Visual Observations

This discussion implies that both components of the system are visually resolved during the occultation; if the components are too close together to be resolved, then the observed effect has been determined2 to depend both on the apparent separation of the components and on their relative brightness. An analysis of a large number of occultation observations that had been made over some 35 years showed that for more than 420 of these observations the observer reported an anomalous event. The observers recorded these occultation events as not to have occurred instantaneously, to have "faded" either smoothly or in a stepwise fashion. For 160 of these events, it was found during the analysis that the 140 stars involved were in fact close doubles, many of which had been discovered by other techniques at later dates. For many of these known double and binary systems their separations and position angles were sufficiently well known to enable a calculation of the expected time intervals between the occultations of the two components, and whether the brighter or fainter component was occulted first. Intuitively it may be expected that for components of similar magnitude and for close doubles where the two occultations follow in rapid succession, the event may appear gradual, taking a slightly longer time to complete than the more normal instantaneous disappearance or reappearance. However, for wider pairs, or where the difference in magnitude of the components is large, the event might be expected to appear more dramatic, with a clear drop or step in brightness after the occultation of the first component. This expectation is borne out by the data, as shown in Figure 18.2, where for each of the observations the calculated event duration is plotted against the computed brightness-change after the occultation of the first component. The observers' comments from the original observation records have been interpreted as either "gradual" or "step" event, and these are used to code the observation symbol on the plot; circles for "gradual" and crosses for "steps". It is clearly seen that the observations are split into two classes according to whether there was a large change in brightness or long duration (step observed), or subtle change in brightness or short duration (gradual event). These results may then be used as a rough guide to interpret further visual observations of occultations, where a non-instantaneous event is observed.

The analysis discussed above concluded that a further 130 stars from Robertson's Zodiacal catalogue3 were possibly undiscovered close doubles and would warrant closer study by say speckle interfer-ometry, or high-speed photometric observation of future lunar occultations. At least one star, in the Praesepe cluster, from the target list given by Appleby2 has later been confirmed as double as a result of this work.4

Given in Table 18.1 is a small subset of this target list showing just those stars that have been observed to fade at occultation on at least three occasions. The stars are identified by their HD, ZC and SAO numbers and visual magnitude.

Figure 18.2.

Observed event for known double stars as a function of calculated duration and brightness change; circle = "gradual", cross = "step".

Figure 18.2.

Observed event for known double stars as a function of calculated duration and brightness change; circle = "gradual", cross = "step".

Table 18.1

List of stars that have been observed to fade on

at least three

occasions.

For an on-line catalogue

of stars

which can be occulted by the Moon see the website of Paul

Schlyter.7

HD

ZC

SAO

Mv

16302

387

75476

6.9

22017

516

93487

7.3

23288

536

76126

5.4

27934

656

76601

4.4

65736

1203

97468

7.1

88802

1500

118181

8.1

89307

1506

99049

7.1

120235

1 978

139559

6.6

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