Andrew Yee
February 28th 06, 02:55 PM
ESO Education and Public Relations Dept.
--------------------------------------------------------------
Text with all links and the photos are available on the ESO
Website at URL:
http://www.eso.org/outreach/press-rel/pr-2006/pr-09-06.html
--------------------------------------------------------------
Contacts
Pierre Kervella
Observatoire de Paris, France
Phone: +33 1 45 07 79 66
Antoine Mérand
Observatoire de Paris & CHARA Array, Mt Wilson Observatory
Phone: +1 626 796 4130
For Immediate Release: 28 February 2006
ESO Science Release 09/06
Cepheids and their 'Cocoons'
Interferometry Helps Discover Envelopes Around Supergiant Stars
Using ESO's Very Large Telescope Interferometer (VLTI) at Cerro
Paranal, Chile, and the CHARA Interferometer at Mount Wilson,
California, a team of French and North American astronomers has
discovered envelopes around three Cepheids, including the Pole
star. This is the first time that matter is found surrounding
members of this important class of rare and very luminous stars
whose luminosity varies in a very regular way. Cepheids play a
crucial role in cosmology, being one of the first "steps" on
the cosmic distance ladder.
The southern Cepheid L Carinae was observed with the VINCI and
MIDI instrument at the VLTI, while Polaris (the Pole Star) and
Delta Cephei (the prototype of its class) were scrutinised with
FLUOR on CHARA, located on the other side of the equator. FLUOR
is the prototype instrument of VINCI. Both were built by the
Paris Observatory (France).
ESO PR Photo 06/06
Model Image of Cepheid L Carinae
Caption: Two model images of the Cepheid star L Carinae as
deduced from the interferometric observations: in the near-
infrared from VINCI measurements (left) and in the mid-
infrared from MIDI (right). In both cases, an envelope is
found to surround the star. The contribution from the
envelope is about 5% in the near-infrared and significantly
more in the thermal infrared. As L Carinae is 17 000 times
brighter than the Sun, this means that its envelope alone
is several hundred times brighter than our Sun!
For most stars, the observations made with the interferometers
follow very tightly the theoretical stellar models. However,
for these three stars, a tiny deviation was detected, revealing
the presence of an envelope.
"The fact that such deviations were found for all three stars,
which however have very different properties, seems to imply
that envelopes surrounding Cepheids are a widespread
phenomenon", said Pierre Kervella, one of the lead authors.
The envelopes were found to be 2 to 3 times as large as the
star itself. Although such stars are rather large -- about
fifty to several hundreds of solar radii -- they are so far
away that they can't be resolved by single telescopes. Indeed,
even the largest Cepheids in the sky subtend an angle of only
0.003 arc second. To observe this is similar to viewing a
two-storey house on the Moon.
Astronomers have thus to rely on the interferometric technique,
which combines the light of two or more distant telescopes,
thereby providing the angular resolution of a unique telescope
as large as the separation between them. With the VLTI, it is
possible to achieve a resolution of 0.001 arc second or less.
"The physical processes that have created these envelopes are
still uncertain, but, in analogy to what happens around other
classes of stars, it is most probable that the environments
were created by matter ejected by the star itself", said
Antoine Mérand, lead-author of the second paper describing
the results.
Cepheids pulsate with periods of a few days. As a consequence,
they go regularly through large amplitude oscillations that
create very rapid motions of its apparent surface (the
photosphere) with velocities up to 30 km/s, or 108 000 km/h!
While this remains to be established, there could be a link
between the pulsation, the mass loss and the formation of
the envelopes.
Notes
Cepheids are commonly used as distance indicators, thanks to
the existence of a basic relation between their intrinsic
brightness and their pulsation period. By measuring the period
of a Cepheid star, its intrinsic brightness can be deduced and
from the observed apparent brightness, the distance may then
be calculated. As they are intrinsically very bright stars,
and can be observed in distant galaxies, this remarkable
property has turned these yellow supergiant stars into primary
'standard candles' for extragalactic distance estimations (see
ESO PR 25/04).
L Carinae is the brightest Cepheid in the sky, and also the one
that presents the largest apparent angular diameter. This is a
massive supergiant star, having about 10 times the mass of the
Sun and a radius approximately 180 times that of the Sun.
Polaris is a peculiar star as it is located very close to the
North celestial pole (hence its name). It is classified as a
Cepheid, but it shows very weak pulsations compared to the
other stars of its class. Delta Cephei is the prototype of the
Cepheids. It was discovered to be a variable star in the 18th
century by the English amateur John Goodricke, and it is still
one of the brightest members of the Cepheid class. Its short
period is characteristic of a relatively small supergiant,
with a radius of "only" 43 times that of the Sun.
These results are published in two articles to appear in
Astronomy and Astrophysics: "Extended envelopes around Galactic
Cepheids I. L Car from near and mid-infrared interferometry
with the VLTI" by P. Kervella et al., and "Extended envelopes
around Cepheids II. Near infrared interferometric observations
of Polaris and delta Cep using CHARA/FLUOR" by Antoine Mérand
et al. The first paper is available as a PDF file from the
editor's web site.
The team is composed of Pierre Kervella, Antoine Mérand, Vincent
Coudé du Foresto , Guy Perrin (LESIA, Paris Observatory, France),
Stephen T. Ridgway (NOAO, Tucson, US and CHARA, Georgia, US),
Jason P. Aufdenberg (NOAO, Tucson, US), Theo A. ten Brummelaar,
Harold A. McAlister, Laszlo Sturmann, Judit Sturmann, Nils H.
Turner and David H. Berger (CHARA, Georgia, US).
The Center for High Angular Resolution Astronomy (CHARA) Array
interferometer is operated by the Georgia State University,
United States.
ESO Media Contacts are on the Public Affairs Dept. Contact page.
National contacts for the media:
Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Finland: Ms. Riitta Tirronen, +358 9 7748 8369
Denmark: Dr. Michael Linden-Vřrnle, +45-33-18 19 97
France: Dr. Daniel Kunth, +33-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Dr. Leopoldo Benacchio, benacchio @ inaf.it
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25
--------------------------------------------------------------
ESO Press Information is available on the WWW at
http://www.eso.org/outreach/press-rel/
--------------------------------------------------------------
(c) ESO Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
--------------------------------------------------------------
--------------------------------------------------------------
Text with all links and the photos are available on the ESO
Website at URL:
http://www.eso.org/outreach/press-rel/pr-2006/pr-09-06.html
--------------------------------------------------------------
Contacts
Pierre Kervella
Observatoire de Paris, France
Phone: +33 1 45 07 79 66
Antoine Mérand
Observatoire de Paris & CHARA Array, Mt Wilson Observatory
Phone: +1 626 796 4130
For Immediate Release: 28 February 2006
ESO Science Release 09/06
Cepheids and their 'Cocoons'
Interferometry Helps Discover Envelopes Around Supergiant Stars
Using ESO's Very Large Telescope Interferometer (VLTI) at Cerro
Paranal, Chile, and the CHARA Interferometer at Mount Wilson,
California, a team of French and North American astronomers has
discovered envelopes around three Cepheids, including the Pole
star. This is the first time that matter is found surrounding
members of this important class of rare and very luminous stars
whose luminosity varies in a very regular way. Cepheids play a
crucial role in cosmology, being one of the first "steps" on
the cosmic distance ladder.
The southern Cepheid L Carinae was observed with the VINCI and
MIDI instrument at the VLTI, while Polaris (the Pole Star) and
Delta Cephei (the prototype of its class) were scrutinised with
FLUOR on CHARA, located on the other side of the equator. FLUOR
is the prototype instrument of VINCI. Both were built by the
Paris Observatory (France).
ESO PR Photo 06/06
Model Image of Cepheid L Carinae
Caption: Two model images of the Cepheid star L Carinae as
deduced from the interferometric observations: in the near-
infrared from VINCI measurements (left) and in the mid-
infrared from MIDI (right). In both cases, an envelope is
found to surround the star. The contribution from the
envelope is about 5% in the near-infrared and significantly
more in the thermal infrared. As L Carinae is 17 000 times
brighter than the Sun, this means that its envelope alone
is several hundred times brighter than our Sun!
For most stars, the observations made with the interferometers
follow very tightly the theoretical stellar models. However,
for these three stars, a tiny deviation was detected, revealing
the presence of an envelope.
"The fact that such deviations were found for all three stars,
which however have very different properties, seems to imply
that envelopes surrounding Cepheids are a widespread
phenomenon", said Pierre Kervella, one of the lead authors.
The envelopes were found to be 2 to 3 times as large as the
star itself. Although such stars are rather large -- about
fifty to several hundreds of solar radii -- they are so far
away that they can't be resolved by single telescopes. Indeed,
even the largest Cepheids in the sky subtend an angle of only
0.003 arc second. To observe this is similar to viewing a
two-storey house on the Moon.
Astronomers have thus to rely on the interferometric technique,
which combines the light of two or more distant telescopes,
thereby providing the angular resolution of a unique telescope
as large as the separation between them. With the VLTI, it is
possible to achieve a resolution of 0.001 arc second or less.
"The physical processes that have created these envelopes are
still uncertain, but, in analogy to what happens around other
classes of stars, it is most probable that the environments
were created by matter ejected by the star itself", said
Antoine Mérand, lead-author of the second paper describing
the results.
Cepheids pulsate with periods of a few days. As a consequence,
they go regularly through large amplitude oscillations that
create very rapid motions of its apparent surface (the
photosphere) with velocities up to 30 km/s, or 108 000 km/h!
While this remains to be established, there could be a link
between the pulsation, the mass loss and the formation of
the envelopes.
Notes
Cepheids are commonly used as distance indicators, thanks to
the existence of a basic relation between their intrinsic
brightness and their pulsation period. By measuring the period
of a Cepheid star, its intrinsic brightness can be deduced and
from the observed apparent brightness, the distance may then
be calculated. As they are intrinsically very bright stars,
and can be observed in distant galaxies, this remarkable
property has turned these yellow supergiant stars into primary
'standard candles' for extragalactic distance estimations (see
ESO PR 25/04).
L Carinae is the brightest Cepheid in the sky, and also the one
that presents the largest apparent angular diameter. This is a
massive supergiant star, having about 10 times the mass of the
Sun and a radius approximately 180 times that of the Sun.
Polaris is a peculiar star as it is located very close to the
North celestial pole (hence its name). It is classified as a
Cepheid, but it shows very weak pulsations compared to the
other stars of its class. Delta Cephei is the prototype of the
Cepheids. It was discovered to be a variable star in the 18th
century by the English amateur John Goodricke, and it is still
one of the brightest members of the Cepheid class. Its short
period is characteristic of a relatively small supergiant,
with a radius of "only" 43 times that of the Sun.
These results are published in two articles to appear in
Astronomy and Astrophysics: "Extended envelopes around Galactic
Cepheids I. L Car from near and mid-infrared interferometry
with the VLTI" by P. Kervella et al., and "Extended envelopes
around Cepheids II. Near infrared interferometric observations
of Polaris and delta Cep using CHARA/FLUOR" by Antoine Mérand
et al. The first paper is available as a PDF file from the
editor's web site.
The team is composed of Pierre Kervella, Antoine Mérand, Vincent
Coudé du Foresto , Guy Perrin (LESIA, Paris Observatory, France),
Stephen T. Ridgway (NOAO, Tucson, US and CHARA, Georgia, US),
Jason P. Aufdenberg (NOAO, Tucson, US), Theo A. ten Brummelaar,
Harold A. McAlister, Laszlo Sturmann, Judit Sturmann, Nils H.
Turner and David H. Berger (CHARA, Georgia, US).
The Center for High Angular Resolution Astronomy (CHARA) Array
interferometer is operated by the Georgia State University,
United States.
ESO Media Contacts are on the Public Affairs Dept. Contact page.
National contacts for the media:
Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Finland: Ms. Riitta Tirronen, +358 9 7748 8369
Denmark: Dr. Michael Linden-Vřrnle, +45-33-18 19 97
France: Dr. Daniel Kunth, +33-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Dr. Leopoldo Benacchio, benacchio @ inaf.it
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25
--------------------------------------------------------------
ESO Press Information is available on the WWW at
http://www.eso.org/outreach/press-rel/
--------------------------------------------------------------
(c) ESO Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
--------------------------------------------------------------