Mid‐Infrared Observations of Planetary Nebula BD +30 3639: Evolution and Distribution of Unidentified IR Band Carriers1

Abstract

We carried out high spatial resolution N-band imaging and spectroscopic observations of the planetary nebula BD +30 3639 with the Cooled Mid-Infrared Camera and Spectrometer (COMICS) mounted on the Subaru telescope. Mid-infrared images reveal a rectangle shell structure of the nebula extending over 4'', which is supposed to be formed by the superwind from the central star. We have detected [Ne II] 12.8 μm and the unidentified infrared (UIR) bands at 8.6 and 11.2 μm, together with the broad plateau emission in 11-13 μm. In addition, the spectra clearly indicate the presence of silicate absorption around 10 μm for the first time, which increases sharply at the outside of the shell. This can be attributed to silicate grains that were formed in the O-rich phase of the central star, which is now C-rich. The present data indicate that these silicate grains are located in a very thin shell, suggesting that BD +30 3639 underwent a short period (~100 yr) of a high mass-loss rate (~3 × 10−3 M☉ yr−1) at the last epoch of the O-rich phase. N-band spectra show that the distribution of [Ne II] is not clearly different from that of the UIR band and suggest that the UIR band carriers coexist with the ionized gas to some extent. This may be attributed to a slow destruction of the band carriers in the ionized region of the high electron density. The 10 μm continuum and the 11-13 μm plateau emission are dominant in the shell region, while the UIR 11.2 μm band becomes stronger in the outside of the shell, suggesting a change in the dust composition or the dust size distribution between the shell and the outside of the shell region. On the contrary, the peak wavelength of the UIR 11.2 μm band agrees with the class B spectrum, which is often seen in planetary nebulae, and does not change between the shell and the outside of the shell region. These facts suggest that the UIR band carriers are originally formed in the carbon-rich stellar wind and partially destroyed in the shell by shocks. The change of the UIR band spectrum from class B to A, which is generally seen in H II regions, should take place outside of the nebula.