CW Leo: losing weight fast, and for good!
Credit: Image courtesy of N. Mauron
Groupe d'Astrophysique, Univ. of Montpellier, France
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.
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.
Technical description:
