A study of hydroxyl masers in the circumstellar envelopes of long period variable stars

Citation

Cimerman, Menachem (1979) A study of hydroxyl masers in the circumstellar envelopes of long period variable stars. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zsta-x423. https://resolver.caltech.edu/CaltechETD:etd-09042008-141513

Abstract

The 18 cm wavelength maser radiation from long period variable (LPV) stars is emitted by hydroxyl (OH) molecules in the expanding circumstellar envelopes. The four possible transitions within the ground state of the OH molecule are divided into two groups: mainlines at 1665 and 1667 MHz and satellite lines at 1612 and 1720 MHz. Three of these lines are observed in maser radiation from LPV stars: the 1612 MHz line and the mainlines. The observed properties of the 1612 MHz radiation are easier to interpret than those of the mainlines due to the quantum mechanical symmetries of the OH molecule. This investigation tackles the problem of the OH mainline masers, first by radio observations of two LPV sources with unique OH properties, and then by a theoretical analysis.

The U Ori OH masers were monitored following the drastic events in these masers in early 1974. The events consisted of the sudden appearance of strong 1612 MHz radiation, coincident with the disappearance of the 1667 MHz radiation. They are interpreted in light of the monitored evolution of the 1612 MHz radiation, the decline in its output level, and the subsequent re-emergence of a 1667 MHz line. The conclusion is that the events are the outcome of a short-lived disturbance in the stellar photosphere, most likely a shock-wave, which affected the maser through radiative coupling between the photosphere and the OH molecules.

Observations of the OH maser in W Hya are presented, which suggest an explanation of their unique polarization characteristics. The maser lines may be formed at a distance of ~ 10[superscript 14] cm from the stellar surface in the presence of a magnetic field of ~ 9 milligauss. This magnetic field is coupled to the ionized component of the outflowing circumstellar envelope. In the W Hya case the magnetic field is evident in the strong polarization, Zeeman pattern and differential magnetorotation. However, it may exist unnoticed in other OH sources, and contribute to the pumping mechanism.

The pumping of OH masers in late-type-variables by direct stellar radiation of 2.8μ is studied. Two pumping schemes are suggested based on line coincidences between OH V=0-1 transitions and strong H[subscript 2]0 υ[subscript 3] transitions. The water molecules perturb the stellar radiation by absorption, and thus cool the coinciding OH transition. It is demonstrated that this cooling is capable of producing strong OH masers in the circumstellar envelopes of late-type-variables. This model can account for the existence of mainline (ΔF=0) OH masers, which is hard to explain otherwise.

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