Spitzer mid-infrared observations of seven bipolar planetary nebulae

Abstract

We have investigated the mid‐infrared (MIR) and visual structures of seven bipolar planetary nebulae (BPNe), using imaging and spectroscopy acquired using the Spitzer Space Telescope (SST), and the Observatorio Astronomico Nacional in Mexico. The results show that the sources are more extended towards longer MIR wavelengths, as well as having higher levels of surface brightness in the 5.8‐ and 8.0‐μm bands. It is also noted that the 5.8 μm/4.5 μm and 8.0 μm/4.5 μm flux ratios increase with increasing distance from the nuclei of the sources. All of these latter trends may be attributable to emission by polycyclic aromatic hydrocarbons (PAHs) and/or warm dust continua within circumnebular photodissociation regions (PDRs). A corresponding decrease in the flux ratios 8.0 μm/5.8 μm may, by contrast, arise due to changes in the properties of the PAH emitting grains. We note evidence for 8.0 μm ring‐like structures in the envelope of NGC 2346, located in a region beyond the minor axis limits of the ionized envelope. An analysis of the inner two rings shows that whilst they have higher surface brightnesses at longer MIR wavelengths, they are relatively stronger (compared to underlying emission) at 3.6 and 4.5 μm. There is also evidence for point‐reflection symmetry along the major axis of the outflow. Finally, the fall‐off in surface brightness along the nebular minor axis suggests that progenitor mass‐loss rates were more‐or‐less constant. NGC 6905 shows evidence for a centrally located and unresolved MIR emission excess. We show that whilst flux ratios are inconsistent with stellar and ionized gas components of emission, the emission can be explained in terms of shock‐excited H2, or grain continuum and PAH emission bands. We provide the deepest image so far published of the outer visual structure of NGC 6445, and compare these results with Spitzer imaging at 8.0 μm. The distributions are quite distinct, and may indicate collimation of the outer ionized emission by a cylindrical PDR. We finally note that the MIR colours of the lobes and nuclei of the sources are radically different. It is also shown that 3.6‐μm emission in BPNe differs from that for the generality of planetary nebulae. We discuss various mechanisms which may contribute to these differences of colour.

Extended Structures of Planetary Nebulae Detected in H2 Emission∗

“…Previous optical imagery of this PN reveals a bright, central ring-shaped morphology and open bipolar lobes The physical structure and internal kinematics were studied by Vázquez et al (2004). Despite these multiwavelength studies, the properties and exact structure of NGC 6445 are still poorly defined (Phillips & Ramos-Larios 2010). Our NOT ALFOSC wide-field optical images show that NGC 6445 has an irregularly shaped central region with a size of ∼40″×50″, where the [O III] emission dominates, while the [N II] emission is much more extended and defines an overall bipolar morphology ( Figure 1).…”

“…There are very faint, extended structures in the H 2 emission, which are almost overwhelmed by bright field stars but are still marginally visible in the color-composite image, where the extended H 2 emission seems to be limb-brightened. The Spitzer mid-IR images show that NGC 6445 is more extended in the 8.0 μm emission (Phillips & Ramos-Larios 2010), which fills the halo region defined by the seemingly limb-brightened H 2 emission ( Figure 6, right).…”

Spitzer mid-infrared spectroscopic observations of planetary nebulae

“…Based on observations of its density, the nebula appears to be evolved. The mid-IR Spitzer image by Phillips & Ramos-Larios (2010) shows differences in the optical and mid-IR distributions, and the presence of strong 8.0 µm emission outside the central nebula. They argue the possibility that this extended mid-IR emission derives from a cylinder of neutral material that would be responsible for the large-scale collimation of the optical structures.…”

“…Mid-infrared (mid-IR) spectroscopy is a powerful tool for the study of the thermal and molecular emission from PNe, since such observations are much less affected by dust extinction. Mid-IR studies of PNe have focused on imaging surveys (e.g., Hora et al 2004;Phillips & Ramos-Larios 2008;Zhang & Kwok 2009;Phillips & Ramos-Larios 2010;Zhang, Hsia & Kwok 2012), but mid-IR spectroscopy of PNe has only been carried out for individual objects for which chemical abundances and H2 excitation temperatures were obtained (Cox et al 1998;Matsuura & Zijlstra 2005;Hora et al 2006;Phillips, Ramos-Larios & Guerrero 2011). Currently, mid-IR studies of the excitation temperature of H2 in PNe are scarce.…”

Spitzer observations of planetary nebulae

“…Phillips & Ramos-Larios (2008a) analyzed quantitatively IRAC images of 18 PNe and suggest that PAH emission from PDRs contributes significantly to the observed mid-IR fluxes. Phillips & Ramos-Larios (2010) extended their analysis to seven bipolar PNe, assessing the roles of PAH emission from PDRs and shock excited H 2 emission, and discussing the color variations from the cores to the lobes. Cerrigone et al (2008) analyzed IRAC images, in conjunction with 4.8 and 8.6 GHz radio continuum observations, of the bipolar PN IC 4406, and concluded that there exist at least three dust components at temperatures ranging from 57 to 700 K.…”