Dust changes in Sakurai’s Object: new PAHs and SiC with coagulation of submicron-sized silicate dust into 10 μm-sized melilite grains

Abstract

6–14 μm Spitzer spectra obtained at 6 epochs between April 2005 and October 2008 are used to determine temporal changes in dust features associated with Sakurai’s Object (V4334 Sgr), a low mass post-AGB star that has been forming dust in an eruptive event since 1996. The obscured carbon-rich photosphere is surrounded by a 40-milliarcsec torus and 32 arcsec PN. An initially rapid mid-infrared flux decrease stalled after 21 April 2008. Optically-thin emission due to nanometre-sized SiC grains reached a minimum in October 2007, increased rapidly between 21–30 April 2008 and more slowly to October 2008. 6.3-μm absorption due to PAHs increased throughout. 20 μm-sized SiC grains might have contributed to the 6–7 μm absorption after May 2007. Mass estimates based on the optically-thick emission agree with those in the absorption features if the large SiC grains formed before May 1999 and PAHs formed in April–June 1999. Estimated masses of PAH and large-SiC grains in October 2008, were 3 × 10−9 M⊙ and 10−8 M⊙, respectively. Some of the submicron-sized silicates responsible for a weak 10 μm absorption feature are probably located within the PN because the optical depth decreased between October 2007 and October 2008. 6.9 μm absorption assigned to ∼10 μm-sized crystalline melilite silicates increased between April 2005 and October 2008. Abundance and spectroscopic constraints are satisfied if $\lesssim$2.8 per cent of the submicron-sized silicates coagulated to form melilites. This figure is similar to the abundance of melilite-bearing calcium-aluminium-rich inclusions in chondritic meteorites.