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Title:
Astrophysical water masers in the presence of magnetic fields
Authors:
Deguchi, S.; Watson, W. D.
Affiliation:
AA(Illinois, University, Urbana), AB(Illinois, University, Urbana)
Publication:
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 302, March 15, 1986, p. 750-756. (ApJ Homepage)
Publication Date:
03/1986
Category:
Astrophysics
Origin:
STI
NASA/STI Keywords:
Interstellar Magnetic Fields, Interstellar Masers, Linear Polarization, Water Masers, Hyperfine Structure, Magnetic Flux, Polarization Characteristics, Radiative Transfer
DOI:
10.1086/164036
Bibliographic Code:
1986ApJ...302..750D

Abstract

The fractional linear polarization Q/I caused by the presence of magnetic fields is calculated for the 22 GHz water maser as a function of the intensity of radiation within the maser. Complications arise due to the high angular momentum and hyperfine structure of the states (6/16-5/23) that are involved. The large-velocity-gradient approximation is utilized to treat the radiative transfer. Calculations are performed for the regime in which Q/I greater than or equal to 1/3 can be attained. Minimum values for the strength of the magnetic field causing the polarization can then be inferred, using the requirement that the Zeeman splitting must be greater than the stimulated emission rate of the maser. According to the calculation here, Q/I greater than or equal to 0.5 requires magnetic fields of at least 0.1-1 G in the masing cloud if this is the dominant mechanism. Flaring of the maser emission in certain galactic sources (for example, the Orion outburst since late 1979) is known to be strongly polarized with Q/I up to 0.6. The calculations may also allow useful inferences about the giant water masers, such as that of W49 and those which have recently been detected in the nuclei of other galaxies. The strength of the weak hyperfine components in comparison with that of the strong components is also considered.

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