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 2005 April and 2008 October 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 2008 April 21. Optically thin emission due to nanometre-sized SiC grains reached a minimum in 2007 October, increased rapidly between 2008 April 21-30 and more slowly to 2008 October. 6.3-μm absorption due to PAHs increased throughout. 20 μm-sized SiC grains might have contributed to the 6-7 μm absorption after 2007 May. Mass estimates based on the optically thick emission agree with those in the absorption features if the large SiC grains formed before 1999 May and PAHs formed in 1999 April-June. Estimated masses of PAH and large-SiC grains in 2008 October, were 3 × 10-9M⊙ and 10-8M⊙, 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 2007 October and 2008 October. 6.9-μm absorption assigned to ~10 μm-sized crystalline melilite silicates increased between 2005 April and 2008 October. Abundance and spectroscopic constraints are satisfied if ≤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.