EDP Sciences
Free access
Volume 406, Number 3, August II 2003
Page(s) 981 - 985
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20030828

A&A 406, 981-985 (2003)
DOI: 10.1051/0004-6361:20030828

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

H. U. Käufl1, J. Koller2, 3 and F. Kerber4

1  European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
2  Department of Physics and Astronomy, MS 108, Rice University, 6100 S. Main St, Houston, TX 77005, USA
3  Los Alamos National Laboratory, X-1, PO Box 1663, MS P225, Los Alamos, NM 87545, USA
    e-mail: jkoller@lanl.gov
4  Space Telescope European Coordinating Facility, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
    e-mail: fkerber@eso.org (Based on observations collected at the European Southern Observatory, Chile, proposals 65.L-0702, 67.D-0413, 68.D-0542.)

(Received 25 February 2003 / Accepted 15 May 2003)

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  $\approx$ $2000~L_\odot$ 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.

Key words: stars: AGB and post-AGB -- stars: evolution -- circumstellar matter -- planetary nebulae: individual: Sakurai's Object (V4334 Sgr)

Offprint request: H. U. Käufl, hukaufl@eso.org

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© ESO 2003