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Session 48 - Circumstellar Envelopes.
Display session, Tuesday, January 14
OH spectra from circumstellar shells around proto planetary nebulae such as AFGL2343, and hypergiants such as IRC+10420, exhibit a multitude of features. Some of these exhibit substantial changes in their intensities and velocities. Thus the intensity ratio of the strongest red and blue shifted 1612 MHz features in IRC+10420 changed from its usual 10-30 range between 1978 amp; 1988 to 2.3 by 1993.9, while the maximum 1612 MHz peak has expanded to cover the velocity range from 47 to 42 km/s, during which time the velocity of the largest blue shifted 1667 MHz peak drifted systematically from 40.787 to 40.487 km/s. This last change may be due to competitive gain between lines. To systematize the investigation of such effects, and to render it independent of the realized spectral bins and any underlying change of shape in the spectral baseline, we utilize a Fourier transform shift algorithm to repeatedly rebin the spectrum until we obtain the maximum intensity of each feature from any binning scheme, together with its associated velocity. This methodology provides a more impersonal result than fitting a gaussian to narrow features with few pixels, and obviates most baseline uncertainties.
Our use of this methodology on the largest spectral feature in AFGL2343 suggests that small velocity changes can only be estimated from spectra at the same resolution. Nevertheless, our 0.22 km/s resolution Arecibo spectra confirm the \bigtriangleup V, 1.4 km/s difference in the peak velocities of the 1612 amp; 1667 Mhz lines found by Likkel (ApJ 344, 350) from 0.9 km/s resolution observations, and tentatively suggest that \bigtriangleup V may be increasing. A growth in the 1667 MHz vis a vis the 1612 MHz line is anticipated at an advanced stage in the expansion of a fossil shell, when the OH lines compete for photons (ApJ 338, 234): this is also likely to result in changes in their velocities. We will present results from applying this methodology to the multitude of features discernable in spectra of IRC+10420 between 1985 amp; 1993.
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