Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21341214s&link_type=abstract
American Astronomical Society, AAS Meeting #213, #412.14; Bulletin of the American Astronomical Society, Vol. 41, p.219
Astronomy and Astrophysics
Astronomy
Scientific paper
Intermediate mass stars (0.8 and 8 solar masses) eventually evolve into Asymptotic Giant Branch (AGB) stars. During this evolutionary phase, these stars lose mass as a result of pulsation and low surface gravity. In addition, newly-formed elements from within the star are dredged-up to the surface and are expelled with the mass loss. Once the material has drifted far enough from the star's surface, dust condenses in a circumstellar shell. Late in the AGB phase, mass loss is strong enough to create an optically thick dust shell that can shield molecules such as water from destructive interstellar UV photons. However, as the dust shell expands, the density decreases and eventually the dust shell loses the ability to protect molecules. As water is destroyed, it forms OH radicals leading to observable MASER emission at 1612 MHz which is easily detected by radio telescopes. Since the OH is contained within an expanding shell, we detect two peaks due to the blue- and red-shift of the near and far side of the shell respectively. We have undertaken a long-term (6 yr) temporal study of the OH emission from IRAS19396+2338 to find the phase-shift between the blue- and red-shifted peaks. These will be used to determine the size of the dust shell. In addition, these observations provide very accurate measurements of the star's pulsation period. Such a long-term study inevitably suffers from issues related to changes in calibration of the telescope. By using the ratio of the blue- and red-shifted peaks, rather than the absolute flux values we produce results that are independent of the telescope's calibration. The resulting dust shell sizes and pulsation periods are crucial inputs to radiative transfer models and models of stellar evolution.
I would like to thank the National Science Foundation's REU program for making this work possible.
Lewis Matthew
Smith Anthony A.
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