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|Title:||HARPS-N high spectral resolution observations of Cepheids I. The Baade-Wesselink projection factor of δ Cep revisited||Authors:||Nardetto, N.
Anderson, R. I.
|Issue Date:||2017||Journal:||ASTRONOMY & ASTROPHYSICS||Number:||597||First Page:||A73||Abstract:||Context. 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. <BR /> Aims: We analyze new HARPS-N spectra of δ Cep to measure its cycle-averaged atmospheric velocity gradient in order to better constrain the projection factor. <BR /> Methods: 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<SUB>0</SUB>); (2) the velocity gradient within the atmosphere (f<SUB>grad</SUB>); and (3) the relative motion of the optical pulsating photosphere with respect to the corresponding mass elements (f<SUB>o-g</SUB>). We then measure the f<SUB>grad</SUB> value of δ Cep for the first time. <BR /> Results: When the HARPS-N mean cross-correlated line-profiles are fitted with a Gaussian profile, the projection factor is p<SUB>cc-g</SUB> = 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 f<SUB>grad</SUB> 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 δ Cep and with our projection factor decomposition. Using the observational values of p and f<SUB>grad</SUB> found for the 17 spectral lines, we derive a semi-theoretical value of f<SUB>o-g</SUB>. We alternatively obtain f<SUB>o-g</SUB> = 0.975 ± 0.002 or 1.006 ± 0.002 assuming models using radiative transfer in plane-parallel or spherically symmetric geometries, respectively. Conclusions: The new HARPS-N observations of δ Cep are consistent with our decomposition of the projection factor. The next step will be to measure p<SUB>0</SUB> directly from the next generation of visible interferometers. With these values in hand, it will be possible to derive f<SUB>o-g</SUB> directly from observations. Table A.1 is also available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr (<A href="http://126.96.36.199">http://188.8.131.52</A>) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/597/A73">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/597/A73||URI:||http://hdl.handle.net/20.500.12386/26753||URL:||https://www.aanda.org/articles/aa/abs/2017/01/aa29400-16/aa29400-16.html||ISSN:||0004-6361||DOI:||10.1051/0004-6361/201629400||Bibcode ADS:||2017A&A...597A..73N||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
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