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

TL;DR

This study analyzes multi-epoch infrared spectra of Sakurai's Object to track dust evolution, revealing formation of new PAHs, SiC grains, and coagulation into melilite silicates over several years.

Contribution

It provides the first detailed temporal analysis of dust feature changes, including new insights into dust formation and coagulation processes in a post-AGB star.

Findings

PAHs increased over time in the spectra.

Large SiC grains formed before May 1999.

Melilite silicates coagulated from submicron silicates, similar to meteorite inclusions.

Abstract

6-14 micron 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-micron absorption due to PAHs increased throughout. 20 micron-sized SiC grains might have contributed to the 6-7 micron 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 x 10 -9 Msun and 10 -8 Msun, respectively. Some of the submicron-sized silicates responsible for a weak 10 micron absorption feature are probably located within the PN because the optical depth decreased between October 2007 and October 2008. 6.9 micron absorption assigned to ~10 micron-sized crystalline melilite silicates increased between April 2005 and October 2008. Abundance and spectroscopic constraints are satisfied if about 2.8 per cent 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.