P Cygni in a short S Doradus phase. Spectroscopic and photometric evidences

¹ Institute of Astronomy and Isaac Newton Institute of Chile Bulgarian Branch, Bulgarian National Astronomical Observatory, PO Box 136, 4700 Smoljan, Bulgaria
² Ritter Observatory, The University of Toledo, Toledo, OH 43606, USA
³ Tartu Observatory, Tõravere 61602, Estonia

Corresponding author: N. Markova, rozhen@mbox.digsys.bg

Received: 22 March 2001
Accepted: 2 May 2001

Abstract

We studied the long-term spectral and photometric behaviour of P Cygni over the 13.8 year interval from March 1985 to January 1999. The UBV photometry reveals a slow (~7.4-year), low-amplitude (~0.1 mag) variation in V in which the star becomes redder when it brightens and vice versa. It underwent a possible maximum in the winter of 1985 (between JD 2 446 000 and JD 2 446 200), a minimum in the winter of 1989 (between JD 2 447 500 and JD 2 447 700), a maximum in the fall of 1992 (between JD 2 448 800 and JD 2 449 000), and a minimum in the spring of 1996 (between JD 2 450 100 and JD 2 450 300) The properties of this variation are typical for weak-active S Dor variables in "short S Dor phase" vanGen01. Simultaneous spectroscopic observations show changes in the Hα equivalent width (corrected for the effect of the changing continuum) in positive correlation with the 7.4-year photometric oscillation in the V-band brightness. This result is interpreted as an indication that in P Cygni an increase in the stellar brightness, during the 7.4-year SD phase, is likely accompanied by an increase in the mass-loss rate. In this behaviour P Cygni is similar to S Dor and R71. From simple considerations, it is concluded that the 7.4-year SD cycle is probably a combination of an increasing radius/decreasing effective temperature and an expanding pseudo-photosphere.