Raghvendra Sahai

Multipolar Bubbles and Jets in Low-Excitation Planetary Nebulae: Toward a New Understanding of the Formation and Shaping of Planetary Nebulae*

First results from a Hubble Space Telescope Wide Field Planetary Camera 2 Hα imaging survey of young planetary nebulae (PNs) are reported. The PNs have been selected on the basis of their low excitation characteristics. All objects imaged so far show highly aspherical morphology, with a majority characterized by multipolar bubbles distributed roughly point-symmetrically around the central star. In some objects, bipolar ansae or collimated radial structures are seen, indicating the presence of jets, whereas in others bright structures near the minor axes indicate the presence of disks or torii. The complexity, organization, and symmetry of the above structures lead us to propose that the primary agent for shaping PNs is high-speed collimated outflows or jets that operate during the late asymptotic galactic branch (AGB) and/or early post-AGB evolutionary phase, and not a preexisting equatorial density enhancement as envisioned in the currently popular model. These outflows carve out a complex imprint within an intrinsically spherical AGB circumstellar envelope (CSE). Subsequent expansion of a hot, tenuous stellar wind from the post-AGB star inside the imprinted AGB CSE then produces the observed PN, whose shape and structure depend in detail on how the characteristics of the jets change with time.

Hubble Space Telescope Observations of the SN 1987A Triple Ring Nebula

We have observed SN 1987A with the optically corrected WFPC2 on the Hubble Space Telescope both in emission lines and in the UV and optical continuum. The previously observed outer nebular structure is shown to be part of two closed unresolved loops. These loops were flash-ionized by the supernova itself. They are not caused by limb brightening of an hourglass shell produced by the interaction of the winds from the progenitor. The inner ring is seen to be extended and may be connected to the new outer rings by sheets of material. However, beyond the outer rings, emission is not seen, implying a very low density (n 1000. This density contrast of at least 100 is difficult to reconcile with the conventional picture of the progenitor evolution. Two models for the rings are presented, but each is deficient in important respects. A proper understanding of this system will require new physical insight.

Preplanetary Nebulae: A Hubble Space Telescope Imaging Survey and a New Morphological Classification System

Using the Hubble Space Telescope (HST), we have carried out a survey of candidate preplanetary nebulae (PPNs). We report here our discoveries of objects having well-resolved geometric structures, and we use the large sample of PPNs now imaged with HST (including previously studied objects in this class) to devise a comprehensive morphological classification system for this category of objects. The wide variety of aspherical morphologies which we have found for PPNs are qualitatively similar to those found for young planetary nebulae (PNs) in previous surveys. We also find prominent halos surrounding the central aspherical shapes in many of our objects; these are direct signatures of the undisturbed circumstellar envelopes of the progenitor AGB stars. Although the majority of these have surface brightness distributions consistent with a constant mass-loss rate with a constant expansion velocity, there are also examples of objects with varying mass-loss rates. As in our surveys of young PNs, we find no round PPNs. The similarities in morphologies between our survey objects and young PNs supports the view that the former are the progenitors of aspherical PNs. This suggests that the primary shaping of a PN does not occur during the PN phase via the fast radiative wind of the hot central star, but significantly earlier in its evolution.

YOUNG PLANETARY NEBULAE: HUBBLE SPACE TELESCOPE IMAGING AND A NEW MORPHOLOGICAL CLASSIFICATION SYSTEM

Using Hubble Space Telescope images of 119 young planetary nebulae, most of which have not previously been published, we have devised a comprehensive morphological classification system for these objects. This system generalizes a recently devised system for pre-planetary nebulae, which are the immediate progenitors of planetary nebulae (PNs). Unlike previous classification studies, we have focussed primarily on young PNs rather than all PNs, because the former best show the influences or symmetries imposed on them by the dominant physical processes operating at the first and primary stage of the shaping process. Older PNs develop instabilities, interact with the ambient interstellar medium, and are subject to the passage of photoionization fronts, all of which obscure the underlying symmetries and geometries imposed early on. Our classification system is designed to suffer minimal prejudice regarding the underlying physical causes of the different shapes and structures seen in our PN sample, however, in many cases, physical causes are readily suggested by the geometry, along with the kinematics that have been measured in some systems. Secondary characteristics in our system such as ansae indicate the impact of a jet upon a slower-moving, prior wind; a waist is the signature of a strong equatorial concentration of matter, whether it be outflowing or in a bound Keplerian disk, and point symmetry indicates a secular trend, presumably precession, in the orientation of the central driver of a rapid, collimated outflow.

Shaping Proto-Planetary and Young Planetary Nebulae with Collimated Fast Winds

Using two-dimensional hydrodynamical simulations, we investigate the interaction of a collimated fast wind (CFW) interacting with a spherical asymptotic giant branch (AGB) wind as the mechanism for shaping proto-planetary nebulae (PPNs) and young planetary nebulae. In particular, we compare our simulations with the observations of an evolved PPN with multiple, highly collimated lobes, CRL 618. We characterize our model CFW by three parameters—opening angle, velocity, and mass-loss rate—and explore the dependence of the properties of the shell on the first two. For given opening angle and velocity, the mass-loss rate is chosen to give a shell velocity of about 150 km s-1 at the tip, similar to that seen in CRL 618. In our simulations, the shell dynamics is found to depend on the velocity of the fast wind: we obtain a momentum-driven shell for a 300 km s-1 fast wind and a ballistic bow shock-driven shell for a 1000 km s-1 fast wind. The shell driven by the collimated fast wind is highly collimated, even though the AGB wind is spherical. Time variations in the velocity of the fast wind produce a series of internal shock pairs interacting with the inner surface of the shell. As a result of radial expansion, the density of the internal shocks decreases with distance. Various emission diagnostics have been derived from our simulations. For a 300 km s-1 fast wind, the optical emission arises from both the shocked AGB wind and shocked fast wind, showing one or two bright bowlike structures at the tip of the lobe. However, for a 1000 km s-1 fast wind, since the shocked fast wind is much hotter, it emits mainly in X-ray emission; the optical emission forms only one bowlike structure at the tip associated with the shocked AGB wind. The position-velocity (PV) diagrams derived from our simulations all show a broad range of velocities at the tip. The detailed PV structure and velocity range at the tip depend on the shell dynamics and the relative contributions of the shocked fast wind and shocked AGB wind. We make a detailed comparison of our simulations to the observations of the relatively isolated northwestern (W1) lobe of CRL 618. We find that a 300 km s-1 collimated fast wind with an opening angle of 10° can readily produce a highly collimated lobe similar to the W1 lobe, including the bowlike emission structure at its tip. However, our models have difficulty producing the bright emission structures seen along the body of the lobe. The [S II] λ6716/λ6730 ratios at the tip of the lobe in all of our simulations are similar to that observed at the tip of the W1 lobe. The optical line ratios indicate a temperature stratification in the tip; for both the simulations and observations, however, the temperatures at the tip of the lobe in our simulations are higher than observed. The position-velocity (PV) diagrams derived from our simulations are all qualitatively consistent with the current observations. The collimated fast wind in CRL 618 is unlikely to be steady and is not radiatively driven.

Time-Resolved Observations of Jupiter's Far-Ultraviolet Aurora

Simultaneous imaging and spectroscopic observations of Jupiters far-ultraviolet aurora covering half a jovian rotation were made on 31 May 1994. The Hubble Space Telescope Wide Field Planetary Camera 2 images revealed dramatic and rapidly changing auroral features, including discrete longitudinal structures along the auroral ovals, with variable contrast; a poleward offset in a north oval sector, showing equatorward motion near dusk; emissions polewards of the ovals, apparently co-rotating; and a bright event developing near the dawn limb. Viewing geometry effects explain the rotational intensity modulation observed by the International Ultraviolet Explorer, without intrinsic longitudinal asymmetries.

Saturn's hydrogen aurora: Wide field and planetary camera 2 imaging from the Hubble Space Telescope

Wide field and planetary camera 2/Hubble Space Telescope (WFPC2/HST) images of Saturns far ultraviolet aurora reveal emissions confined to a narrow band of latitudes near Saturns north and south poles. The aurorae are most prominent in the morning sector with patterns that appear fixed in local time. The geographic distribution and vertical extent of the auroral emissions seen in these images provide the first new observational insights into the physical mechanisms that power Saturns aurora since the Voyager encounters 17 years ago.

The “Water-Fountain Nebula” IRAS 16342–3814: Hubble Space Telescope/Very Large Array Study of a Bipolar Protoplanetary Nebula*

We present Hubble Space Telescope (HST) Wide-Field and Planetary Camera 2 images and VLA OH maser emission-line maps of the cold infrared object IRAS 16342-3814, believed to be a protoplanetary nebula. The HST images show an asymmetrical bipolar nebula, with the lobes separated by a dark equatorial waist. The two bright lobes and the dark waist are simply interpreted as bubble-like reflection nebulae illuminated by starlight escaping through polar holes in a dense, flattened, optically thick cocoon of dust, which completely obscures the central star. A faint halo can be seen surrounding each of the lobes. The bubbles are likely to have been created by a fast outflow (evidenced by H2O emission) plowing into a surrounding dense, more slowly expanding, circumstellar envelope of the progenitor asymptotic giant-branch (AGB) star (evidenced by the halo). The IRAS fluxes indicate a circumstellar mass of about 0.7 M☉(D/2 kpc) and an AGB mass-loss rate of about 10-4 M☉ yr-1 (Vexp/15 km s-1)(D/2 kpc)2 (assuming a gas-to-dust ratio of 200). OH features with the largest redshifted and blueshifted velocities are concentrated around the bright eastern and western polar lobes, respectively, whereas intermediate-velocity features generally occur at low latitudes, in the dark waist region. We critically examine evidence for the post-AGB classification of IRAS 16342-3814.

The Starfish Twins: Two Young Planetary Nebulae with Extreme Multipolar Morphology

We present Hα images of two objects, He 2-47 and M1-37, obtained during a Hubble Space Telescope imaging survey of young planetary nebulae (PNs) selected on the basis of their low-excitation characteristics. The two objects show a highly aspherical morphology, characterized by multiple lobes distributed around the central star. Such a morphology has never been seen before in any astrophysical setting. Bright structures near the minor axes indicate the presence of dense equatorial tori (seen edge-on in M1-37 and as partial elliptical rings in He 2-47). In both nebulae, the central star is found to be offset from the geometrical center of symmetry of the waist region. The multiple lobes of He 2-47 and M1-37 have been produced fairly recently (a few hundred years ago), based on rough estimates of their expansion ages. The extreme multipolar morphology of these PNs supports the recent hypothesis of Sahai & Trauger that the primary agents for shaping PNs are high-speed collimated outflows that operate during the late asymptotic giant branch (AGB) and/or early post-AGB evolutionary phase; these outflows change their direction episodically, and/or multiple collimated outflows with different axes operate (quasi)simultaneously. Drastic modifications of existing theories or completely new ideas are needed in order to obtain a full understanding of the salient morphological features of He 2-47 and M1-37.

The Spatio-Kinematical Structure and Distance of the Preplanetary Nebula IRAS 19134+2131

Using the Very Long Baseline Array at six epochs, we have observed H2O maser emission in the preplanetary nebula IRAS 19134+2131 (I19134), in which the H2O maser spectrum has two groups of emission features separated in radial velocity by ~100 km s-1. We also obtained optical images of I19134 with the Hubble Space Telescope to locate the bipolar reflection nebula in this source for the first time. The spatio-kinematical structure of the H 2O masers indicates the existence of a fast, collimated (precessing) flow having a projected extent of ~140 mas and an expansion rate of ~1.9 mas yr-1 on the sky plane, which gives a dynamical age of only ~40 yr. The two detected optical lobes are also separated by ~150 mas in almost the same direction as that of the collimated flow. The good agreement between the extent and orientation of the H2O maser outflow and optical lobes suggests that the lobes have been recently formed along the collimated fast flow. Thus, the circumstellar envelope around the evolved star has apparently been penetrated by the fast flow and has been cleared for the emergence of the starlight in the directions of the fast flow. The positions of all of the detected maser features have been measured with respect to the extragalactic reference source J1925+2106 over one year. Therefore, we analyzed maser feature motions that consist of the combination of an annual parallax, a secular motion following Galactic rotation, and the intrinsic motions within the flow. We obtain an annual-parallax distance to I19134 of D = 8.0 kpc and estimate its location in the Galaxy to be (R, θ, z) = (7.4 kpc, 62° ± 5°, 0.65 kpc). From the mean motion of the blueshifted and redshifted clusters of maser features, we estimate the three-dimensional (3D) secular motion of I19134 to be (VR,Vθ,Vz) = (3, 125, 8) km s-1. From the height from the Galactic plane, z, and the velocity component perpendicular to the Galactic plane, Vz, we estimate a rough upper limit of ~9 M☉ to the stellar mass of I19134s progenitor.