4,091 research outputs found
The Formation of Globules in Planetary Nebulae
We discuss the formation of globules in planetary nebulae, typified by those
observed in the Helix Nebula. We show that the properties of the globules,
their number, mass, separation, and overall geometry strongly support a
scenario in which globules are formed by the fragmentation of a swept-up shell
as opposed to models in which the knots form in the AGB wind. We show that the
RT or other instabilities which lead to the break-up of shells formed in the
nebulae by fast winds or ionization fronts can produce arrays of globules with
the overall geometry and within the mass range observed. We also show that the
presence of a magnetic field in the circumstellar gas may play an important
role in controlling the fragmentation process. Using field strengths measured
in the precursor AGB envelopes, we find that close to the central star where
the fields are relatively strong, the wavelengths of unstable MRT modes are
larger than the shell dimensions, and the fragmentation of the shell is
suppressed. The wavelength of the most unstable MRT mode decreases with
increasing distance from the star, and when it becomes comparable to the shell
thickness, it can lead to the sudden, rapid break-up of an accelerating shell.
For typical nebula parameters, the model results in numerous fragments with a
mass scale and a separation scale similar to those observed. Our results
provide a link between global models of PN shaping in which shells form via
winds and ionization fronts, and the formation of small scale structures in the
nebulae.Comment: 4 pages, 2 figures, to appear in IAU Symp. 234, Planetary Nebulae in
Our Galaxy and Beyond, eds. M. J. Barlow, R. H. Mende
Experimental investigations of the effects of cutting angle on chattering of a flexible manipulator
When a machine tool is mounted at the tip of a robotic manipulator, the manipulator becomes more flexible (the natural frequencies are lowered). Moreover, for a given flexible manipulator, its compliance will be different depending on feedback gains, configurations, and direction of interest. Here, the compliance of a manipulator is derived analytically, and its magnitude is represented as a compliance ellipsoid. Then, using a two-link flexible manipulator with an abrasive cut off saw, the experimental investigation shows that the chattering varies with the saw cutting angle due to different compliance. The main work is devoted to finding a desirable cutting angle which reduces the chattering
Circumstellar chemistry
The study of the outer envelopes of cool evolved stars has become an active area of research. The physical properties of CS envelopes are presented. Observations of many wavelengths bands are relevant. A summary of observations and a discussion of theoretical considerations concerning the chemistry are summarized. Recent theoretical considerations show that the thermal equilibrium model is of limited use for understanding the chemistry of the outer CS envelopes. The theoretical modeling of the chemistry of CS envelopes provides a quantitive test of chemical concepts which have a broader interest than the envelopes themselves
Deep optical imaging of AGB circumstellar envelopes
We report results of a program to image the extended circumstellar envelopes
of asymptotic giant branch (AGB) stars in dust-scattered Galactic light. The
goal is to characterize the shapes of the envelopes to probe the mass-loss
geometry and the presence of hidden binary companions. The observations consist
of deep optical imaging of 22 AGB stars with high mass loss rates: 16 with the
ESO 3.5 m NTT telescope, and the remainder with other telescopes. The
circumstellar envelopes are detected in 15 objects, with mass loss rates > 2E-6
Msun/year. The surface brightness of the envelopes shows a strong decrease with
Galactic radius, which indicates a steep radial gradient in the interstellar
radiation field. The envelopes range from circular to elliptical in shape, and
we characterize them by the ellipticity (E = major/minor axis) of iso-intensity
contours. We find that about 50 percent of the envelopes are close to circular
with E
1.2. We interpret the shapes in terms of populations of single stars and
binaries whose envelopes are flattened by a companion. The distribution of E is
qualitatively consistent with expectations based on population synthesis models
of binary AGB stars. We also find that about 50 percent of the sample exhibit
small-scale, elongated features in the central regions. We interpret these as
the escape of light from the central star through polar holes, which are also
likely produced by companions. Our observations of envelope flattening and
polar holes point to a hidden population of companions within the circumstellar
envelopes of AGB stars. These companions are expected to play an important role
in the transition to post-AGB stars and the formation of planetary nebulae.Comment: 19 pages, 13 figures, color pictures in Appendix, accepted by A&
Neutral atomic carbon in the globules of the Helix
We report detection of the 609u line of neutral atomic carbon in globules of
the Helix nebula. The measurements were made towards the position of peak CO
emission. At the same position, we obtained high-quality CO(2-1) and 13CO(2-1)
spectra and a 135" x 135" map in CO(2-1). The velocity distribution of CI shows
six narrow (1 -> 2 km/sec) components which are associated with individual
globules traced in CO. The CI column densities are 0.5 -> 1.2 x 10^16/cm^2. CI
is found to be a factor of ~6 more abundant than CO. Our estimate for the mass
of the neutral envelope is an order of magnitude larger than previous
estimates. The large abundance of CI in the Helix can be understood as a result
of the gradual photoionisation of the molecular envelope by the central star's
radiation field.Comment: 5 pages, Latex, AAS macros, 3 EPS figures, to appear in Astrophysical
Journal Letter
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