphere, and (3) the relative motion of the
"optical" pulsating photosphere compared to the corresponding mass elements
(hereafter fo-g). Both, (1) and (3) are deduced from geometrical and
hydrodynamical models, respectively, while (2) is derived directly from
observations. The Fe I 4896.439 A line is found to be the best one to use in
the context of IBW and SB methods. A coherent and consistent period-projection
factor relation (hereafter Pp relation) is derived for this specific spectral
line: p = [0.064 +- 0.020] log P + [1.376 +- 0.023]. This procedure is then
extended to derive dynamic projection factors for any spectral line of any
Cepheid. This Pp relation is an important tool for removing bias in the
calibration of the period-luminosity relation of Cepheids. Moreover, it reveals
a new physical quantity fo-g to investigate in the near future.

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The categories of this paper are: cmp-lg
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The categories of this paper are: adap-org
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The categories of this paper are: atom-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: solv-int
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The categories of this paper are: mtrl-th
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The categories of this paper are: quant-ph
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The categories of this paper are: econ
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The categories of this paper are: hep-lat
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The categories of this paper are: cs
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The categories of this paper are: plasm-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: funct-an
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The categories of this paper are: cond-mat
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The categories of this paper are: bayes-an
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The categories of this paper are: nucl-th