6. Light amplitudes
When compared to previous amplitudes, the actual 1987 and 1992 determinations show an increase in the amplitude of (which was at its minimum around 1980). So our 1992 observations show for the first time to have an amplitude higher than since 16 Lac was known as a light variable.
The amplitude of is still low since 1980-1984 (around 6 mmag on the average) and is in our 1987-1992 data clearly lower than that of , while amplitude "seems erratic" as usual (see Fig. 3 in Paper I).
It has been already pointed out that the amplitudes of and were much higher around 1950 and have been decreasing together at similar rates since then. They also were very probably at a lower level at the beginning of this century (i.e. in 1912-1914: Paper I).
The actual increase of the amplitude of means that something has happened that provides in this non radial pulsation mode more energy than a decade ago. In comparison the decrease, or stability, of the amplitude of at a rather low level shows that this (very probable) radial mode keeps some energy to continue, but is not favorized as it used to be 40 years ago. Is there a one century period in the excitation of this () radial mode, or only a time constant of several decades? Only future observations will tell us...
The behavior of seems to be totally independant from and . The amplitude of may vary from 15.5 mmag to almost zero (1.5 mmag) in a time scale of months as happened in 1951.
But if we analyze further these amplitudes, they appear in the past and in our data to be distributed around two values, each one with a rather small rms error: 6 measurements with a mean value at 5.0 mmag (rms error = 1.8 mmag), 7 measurements with a mean value at 12.5 mmag (rms error = 1.7 mmag). The average value is 9.1 mmag, but with a very large rms error (4.2 mmag). This might be the signature of an oscillation (on a sinusoidal time dependant phenomenon, the probability is maximum for the values to be most of the time near their extrema).
On our longest run (1992), if we substract and sine functions, we can obtain the one, night by night for the 11 nights of observation. These 11 data points, as expected, do not present any probability peak in the (orbital) 12 d range. If plotted in phase with the ephemeris of the orbital motion (Pigulski and Jerzykiewicz, 1988), the resulting variation could be represented by a least square sine function with period (Fig.4). This "best fit" sinusoid has an amplitude of 1.2 mmag and its mean is 6.7 mmag. The correlation is rather low, as the extreme observed values (second and eleventh nights: JD 2448894 and 904 respectively) are those with less data. However this sinusoid, which average value is similar to that of Paper I, shows a maximum value at HJD 2448903.206, i.e. at phase 0.15 after the calculated periastron. This phase is determined with a rather low precision. Due to the actual amplitudes of , which is no longer the principal period, this result is not contradictory with the previous 0.25 value, itself also determined with rather low precision (Paper I).
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998