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| Paper: |
Determining Phase Lags for OH/IR Stars |
| Volume: |
135, A Half Century of Stellar Pulsation Interpretations: a Tribute to Arthur N. Cox |
| Page: |
331 |
| Authors: |
West, M. E.; van der Walt, D. J.; Gaylord, M. J. |
| Abstract: |
Time series of OH masers at 1612 MHz can be used to determine the sizes of circumstellar shells of OH/IR stars. This is done by measuring the phase lag which arises from the differerence between the travel time of the OH emission from the front and back of the shell. The 1612 MHz OH masers in the Hart RAO monitor programme were observed fortnightly, and later weekly, which resulted in time series which were well sampled relative to the periods of the OH/IRs. Previous methods used for determining phase lags from the HartRAO OH data were either too simple to model the complexities of the time series or did not produce reliable error estimates. We have now implemented a method based on the discrete correlation function of Edelson & Krolik (1988). Simulated time series were created for points across the spectrum of one long period and one intermediate period OH/IR. The underlying functions used were modulated asymmetric cosines, sampled at the same intervals as the real data. The scatter in the simulated data and the estimated error for each point were generated using Monte Carlo simulations derived from the real data. The simulations test the ability of the method to extract the correct phase lag from simulated data with a known phase lag. These tests give an indication of the accuracy that can be expected when determining phase lags from the real OH data sets using this method. |
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