Browsing by Author "Boboltz, David Allen Jr."

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    R Aquarii: First Detection of Circumstellar SiO Maser Proper Motions
    Boboltz, David Allen Jr.; Diamond, Philip J.; Kemball, A. J. (IOP PUBLISHING LTD, 1997-10)
    We have made the first detection of circumstellar SiO maser proper motions in the envelope of a late-type star. Using the Very Long Baseline Array (VLBA), we have obtained observations at four epochs of the 43 GHz, upsilon = 1, J = 1-0 SiO maser emission toward the Mira variable in the symbiotic binary R Aqr. The maser emission has a ringlike structure approximately 31 mas across with a slight elongation in the north-south direction. We find that the emission changes significantly over a timescale of about 1-2 months with almost no similarity in structure for timescales greater than or similar to 6 months. Our observations show that over a 98 day period the masers have an average inward proper motion of about 1 mas. This contraction of the ring implies an infall velocity of about 4 km s(-1) for the SiO masers in the circumstellar envelope.
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    The Structure and Polarization Properties of the SiO Masers in the Extended Atmosphere of R Aquarii
    Boboltz, David Allen Jr. (Virginia Tech, 1997-06-13)
    Silicon monoxide (SiO) maser emission has been observed towards many late-type stars. The conditions necessary for the formation of SiO masers dictate that they be produced in a region which is inside the silicate dust formation point close to the surface of the star. Very Long Baseline Interferometry (VLBI) has shown that these masers do indeed lie close to the stellar surface at a distance of a few stellar radii. This extended atmosphere is a complex region dominated by stellar pulsations and permeated by circumstellar shocks. This dissertation presents the results of a multi-epoch VLBI study of the v=1, J=1-0, 43-GHz SiO maser emission towards the symbiotic binary R Aquarii. Four epochs of full-polarization observations were recorded using the Very Long Baseline Array a facility the National Radio Astronomy Observatory. The first high-resolution images of the extended atmosphere of a Mira variable in a symbiotic binary have been produced. The SiO masers towards R Aquarii have been found to exhibit a ring-like morphology ~31 mas (~6.8 AU) in diameter. The emission changes significantly over a time period of ~1-2 months with almost no similarity in structure on timescales >6 months. An analysis of the four epochs of observations has provided the first direct evidence of SiO maser proper motions. These observations, taken as the Mira variable approached maximum light, show that over a 98-day period the masers have an average inward proper motion of ~1 mas. This contraction of the maser shell implies an infall velocity of ~4 km/s for the SiO masers during this phase of the stellar pulsation cycle. In addition to the total intensity images, maps of the linear and circular polarization morphology were also produced. These images show that the SiO masers are significantly polarized, and that the polarization structure and intensity change on timescales as short as ~1-2 months. For three of the four epochs, a mean fractional circular polarization of 4% was determined implying a magnetic field strength Bsec(theta) = 13 G. For one of the four epochs, the mean fractional circular polarization was found to be ~14% indicating a magnetic field strength Bsec(theta) = 46 G. The fractional linear polarization is fairly constant for all four epochs with mean values ranging from 20.8-25.0%, and peak values as high as ~83% for isolated maser features. Maps of the linear polarization vectors show an orderly structure over large portions of the maser shell indicating a uniform magnetic field topology in these regions of the extended atmosphere of R Aquarii.
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    A Water Maser Flare in W49N: Amplification by a Rotating Foreground Cloud
    Boboltz, David Allen Jr.; Simonetti, John H.; Dennison, Brian K.; Diamond, Philip J.; Uphoff, J. A. (IOP PUBLISHING LTD, 1998-12)
    We monitored the 22 GHz H2O maser emission toward W49N from December 1989 through May 1990. During this period we observed an outburst in a component at -66.25 km s(-1). The flux density of the flaring component increased by a factor of greater than or similar to 10 to a maximum of 4020 Jy over a period of 24 days and decreased over the following 34 days to 1400 Jy on the last day of monitoring. During the flux increase the line narrowed from about 1.1 to 0.8 k s(-1); it subsequently rebroadened to 1.0 km s(-1) Most interestingly, during the flaring behavior the line center shifted by approximately 0.5 km s(-1) over the 58 day period. To explain the flare, particularly its shifting line center, we present a model of two interacting maser clouds. In the model, saturated maser radiation produced in a background cloud is amplified by an unsaturated, masing foreground cloud. Motion of the foreground cloud across the line of sight produces a flaring line, accompanied by line narrowing. We demonstrate that the observed flare in W49N may be explained by such a model where a rotating foreground cloud passes in front of a non-rotating background cloud. The differential amplification of the background cloud's radiation produces the observed increase in flux density, line narrowing, and systematic shift in center velocity.