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Published April 20, 2008 | Published
Journal Article Open

Submillimeter array imaging of the maser emission from the H30α radio recombination line in MWC 349A

Abstract

We used the Submillimeter Array to map the angular distribution of the H30α recombination line (231.9 GHz) in the circumstellar region of the peculiar star MWC 349A. The resolution was 1.2", but because of high signal-to-noise ratio we measured the positions of all maser components to accuracies better than 0.01", at a velocity resolution of 1 km s^(−1). The two strongest maser components (called high-velocity components) at velocities near –14 and 32 km s^(−1) are separated by 0.048" ± 0.001" (60 AU) along a position angle of 102° ± 1°. The distribution of maser emission at velocities between and beyond these two strongest components were also provided. The continuum emission lies at the center of the maser distribution to within 10 mas. The masers appear to trace a nearly edge-on rotating disk structure, reminiscent of the water masers in Keplerian rotation in the nuclear accretion disk of the galaxy NGC 4258. However, the maser components in MWC 349A do not follow a simple Keplerian kinematic prescription with v ~ r^(−1/2), but have a larger power-law index. We explore the possibility that the high-velocity masers trace spiral density or shock waves. We also emphasize caution in the interpretation of relative centroid maser positions where the maser is not clearly resolved in position or velocity, and we present simulations that illustrate the range of applicability of the centroiding method.

Additional Information

© 2008. The American Astronomical Society. Print publication: Issue 2 (2008 April 20). Received 2007 August 17; accepted 2008 January 5. We thank Henrik Beuther, Chunhua Qi, David Fong, Mark Gurwell, and Jun-Hui Zhao for help with the preparation of the observing script as well as the data calibration and reduction. We are grateful for the astute remarks of the anonymous referee, who urged us to look more deeply into the simulations reported here and made other very useful suggestions. We thank William Danchi for providing digital data for Figures 1 and 4, and Yolanda Gόmez and Daniel Tofoya for providing the digital image for Figure 1. Jack Welch encouraged us to carry out a simulation of centro iding analysis and showed us the BIMA images of MWC 349A, and Vladimir Strelnitski discussed radiative transfer issues and dynamical models with us. We also benefited from discussions with Naama Dror, Yolanda Gόmez, Lisa Harvey-Smith, Luis Rodríguez, Howard Smith, and Daniel Tafoya.

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