We have recently (Gonçalves et al. 2006) shown that previously reported enhanced N abundances in fast, low-ionization emission regions (FLIERs) of PNe, particularly in NGC 7009, may be due to ionization effects. We reached this conclusion through the 3D MOCASSIN photoionization modeling of the bright inner rim of NGC 7009 and its pair of FLIERs, assuming homogeneous elemental abundances throughout the nebula, for N as well as other elements. The (N$^+$/N)/(O$^+$/O) ratio predicted by our models is 0.60 for the rim and 0.72 for the knots, clearly in disagreement with the N$^+$/N=O$^+$/O assumption of the ionization correction factors (ICF) method. Therefore the ICFs are underestimated in both components, rim and knots, but more so in the knots. This effect is partly responsible for the apparent N overabundance claimed for FLIERs.

Based on the above results we are also investigating what would be the effect of the N/O ICF overestimation on the definition of Type I PNe which do not have FLIERs-like structures.

We report on our recent efforts to compute the X-ray emission from the hot, shocked stellar wind gas filling the inner cavity of planetary nebulae. To this end, we updated our 1D hydrodynamics code NEBEL by including a module that computes the heat transfer by thermal conduction across the contact discontinuity separating the hot shocked wind gas from the much cooler nebular material. Given the temperature and density structure of the hot bubble the X-ray emission is computed by means of the CHIANTI code. We find a reasonably close agreement of the computed X-ray luminosities with recent observations of Newton-XMM and Chandra. Our simulations also predict how the X-ray emission depends on the wind luminosity and the stellar parameters.

New results on the time variation of the radial abundance gradients in the galactic disk are presented on the basis of four different samples of planetary nebulae. These comprise both smaller, homogeneous sets of data, and larger but non-homogeneous samples. Four different chemical elements are considered, namely, O, S, Ar, and Ne. Other objects such as open clusters, cepheids and HII regions are also taken into account. Our analyses support our earlier conclusions in the sense that, on the average, the radial abundance gradients have flattened out during the last 6 to 8 Gyr, with important consequences for models of the chemical evolution of the Galaxy.

We present Spitzer/IRS spectra of a small sample of galactic OHPNe. This is a rare class of transition sources displaying both radio continuum and OH maser emission at 1612 MHz. Our observations show that they are heavily obscured O-rich stars whose mid-infrared spectra are dominated by the simultaneous presence of strong and broad amorphous silicate absorption features together with crystalline silicate emission features which sometimes appear also in absorption. Three of the sources observed are non-variable, confirming their post-AGB status, while another two seem to be still strongly variable. We propose that OHPNe represent the youngest population of high-mass PNe in the Galaxy.

The extreme carbon star, AFGL 3068, is losing mass at a rate in excess of 10$^{-4}$ M$_{\odot}$ yr$^{-1}$, and has so far been detected only in the infrared because it is hidden by a thick dust photosphere having a color temperature of $\sim$300K. Using the ACS camera on HST, we have imaged AFGL 3068 with broad-band filters at 0.6 and 0.8 $\mu$m and find a thin, apparently continuous spiral arc winding 4 or 5 times around the location of the star, from angular radii of 2 to 10 arcsec. We interpret this as the projection of nested spiral shells such as were predicted to occur when the mass-losing star is a member of a binary system. In this case, the illumination is presumably provided by ambient galactic starlight. Subsequent near-IR observations with the NIRC2 camera on the Keck II telescope using adaptive optics reveal that AFGL 3068 has two components separated by 0.11 arcsec, or 109 AU at a distance of 1 kpc. One very red component is presumably the mass-losing carbon star, while the other component is apparently a much bluer companion. Assuming each component has mass M(M$_{\odot}$), and ignoring the projection of the separation vector, we find the binary period to be 810 M$^{-0.5}$ yrs, strikingly comparable to the 710-yr separation of the shells obtained from the known outflow velocity of 14.7 km s$^{-1}$.

High-resolution 905-1187Å spectra of the central star of NGC 7009 obtained with the FUSE satellite provide new constraints on the mass loss and atmosphere parameters. The most prominent spectral feature is a very strong P-Cygni profile of O VI 1032-1038. The only other wind lines are S VI 933-44 (FUSE) and N V 1238-42 and O V 1371 from IUE spectra. C IV 1548-50 and P V 1118-28 are not present, indicating that the wind is very highly ionized. A lower limit on the mass loss rate ($\dot{M}>10^{-8} M_{\odot} $y$^{-1}, v_{\infty}=2450$ km s$^{-1}$) has been derived from an SEI analysis of these wind lines. Preliminary identification of several photospheric spectral features include several strong Fe VII and O VI lines between 1100 and 1170 Å.

Mass loss on the Asymptotic Giant Branch provides the origin of planetary nebulae. This paper reviews several relevant aspects of AGB evolution: pulsation properties, mass loss formalisms and time variable mass loss, evidence for asymmetries on the AGB, binarity, ISM interaction, and mass loss at low metallicity. There is growing evidence that mass loss on the AGB is already asymmetric, but with spherically symmetric velocity fields. The origin of the rings may be in pulsational instabilities causing mass-loss variations on time scales of centuries.

We present ELSA, a new modular software package, written in C, to analyze and manage spectroscopic data from emission-line objects. In addition to calculating plasma diagnostics and abundances from nebular emission lines, the software provides a number of convenient features including the ability to ingest logs produced by IRAF's splot task, to semi-automatically merge spectra in different wavelength ranges, and to automatically generate various data tables in machine-readable or LaTeX format. ELSA features a highly sophisticated interstellar reddening correction scheme that takes into account temperature and density effects as well as He II contamination of the hydrogen Balmer lines. Abundance calculations are performed using a 5-level atom approximation with recent atomic data, based on R. Henry's ABUN program. Downloading and detailed documentation for all aspects of ELSA are available at the following URL:

We present our recent results of the high dispersion spectroscopy of a Galactic halo planetary nebula, K 648, in the globular cluster M 15. The position-velocity diagrams of our spectra indicate the existence of collimated outflows in K 648. In the [N II$\lambda$6583 diagram, we found high velocity components with a velocity $\sim$60 km s$^{-1}$ in the central region of K648.

We report the detection of nebular emission lines in the optical and mid-infrared spectra of IRAS 17347–3139, a heavily obscured OH/IR star which may be rapidly evolving from the AGB to the PN stage. The presence of emission lines is interpreted as a clear indication that the ionization of its circumstellar envelope has already started. This source belongs to the rare class of objects known as ‘OHPNe’ displaying both OH maser and radio continuum emission. However, unlike the rest of stars in this class, prominent C-rich dust features are detected in its mid-infrared spectrum, which makes the analysis of this star particularly interesting.

We present preliminary results from the first X-ray gratings spectrometer observations of a planetary nebula (PN). We have used the Chandra X-ray Observatory Low Energy Transmission Gratings Spectrometer (LETGS) to observe the bright, diffuse X-ray source within the well-studied BD +30°3639. The LETGS spectrum of BD +30°3639 displays prominent and well-resolved emission lines of H-like C, O, and Ne and He-like O and Ne. Initial modeling indicates a plasma temperature $T_X \sim 2.5\times10^6$ K and abundance ratios of C/O $\sim20$, N/O $\stackrel{<}{\sim}1$, Ne/O $\sim4$, and Fe/O $\stackrel{<}{\sim}0.1$. These results suggest that the X-ray-emitting plasma is dominated by the shocked fast wind from the emerging PN core, where this wind gas likely originated from the intershell region of the progenitor asymptotic giant branch star.

Planetary nebulae are useful kinematic tracers of the stars in all galaxy types. I review recent observationally-driven developments in the study of galaxy mass profiles. These have yielded surprising results on spiral galaxy disk masses and elliptical galaxy halo masses. A key remaining question is the coupling between PNe and the underlying stellar populations.

We have observed the bipolar post-AGB candidate OH 231.8+4.2, using the mid-infrared interferometer MIDI and the infrared camera with the adaptive optics system NACO on the Very Large Telescope. The NACO images at 2.12 and 3.8 $\mu$m show a bipolar outflow and a flared disk or torus. An unresolved core ($<$200 mas in FWHM) is found at the centre of OH 231.8+4.2 in the 3.8 $\mu$m image. This compact source is resolved with the interferometer. The fringes from the four baselines consistently show the presence of a compact circumstellar material with an inner radius of 30–40 mas, which is equivalent to 40–50 AU at 1.3 kpc. This clearly shows that the mid-infrared compact source is not the central star (3 AU) but circumstellar material.

The results of long term observations of photometric ($UBV$) and spectral variability of several PNe are presented. For the most variable nebulae, IC 4997 and NGC 6572, showing both photometric and spectral changes in time, the main physical parameters ($T_{e}$, $N_{e}$ and $T_{*}$) and their time variations were estimated and discussed.

We present a $K\prime$-band speckle image and $HK$-band polarimetric images of the proto-planetary nebula Frosty Leo obtained using the 6 m SAO telescope and the 8 m Subaru telescope, respectively. Our speckle image revealed clumpy structures in the hourglass-like bipolar nebula. The polarimetric data, for the first time, detected an elongated region with small polarizations and polarization vector alignment on the east side of the central star. We have performed radiative transfer calculations to model the dust shell of Frosty Leo. We found that micron-size grains in the equatorial dense region and small grains in the bipolar lobes are required to explain the total intensity images, the polarization images, and the spectral energy distribution.

We present the results of time-resolved photometry and spectroscopy of the central star of the planetary nebula NGC 6026 and time-resolved photometry of the central star of the planetary nebula NGC 6337. The results of period analysis give an orbital period of 0.528088 days for NGC 6026 and a photometric period of 0.173474 days for NGC 6337. In the case of NGC 6337 it appears that the photometric period accurately reflects the orbital period and that the variability is the result of an irradiated hemisphere on the cool companion. For NGC 6026 however, radial velocities from spectroscopy show that the orbital period is twice the photometric period. In this case, the photometric variability is due to an ellipsoidal effect in which one of the stars fills, or nearly fills, its Roche lobe. Based on the data and modeling using the Wilson-Devinney code, we discuss the physical parameters of the two systems. We also relate the physical parameters to the shape and orientation of the nebulosity.

We present Spitzer/MIPS far-infrared (FIR) mapping of O-rich AGB star, R Hya.

Molecular line emission in the mm- and submm-wave domains is found to be a very useful tool to study planetary nebulae (PNe). Molecular lines, particularly the low-$J$ transitions of carbon monoxide, are known to probe most of the nebular material in young PNe and protoplanetary nebulae (PPNe). Many quantitative results on these objects have been so obtained, including general structure, total mass and density distribution, kinetic temperatures, velocity fields, etc. In more evolved PNe, however, the molecular line intensity (or even detection) is quite uncertain, due to photodissociation. Molecular lines often trace just certain parts of these nebulae, like equatorial tori or axial clumps. Other molecules are in general more affected by photodissociation than CO, but photo- or shock-induced chemistry produces high abundances of radicals and large molecules in some intermediate-evolution objects. The presence of rotating disks in some objects is finally discussed.

While photoionization codes have been carefully intercompared, a fundamental need for clean tests against real nebulae remains. NGC 2610 is a high-excitation planetary nebula which, even at HST resolution, is smooth and symmetric. Helium is He$^{++}$ throughout this nebula, which has a high electron temperature (20,000 K) resulting in strong UV lines. It is the best object we know of to test the performance of photoionization codes without the complication of low ionization knots or filaments. Its large angular diameter (40$\prime\prime$) allows spatial gradients to be observed. In 2001 and 2003, we obtained HST STIS long-slit observations to test against models. Observed lines cover wavelengths from 1240Å to 6563Å. Interstellar reddening is small.

We have constructed photoionization models of this nebula, and compare one with our observations. Most lines are in good agreement. The most discordant line is [Ne IV] $\lambda$2424, which is observed to be twice as strong as predicted. Collisional excitation of H$^0$ is the most important coolant, responsible for 30% of the total. Observations of the Balmer decrement in this nebula can put useful constraints on H$^0$ collision strengths.

We have observed SBS 1150+599A spectroscopically in the UV using HST to derive C/H $\sim$ 7.6 and N/H $\sim$ 7.0 for the first time. The central star temperature is now better constrained to $\sim$130,000K, but still is not well determined. This uncertainty dominates the error in O/H, which has been the subject of debate, yet with these data, SBS 1150+599A has the lowest O/H of any PN. Furthermore, the physics of this object are so extreme that minor differences in atomic modeling impact the composition analysis strongly. We also find that the binary central star, based on photometric and kinematic variations, exhibits CV-like properties with an amplitude of 13% and an orbital period of 3.924 hours.