Katherine Y. L. Su
University of Calgary
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Featured researches published by Katherine Y. L. Su.
The Astrophysical Journal | 1998
Sun Kwok; Katherine Y. L. Su; Bruce J. Hrivnak
We report Hubble Space Telescope Wide Field Planetary Camera 2 imaging of the bipolar proto-planetary nebula IRAS 17150-3224. We find a series of concentric arcs superposed on the two reflection nebular lobes, similar to those observed in AFGL 2688. This suggests that periodic mass-loss enhancement is a common phenomenon in the asymptotic giant branch (AGB) phase. The near circular shape of the arcs suggests that mass loss is spherically symmetric during the AGB phase and that the bipolar morphology develops only after the end of the AGB phase. The possible evolution in morphology from proto-planetary nebulae to planetary nebulae is discussed.
The Astrophysical Journal | 1998
Katherine Y. L. Su; Kevin Volk; Sun Kwok; Bruce J. Hrivnak
The bipolar proto-planetary nebula IRAS 17441-2411 (the Silkworm Nebula) was observed with the Hubble Space Telescope. By fitting the observed spectral energy distribution and the V-band image by a two-dimensional radiation transfer model, we derive the properties of the circumstellar wind and a proposed circumstellar disk. We suggest that bipolar structures are probably more common than actually observed, because objects with similar intrinsic structures may simply appear stellar because of their orientation in the sky.
The Astrophysical Journal | 1999
Bruce J. Hrivnak; Sun Kwok; Katherine Y. L. Su
We report the discovery of two new, bipolar proto-planetary nebulae (PPNs). Both are cool IRAS sources for which we have confirmed optical counterparts by our 10 μm observations. Ground-based visible and infrared photometry was combined with the IRAS photometry and spectroscopy to produce their spectral energy distributions (SEDs). These SEDs look like those of other PPNs, in particular those of bipolar PPNs. The central stars of both objects are highly reddened and have color temperatures ~3000-4000 K. The nebulosities are dominated by scattered light, not emission lines as in planetary nebulae. IRAS 16594-4656 appears to possess the 21 μm emission feature seen previously in a dozen carbon-rich PPNs, along with the 8 μm polycyclic aromatic hydrocarbon feature. Published millimeter-wave observations support the notion that it is carbon-rich, while IRAS 17245-3951 appears to be oxygen-rich. These facts confirm that these two objects are PPNs in transition between the asymptotic giant branch and planetary nebula phases. Hubble Space Telescope imaging reveals that they are indeed bipolar nebulae. IRAS 17245-3951 clearly displays two lobes separated by a dust lane; thus it is viewed nearly edge-on. Two jetlike features are seen in the southern lobe of IRAS 17245-3951, similar to the base of the searchlight beams seen in AFGL 2688. IRAS 16594-4656 appears to be a bipolar nebulae viewed at an intermediate orientation, and both the lobes and the central star can be seen. IRAS 16594-4656 therefore gives us our first clear example of the apparent morphology of a bipolar PPN viewed at an intermediate orientation. The addition of these objects to the list of bipolar PPNs confirms that such bipolar morphologies develop early in post-AGB evolution.
The Astrophysical Journal | 2000
Sun Kwok; Bruce J. Hrivnak; Katherine Y. L. Su
We report the discovery of a collimated bipolar outflow emerging from a visible disk around the proto-planetary nebula IRAS 17106-3046. The radius of the disk, estimated to be 2500 AU at a distance of 4 kpc, is too large for it to be a Keplerian disk created by accretion. The radial intensity profile of the disk suggests that it is consistent with an expanding torus. In contrast to the open-end, butterfly-like lobes commonly seen in bipolar planetary nebulae, the lobes of IRAS 17106-3046 have pointed ends, suggesting a recently formed jet breaking out of the lobes. IRAS 17106-3046 therefore could represent the earliest stage of the bipolar-shaping process during the transition from an asymptotic giant branch star to a planetary nebula.
The Astrophysical Journal | 1999
Bruce J. Hrivnak; Philip P. Langill; Katherine Y. L. Su; Sun Kwok
Subarcsecond (075) V and I images have been obtained of 13 new proto-planetary nebulae (PPNs). We find nine of them to be resolved, with sizes up to 3, and two others to perhaps also be extended. Of the nine, four appear to be elliptical in shape. Thus an aspherical morphology is demonstrated to commonly exist early in the transition between the asymptotic giant branch and planetary nebula phases. The results of this larger observational study, together with the previously published imaging studies, indicate that most PPNs are elliptical (or bipolar) in shape.
arXiv: Astrophysics | 2003
Katherine Y. L. Su; Kevin Volk; Sun Kwok
Resent high-resolution optical imaging has directly revealed reflection nebulosity around proto-planetary nebulae (PPNs), the transition objects between asymptotic giant branch (AGB) stars and planetary nebulae (Sahai et al. 1998, Su et al. 1998, Ueta et al. 2000, Su et al. 2001). The existence of bipolar nebulae observed in the PPN phase suggests the presence of asymmetry in the AGB circumstellar dust shell. In order to model these objects, a self-consistent radiation transfer model is necessary. As a first attempt, we construct an approximate two-dimensional dust radiative transfer model to simultaneously fit the spectral energy distribution (SED) and images of a centrally-heated dust envelope.
Archive | 2001
Bruce J. Hrivnak; Sun Kwok; Katherine Y. L. Su
While AGB stars display spherical shapes and winds, planetary nebulae appear asymmetric. By studying proto-planetary nebulae (PPNs), the transitional objects between these two stages of stellar evolution, we can learn about when and how these asymmetric shapes develop. We describe in particular our HST imaging studies of PPNs in the visible and near-IR. They commonly reveal a bipolar morphology, in some cases with circumstellar, concentric arcs and in one case with a visible collimating disk. Mechanisms for shaping the nebula are reviewed, and the negative results of a radial velocity study to search for binary central stars of PPNs are discussed.
Archive | 2005
Bruce J. Hrivnak; Nicholas D. Smith; Katherine Y. L. Su
Archive | 2004
Bruce J. Hrivnak; Debbie M. Kelly; Katherine Y. L. Su
Archive | 2003
Bruce J. Hrivnak; Sun Kwok; Kenneth H. Hinkle; Doug Kelly; Katherine Y. L. Su