Observed evolutionary changes in the visual magnitude of the luminous blue variable P Cygni

Abstract

Historical observations of the star P Cygni (B1 Ia(+)) show that thestar has steadily increased its visual brightness between 1700 and 1988by 0.15 +/- 0.02 magn/century. If we assume that the luminosity of thestar remained constant, (as predicted by evolutionary tracks of massivestars) the increase in brightness must be due to a steady decrease ofthe Bolometric Correction and hence of Teff at a rate of 6 +/- 1 percentper century. This is the first time ever that the evolution of a starhas been observed in secular photometric variations. The redwardevolution of P Cyg in the HR-diagram occurs on a Kelvin-Helmholtz timescale for core-contraction and envelope expansion, but the observed rateof change is a factor of two faster than predicted by the evolutionarytracks. The discrepancy can be explained by either assuming thatdynamical effects speed up the post main-sequence evolution of massivestars or that the core mass of the star is smaller than predicted. Webriefly discuss the absolute bolometric magnitude of P Cygni during theoutbursts in the seventeenth century derived from the observed color andthe visual brightness.