The Circumstellar Environment of Evolved Stars as Revealed by Studies of Circumstellar Water Masers

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Universal-Publishers, 1996 - 528 pages
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The dissertation presents the results of a multi-epoch very long baseline interferometric study of water masers located in the extended atmospheres of evolved stars. The research was performed using the Very Long Baseline Array and Very Large Array of the National Radio Astronomy Observatory. Optical monitoring of the stars was provided by the American Association of Variable Star Observers, the Variable Star Network and Dr. Bill Neely of the NF/Observatory. Water masers of found to exist in a region where a population inversion of the rotation transition of 22 GHz can be maintained by collisional pumping. The masers are identified as individual pockets of gas, which have the good velocity coherence and may be imaged using radio interferometry. Stellar winds are initiated in these sources by dust formation and acceleration of the gas through momentum coupling. The typical wind speeds in the region of the water masers are 10 to 20 km/s The water masers are followed through several epochs of observation and exhibit proper motions consistent with the assumed source distance and the measured outflow of velocity in the water maser region. Estimates of the distance to the sources using statistical approximation are in agreement with the currently accepted distances to the stars. A detailed kinematic model is used to describe the flow motions of the gas in the maser region. The regions are found to be complex and not well modeled by uniform radial outflow, radial outflow with rotation, or radial with acceleration. The reasons for this are explored and include anisotropic velocity fields induced through non-uniform dust formations near the star and incomplete sampling of the outflow due to a lack of detected masers. Possibilities for future work in the subject are described and include more sophisticated modeling, more sensitive observations, and analysis of other maser species.
 

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Table des matières

35 NML Cygni
182
352 Results
191
353 Modeling
210
354 Interpretations
211
36 U Herculis
213
362 Results
218
363 Modeling
228
364 Interpretations
231

14 Dissertation Goals
30
141 Proper Motions
31
142 Stellar Shell Morphology
32
OBSERVATIONS AND REDUCTIONS
35
211 AAVSO data
36
212 VSNET DATA
38
22 Observations with the VLA
42
221 Observational Parameters
43
23 Observations with the VLBA
45
SkytoTape
47
233 Correlation
51
234 Correlation specifics for this dissertation
55
235 Calibration of Spectral Line VLBI Data
56
236 Mapping of Spectral Line VLBI Data
59
237 Measurement of Maser Spot Locations
60
238 Errors in Measurement of Spot Locations
61
SOURCE HISTORIES RESULTS AND INTERPRETATIONS
66
32 S Persei
73
321 History
74
322 Results
79
323 Modeling
103
324 Interpretations
109
33 VX Sagittarii
114
331 History
115
332 Results
120
333 Modeling
142
334 Interpretations
144
34 VY Canis Majoris
149
342 Results
155
343 Modeling
180
344 Interpretations
181
37 RX Bootis
234
371 History
236
372 Results
239
373 Modeling
249
374 Interpretations
252
38 IK Tauri
253
382 Results
257
383 Modeling
269
384 Interpretations
271
CONCLUSIONS AND DISCUSSION
274
42 Future Work
284
Bibliography
290
The Simple TwoElement Very Long Baseline Interferometer
303
VLBA Data Reduction
310
Mapping Procedures
318
C1 The First Step
320
C2 Mapping the Cube
322
C3 The Problems
323
Ellipsoidal Shell Modeling
325
D2 The Model
326
D3 The Process
330
D4 Acknowledgments
332
Ellipsoidal Shell Modeling
334
E2 S Persei Models
335
E3 VX Sagittarii Models
358
E4 NML Cygni Models
374
E5 U Herculis Models
384
E6 RX Bootis Models
397
Component Tables
410
F2 Tables of mean proper motions
491
Droits d'auteur

Expressions et termes fréquents

Fréquemment cités

Page 3 - The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under contract with the National Science Foundation.
Page 11 - Z=p/(2xmkT) 1/2 where p is the partial pressure of the molecular species, m is the mass of the molecule, k is the Boltzmann constant, and T the absolute temperature (16). Under ambient conditions, Z is in the order of 3 x 10 collisions per second and cm for a pressure of 1 bar.

À propos de l'auteur (1996)

Dr. Kevin Marvel is currently serving as Associate Executive Officer for policy programs with the American Astronomical Society. In his current position, he lobbies Congress and the administration for increased funding for basic research and directs the AAS career services program.

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