The expansion of the outer circumstellar shell of P Cygni

The expansion of a faint, outer irregular shell to the circumstellar nebulosity of the luminous blue variable P Cygni has been measured with the Manchester Echelle Spectrometer combined with the high-altitude San Pedro Martir 2.1-m UNAM telescope. This is the first ever measurement of the expansion velocity of such a historic remnant of a luminous blue variable star eruption. The faintness of the outer shell and the high brightness and close proximity of the star make it a particularly difficult observation. The high optical luminosity of the spectrometer and the clarity of the sky have combined to permit a successful measurement. A radial expansion velocity of 185 +/- 10 km s(-1) is implied, which suggests an age of 2100 +/- 200 yr for this 1.6 arcmin diameter outer shell, if we assume a distance of 1.8 kpc to P Cygni.