J/A+A/597/A73delta 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 (SIMBAD/NED BibCode)ADC_Keywords: Stars, variable ; Spectroscopy ; Line Profiles ; Stars, distancesKeywords: stars: oscillations - techniques: spectroscopy - stars: individual: delta Cep - stars: distances - stars: atmospheres - stars: variables: CepheidsAbstract: 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 (p_{0}), (2) the velocity gradient within the atmosphere (f_{grad}), and (3) the relative motion of the optical pulsating photosphere with respect to the corresponding mass elements (f_{o}-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 p_{cc-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 f_{grad}found for the 17 spectral lines, we derive a semi-theoretical value of f_{o-g}. We alternatively obtain f_{o-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 T_{0}=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: -------------------------------------------------- RA (2000) DE Designation(s) -------------------------------------------------- 22 29 10.27 +58 24 54.7 delta Cep = V* del Cep --------------------------------------------------File Summary:

FileName Lrecl Records Explanations

ReadMe 80 . This file table1.dat 36 103 HARPS-N Radial Velocity measurements of delta Cep

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

Bytes Format Units Label Explanations

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

Acknowledgements: Nicolas Nardetto, Nicolas.Nardetto(at)oca.eu(End)Nicolas Nardetto [Nice, France], Patricia Vannier [CDS] 08-Nov-2016

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