Published by
EDP Sciences
EDP Sciences Journals List
Subscriber Authentication Point
Home All issues Volume 489/ 3 Article
Press Release
Free access
Issue A&A
Volume 489, Number 3, October III 2008
Page(s) 1255 - 1262
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:200809587
Published online 16 April 2008



A&A 489, 1255-1262 (2008)
DOI: 10.1051/0004-6361:200809587

High-resolution spectroscopy for Cepheids distance determination

III. A relation between $\gamma$-velocities and $\gamma$-asymmetries
N. Nardetto1, A. Stoekl2, D. Bersier3, and T. G. Barnes4, 5

1  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
    e-mail: nardetto@mpifr-bonn.mpg.de
2  CRAL, Université de Lyon, CNRS (UMR5574), École Normale Supérieure de Lyon, 69007 Lyon, France
3  Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
4  University of Texas at Austin, McDonald Observatory, 1 University Station, C1402, Austin, TX 78712-0259, USA
5  currently on assignment to the National Science Foundation, 4201 Wilson Boulevard, Arlington, VA 22230, USA

Received 15 February 2008 / Accepted 1 April 2008

Abstract
Context. Galactic Cepheids in the vicinity of the Sun have a residual line-of-sight velocity, or $\gamma$-velocity, which shows a systematic blueshift of about 2 km s-1 compared to an axisymmetric rotation model of the Milky Way. This term is either related to the space motion of the star and, consequently, to the kinematic structure of our Galaxy, or it is the result of the dynamical structure of the Cepheids' atmosphere.
Aims. We aim to show that these residual $\gamma$-velocities are an intrinsic property of Cepheids.
Methods. We observed eight galactic Cepheids with the HARPS (High Accuracy Radial velocity Planetary Search project developed by the European Southern Observatory.) spectroscope, focusing specifically on 17 spectral lines. For each spectral line of each star, we computed the $\gamma$-velocity (resp. $\gamma$-asymmetry) as an average value of the interpolated radial velocity (resp. line asymmetry) curve.
Results. For each Cepheid in our sample, a linear relation is found between the $\gamma$-velocities of the various spectral lines and their corresponding $\gamma$-asymmetries, showing that residual $\gamma$-velocities stem from the intrinsic properties of Cepheids. We also provide a physical reference to the stellar $\gamma$-velocity: it should be zero when the $\gamma$-asymmetry is zero. Following this definition, we provide very precise and physically calibrated estimates of the $\gamma$-velocities for all stars of our sample [ in km s-1] : -11.3 $\pm$ 0.3 [R TrA], -3.5 $\pm$ 0.4 [S Cru], -1.5 $\pm$ 0.2 [Y Sgr], 9.8 $\pm$ 0.1 [ $\beta$ Dor] , 7.1 $\pm$ 0.1 [ $\zeta$ Gem] , 24.6 $\pm$ 0.4 [RZ Vel], 4.4 $\pm$ 0.1 [ $\ell$ Car] , 25.7 $\pm$ 0.2 [RS Pup]. Finally, we investigated several physical explanations for these $\gamma$-asymmetries like velocity gradients or the relative motion of the line-forming region compared to the corresponding mass elements. However, none of these hypotheses seems to be entirely satisfactory to explain the observations.
Conclusions. To understand this very subtle $\gamma$-asymmetry effect, further numerical studies are needed. Cepheids' atmosphere are strongly affected by pulsational dynamics, convective flows, nonlinear physics, and complex radiative transport. Hence, all of these effects have to be incorporated simultaneously and consistently into the numerical models to reproduce the observed line profiles in detail.


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



© ESO 2008

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.