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| PRI (MPIfR) 09/2008 (3) | Press Release | September 19, 2008 |
Since their discovery in 1912 by Henrietta Leavitt, Cepheids have been used as distance indicators. But, combined with velocity measurements, they are also extremely precious to probe the kinematical structure of the Milky Way. A group of astrophysicist led by Nicolas Nardetto from Max Planck Institute for Radio Astronomy has used high-precision spectroscopy with ESO's HARPS instrument to show that the kinematical structure of our Milky Way, derived from Cepheid properties, is probably more simple than previously expected.
Their results have been published in a recent paper in
"Astronomy & Astrophysics".
Figure 1:
Artist's impression of the environment of the sun within our galaxy, the Milky Way. The figure shows the local spiral arm, called Orion arm and its two neighbours, the Perseus and Sagittarius spiral arms. The positions of a number of prominent stars in the solar neighbourhood are indicated(white dots).
The blue dots mark the positions of the eight Cepheids used in the investigation.
Image: ESO PR 30-08, September 19, 2008 (Click image for higher resolution).
Since Shapley's discovery of the centre of our Galaxy, and our position within it, and later the Hubble's discovery of the expansion of the universe, the motion of Galactic Cepheids is misleading and sows discord among researchers. Indeed, Galactic Cepheids seem to fall on the sun with a systematic velocity of about 2 km/s.
A multi-decade debate was then to determine if this phenomenon was truly related to the space motion of the stars and, consequently, to a complicated kinematical structure of our Galaxy, or if it was the result of the dynamical structure of Cepheids' atmosphere.
Figure 2:
HARPS spectrograph at ESO during laboratory tests. The vacuum tank is open so that some of the high-precision components inside can be seen. The large optical grating by which the incoming stellar light is dispersed is visible on the top of the bench; it measures 200 x 800 mm.
Image: ESO (PR photo 08a/03, March 27, 2003).
(Click image for higher resolution).
Indeed, they found that residual line-of-sight velocity of Cepheids are significantly linked to the chemical species considered, and more specifically, to the line-forming region within the pulsating atmosphere. For the first time, it was possible to provide "physically calibrated" space motions of Cepheids. This result, generalized to all Cepheids would mean that the rotation of the Milky Way is more simple than expected, certainly axi-symmetric.
"Our new findings solve a puzzle which has existed in astronomy for decades",
says Nicolas Nardetto.
"High resolution spectroscopy for Cepheids distance determination.
III. A relation between gamma-velocities and gamma-asymmetries",
N. Nardetto, A. Stoekl, D. Bersier, T. G. Barnes, Astronomy & Astrophysics
(DOI 10.1051/0004-6361:200809587).
Pinning down the Milky Way's spin,
ESO Press Release, September 19, 2008.
Max Planck Institute for Radio Astronomy (MPIfR).
Infrared Interferometry Group at MPIfR.
European Southern Observatory (ESO).
High Accuracy Radial velocity Planet Searcher (HARPS) at ESO/La Silla.
Cepheid Variable (wikipedia).
Dr. Nicolas Nardetto,
Departamento de Astronomia, Universidad de Concepcion, Casilla, Chile.
Fon: +56-41-2203366
E-mail: nnardetto (at)
astro-udec.cl
Dr. Thomas Driebe,
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49-228-525-353
Fax: +49-228-525-229
E-mail: tdriebe (at)
mpifr.de
Dr. Norbert Junkes,
Public Outreach,
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49-228-525-399
Fax: +49-228-525-438
E-mail: njunkes (at)
mpifr.de