J/A+A/597/A73 delta Cep HARPS-N radial velocities (Nardetto+, 2017)
HARPS-N high spectral resolution observations of Cepheids.
I. The Baade-Wesselink projection factor of delta Cep revisited.
Nardetto N., Poretti E., Rainer M., Fokin A., Mathias P., Anderson R.I.,
Gallenne A., Gieren W., Graczyk D., Kervella P., Merand A., Mourard D.,
Neilson H., Pietrzinski G., Pilecki B., Storm J.
<Astron. Astrophys. 597, A73 (2017)>
=2017A&A...597A..73N 2017A&A...597A..73N (SIMBAD/NED BibCode)
ADC_Keywords: Stars, variable ; Spectroscopy ; Line Profiles ; Stars, distances
Keywords: stars: oscillations - techniques: spectroscopy -
stars: individual: delta Cep - stars: distances - stars: atmospheres -
stars: variables: Cepheids
Abstract:
The projection factor p is the key quantity used in the
Baade-Wesselink (BW) method for distance determination; it converts
radial velocities into pulsation velocities. Several methods are used
to determine p, such as geometrical and hydrodynamical models or the
inverse BW approach when the distance is known.
We analyze new HARPS-N spectra of delta Cep to measure its
cycle-averaged atmospheric velocity gradient in order to better
constrain the projection factor.
We first apply the inverse BW method to derive p directly from
observations. The projection factor can be divided into three
subconcepts: (1) a geometrical effect (p0), (2) the velocity
gradient within the atmosphere (fgrad), and (3) the relative motion
of the optical pulsating photosphere with respect to the corresponding
mass elements (fo-g). We then measure the fgrad value of delta Cep
for the first time.
When the HARPS-N mean cross-correlated line-profiles are fitted with a
Gaussian profile, the projection factor is
pcc-g=1.239±0.034(stat.)±0.023 (syst.). When we consider the
different amplitudes of the radial velocity curves that are associated
with 17 selected spectral lines, we measure projection factors ranging
from 1.273 to 1.329. We find a relation between fgrad and the line
depth measured when the Cepheid is at minimum radius. This relation is
consistent with that obtained from our best hydrodynamical model of
delta Cep and with our projection factor decomposition. Using the
observational values of p and fgrad found for the 17 spectral lines,
we derive a semi-theoretical value of fo-g. We alternatively obtain
fo-g=0.975±0.002 or 1.006±0.002 assuming models using radiative
transfer in plane-parallel or spherically symmetric geometries,
respectively.
The new HARPS-N observations of delta Cep are consistent with our
decomposition of the projection factor. The next step will be to
measure p0 directly from the next generation of visible
interferometers. With these values in hand, it will be possible to
derive fo-g directly from observations.
Description:
The columns give, respectively, the BJD, the pulsation phase, the hour
angle (HA), and the radial velocity, with its corresponding
uncertainty. In order to calculate the pulsation phase of each
spectrum, we used P=5.366208d and T0=2457105.930d (see the paper),
the time corresponding to the maximum approaching velocity determined
from the HARPS-N radial velocities. The radial velocity RVcc-g is
derived applying a Gaussian fit of the cross-correlated line-profile
(calculated using a F5I spectral type template).
Objects:
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RA (2000) DE Designation(s)
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22 29 10.27 +58 24 54.7 delta Cep = V* del Cep
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 36 103 HARPS-N Radial Velocity measurements of delta Cep
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See also:
J/A+A/584/A80 : Pulsation model data for delta Cep and eta Aql (Merand+, 2015)
J/A+A/593/A45 : delta Cep VEGA/CHARA observing log (Nardetto+, 2016)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 11 F11.3 d BJD Barycentric Julian Date
14- 18 F5.3 --- phi The pulsation phase
21- 28 F8.4 km/s RVcc-g Cross-correlated radial velocity
31- 36 F6.4 km/s e_RVcc-g Uncertainty on the cross-correlated radial
velocity
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Acknowledgements:
Nicolas Nardetto, Nicolas.Nardetto(at)oca.eu
(End) Nicolas Nardetto [Nice, France], Patricia Vannier [CDS] 08-Nov-2016