Published by
EDP Sciences
EDP Sciences Journals List
Subscriber Authentication Point
Home All issues Volume 502/ 3 Article
Free access
Issue A&A
Volume 502, Number 3, August II 2009
Page(s) 951 - 956
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361/200912333
Published online 15 June 2009

A&A 502, 951-956 (2009)
DOI: 10.1051/0004-6361/200912333

High-resolution spectroscopy for Cepheids distance determination

V. Impact of the cross-correlation method on the p-factor and the $\gamma$-velocities  
N. Nardetto1, W. Gieren1, P. Kervella2, P. Fouqué3, J. Storm4, G. Pietrzynski1, 5, D. Mourard6, and D. Queloz7

1  Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
    e-mail: nnardetto@astro-udec.cl
2  Observatoire de Paris-Meudon, LESIA, UMR 8109, 5 Place Jules Janssen, 92195 Meudon Cedex, France
3  Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique, UMR 5572, Université Paul Sabatier, Toulouse 3, 14 avenue Edouart Belin, 31400 Toulouse, France
4  Astrophysikalisches Institut Postdam, An der Sternwarte 16, 14482 Postdam, Germany
5  Warsaw University Observatory, AL. Ujazdowskie 4, 00-478 Warsaw, Poland
6  OCA/CNRS/UNS, Dpt. Fizeau, UMR6525, Avenue Copernic, 06130 Grasse, France
7  Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland

Received 15 April 2009 / Accepted 14 May 2009

Abstract
Context. The cross correlation method (hereafter CC) is widely used to derive the radial velocity curve of Cepheids when the signal to noise ratio of the spectra is low. However, if it is used with an inaccurate projection factor, it might introduce some biases in the Baade-Wesselink (BW) methods of determining the distance of Cepheids. In addition, it might affect the average value of the radial velocity curve (or $\gamma$-velocity) important for Galactic structure studies.
Aims. We aim to derive a period-projection factor relation (hereafter Pp) appropriate to be used together with the CC method. Moreover, we investigate whether the CC method can explain the previous estimates of the “K-term” of Cepheids.
Methods. We observed eight galactic Cepheids with the HARPS spectrograph. For each star, we derive an interpolated CC radial velocity curve using the HARPS pipeline. The amplitudes of these curves are used to determine the correction to be applied to the semi-theoretical projection factor. Their average value (or $\gamma$-velocity) are also compared to the center-of-mass velocities derived in previous works.
Results. The correction in amplitudes allows us to derive a new Pp relation: p = [ -0.08 $\pm$ 0.05] log P + [ 1.31 $\pm$ 0.06] . We also find a negligible wavelength dependence (over the optical range) of the Pp relation. We finally show that the $\gamma$-velocity derived from the CC method is systematically blue-shifted by about 1.0 $\pm$ 0.2 km s-1 compared to the center-of-mass velocity of the star. An additional blue-shift of 1.0 km s-1 is thus needed to totally explain the previous calculation of the “K-term” of Cepheids (around 2 km s-1).
Conclusions. The new Pp relation we derived is a reliable tool for distance scale calibration, and especially to derive the distance of LMC Cepheids with the infrared surface brightness technique. Further studies should be devoted to determining the impact of the signal to noise ratio, the spectral resolution, and the metallicity on the Pp relation.


Key words: techniques: spectroscopic -- stars: atmospheres -- stars: oscillations -- stars: variables: Cepheids -- stars: distances



© ESO 2009

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.
  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.