The evolution of V4334 Sgr (Sakurai's object) in the Mid-Infrared

Since Sakurai's object (V4334 Sgr) underwent a late helium flash in 1995, its fast evolution has been monitored with great effort. We present TIMMI and TIMMI 2 photometric observations covering a spectral range from 5 to 20 $\rm{\mu}$m with a total of 8 filters. We found drastic changes in the spectral energy distribution indicating a drop of temperature in the dust envelope from 1000 to 600 K in half a year only. Combined with optical observations reported elsewhere, we conclude that the phase of dust formation has probably stopped in mid-2001 and that the dusty envelope has been expanding only, ever since. An expansion velocity of about 1000 km s-1 is required to explain the rapid cooling. Other optical and near-infrared observations support the existence of such high velocity flows in the envelope of V4334 Sgr. An estimate for the bolometric luminosity of ≈$2000~L_\odot$ is derived for a distance of 1 kpc. Taking into account theoretical models a distance of $2 \pm 1$ kpc to V4334 Sgr is suggested. As a consequence of its fast final helium flash evolution, Sakurai's object may start to destroy its surrounding dust in the not too distant future.