The Deep Fades of AB Aurigæ

AB Aur is classified as a type Ina variable star with a spectral class of B9 IV and ordinarily hovers around mag 6.7 - 6.9 with occasional fades to mag 7.1 or 7.2. However, in 1975 and 1997 rapid deep fades to below mag 8.0 were observed. The 1975 fade was reported by Colin Henshaw and Ian Middlemist, and the 1997 fade by Melvyn Taylor and myself. The following is an extract from a report by Colin Henshaw on the 1975 fade reprinted with permission from Light Curve 1 No 2 (March 1976):

 Over the past five years this star has been observed fairly intensively
 but my observations have shown that it has done very little. In fact I
 considered this star so boring that for some time I was seriously
 considering dropping it from my programme; it never seemed to do anything.
 However, on November 29th 1975, these years of patient observation paid off.
 During a routine observing sessionI was due to observe AB Aurigæ, since I
 had not observed it for 19 days. To my amazement the configuration of the
 stars in that field had changed; AB Aurigæ had faded to mag 8.4 and was no
 longer easy to see.
 In fact it formed a close double system with the well known irregular star
 SU Aurigæ. The faintness of AB Aurigæ made SU Aurigæ much easier to see.
 Confirmation came two days later when Ian Middlemist telephoned me to say
 he had seen the star at mag 8.4 on November 30th. He commented that the
 fade bore similarities to those of R Cr B stars. Further examination of
 the observations that were received, have revealed that the fade did in
 fact occur on November 29th and that it occurred very rapidly.
 My observation at mag 8.4 was made at 1505 GMAT, but Ian observed it at
 7.8 at 0830 GMAT. By December 2nd the star was back to normal again.
 Because of the bad weather that prevailed over Europe at the end of
 November we were the only observers who recorded the fade.

On the evening of November 30th at 0721 GMAT I saw AB Aur at mag 7.1. I immediately thought 'that's equal to the faintest that I have previously seen it, so it's worthwhile coming back to recheck the star later in the evening'. At 1040 GMAT I observed it at mag 7.5, it had faded a further 0.4 mag in just over 3 hours! After alerting Gary Poyner I telephoned Melvyn Taylor who responded in a sleepy voice “you're going to tell me AB Aur is fading”. Melvyn advised me that he had also seen a rapid fade from its normal brightness of 6.8 at 0508 GMAT to 7.5 at 0958 GMAT, corresponding to a fade of 0.7 mag in just under 5 hours! Unfortunately the sky clouded over shortly afterwards and no one was able to secure observations into the morning.

The following night (December 1st) was partly clear and it was apparent that AB Aur had reached its minimum. Melvyn made it mag 8.2 in the late evening whilst I saw it at mag 8.0 in both the evening and morning. I concede however, that it may have been slightly fainter, because with the low power binoculars that I was using I was probably estimating the total combined light of both AB Aur and the nearby type Inb variable star SU Aur. Subsequent observations with a telescope indicate that SU Aur was probably about mag 9.5, therefore effectively brightening AB Aur by around 0.2 mag. This then ties in neatly with Melvyn's observations at mag 8.2.

Fortunately, the night of December 2nd was completely clear, and as soon as darkness fell it was obvious that AB Aur was already starting to recover. In the early evening at 0533 GMAT Melvyn made it mag 7.8 whilst by the late morning I saw it at mag 7.4 at 1821 GMAT. It seemed to me that the recovery had significantly slowed down during the morning hours. The rate of rise was slow compared to the fall observed on November 30th, but nevertheless a rate of change of 0.4 mag in 12 hours is still quite rapid for this class of variable.

Unfortunately the whole of the UK was clouded out on the night of December 3rd. As darkness fell on the evening of December 4th though, Guy Hurst and I secured estimates of 6.9 and 6.8 respectively. This signaled the end of the event, although in reality it had probably actually ended around 24 hours earlier

Since the end of the 1997 fade there has been more activity seen in AB Aur than usual. During the middle part of December several observers reported the star to have dropped back to mag 7.1. Maybe the deep fade has unsettled the normal system of the star for a while. The following rates of change have been calculated from both the 1975 and 1997 fades:

Rate of fade
Observer(s)	Date	Mag change	Time	Change 0.1mag (rate)
Middlemist/Henshaw	29-11-75	7.8 - 8.4	0625	64 minutes
Taylor	30-11-97	6.8 - 7.5	0450	41 minutes
Toone	30-11-97	7.1 - 7.5	0319	50 minutes
Rate of rise
Observer	Date	Mag change	Time	Change 0.1 mag (rate)
Taylor	2-12-97	7.8 - 7.5	0251	57 minutes
Toone	2-12-97	7.74 - 7.36	1031	166 minutes
Poyner	2-12-97	7.9 - 7.3	0637	66 minutes

The similarity of the two deep fades is striking. They occurred almost exactly at the same time of year (22 years apart) with most of Europe clouded out. The depth and width of each fade was roughly measured as the same at 1.2 magnitudes and 70 hours respectively.

These sudden fades of AB Aur are quite extraordinary and difficult to explain. Are we dealing with eclipses with a period of 22 years or some fraction thereof, or is the light output from the star being interfered with by the star's associated nebula? These are questions for professional astronomers to answer. In the meantime we amateurs need to monitor this star closely to establish exactly how frequently these deep fades occur. I would recommend that this star is treated in the same way that eruptive variables are with observations on every clear night. If the star appears fainter than the nearby comparison E (7.5) then an alert should go out. Remember though the star may not vary much for many years so try not to lose patience. This long term inactivity certainly contributes to the excitement when you see it fading rapidly for the first time as our vigilant observers found both in 1975 and 1997.