The Cepheid Star RS Pup
Light Echoes in RS Pup
RS Pup is the only Cepheid to be embedded in a large nebula , which is made of very fine dust that reflects some of the light emitted by the star.
Because the luminosity of the star changes in a very distinctive pattern, the presence of the nebula allows the astronomers to see light echoes and use them to measure the distance of the star.
"The light that travelled from the star to a dust grain and then to the telescope arrives a bit later than the light that comes directly from the star to the telescope," explains Kervella. "As a consequence, if we measure the brightness of a particular, isolated dust blob in the nebula, we will obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time."
This delay is called a 'light echo', by analogy with the more traditional echo, the reflection of sound by, for example, the bottom of a well.
By monitoring the evolution of the brightness of the blobs in the nebula, the astronomers can derive their distance from the star: it is simply the measured delay in time, multiplied by the velocity of light (300 000 km/s). Knowing this distance and the apparent separation on the sky between the star and the blob, one can compute the distance of RS Pup.
From the observations of the echoes on several nebular features, the distance of RS Pup was found to be 6500 light years, plus or minus 90 light years.
"Knowing the distance to a Cepheid star with such an accuracy proves crucial to the calibration of the period-luminosity relation of this class of stars," says Kervella. "This relation is indeed at the basis of the distance determination of galaxies using Cepheids."
RS Pup is thus distant by about a quarter of the distance between the Sun and the Centre of the Milky Way. RS Pup is located within the Galactic plane, in a very populated region of our Galaxy.
"The long-period Galactic Cepheid RS Puppis - I. A geometric distance from its light echoes", P. Kervella et al. is in press in Astronomy and Astrophysics.
The team is composed of Pierre Kervella and Guy Perrin (LESIA, Observatoire de Paris, France), Antoine Mérand (Center for High Angular Resolution Astronomy, Atlanta, Georgia, USA), László Szabados (Konkoly Observatory, Budapest, Hungary), Pascal Fouqué (Observatoire Midi-Pyrénées, Toulouse, France), David Bersier (Liverpool John Moores University, UK), and Emanuela Pompei (ESO).
Observatoire de Paris-Meudon, France
E-mail: Pierre.Kervella (at) obspm.fr
Phone: +33 1 45 07 79 66