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We have determined the gas kinematics, diagnostic and ionic radial profiles, spatial structure, and evolutionary phase of NGC 6720 (the Ring Nebula) by means of tomography and a three-dimensional recovery technique applied to long-slit high-resolution spectra. The main shell of the Ring Nebula is a triaxial ellipsoid (radii of 0.10, 0.13, and 0.20 pc) seen nearly pole-on and expanding in an approximately ballistic fashion (Vexp = 0.65 km s-1 arcsec-1). The central star characteristics [log(L*/L?) 2.3, T* 120,000 K], combined with the nebular age of 7000 yr, indicate that the M* 0.61-0.62 M? post-AGB star is approaching the white dwarf cooling sequence. The equator of the Ring Nebula is optically thick and much denser than the optically thin poles. The inner halo surrounding NGC 6720 represents the pole-on projection of the AGB wind at high latitudes (circumpolar) directly ionized by the central star, whereas the outer, fainter, and circular halo is the projection of the recombining AGB wind at mean to low latitudes, shadowed by the main nebula. The spatio-kinematical properties of the Ring Nebula and the origin of the dense knots commonly observed in late-stage planetary nebulae are critically compared with the predictions of radiation-hydrodynamic and wind interaction models.

We studied the brightness and spectral evolution of the young eruptive star V1647 Ori during its recent outburst in the period 2004 February-2006 September. We performed a photometric follow-up in the bands V, RC, IC, J, H, and Ks, as well as visible and near-IR spectroscopy. The main results derived from combining our data with those published by other authors are as follows: the brightness of V1647 Ori stayed more than 4 mag above the preoutburst level until 2005 October, when it started a rapid fading. In the high state we found a periodic component in the optical light curves with a period of 56 days. The delay between variations of the star and variations in the brightness of clumps of nearby nebulosity corresponds to an angle of 61° ± 14° between the axis of the nebula and the line of sight. The overall appearance of the infrared and optical spectra did not change in the period 2004 March-2005 March, although a steady decrease of H I emission-line fluxes could be observed. In 2006 May, in the quiescent phase, the He I 1.083 μm line was observed in emission, contrary to its deep blueshifted absorption observed during the outburst. The J - H and H - Ks color maps of the infrared nebula reveal an envelope around the star whose largest extension is about 18'' (0.03 pc). The color distribution of the infrared nebula suggests reddening of the scattered light inside a thick circumstellar disk. Comparison of the Ks and Hα images of McNeil's Nebula, the conical nebulosity illuminated by V1647 Ori, shows that HH 22A, the Spitzer infrared source, and the bright clump C of the nebula may be unrelated objects. We show that the observed properties of V1647 Ori could be interpreted in the framework of the thermal instability models of Bell and coworkers. V1647 Ori might belong to a new class of young eruptive stars, defined by relatively short timescales, recurrent outbursts, a modest increase in bolometric luminosity and accretion rate, and an evolutionary state earlier than that of typical EXors.

We have analysed the magnitudes, kinematics and positions of a complete sample of 320 PNs in the elliptical galaxy NGC 4697. We show (i) that the PNs in NGC 4697 do not constitute a single population that is a fair tracer of the distribution of all stars. The radial velocity distributions, mean velocities, and dispersions of bright and faint subsamples differ with high statistical confidence. (ii) Using the combined data for PNs brighter than 26.2, we have identified a subpopulation of PNs which is azimuthally unmixed and kinematically peculiar, and which thus neither traces the distribution of all stars nor can it be in dynamical equilibrium with the galaxy potential. (iii) The planetary nebula luminosity functions (PNLF) of two kinematic subsamples in NGC 4697 differ with 99.7% confidence, ruling out a universal PNLF. We estimate that the inferred secondary PN population introduces an uncertainty in the bright cutoff magnitude of ~ 0.15 mag for this galaxy. -- We argue that this secondary PN distribution may be associated with a younger, >~ 1 Gyr old stellar population, perhaps formed in tidal structures that have now fallen back onto the galaxy, as has previously been suggested for the X-ray point sources in this galaxy, or coming from a more recent merger/accretion with a red galaxy. The use of PNs for extragalactic distance determinations is not necessarily compromised, but their use as dynamical tracers of dark halos will require deep observations and careful analysis of large PN samples., 12 pages, 11 figures, Accepted for publication in The Astronomical Journal

Near-IR imaging in the direction of the IRAS 15408-5356 point source, which is associated with the RCW 95 H II region, revealed a young and massive stellar cluster. We detected 136 member candidates up to our completeness limit, embedded in an infrared nebula and concentrated in an area of about 3 pc2. About 60% of the candidates detected in all three of the J, H, and K bands present infrared excess at 2.2 μm. The UV photons provided by the most massive stars are enough to explain the observed free-free emission at radio wavelengths and the integrated infrared luminosity produced by the heated dust. The near-IR counterpart of the IRAS point source was identified using a more accurate position from the Midcourse Space Experiment (MSX) catalog; it coincides with the peak of emission in the MSX mid-IR bands. The measured integrated flux density of the infrared nebula at the K band is compatible with the expected free-free emission derived from the radio data, but an excess at the J and H bands was detected and may be due to either nonhomogeneous absorption across the nebula or to the presence of scattered stellar light, more prominent at smaller wavelengths.

New spatiokinematic observations were undertaken of the planetary nebula NGC 1514 in the [O III] line at 5007 A using an imaging Fabry-Perot spectrometer. Our results show an inner ellipsoidal shell and polar blobs that do not conform to bipolar morphology. It is argued that the nebula is a descendant of a common envelope binary system the periodicity of which is estimated to be about 10 days, with a progenitor mass of 4.5 M⊙.

We report long-slit spectroscopic mapping of the η Carinae nebula obtained using the Space Telescope Imaging Spectrograph. The observations reveal the presence of a previously unknown bipolar emission nebula (roughly ±2'' along its major axis) embedded within the well-known and larger Homunculus Nebula. A preliminary analysis suggests that this embedded nebula may have originated from a minor eruption event circa 1890, 50 years after the formation of the larger Homunculus.

HST emission-line images of five of the arguably closest planetary nebulae have shown that there is a progression of characteristics of their knots. This progression begins with dark tangential structures showing no alignment with the central star and location near the main ionization front. At the end of the progression in the largest nebulae, the knots are located throughout much of the ionized zone, where they are photoionized on the side facing the central star and accompanied by long tails well aligned radially. This modification of characteristics is what would be expected if the knots were formed near or outside the main ionization front, obtaining densities high enough to lead to their being only partially ionized as they are fully illuminated by the Lyman continuum (Lyc) radiation field. Their expansion velocities must be lower than that of the main body of the nebular shell. Their forms are altered by exposure to the radiation field from the star, although it is not clear as to the relative role of radiation pressure acting on the dust component vis-a-vis ionization shadowing. The one object that does not fit into this sequence is NGC 2392, which is the most complex nebula in our sample. In this case the inner part of the nebula is composed of a series of loops of material, some being ionization bounded, which cover only a small fraction of the area illuminated by the star. This complex structure may be what gives rise to the large variations in electron temperature inferred from low spatial resolution observations. Cometary-form knots are seen in the outer part of this object, with these objects closely resembling those found in the largest nebula in our sample, NGC 7293.

We present scanning Fabry-Perot observations of the planetary nebula NGC 2346 at the near-infrared vibrationally excited line H2 S(1) 1–0 at 2.122 μm. The kinematics matches a model of two ellipsoids with an outflow velocity proportional to the distance of the gas from the central star and moving radially outward. The stronger emission is located in an equatorial torus expanding at 16 km s-1. It is likely that a shock between 6 and 16 km s-1, depending on the assumed velocity of the red giant envelope (0 to 10 km s-1), excites the molecular gas. Depending on those limiting values for the shock velocity, the density of the preshock gas falls in the range (0.3–1.7) × 104 cm-3, and consequently the mass of the molecular gas in the nebula is estimated to be between 0.34 and 1.85 M⊙, much larger than the mass of the ionized gas, and marginally in agreement with the estimate coming from CO observations alone.

Hubble Space Telescope WFPC2 images made as planned parallel observations have produced emission-line images of fields in the outer portions of the Orion Nebula and near the center of the companion H II region, M43. Examination of these images have uncovered three new bright proplyds and one silhouette proplyd. Two of the bright proplyds lie within M43 and are photoionized by its central star, NU Ori. The new bright proplyd in the Orion Nebula shows a monopolar microjet, and analysis of its size and surface brightness indicates that it lies well in the foreground. Symmetric shocks indicate bipolar flow around the K8e star V1348 Ori (304-539) near θ2 Ori C. Evidence for multiple outflows from a source southeast of 036-927, well south of the bright bar feature of the nebula, is indicated. A bright reflection nebula was found around the B1.5 Vp star LP Ori (098-753). The form of this object indicates that this star is moving within the veil of neutral material that lies in front of M42.

Multiepoch HST/WFPC2 images of the massive eclipsing binary RY Scuti reveal that the compact ionized rings in its circumstellar nebula are expanding. Radio images of the free-free continuum at 15 GHz obtained with the Very Large Array confirm the expansion rate and extend the temporal baseline. Combined Hα and radio proper-motion measurements of the nebula's outer edge across the major axis suggest that the nebula is roughly 120 yr old, although complications exist due to nonuniform motion. The linear expansion rate supports a shell ejection hypothesis for the origin of RY Scuti's double-ring nebula and has important implications for the mass-loss history and evolutionary state of the stellar components. Historical observations lead us to speculate that at least one of the components in the binary system is an S Doradus variable or a close relative of this class of stars. [S III] λ9532 emission traces higher excitation than Hα, but it shows only minor intensity variations with position in the nebula. The width of the ionization fronts at the limb-brightened edges of the nebula are nearly unresolved in our WFPC2 images, validating proper-motion measurements that combine both Hα and radio continuum emission.

We have compared radio images from the Australia Telescope Compact Array and X-ray images from ROSAT of an area about 560 pc in diameter around the very extended emission nebula N19 on the southwestern edge of the bright ridge of the Small Magellanic Cloud. These data have allowed us to identify tentatively seven supernova remnants in this region. This number is large enough to suggest that the area has recently undergone some starburst activity. N19 itself appears to contain two overlapping supernova remnants within the extended H II region.

K'-band polarimetric images of star-forming regions S187 IR and S233 are presented. In S187 IR, a bipolar, near-infrared nebula is observed around IRAS 01202+6133, with the southern part being bright and knotty, and the northern part being faint and filamentary. The polarization pattern shows that the nebula is illuminated by a single near-infrared source, which is associated with IRAS 01202+6133. A polarization disk is found around this source with the disk plane roughly perpendicular to the axis of the bipolar nebula. We conclude that this source is the driving source of the outflow in the region. Two nebulae are detected in the S233 region. Associated with a compact infrared cluster, S233 A is roughly round in shape and shows a weak polarization pattern. The S233 B nebula shows roughly elongated geometry in the east-west direction. The polarization vectors divide it into three parts. Each part has distinctly different polarization from the others. The northeastern part of S233 B exhibits a centrosymmetric polarization pattern around a centroid, at which we locate a deeply embedded source (DES) undetected in the K' band and shorter wavelengths. The DES is likely the exciting source of water masers and outflows in the S233 B region. The western part of the nebula shows a parallel polarization pattern, but the southeastern part displays very little polarization. Comparison of the morphology and polarization of the nebulae in different star-forming regions suggests a close relationship between the evolutionary sequence of the young stellar objects and the morphology of the associated nebulae.

IC 349 is a small, fan-shaped reflection nebula located only 30'' from 23 Tau; its nucleus is, by a factor 15, the brightest area of the Pleiades nebulosity. We propose that IC 349 is a fragment of the Taurus-Auriga molecular cloud that has been encountered by the Pleiades in that cluster's southward motion and is being illuminated and shaped by the radiation field of 23 Tau. New Hubble Space Telescope multicolor imagery and the structure, colors, and surface brightness of IC 349 are discussed in terms of that hypothesis. What is known of the proper motion of the nebula, what can be inferred of the properties of the nebula from its color, and what is expected from radiation pressure theory appear to be compatible with this cloudlet-encounter hypothesis.

We have observed an ionized halo surrounding the planetary nebula Abell 36. It is barrel-shaped, with dimensions 4° × 5°, which is 17 pc × 21 pc at a distance of 240 pc. With an average Hα surface brightness of 0.8 ± 0.2 rayleighs, the halo's total Hα flux, 8.3 × 10-10 ergs cm-2 s-1, exceeds that of the previously known inner part of the nebula by a factor of 30. The ionized mass of the halo is 48 M⊙, where is the filling factor of the ionized gas. Velocity-resolved Hα spectra indicate that the halo is ambient interstellar matter and not matter ejected from the planetary nebula. The edges of the halo are evident in both 100 μm emission and red optical continuum.

The optical appearance of the Orion Nebula is significantly altered by the presence of interstellar extinction. Since Orion is high above the Galactic plane and is nearby, most of this extinction is due to material immediately in front of the associated star cluster. This extinction fundamentally alters the optical appearance of the Orion Nebula. We have quantitatively determined the extinction correction by comparing the surface brightness of the nebula at a resolution of about 17 in the radio continuum and the Hα emission line. The results compare well with new determinations made from Hα and Hβ line ratios. Extinction-corrected optical images are generated and discussed. The most important extinction feature, the dark bay that obscures the east side of the nebula, has a mass of about 3 M⊙ and possesses numerous small knots of material in addition to a feature that appears to be a shock. We also find that the thin main emitting layer on the front of the parent Orion molecular cloud is marked by numerous walls and plateaus. Only one such structure had been previously known, the one that produces a bright bar across the southeast portion of the nebula. The sharpest structure is associated with the complex of molecular outflow and infrared sources near the region called Orion-S and is likely to be the result of uncollimated outflow from an imbedded young star.

New deep narrowband images of the Orion Nebula obtained with WFPC2 on the Hubble Space Telescope (HST) and spectra taken with the HIRES spectrometer at the Keck Observatory are presented. We report eight new circumstellar disks seen in silhouette against the background nebular light and about 30 dark disks embedded within the bright proplyds rimmed by ionization fronts. Deep narrowband λ6300 A images reveal skins of glowing [O I] emission associated with several disks embedded within bright proplyds. [O I] emission also surrounds one dark disk not surrounded by an ionization front; this object may be embedded within the photon-dominated, mostly neutral region behind the ionization front of the Orion Nebula. The intensity and morphology of the [O I] emission provides support for the photon-dominated–region models of externally irradiated circumstellar disks in which soft UV powers photoablation of the disk surface. Dozens of outflows powered by young stars have been discovered on the new images. More than 20 stellar jets emerge from the externally irradiated circumstellar disks or their associated young stars embedded within the Nebula. Most are one-sided (monopolar) subarcsecond-scale microjets, too small to be seen on ground-based images against the bright background nebular light. Additionally, wide-angle winds from 10 young stars in the outskirts of the Nebula power large-scale bow shocks facing the Trapezium OB stars. These shocks may be produced by wind-wind interactions where the T-Tauri winds interact with the outflow of plasma from the core of M42. The largest such structure, associated with the star LL Ori, contains a number of compact high–proper-motion clumps moving almost tangentially to the bow shock. The new data are combined with older HST images to determine proper motions for many nebular features. Neither the LL Ori type bow shocks in the outskirts of the nebula nor the Hα + [O III] arcs that surround many proplyds near the Trapezium show measurable proper motions and are therefore stationary structures. However, most other bow-shaped features not centered on young stars exhibit large proper motions, with velocities ranging from 50 to 300 km s-1. The sources of many of these moving features remain unknown. The proper-motion survey of the nebular core reveals the presence of about a dozen new large-scale (>0.1 pc) outflow complexes. Many of these new outflows originate from the vicinity of the high-luminosity OMC-1S infrared and submillimeter source complex located southwest of the Trapezium. These supersonic features provide evidence that stellar outflows inject large amounts of kinetic energy into the nebula. However, a quantitative analysis indicates that their total power is small compared with the power in the plasma flowing away from the main nebular ionization front.

Linear features of up to 0.25 pc length are seen in both the Helix Nebula and the Orion Nebula. These features are most evident in emission-line ratio images. It is demonstrated that in both objects the features are along projections of lines between the dominant star and an optically thick compact feature (the cometary knots in the Helix and the proplyds in Orion). In general, these features are caused by shadowing of Lyman continuum radiation from the photoionizing star, although in the case of the Helix features, extinction by entrained dust plays a role. Some of the rays have neutral hydrogen cores. It is shown that the characteristics of these linear features can be used to analyze the three-dimensional structure of the nebulae. If the most recent model for these rays is correct and the electron temperature of the shadowed gas is only two-thirds that of the ambient material, then these rays play a small but measurable role in solving the t2 problem.

The planetary nebula M2-9 has drastically changed its shape since its discovery by Minkwoski. Although the outline of the nebula seems to be stationary, most of the knots and bright features (N1, N2, S1, and S2) have moved laterally from the west to the east edges of both lobes. These features and their changes have reflection, not point, symmetry. We have compiled high-quality CCD images in Hα and [O III] obtained every 2–5 years since 1985 to monitor position and morphological changes in the individual knots. Our results show that the recent structural changes are more complex than suggested previously. The pattern of changes resemble a rotating corkscrew-like pattern, as if a precessing ionization/excitation beam inscribes the knots and filaments on the pencil-shaped lobe edges. The beam flow speed is no higher than 0.01c, so the beam is not a light beam. Its rotation period is about 120 yr. The corkscrew pattern and its apparent speed render many simple models implausible. The model of beads sliding along a helical wire is easily ruled out by observations. A model that may work is a combination of a beam and a spray of energetic particles, both invisible and moving radially at about 1000 km s-1, which shock-heats and ionizes the walls of the lobes as the beams precess.

An analysis is reported of the low-resolution optical spectra of the very low excitation emission-line object Lin 593. It is demonstrated that this object has a hot star that ionizes the surrounding nebula, thus creating a Stromgren sphere. The hot star is the source of the observed continuum, while the lines arise from the nebula ionized by the photons from the hot star. From the measurements of line emission and continuum, the chemical abundance and the physical parameters of the hot star as well as those of the ionized nebula were obtained. It is shown that the chemical abundance of Lin 593 follows the SMC abundance as derived from previous analysis of similar objects, except for oxygen, which presents a low abundance compared to the SMC average abundance pattern. It is also shown that the hot star is a main-sequence star with spectral type in the range O9.5–B0.5 and, finally, that the ionized nebula of Lin 593 forms a H II region for which several parameters were obtained, characterizing it as a class III–IV, classical-dense, H II region.

We report the discovery of three proplyd-like structures in the giant HII region NGC 3603. The emission nebulae are clearly resolved in narrow-band and broad-band HST/WFPC2 observations in the optical and broad-band VLT/ISAAC observations in the near-infrared. All three nebulae are tadpole shaped, with the bright ionization front at the head facing the central cluster and a fainter ionization front around the tail pointing away from the cluster. Typical sizes are 6,000 A.U. x 20,000 A.U. The nebulae share the overall morphology of the proplyds (``PROto PLanetarY DiskS'') in Orion, but are 20 to 30 times larger in size. Additional faint filaments located between the nebulae and the central ionizing cluster can be interpreted as bow shocks resulting from the interaction of the fast winds from the high-mass stars in the cluster with the evaporation flow from the proplyds. The striking similarity of the tadpole shaped emission nebulae in NGC 3603 to the proplyds in Orion suggests that the physical structure of both types of objects might be the same. We present 2D radiation hydrodynamical simulations of an externally illuminated star-disk-envelope system, which was still in its main accretion phase when first exposed to ionizing radiation from the central cluster. The simulations reproduce the overall morphology of the proplyds in NGC 3603 very well, but also indicate that mass-loss rates of up to 10^-5 Mo/yr are required in order to explain the size of the proplyds. (abbreviated)

We report high angular resolution observations of complex mid-infrared (4.8?18 ?m) structure in the ? Carinae Nebula. This massive (~120 M?), luminous star is surrounded by a large bipolar nebula extending as much as ~22'' in the mid-infrared. Our observations reveal the presence of several condensations within the complex core known as the Homunculus, as well as what may be a toroidal structure visible at 18 ?m. This structure may be related to a similar torus seen at near-infrared wavelengths. Our high angular resolution photometry reveals for the first time the spatial variation of the silicate feature in the nucleus of the ? Car Nebula. Finally, we present the first high-resolution mid-infrared color-temperature and optical depth maps for the nebula and discuss their significance in relation to the origin of the emission and evolutionary status of the star.

We present HST Wide Field Planetary Camera 2 images of the very massive eclipsing binary RY Scuti. The HST Hα image shows a very complex ionized nebula roughly 1'' in size. Several interesting structures are revealed, including what appears to be a pair of concentric ionized rings above and below the equatorial plane of the system, located at the inner edge of a more extended dust torus. We reexamine some essential physical characteristics of the nebula, such as a possible magnetic field and the complicated mass-loss geometry of this near-Eddington limit contact binary. The effects of a rotating illumination source are considered to explain the brightness distribution in the rings. We suggest a model for the origin of the mass and detailed structure in the nebula. This model invokes a previous mass ejection that is shaped by nonspherical interacting winds and possibly magnetic fields.

A survey of extended objects in the Large Magellanic Cloud was carried out on the ESO/SERC R and J Sky Survey Atlases, checking entries in previous catalogs and searching for new objects. The census provided 6659 objects including star clusters, emission-free associations and objects related to emission nebulae. Each of these classes contains 3 subclasses with intermediate properties, which are used to infer total populations. The survey includes cross-identifications among catalogs and we present 3246 new objects. We provide accurate positions, classification, homogeneous measurements of sizes and position angles, as well as information on cluster pairs and hierarchical relation for superimposed objects. This unification and enlargement of catalogs is important for future searches of fainter and smaller new objects. We discuss the angular and size distributions of the objects of the different classes. The angular distributions show two off-centered systems with different inclinations, suggesting that the LMC disk is warped. The present catalog together with its previous counterpart for the SMC and the inter-Cloud region provide a total population of 7847 extended objects in the Magellanic System. The angular distribution of the ensemble reveals important clues on the interaction between the LMC and SMC.

We present near-infrared images of the star-forming region AFGL 5157 in the JHK' broadband filters and H-2 upsilon = 1-0 S(1) narrowband filter. The images reveal a dense cluster of stars and infrared nebulosities associated with previously known infrared sources. Of 54 near-infrared sources detected in the nebula, NGC 1985, 12 exhibit infrared excesses typical of T Tauri stars, Herbig Ae/Be stars, and protostars. The magnitude and color distribution of the cluster of stars in the nebula are found to be different from those outside the nebular region. The K'-magnitude distribution of the cluster is quite flat, while the noncluster is peaked toward the low magnitude. The [H - K'] color of the cluster also displays 0.3 mag redder than that of the noncluster. The infrared nebula displays a bright nucleus with two spirals extended to the north and south. In light of the color properties of the nebula, we propose a shell model for the nebular structure that could be formed by star-forming activity of the central cluster. Many sources with infrared excesses are found to be embedded in the shell structure. Twelve shocked knots in H-2 emission are observed in the region. The nonaxisymmetric distribution of the knots indicates the presence of multiple outflows in the region. Although we failed to identify the powering sources responsible for some of the HH-like objects, the relationship of the H-2 emission with infrared sources shows that there must be several spatially separated sources exciting the shocked H-2 emission as well as the previously observed H2O masers and molecular outflow. Diffuse H-2 emission is also detected on the shell structure, which supports the shell model of the nebula. This diffuse emission could result from fluorescence by relatively evolved stars in the cluster.

We present high-resolution, spatially resolved spectra of the nebula surrounding Sakurai's object. We find the expansion velocity of the nebula to be 30.9 ± 0.7 km s-1, and the nebula extent to be 44'' in diameter. We discuss the wide range of reported distances to Sakurai's object and the dilemma that these present, since some evidence strongly favors a short distance (1.1 kpc) while other data strongly favor a longer distance (5–8 kpc). We also present spectra of Sakurai's star centered near the λ4216 CN band that suggests the star had cooled significantly between 1996 October and 1997 May, but had not cooled further by 1997 July.

The structure and conditions within the Helix Nebula have been determined from emission-line images in Hβ, [O III], and He II and using spectra tracing a radial from the central star almost out to the northern boundary of the optical object. The nebula is approximately like a thick disk, rather than the torus suggested by low-ionization ions. The central region, previously thought to be a cavity, is filled with He+2 with a total gas density comparable to the main ring. The electron temperature in the outer part was determined from [N II] line ratios to be 9400 ± 200 K, while [O III] lines gave 11700 ± 700 K for the bulk of the main ring of intermediate-ionization material. The Hα/Hβ ratio is anomalously low in the central portion, suggesting that the electron temperature there is very high. The likely source of this elevated central temperature is heating by photoelectric electrons from grains mixed in with the nebular gas, since this process depends directly on the distance from the central star and becomes relatively more important than photoionization heating at the low nebular densities that apply for the helix. However, it is unlikely that the two-phase condition that can exist when photoelectric heating dominates has produced the cometary knots that freely populate this nearby planetary nebula.

The Maria Mitchell Observatory, in collaboration with the Astrokolkhoz Observatory, started a program of photometric monitoring of UX Ori-type stars (UXORs) with narrowband interference filters (IFs; augmented with the traditional broadband filters) aimed at separating the Hα emission variations from those of the continuum. We present the method of separation and the first results for RR Tau obtained in two seasons, each roughly 100 days long (2010 Winter-Spring and 2010 Fall-2011 Spring). We confirm the conclusion from previous studies that the Hα emission in this star is less variable than the continuum. Although some correlation between the two is not excluded, the amplitude of Hα variations is much smaller (factors of 3-5) than that of the continuum. These results are compatible with Grinin's model of UXORs, which postulates the presence of small obscuring circumstellar clouds as the cause of the continuum fading, as well as the presence of a circumstellar reflection/emission nebula, larger than the star and the obscuring clouds, which is responsible for Hα emission and the effect of the color reversal in deep minima. However, the results of both our broadband and narrowband photometry indicate that the obscuration model may be insufficient to explain all of the observations. Disk accretion, the presence of stellar or (proto) planetary companion(s), as well as the intrinsic variations of the star, may contribute to the observed light variations. We argue, in particular, that the Hα emission may be more closely correlated with the intrinsic variations of the star than with the much stronger observed variations caused by the cloud obscuration. If this hypothesis is correct, the close monitoring of Hα emission with IFs, accessible to small-size telescopes, may become an important tool in studying the physical nature of the UXORs' central stars.

Long-slit spectra of two peripheral regions around the Crab Nebula show no H(alpha) emission down to a flux level of 1.5 x 10(exp -7)erg/sq cm s sr (0.63 Rayleigh), corresponding to an emission measure limit of 4.2 cm(sup - 6) pc (3(sigma)) assuming A(sub V)= 1.6(sup m) and T(sub e)=7000 K. This is below the flux levels reported by Murdin & Clark (Nature, 294, 543 (198 1)) for an H(alpha) halo around the Crab. Narrow H(beta) emission as described by Murdin (MNRAS, 269, 89 (1994)) is detected but appears to be Galactic emission unassociated with the remnant. A review of prior searches indicates no convincing observational evidence to support either a high- or low-velocity envelope around the remnant. Spectral scans confirm a well-organized, N-S expansion asymmetry of the filaments with a approx. 500 km/s central velocity constriction as described by MacAlpine et al. (ApJ, 342, 364 (1989)) and Lawrence et (it. (AJ, 109, 2635 (1995)] but questioned by Hester et al. (ApJ, 448, 240 (1995)). The velocity pinching appears to coincide with an cast-west chain of bright [O III] and helium-rich filaments. This expansion asymmetry might be the result of ejecta interaction with a disk of circumstellar matter, but such a model may be inconsistent with H and He filament abundances in the velocity constriction zone. A re-analysis of the remnant's total mass suggests that the filaments contain 4.6 +/- 1.8 M(solar) in ionized and neutral cas, about twice that of earlier estimates. For a 10M(solar) progenitor, this suggests that approx.equals 4M(solar) remains to be detected in an extended halo or wind.

We have surveyed two PG 1159 class stars for the presence of ancient planetary nebulae by direct Her imaging. While we easily found an 11' diameter nebula around PG 1520+525, no nebula was detected around PG 1424+535. This nebula is the tenth member of the class of planetary nebulae surrounding PG 1159 stars. It now seems extremely likely that planetary nebulae are the direct progenitors of PG 1159 stars and the hydrogen deficient white dwarfs. Nevertheless, not all PG 1159 stars have detectable nebulae around them, probably because small differences in nebular ejection velocities and central star masses have a disproportionately large effect on the detectability of the nebulae. A 25% increase in expansion velocity or a 4% decrease in central star mass reduces the surface brightness by a factor of 2 by the time the star reaches the PG 1159 state. The nebula around PG 1520+525 is limb brightened to the south. While possibly due to an interaction with the ISM, the case is weak because the central star fails to exhibit a high proper motion and because the ISM density is not expected to be sufficient to induce the observed effect at the Galactic height (similar to 0.6 kpc) of the nebula. If an interaction is present, then the ISM in the vicinity of the nebula has a density similar to 0.05 cm(-3). We consider arguments that the distance to PG 1520+525 has been overestimated by the model atmosphere analysis method. At a distance of 1.1 kpc, the kinematic age of the nebula is three times the predicted post-AGB lifetime of the exciting star, the nebula is among the very largest known in the Galaxy, and the similar to 1.4 M(circle dot) nebula is abnormally massive. We find that a distance of 0.8 kpc violates none of the observational constraints by more than 1 sigma and, therefore, is preferred. (C) 1995 American Astronomical Society.

We report new high-resolution near-infrared imaging observations of the planetary nebula J900, in broadband J, H, and K, and narrowband 2.122 micrometers (for H2 1-0 S(1)), Brackett-gamma, and continuum filters. The H2 images reveal a remarkable set of outward-extending features not detected in any previous observations of theis object. In the northwest lobe of the inner bipolar region, the H2 emission, Brackett-gamma emission, and underlying continuum are spatially resolved. The relative positions of the peaks of the line emission suggest an ionization front seen edge-on. The continuum emission, apparently located nearer to the central star, is likely due to transiently heated small dust grains. The extended H2 emission features are probably excited either by oblique shocks caused by material ejected from the nebula, or by radiation leaking from holes in the opacity surrounding the ionized shell.

The object AFGL 2688 is a bipolar nebula surrounding a post-asymptotic giant branch star and is in transition toward becoming a planetary nebula. We present new NIR images in the I, J, H, and K bandpasses. These images reveal a wealth of structure in the nebula not previously seen. We find that the northern lobe is not smooth, but clumpy. There is evidence in the I-band image for periodic variations in mass-loss rate. In addition, we present narrowband images centered on the V = 1 - 0 S(1) line of molecular hydrogen and nearby continuum. The narrowband images reveal that broadband flux found in the equatorial region is dominated by line emission from vibrationally excited molecular hydrogen. H2 emission is also present in the lobes. Our K-band images show the equatorial emission to be in the form of a ring or torus extending around the object at a distance R about 7 x 10 exp 16 (D/1 kpc) cm. Mechanisms for exciting the equatorial H2 emission are discussed; we conclude that shocks are the only plausible source. We have analyzed the broadband images using models which assume single scattering of photons from the central star. We find that the bipolar axis is inclined to the plane of the sky by i about 5 deg. The images are best reproduced by models in which the density of scattering dust decreases fairly rapidly with increasing stellar latitude, with little or no dust near the poles. Furthermore, the optical depth is found to have very little wavelength dependence, which implies that the dust in AFGL 2688 is different from that in the interstellar medium.

The young planetary nebula Hubble 12 is observed using near-IR and UV spectrophotometry. The brightness of the O I lines, which is greater than in any other planetary nebula yet measured, indicates that fluorescent excitation by stellar continuum is the principal mechanism generating these lines. Extinction, electron density, and electron temperature are determined using infrared measurements combined with UV data and published optical observations. The range in extinction, density, and temperature implies that, within the ionized region, pockets of emission with distinctly different conditions exist. Logarithmic abundances for helium, oxygen, and sulfur are presented.

We present a new longslit CCD spectrum and Hα imaging of the nebula MA 1 in the Local Group spiral galaxy M33, one of McCall's four «Rosetta Stone» extragalactic H II regions. Our Hα image shows MA 1 to be a complex of arcs, loops, and filaments having a diameter of about 45 arcsec. Our spectrum does not confirm McCall's suggestion that MA 1 is a matter-bounded nebula; rather, the spatial profile of the emission lines shows that [O II] is quite weak near the central stellar object, but becomes more nearly normal in strength in the outer parts of the nebula

IN Com, the central star of the planetary nebula LT-5, is concluded to be a triple system as a result of photometric and spectroscopic examination. Data from UBV photometric and high-dispersion spectroscopic studies conducted between 1984 and 1989 are analyzed by means of a light curve and radial velocity studies. The radial velocity studies show that a single-lined binary with a period of 1.7545 days comprises the inner system. Further analysis reveals that it is an evolved G-type star with an active chromosphere. The G star has a low-mass companion, and the third body is a hot subdwarf that excites the surrounding planetary nebula. A change in the observed systemic velocities with a period of approximately 2000 days points toward the existence of the third body. Photospheric and chromospheric disturbances on the surface of the G star account for the changeable period of light variance on the central star. The masses of all the components are calculated, and the rotational velocity of the primary component and the orbital inclination of the inner system are determined. The statistical relation between distance and apparent diameter for nebulae of this type suggests that the distance d = 100-170 pc is accurate. 26 refs.

The polarization characteristics of stars in the stages of evolution from red giant to planetary nebula are investigated. Polarization is found to be a characteristic of the majority of these stars. The maximum observed polarization increases with age as the star evolves up the asymptotic giant branch (AGB) to the protoplanetary nebula phase, where the polarization reaches a maximum. The polarization then decreases as the star further evolves into a planetary nebula. These results indicate that aspherical mass loss is likely to be a continual feature of the late stages of stellar evolution, maintaining a clear continuity throughout the life of a star from the moment it first develops a measurable dust shell. The aspherical morphology seen in planetary nebulae has its origin in an intrinsic property of the star that is present at least as early as its arrival at the base of the AGB. 77 refs.