3,289 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
The Shapes of AGB Envelopes as Probes of Binary Companions
We describe how the large scale geometry of the circumstellar envelopes of
asymptotic giant branch stars can be used to probe the presence of unseen
stellar companions. A nearby companion modifies the mass loss by
gravitationally focusing the wind towards the orbital plane, and thereby
determines the shape of the envelope at large distances from the star. Using
available simulations, we develop a prescription for the observed shapes of
envelopes in terms of the binary parameters, envelope orientation, and type of
observation. The prescription provides a tool for the analysis of envelope
images at optical, infrared, and millimetre wavelengths, which can be used to
constrain the presence of companions in well observed cases. We illustrate this
approach by examining the possible role of binary companions in triggering the
onset of axi-symmetry in planetary nebula formation. If interaction with the
primary leads to axi-symmetry, the spherical halos widely seen around newly
formed nebulae set limits on the companion mass. Only low mass objects may
orbit close to the primary without observable shaping effects: they remain
invisible until the interaction causes a sudden change in the mass loss
geometry.Comment: 11 pages, 7 figures, to appear in MNRA
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