Birth and early evolution of a planetary nebula

The final expulsion of gas by a star as it forms a planetary nebula --- the ionized shell of gas often observed surrounding a young white dwarf --- is one of the most poorly understood stages of stellar evolution. Such nebulae form extremely rapidly (about 100 years for the ionization) and so the formation process is inherently difficult to observe. Particularly puzzling is how a spherical star can produce a highly asymmetric nebula with collimated outflows. Here we report optical observations of the Stingray Nebula which has become an ionized planetary nebula within the past few decades. We find that the collimated outflows are already evident, and we have identified the nebular structure that focuses the outflows. We have also found a companion star, reinforcing previous suspicions that binary companions play an important role in shaping planetary nebulae and changing the direction of successive outflows.Comment: 9 pages + 3 figures. To appear in Nature, 2 April 199

Two Subclasses of Proto-Planetary Nebulae: Model Calculations

We use detailed radiative transfer models to investigate the differences between the Star-Obvious Low-level-Elongated proto-planetary nebulae (SOLE PPNs) and DUst-Prominent Longitudinally-EXtended proto-planetary nebulae (DUPLEX PPNs) which are two subclasses of PPNs suggested by Ueta, Meixner, & Bobrowsky (2000). We select one SOLE PPN, HD 161796, and one DUPLEX PPN, IRAS 17150$-$3224, both of which are well studied and representative of their PPN classes. Using an axisymmetric dust shell radiative transfer code, we model these two sources in detail and constrain their mass-loss histories, inclination angles and dust composition. The physical parameters derived for HD 161796 and IRAS 17150$-$3224 demonstrate that they are physically quite different and that their observed differences cannot be attributed to inclination angle effects. If these calculations reflect a more general truth about SOLE vs. DUPLEX PPNs, then these two subclasses of PPNs are physically distinct with the SOLE PPNs derived from low mass progenitors and DUPLEX PPNs derived from high mass progenitors.Comment: Accepted by ApJ. 8 fig

An HST Snapshot Survey of Proto-Planetary Nebulae Candidates: Two Types of Axisymmetric Reflection Nebulosities

We report the results from an optical imaging survey of proto-planetary nebula candidates using the HST. We exploited the high resolving power and wide dynamic range of HST and detected nebulosities in 21 of 27 sources. All detected reflection nebulosities show elongation, and the nebula morphology bifurcates depending on the degree of the central star obscuration. The Star-Obvious Low-level-Elongated (SOLE) nebulae show a bright central star embedded in a faint, extended nebulosity, whereas the DUst-Prominent Longitudinally-EXtended (DUPLEX) nebulae have remarkable bipolar structure with a completely or partially obscured central star. The intrinsic axisymmetry of these proto-planetary nebula reflection nebulosities demonstrates that the axisymmetry frequently found in planetary nebulae predates the proto-planetary nebula phase, confirming previous independent results. We suggest that axisymmetry in proto-planetary nebulae is created by an equatorially enhanced superwind at the end of the asymptotic giant branch phase. We discuss that the apparent morphological dichotomy is caused by a difference in the optical thickness of the circumstellar dust/gas shell with a differing equator-to-pole density contrast. Moreover, we show that SOLE and DUPLEX nebulae are physically distinct types of proto-planetary nebulae, with a suggestion that higher mass progenitor AGB stars are more likely to become DUPLEX proto-planetary nebulae.Comment: 27 pages (w/ aaspp4.sty), 6 e/ps figures, 4 tables (w/ apjpt4.sty). Data images are available via ADIL (http://imagelib.ncsa.uiuc.edu/document/99.TU.01) To be published in Ap

Dealing with Disbelieving Students on Issues of Evolutionary Processes and Long Time Scales

Sooner or later, the Astronomy 101 or astrobiology instructor encounters a student who disbelieves, or is at least skeptical of, factual information presented about the age of the Earth, the age of the universe, astrobiology, or biological evolution. Understanding the evidence and current state of our scientific knowledge about these subjects is important for the Astro 101 instructor faced with individual skeptical students. This understanding is also vitally important for those Astro 101 students who are future teachers and have preconceptions that could have a major impact on the thinking of large numbers of future students. This article contains a summary of different types of pseudoscientific beliefs that students have and suggests ways to approach these subjects so that skeptical learners are more likely to consider the facts presented in the astronomy class. Also included are some useful approaches for dealing with the more recent creationist ideas and tactics, such as “irreducible complexity” and “intelligent design”—especially now that President George W. Bush has expressed support for intelligent design. An appendix catalogs the different forms of creationism and lists some typical questions that their proponents might ask in class, along with suggested answers

THE DUST RING OF LUMINOUS BLUE VARIABLE CANDIDATE HD 168625: INFRARED OBSERVATIONS AND MODEL CALCULATIONS1

We present a 2.218 lm image from the Hubble Space Telescope/Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and a 55 lm image from ISOPHOT of the dust ring surrounding the luminous blue variable (LBV) candidate HD 168625, together with new temperature and optical depth maps derived from mid-IR images. The shell is detached from the star in the near-IR, and substructure in the overall toroidal shell is visible. The far-IR image constrains the extent of the dust shell to 2500 in diameter, providing an upper radius limit for modeling. The temperature maps and the NICMOS image show evidence for very small transiently heated dust grains in the shell. The opacity maps show higher optical depth in the limbs, consistent with interpretation of the dust shell as an equatorially enhanced torus inclined 60 with respect to the observer. An overall trend in the dust emission location with wavelength is observed and interpreted as a variation with respect to location in the nebula of either the dust grain size distribution or gas-to-dust mass ratio. Radiative transfer calculations using 2-Dust indicate that a mass-loss event occurred5700 yr ago with a rate of ð1:9 0:1Þ 104 M yr1, creating a dust torus that currently has a V 0:22 in the equatorial plane and a dust mass of ð2:5 0:1Þ 103 M. Using published values for the gas mass, we nd a gas-to-dust mass ratio of 840, which is 4 times higher than current estimates for the interstellar medium. In addition to