Thesis Summary:

Hubble Space Telescope Observations of Proto-Planetary Nebulae

Kate Y. L. Su

One of the main unsolved problems in the study of stellar evolution is to explain the rich variety of morphologies in planetary nebulae (PNs). I aim to provide a better understanding on the formation processes of the planetary nebulae in this thesis. It is believed that Asymptotic Giant Branch (AGB) stars, which have initial mass ranging from 1-8 Msun, are the progenitors of PNs. Since the first PN catalog was published by Curtis in 1918, astronomers have been puzzled by the rich variety of morphologies observed in PNs. The majority of PNs are asymmetric (~ 90% of the PNs are elliptical, bipolar, and point-symmetric). The Generalized Interacting Stellar Wind (GISW) model (Balick 1987) can explain the origin of asymmetry PNs if there exists an asymmetry in the AGB remnant, then the spherically symmetric fast wind can exploit this asymmetry to create various morphologies in PN phase. However, the cause of this asymmetry in the AGB remnant is poorly understood given the fact that the mass-losing process in the AGB phase is spherically symmetric in general. Studying the transition objects, Proto-Planetary Nebulae (PPNs) holds the key to understand the origin of the pre-existent asymmetry.

Fig 1: RGB color image of PPN IRAS 17150-3224. Red was taken with NICMOS F212N (H2) filter, and green and blue were taken with WFPC2 F606W and F814W filters respectively.

In this thesis, I present a high angular resolution imaging study on nine PPNs with the Hubble Space Telescope. Seven of the PPNs show prominent bipolar nebulosity (see Fig 1 as an example) with various complexities such as point-symmetric structures and evidences of collimated outflows, suggesting that the dust shells in PPNs possess significant departures from spherical symmetry which is usually found in the dust shells in the preceding AGB phase. The images were obtained under three-year HST WFPC2 and NICMOS imaging programs¹. Color images are constructed for those PPNs that have images available at more than one wavelength. In our color analysis, the colors of the reflection lobes are bluer than the colors of the equatorial regions, suggesting that the bipolarity is due to a reflection nebula seen in scattered stellar light. The color of the central stars, when detected, is always the reddest, suggesting the stars are embedded in dense dust shells. The 2 mm polarization study shows that the nebulosity is highly polarized with centrosymmetric polarization patterns, consistent with the nebulosity being a reflection nebula, and the central star as the illuminating source.

Fig 2:Cross-section cut of model axi-symmetric density distribution in the PPN circumstellar dust shell.

Assuming an axi-symmetric density distribution (Fig 2) in the circumstellar dust shell, a two-dimensional radiation transfer model was constructed to fit the observed image as well as the entire spectral energy distributions (SEDs). The total optical depth of the dust shell is well constrained by the absorption dust feature (9.7mm silicate feature), and the orientation of the object on the sky is well determined by comparing the flux ratio of the bipolar reflection lobes in both the observed and model images. The numerical model (as called NONSP) has successfully simulated the bipolar morphology, searchlight beams and dark lane structures in the observed images as well as obtained satisfactory SED fittings (see Fig 3). The NONSP model has shown that an object's orientation on the sky and the degrees of asymmetry in its circumstellar dust shell have a great impact on the optical morphology and spectral energy distribution.

Deep, high-resolution imaging has helped to detect circumstellar arc structures in an increasing number of PPNs (a total of 5 objects and 4 in this thesis: IRAS 17150-3224, IRAS 17441-2411, IRAS 16594-4656, IRAS 20028+3910, and AFGL 2688; see Table below for details.). The presence of these circular arcs in PPNs with a variety of viewing orientations implies that the arcs are illuminated portions of spherical shells, viewed in scattered light. The arcs are likely formed by density enhancements of the mass loss during the preceding AGB phase. The separations of the arcs translate to time scales of a few hundred years, which cannot be explained by the stellar pulsation period (~1 yr) or the nuclear thermal pulse period (~ 10⁴ yr). Similar arc and ring structures are also found in an AGB star (IRC+10216) and three PNs (NGC 6543, NGC 7027, and Hb 5). The persistence of these rings from the AGB to the PPN to the PN phase suggests that most parts of the AGB envelope are not affected during this transition, and that the shaping of PNs can be accomplished by wind interactions over a small solid angle.

A collimated bipolar outflow emerging from a visible disk around the proto-planetary nebula IRAS 17106-3046 (Fig 4a) is discovered. The radial intensity profile (Fig 4b) of the disk suggests that it is consistent with an expanding disk (rho propto r^-1) with an outer constant expanding envelope (rho propto r^-2). A recently formed jet breaking out of the bipolar lobe is also seen in the system.

My observational and simulation results suggest that the asymmetry needed to explain the origin of asymmetric PNs is already present in PPNs. Hence, the asymmetry in circumstellar dust shells must have been developed quickly in the very late AGB phase or the early PPN phase. The asymmetric dust shells play an important role on both its optical image and SED (see Fig 5a). The orientation of the asymmetry on the sky greatly affects the optical morphology: the object appears a bipolar reflection nebula when it is viewed at edge-on, but a bright point-like source with extended halo when it is viewed at pole-on (see Fig 5b).

The optical flux of a PPN, which originates from the scattered light and the reddened photosphere, differs greatly when it is viewed at different orientations while the infrared flux is the same since it is optically thin at longer wavelengths (see the SED above). The flux ratio between the infrared and the optical components in the SED is a first-order indicator of a PPN's orientation. The edge-on systems tend to possess high flux ratio while the pole-on systems have the flux ratio of near unity. However, the optical brightness is greatly dependent on the scattering process. The degree of asymmetry (such as the opening angle of a cavity) is another important factor that affects the flux ratio. As demonstrated in the NONSP model simulations, the flux ratio is sensitive to objects' orientations only when the opening angle is small. When the opening angle is large, the flux ratio calculated from the SEDs cannot easily differentiate objects' inclination angles.

The results from my thesis study on the nine PPNs are consistent with the theory, which explains the origin of optical bipolarity in PPNs; i.e., asymmetric circumstellar dust shells exist in the PPN phase. In contrast to the spherically symmetric circumstellar dust shells in the preceding AGB stars, dust shells in the PPN phase possess significant departure from the spherical symmetry. The orientation of the asymmetry and the degree of the deviation from the spherical symmetry greatly affect the observed optical morphology seen in PPNs. Based on my observational and numerical results, I conclude that the asymmetry needed to explain the origin of asymmetric PNs is already present in PPNs. Namely, the asymmetry must have been developed quickly in the very late AGB phase or the early PPN phase. The detection of concentric arcs in PPNs and PNs and the fact that the innermost arcs have typical dynamical ages of 103 years further constraints the shaping timescale. The existence of the arcs also suggests that most of the AGB envelope is not disturbed by the fast outflow which is likely responsible for the optical bipolarity in PPNs, and the fast outflow could be confined to a limited solid angle.