V. Bujarrabal

Unraveling the Structure of Aspherical Proto-Planetary Nebulae. I. Hubble Space Telescope Imaging and Hydroxyl Maser Line Observations of Roberts 22

We have obtained high-resolution wide-band, narrowband, and polarimetric images of the bipolar proto–planetary nebula Roberts 22 with the Wide Field Planetary Camera 2 on the Hubble Space Telescope. OH maser-line emission has also been observed using the Australia Telescope Compact Array. The wide-band images at 0.6 μm show bright bipolar lobes shaped like a butterflys wings, separated by a dark equatorial body of dense dust that completely obscures the central star. The material within each lobe appears to be organized into an amazingly complex pattern of a very large number of intersecting loops and filaments. The bright lobes are surrounded by a fainter halo whose inner region contains multiple thin shell structures, reminiscent of those seen in the prototype proto–planetary nebula, CRL 2688 (Egg Nebula). The halo can be traced to a distance of about 25 from the nebular center. The radial surface brightness of the halo is inconsistent with that expected for time-invariant mass loss at a constant expansion velocity. A simple model of the radial scattered light distribution supports the hypothesis that the bright lobes are cavities with thin (<05), dense walls that are optically thick in scattered light. The amount of mass in the cavity walls is quite large (0.3 M⊙) and the dynamical timescale for their formation is short (440 yr). The bright lobes show high fractional polarization (40%–50%) and a centrosymmetric polarization pattern, typical of a singly scattering reflection nebula. In contrast, the polarization in the dark equatorial region is small, and the polarization vectors do not show a simple orientation pattern. The polarization data have been compared with published models to set rough constraints on the dust albedo and scattering phase function and the nebular optical depths. The OH maser-line emission at 1665 and 1667 MHz is found to be concentrated in the dark equatorial region; the kinematics of the OH features indicates that they arise in low-latitude outflows. The nebular morphology in Roberts 22 is characterized by a high degree of point symmetry, which may result from the interaction of collimated bipolar outflows with the progenitor AGB envelope.

The molecular envelope of Mira

The CO J = 2 - 1 and J = 0 emission of the circumstellar envelope of Mira has been mapped. Three velocity components are found at 44.0, 46.8, and 48.1 km/s. The emission is interpreted as arising from a standard circumstellar envelope and a less extended bipolar outflow. The spherical circumstellar envelope has an expansion velocity of 3 km/s, a mass-loss rate of 1 x 10 to the -7th solar mass/yr, and a total molecular mass of 0.0004 solar mass. 29 refs.

A Highly Collimated Bipolar Outflow in a Proto-planetary Nebula: Hubble Space Telescope Imaging of Hen 401

We have obtained high-resolution wide- and narrowband images of the bipolar proto-planetary nebula Hen 401 with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. Two very long (145), cylindrical-shaped bipolar outflow lobes are seen in reflected light, each with a length/width ratio of ≈7, probably the largest seen in a proto-planetary nebula so far. The lobes are limb-brightened, indicating that they are optically thin to scattering. The central star, resolved for the first time from the surrounding nebulosity, is girdled by an equatorial torus and a bipolar skirtlike structure, both of which are coaxial with the lobes. A faint halo around the lobes marks the presence of an asymptotic giant branch (AGB) circumstellar envelope. We find Hα emission from photoionized gas in the vicinity of the central star, and we tentatively detect two small shock-emitting blobs located along the nebular axis about ±62 from the central star. A comparison of the lobe morphology with theoretical models indicates that the highly collimated lobes of Hen 401 result from the momentum-driven shock interaction of a high-velocity bipolar jet with the circumstellar envelope of the progenitor AGB star.

Massive Expanding Torus and Fast Outflow in Planetary Nebula NGC 6302

We present interferometric observations of 12CO and 13CO J = 2–1 emission from the butterfly-shaped, young planetary nebula NGC 6302. The high angular resolution and high sensitivity achieved in our observations allow us to resolve the nebula into two distinct kinematic components: (1) a massive expanding torus seen almost edge-on and oriented in the north-south direction (roughly perpendicular to the optical nebula axis), which exhibits very complex and fragmented structure; and (2) high-velocity molecular knots moving at more than 20 km s−1 and located in the optical bipolar lobes. These knots show a linear position-velocity gradient (Hubble-like flow), which is characteristic of fast molecular outflow in young planetary nebulae. From the low but variable 12CO/13CO J = 2–1 line intensity ratio, we conclude that the 12CO J = 2–1 emission is optically thick over much of the nebula. Using the optically thinner line 13CO J = 2–1, we estimate a total molecular gas mass of ~0.1 M☉, comparable to the ionized gas mass; the total gas mass of the NGC 6302 nebula, including the massive ionized gas from the photon dominated region, is found to be ~0.5 M☉. From radiative transfer modeling, we infer that the torus is seen at an inclination angle of 75° with respect to the plane of the sky, and is expanding at a velocity of 15 km s−1. Comparison with recent observations of molecular gas in NGC 6302 is also discussed.

Modeling the physical and excitation conditions of the molecular envelope of NGC 7027

Context. The link between the shaping of bipolar planetary nebulae and the mass ejection activity of their central stars is still poorly understood. Appropriately characterizing the evolution of shells ejected during the late stages of stellar evolution and the interaction between these shells is fundamental to gain insight into the mechanism of nebular shaping. It must include the study of the molecular emission, which tracks the mass-loss history during the late asymptotic giant branch (AGB) and post-AGB stages, when the nebula is being actively shaped. Aims. The Heterodyne Instrument for the Far Infrared (HIFI) aboard Herschel is an invaluable tool because it opens a new window (most of the sub-mm and far-infrared range is only accessible from space) from which to probe warm molecular gas (∼50–1000 K). This paper presents a radiative transfer, spatio-kinematic modeling of the molecular envelope of the young planetary nebula NGC 7027 in several high- and low-J 12 CO and 13 CO transitions observed by Herschel/HIFI and the IRAM 30-m radio telescope, and discusses the structure and dynamics of the molecular envelope. Methods. We developed a code that, used along with the SHAPE software, implements spatio-kinematic modeling with accurate non-LTE calculations of line excitation and radiative transfer in molecular species. We used this code to build a relatively simple “Russian doll” model to account for the physical and excitation conditions of the molecular envelope of NGC 7027. Results. The model nebula consists of four nested, mildly bipolar shells plus a pair of high-velocity blobs. The innermost shell is the thinnest and shows a significant increase in physical conditions (temperature, density, abundance and velocity) compared to the adjacent shell. This is a clear indication of a shock front in the system, which may have played a role in shaping the nebula. Each of the high-velocity blobs is divided into two sections with considerably different physical conditions. The striking presence of H2 Oi n NGC 7027, a C-rich nebula, is likely due to photo-induced chemistry from the hot central star, although formation of water by shocks cannot be ruled out. The computed molecular mass of the nebula is 1.3 M� , compatible with that derived from previous works.

Gomez's Hamburger (IRAS 18059-3211): a pre main-sequence A-type star

Aims. We study the nature of Gomezs Hamburger (IRASu200918059-3211; GoHam), a nebula that has been proposed to be a post-AGB object. Such a classification has not been confirmed; instead, we argue that it will be a key object in the study of disks rotating around young stars. Methods. We present high-resolution SMA maps of COxa0 J xa0= 2-1 in Gomezs Hamburger. The data are analyzed by means of a code that simulates the emission of a nebula showing a variety of physical conditions and kinematics. Results. Our observations clearly show that the COxa0emitting gas in Gomezs Hamburger forms a spectacular disk in keplerian rotation. Model calculations undoubtly confirm this result. The central (mainly stellar) mass is found to be high, ~4xa0

The physical conditions in Gomez's Hamburger (IRAS 18059-3211), a pre-MS rotating disk

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Very Large Telescope observations of Gomez’s Hamburger: Insights into a young protoplanet candidate

u2000for a distance of 500xa0pc. The mass and (relatively low) luminosity of the source are, independent of the assumed distance, very different from those possible in evolved stars. This object is probably transitional between the pre-MS and MSxa0phases, still showing interstellar material around the central star or stellar system.

Early results from ChanPLaNS: Mystery of hard X-ray emitting CSPNe

Aims. We aim to study the structure, dynamics, and physical conditions of Gomez’s Hamburger (IRAS 18059-3211; GoHam), and in particular confirm that GoHam mainly consists of a flaring disk in Keplerian rotation around a young, probably pre-MS star. Methods. We present high resolution SMA maps of 12 CO J = 2−1, 13 CO J = 2−1, 12 CO J = 3−2, and C 17 O J = 3−2, as well as data on 12 CO J = 6−5 and the continuum flux at these wavelengths. Spatial resolutions as high as 1 �� are attained. Except for the C 17 O data, the dynamical ranges are larger than 10. The maps are compared with a numerical model, which simulates the emission of a rotating disk with the expected general properties of such objects, and a very satisfactory fitting of our maps is obtained. The meaning and reliability of our results are thoroughly discussed. Results. Our observations allow measurement of the main properties of GoHam on scales of between ∼1 �� (∼5 × 10 15 cm, for the assumed distance, 300 pc) and the total extent of the nebula, 14 �� . We are able to measure the global structure of the gas-rich disk, which is found to be flaring, and its dynamics, which is clearly dominated by Keplerian rotation, with a small degree of turbulence. The combination of different lines, in particular of different opacities, allows us to estimate reasonably the distributions of gas temperature and density. We clearly find a significant and sharp increase in temperature at large distances from the equator, accompanied by a decrease in density of the same order. Finally, we identify a condensation in the southern part of the disk that has no counterparts in the northern nebula. This condensation is quite extended (about 5 × 10 15 cm), contains a significant amount of mass (roughly, ∼6 × 10 −3 M� ), and seems to be associated with a detectable distortion of the global rotation kinematics. We discuss several possible

Low-Excitation Atomic Gas in PPNe: Iso Observations of Fir line Emission in O-Rich Objects

Planets are thought to form in the gas and dust disks around young stars. In particular, it has been proposed that giant planets can form through the gravitational instability of massive extended disks around intermediate-mass stars. However, we still lack direct observations to constrain this mechanism. We have spatially resolved the 8.6 and 11.2