CFHT, Current Image of the Week


October 2nd, 2000

CW Leo: losing weight fast, and for good!

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CW Leo: losing weight fast, and for good!

Credit: Image courtesy of N. Mauron
Groupe d'Astrophysique, Univ. of Montpellier, France

(for more details see: Mauron & Huggins, 1999, A&A, 349, 203-208, and
Mauron & Huggins, 2000, A&A, 359, 707-715.)

This week's image dramatically captures CW Leo, better known as IRC+10 216, as it massively sheds its outer stellar envelope. The fine details of this deep exposure reveals the history of the mass-loss episodes that occured during the last 8000 years in the star.

CW Leo is now in a late phase of its evolution, a phase known as the "Asymptotic Giant Branch". To make a long story short, a star like the Sun will typically swell and become large enough to encompass Earth's orbit as it reaches the final stages of its life. Its outer layers will become tenuous and cold and will likely be driven off by various mechanisms, among which radiation pressure is probably important.

At the end of this phase, a large fraction of the stellar mass will have been expelled, forming an envelope or a shell made of dust and gas that propagates outward. This is what can be seen in this week's V-band image.

The authors captured this dramatic image of the mass-loss from CW Leo in February 1998. This star is one of the rare cases where the mass-loss phenomenon can be studied in such details. Two views in different filters are presented here in V-band (left panel) and B-band (right panel). Interestingly, the massive dust envelope is detectable because it reflects (or scatters) the light from the Galaxy! The envelope is so opaque that the light from the central star does not reach the outside easily in the optical. It is absorbed and re-emitted at longer wavelengths that our eyes cannot see.

The structure of the envelope contains the history of the poorly understood mass-loss episodes. CW Leo's envelope is roughly spherical, but appears made of several concentric shells, each shell being incomplete. The envelope is detected more than 200arcsec away from the center, tracing the mass loss activity over the last 8000 years, if the actual expansion rate (14km/sec) is constant! Because CW Leo is representative of carbon-rich AGB stars, these results strongly suggest that mass-loss in AGB stars is not smooth and homogeneous.

Technical description:

The observations were carried out on February 17-18 with the MOS focal reducer in the B- and V-bands. The circumstellar envelope was exposed for 80 minutes in V (4 X 20min) and 40 minutes in B (2 X 20min). The image is deep and the surface brightness of the halo around CW Leo is 25.1mag arcsec-2 in V! This is only 4% of the sky surface brightness!

The complete field of view in the image presented here is 131" x 131" with IRC+10 216 located at the center. North is up and East to the left. Because of the depth of the image, many background galaxies are also detected and visible in this larger size image, especially to the East (left) of CW Leo.

next week: The double barred galaxy NGC 3504

editors: François Ménard & Jean-Charles Cuillandre
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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.