The following document lists the file abstract/BMONTESI_FGSGE_IR.abs
from catalogue VI/111.
A plain copy of the file
(without headers/trailers) may be downloaded.
FG Sge is at present undergoing the very short phase of stellar evolution which connects Asymptotic Giant Branch stars to Planetary Nebula Nuclei. At the beginning of the century the star had an effective temperature above 50000 K, whereas in 1990 the spectral type corresponded to F6-7 I with an effective temperature of 6500 K (Montesinos et al. 1990). The star seems to have returned to the position in the HR diagram where it ejected a planetary nebula about 6000 years ago. From 9 August 1992 the V magnitude of FG Sge started decreasing dramatically, from V=9.2 down to V=13.5 at the end of October. The variation in the colour indices U-B and B-V seemed to indicate that the central star was becoming bluer and hotter, but an IUE spectrum taken on 11 September 1992 does show the same slope as that from previous observations, suggesting that by that time the spectral type of FG Sge had not changed substantially. A condensation of dust in a very dense envelope or the ocultation of the star by dense, cold clouds of dust might be the origin of the decrease in brightness. The star, after being in a pulsating phase for a few decades, is probably to be undergoing a R-CrB-like phase. FG Sge was detected by IRAS in the 60 micron band, with a flux of 0.62 Jy. Given the enhanced sensitivity of ISO, and in particular of ISOPHOT in comparison with IRAS, we propose photometric observations in the IRAS bands to obtain the far infrared energy distribution of this star and then estimates for the temperature of the surrounding clouds.