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Astron. Astrophys. 317, 761-768 (1997)

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2. Observations

Lockwood (1972) observed 292 M- and S-type Mira variables from 1969 to 1971 at the Kitt Peak National Observatory, with a five-colour narrow-band photometric system. He obtained 1795 individual sets of five-colour measurements, at different phases, and, for some stars, during several cycles. The photometric system used was based on Wing's system (Wing 1967a), which measured 27 colours. Lockwood reduced the number of bandpasses to five: two continuum or pseudo continuum regions, one VO and two TiO bands. Table 1 gives properties of the five-colour system.


Table 1. Properties of the five-colour system

The significance of the Lockwood measurements lies in the fact that only few sets of regular observations of an important number of Miras are available. Moreover, the narrow bands allow a precise study of TiO and VO behaviour with phase. The presence of these molecules in the atmosphere depends strongly on temperature. So it is possible to use TiO and VO indices to obtain temperature estimates.
Filter 104 measures a region relatively free of molecular absorption. Wing (1967b) considered this bandpass to give a reliable measurement of the continuum. Filter 87 should have been a continuum measurement too, but this band becomes contaminated by TiO bands in the later M stars. This effect makes the continuum measurement very unreliable and has prevented a colour temperature determination as Lockwood expected from the 87-104 index. Filters 78 and 88 measure several bands of TiO as indicated in Table 1, and filter 105 measures the [FORMULA] v=0 transition of the VO A-X system.

The median standard errors are less than 0.025 mag. The effect of interstellar and circumstellar reddening is supposed to be small in the near-infrared around 10 000 Å. But this hypothesis, especially concerning the circumstellar extinction, might be erroneous for some stars and we must be aware of it.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998