Light echoes whisper the distance to a star (Forwarded)

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Science Contact:

Pierre Kervella
Observatoire de Paris-Meudon, France
Phone: +33 1 45 07 79 66

Embargoed till 11 February 2008, 00:01 GMT

ESO Science Release 05/08

Light echoes whisper the distance to a star

Astronomers calibrate the distance scale of the Universe

Taking advantage of the presence of light echoes, a team of astronomers
have used an ESO telescope to measure, at the 1% precision level, the
distance of a Cepheid -- a class of variable stars that constitutes one of
the first steps in the cosmic distance ladder.

"Our measurements with ESO's New Technology Telescope at La Silla allow us
to obtain the most accurate distance to a Cepheid," says Pierre Kervella,
lead-author of the paper reporting the result.

Cepheids [1] are pulsating stars that have been used as distance
indicators since almost a hundred years. The new accurate measurement is
important as, contrary to many others, it is purely geometrical and does
not rely on hypotheses about the physics at play in the stars themselves.

The team of astronomers studied RS Pup, a bright Cepheid star located
towards the constellation of Puppis ('the Stern') and easily visible with
binoculars. RS Pup varies in brightness by almost a factor of five every
41.4 days. It is 10 times more massive than the Sun, 200 times larger, and
on average 15 000 times more luminous.

RS Pup is the only Cepheid to be embedded in a large nebula [2], 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.

More Information

"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).

Notes

[1]: Cepheids are rare and very luminous pulsating stars whose luminosity
varies in a very regular way. They are named after the star Delta Cephei
in the constellation of Cepheus, the first known variable star of this
particular type and bright enough to be easily seen with the unaided eye.
Almost a century ago, in 1912, American astronomer Henrietta Leavitt
published a relation between the intrinsic brightness and the pulsation
period of Cepheids, the longer periods corresponding to the brighter
stars. This relation still plays today a central role in the extragalactic
distance scale.

[2]: The nebula around RS Pup was discovered in 1961 by Swedish astronomer
Bengt Westerlund, who later became ESO Director in Chile (1970-74).
Shortly after, in 1972, the American astronomer Robert Havlen, then
visiting ESO Chile, published the first study of the nebula in the then
rather young European journal Astronomy & Astrophysics.

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