FLIERs and Other Microstructures in Planetary Nebulae. IV. Images of Elliptical PNs from the Hubble Space Telescope

?????We report new results from high spatial resolution Wide Field Planetary Camera 2 imaging studies of FLIERs and other microstructures in the planetary nebulae NGC 3242, 6826, 7009, and 7662. Most FLIERs have head-tail morphologies, with the tails pointing outward from the nucleus. Ionization gradients that decrease with distance from the nebular center are ubiquitous. These are consistent with an ionization front in neutral knots of density ?104 cm-3. Can neutral knots account for the properties of FLIERs? We compare two broad classes of possible explanations for FLIERs with the new images: high-speed bullets ramming through the shells of planetary nebulae, and photoevaporated gas swept by winds into head-tail shapes. Both classes of models fail basic consistency tests. Hence an entirely new conceptual paradigm is needed to account for the phenomenology of FLIERs

HST Measurements of the Expansion of NGC 6543: Parallax Distance and Nebular Evolution

The optical expansion parallax of NGC 6543 has been detected and measured using two epochs of HST images separated by a time baseline of only three years. We have utilized three separate methods of deriving the angular expansion of bright fiducials, the results of which are in excellent agreement. We combine our angular expansion estimates with spectroscopically obtained expansion velocities to derive a distance to NGC 6543 of 1001$\pm$269 pc. The deduced kinematic age of the inner bright core of the nebula is 1039$\pm$259 years; however, the kinematic age of the polar caps that surround the core is larger - perhaps the result of deceleration or earlier mass ejection. The morphology and expansion patterns of NGC 6543 provide insight into a complex history of axisymmetric, interacting stellar mass ejections.Comment: Accepted for publication in AJ. 18 pages. 6 figure

Hubble Space Telescope Expansion Parallaxes of the Planetary Nebulae NGC 6578, NGC 6884, NGC 6891, and IC 2448

We have combined two epochs of Hubble Space Telescope WFPC2 imaging data with ground-based expansion velocities to determine distances to three planetary nebulae (NGC 6578, NGC 6884, and IC 2448). We used two variants of the expansion parallax technique--a gradient method and a magnification method--to determine the distances. The results from the two methods agree to within the errors. A fourth nebula was included in the study (NGC 6891), but the expansion was too small to determine the distance, and only a lower limit was obtained. This is the first paper in a series which will examine at least 24 nebulae in total.Comment: 26 pages, 6 figures, figure 2 is color. accepted AJ, March 200

Flare stars

Flare Star

A Conversation with Yervant Terzian

Additional content can be found at http://hdl.handle.net/1813/14723.1_7ivamug

Close galaxy pairs in low and medium density regions: the Southern sky

We extend to the southern hemisphere a continuing program of optical and HI observations of galaxy pairs (Chengalur \etal 1993, 1994, 1995 and Nordgren \etal 1997). These pairs are drawn from published redshift catalogs and represent a complete sample. We present new data of 15 pairs observed with the Palomar 5-meter telescope, Mount Stromlo Siding Spring 40-inch telescope, Australia Telescope Compact Array and VLA D synthesis array. These galaxy pairs are all defined as close pairs (projected separations < 100 kpc). HI companions are found near five of 15 pairs

Fast, low-ionization emission regions and other microstructures in planetary nebulae

Planetary nebulae (PNs) have highly articulated structures, called "microstructures", little of which has been explored with spectrophotometry at high spatial resolution. We report spectrophotometric observations of microstructures characterized by both high SNR and high spatial resolution (1."5)along the symmetry axes of NGC 3242, NGC 7662, and IC 2149. Of particular interest in these PNs are pairs of low-ionization and/or high-velocity ansae and jets (fast, low-ionization emission regions (FLIERs) on nearly opposite sides of their respective central stars, or nuclei. We present deep optical spectra of FLIERs in all three PNs and discuss their consequences. The strong low-ionization lines of O°, N<SUP>+</SUP>, O<SUP>+</SUP>, and S<SUP>+</SUP>, substantial space velocities, and temperatures and densities of FLIERs are not well explained either by models of edge-on ionization fronts or by models built on analogies to shock-excited Herbig-Haro objects

The 1.0 megaparsec galaxy pair sample in low-density regions

Using complete redshift catalogs, we have compiled a list of galaxy pair's based solely on a pair's projected separation, r<SUB>p</SUB>, and velocity difference, ΔV. We have made high-velocity precision H I observations of each galaxy in the sample and have reported these in the literature. Due to the nature of the redshift catalogs, we are able to quantitatively evaluate the effects of isolation and number density of surrounding galaxies on each pair in the sample. For the close galaxy pairs (r<SUB>p</SUB> &lt; 100 kpc), the degree of isolation (a measure of the number of near neighbors) has little effect on the median ΔV. This median is about 55 km s<SUP>−1</SUP> for the 25 close pairs (if medium-density close pairs are omitted ΔV is even smaller, but the difference is not statistically significant). The effect of isolation is strong for the entire sample of galaxy pairs with separations as large as 1.0 Mpc. For these larger separation pairs, relaxation of strict isolation requirements introduces small groups into the sample, which dramatically increases the median ΔV. We find little evidence of an increase in the median ΔV with decreasing r<SUB>p</SUB>, nor with increasing total luminosity. For our isolated pairs in low-density regions, the overall median ΔV is only 30 km s<SUP>−1</SUP>. For similar separations and isolation criteria, galaxy satellites with larger luminosity ratios (i.e., less dynamical friction) in higher density regions have ΔV approximately twice as large. We conjecture that our orbits are highly eccentric, so that the indirect effect of dynamical friction leads to predominantly small ΔV. However, the halos of our galaxies may also be of low density (although highly extended)