Spatially resolved mid-infrared observations of the circumstellar environment of the born-again object FG Sge

TL;DR

This study spatially resolved the circumstellar dust environment of FG Sge using mid-infrared interferometry, revealing a compact dust shell with specific properties and contrasting PAH emission features with previous observations.

Contribution

First spatially resolved mid-infrared observations of FG Sge's circumstellar environment, providing detailed dust shell modeling and insights into its composition and mass-loss history.

Findings

Circumstellar dust shell has a FWHM of ~10.5 mas.

Dust shell characterized by steep density profile and specific inner radius.

Absence of PAH emission in MIDI spectrum contrasts with earlier Spitzer data.

Abstract

FG Sge has evolved from the hot central star of the young planetary nebula Hen 1-5 to a G-K supergiant in the last 100 years. It is one of the three born-again objects that has been identified as of yet, and they are considered to have undergone a thermal pulse in the post-asymptotic giant branch evolution. FG Sge was observed with MIDI at the Very Large Telescope Interferometer at baselines of 43 and 46 m between 8 and 13 micron. The circumstellar dust environment of FG Sge was spatially resolved, and the Gaussian fit to the observed visibilities results in a full width at half maximum of ~10.5 mas. The observed mid-infrared visibilities and the spectral energy distribution can be fairly reproduced by optically thick (tauV ~ 8) spherical dust shell models consisting of amorphous carbon with an inner radius Rin of ~30 Rstar (corresponding to a dust temperature of 1100 +/- 100 K). The dust shell is characterized with a steep density profile proportional to r^{-3.5+/-0.5} from the inner radius Rin to (5-10) x Rin, beyond which it changes to r^{-2}. The dust mass is estimated to be ~ 7 x 10^{-7} solar mass, which translates into an average total mass-loss rate of ~9 x 10^{-6} solar mass/yr as of 2008 with a gas-to-dust ratio of 200 being adopted. In addition, the 8--13 micron spectrum obtained with MIDI with a field of view of 200 mas does not show a signatureof the polycyclic aromatic hydrocarbon (PAH) emission, which is in marked contrast to the spectra taken with the Spitzer Space Telescope six and 20 months before the MIDI observations with wide slit widths of 3.6--10 arcsec. This implies that the PAH emission originates from an extended region of the optically thick dust envelope, i.e., from the material ejected before the central star became H-deficient.