Long-term Monitoring of the
H Emission Line Strength
of the LBV P Cygni

The application of CCD technology gives an exciting opportunity for the amateur spectroscopist to provide a modest contribution to the professional study of Be-star phenomena and the variability of emission-line stars in general. Especially the long-term monitoring of the time evolution of the H emission is one of the possibilities which has already lead to interesting results with a relatively modest instrumental expenditure. Stars including  Cas, P Cyg, and VV Cep, just to name a few, are on the top of my priority list to obtain long-term observations over a period of several years. As an example of such monitoring, the behavior of the H emission of P Cygni is presented here. Although this star is not a classical Be star, but an LBV prototype, these observations demonstrate the potential of amateur spectroscopy.

The equipment used to obtain the results presented here is a configuration of a Maksutov-type reflector (f = 1000 mm, aperture = 100 mm) and an objective prism made of F₂ flint glass of the same aperture with a refracting angle of 30°. This instrument's linear dispersion is about 6 Å/pixel at H. The spectrum is recorded on a CCD camera. The chip is an FT800P manufactured by Philips Co. with 386×290 pixels. The camera has a 6.4×4.8 mm field size and offers a resolution of /2000 at H. The equivalent width W was calculated using the standard formula but with the sign convention changed to use positive numbers for emission lines. The integration interval over the emission maximum is 55 Å. Since the blue-shifted absorption component is not resolved, it is included in the measured W. The exposure times of the spectra are 128 seconds, giving a S/N on the order of 200.

The observation period covered so far extends from September 1994 until January 1999 (see Figure 1), during which the equivalent width varied from 60 to 110 Å. The data over nearly 5 years document the slow passage through a minimum in the equivalent width, i.e., in the emission line strength. Superimposed, a quasi-periodic microvariation is seen on time scales from weeks to months. A period analysis by means of Lomb-Scargle periodograms, however, detects no statistically significant (3 ) periodic variations in the data record (Kaufer, priv. comm.).

This result obviously encourages continuation of the monitoring of P Cygni in the same patient way for some further years in order to search for H variability in a much larger and continuous data record.

Figure 1. Time variability of the H equivalent width of P Cyg.