Issue |
A&A
Volume 386, Number 2, May I 2002
|
|
---|---|---|
Page(s) | 633 - 645 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20020220 | |
Published online | 15 May 2002 |
The structure and dynamics of the molecular envelope of M 2-56 *
1
Observatorio Astronómico Nacional (IGN), Apdo. 1143, 28800 Alcalá de Henares, Spain e-mail: carrizo,bujarrabal,j.alcolea@oan.es
2
Jet Propulsion Laboratory, MS 183-900, California Institute of Technology, Pasadena, CA 91109, USA e-mail: sanchez@eclipse.jpl.nasa.gov
3
IRAM, 300 rue de la Piscine, 38406 St Martin d'Hères, France e-mail: neri@iram.fr
Corresponding author: A. Castro-Carrizo, carrizo@oan.es
Received:
31
October
2001
Accepted:
7
February
2002
M 2-56 is a protoplanetary nebula (PPN) in which strong shocks
are taking place, therefore, useful to study the post-AGB wind interaction.
It is well known that molecular observations allow studying the mass
distribution of PPNe, even in those regions that have been recently
shocked.
We present high-resolution maps of the emission of
12CO J = 2-1 and J = 1-0 in M 2-56.
Such maps show a bipolar, molecular nebula that extends
~28 ´´ along the symmetry axis.
The nebula is composed of two contiguous, incomplete shells located
along the symmetry axis, which has an inclination of
~17o with respect to the plane of the sky.
Those empty lobes intersect in the center of the nebula, where
there is a small and dense ring perpendicular to the axis.
This central ring expands radially at about 8 and seems to be
the remnant of the circumstellar envelope of the AGB star, that has
not been accelerated by the interaction with the fast post-AGB jets.
The radius of the central ring is of ~ cm, for a
distance of 2.1 kpc (deduced from an analysis of the main properties
of the object).
At ~
cm from the nebular center, the tips of the
lobes reach axial expansion velocities of ~200 .
We have developed a model for the spatio-kinematical distribution and the
excitation conditions of the molecular gas in M 2-56.
From the best fitting of the observations with the predictions of the model
for both lines, we have estimated the physical conditions of the molecular
nebula.
It is found that the density varies from
to
cm-3 from the nebular center to the lobe tips,
and that the part of the lobes that has not been detected is probably
composed of photodissociated gas, due to the effect of interstellar
photons on low-density regions.
The rotational temperature is estimated to be approximately constant,
~13-16 K.
For the assumed geometry, a velocity field composed by a dominant radial
component plus an axial contribution has been deduced.
The emission of both lines is found to be optically thin, and therefore
probes the whole molecular gas, which has a mass of ~0.05 .
The “scalar” momentum and the kinetic energy of the different regions
of the molecular nebula have been calculated, finding that
the high momentum won by the gas in the post-AGB phase cannot have
been supplied by the radiation pressure mechanism.
Although the central star of M 2-56 is not very hot yet
(~20 000 K), this PPN has a large kinematical age, between
1000 yr and 1700 yr, in comparison with other PPNe that have
hotter central stars.
M 2-56 may not be a typical PPN, but an intermediate object
between the known low-mass post-AGB nebulae and the standard
PPNe.
Key words: stars: individual: M 2-56 / circumstellar matter / stars: AGB and post-AGB / radio lines: stars / stars: mass-loss
© ESO, 2002
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