Multipolar Planetary Nebulae: Not as Geometrically Diversified as Thought

Planetary nebulae (PNe) have diverse morphological shapes, including point-symmetric and multipolar structures. Many PNe also have complicated internal structures such as torus, lobes, knots, and ansae. A complete accounting of all the morphological structures through physical models is difficult. A first step toward such an understanding is to derive the true three-dimensional structure of the nebulae. In this paper, we show that a multipolar nebula with three pairs of lobes can explain many of such features, if orientation and sensitivity effects are taken into account. Using only six parameters - the inclination and position angles of each pair - we are able to simulate the observed images of 20 PNe with complex structures. We suggest that the multipolar structure is an intrinsic structure of PNe and the statistics of multipolar PNe has been severely underestimated in the past.Comment: 36 pages, 5 figures, 2 table

H2O maser motions and the distance of the star forming region G192.16-3.84

We present the results of astrometic observations of H2O masers associated with the star forming region G192.16-3.84 with the VLBI Exploration of Radio Astrometry (VERA). The H2O masers seem to be associated with two young stellar objects (YSOs) separated by \sim1200 AU as reported in previous observations. In the present observations, we successfully detected an annual parallax of 0.66 \pm 0.04 mas for the H2 O masers, which corresponds to a distance to G192.16-3.84 of D = 1.52 \pm 0.08 kpc from the Sun. The determined distance is shorter than the estimated kinematic distance. Using the annual parallax distance and the estimated parameters of the millimeter continuum emission, we estimate the mass of the disk plus circumstellar cloud in the southern young stellar object to be 10.0+4.3M\cdot. We also estimate the galactocentric distance and the peculiar motion -3.6 of G192.16-3.84, relative to a circular Galactic rotation: R\star = 9.99 \pm 0.08 kpc, Z\star = -0.10 \pm 0.01 kpc, and (U\star,V\star,W\star)=(-2.8\pm1.0,-10.5\pm0.3,4.9\pm2.7)[kms-1]respectively. The peculiar motion of G192.16-3.84 is within that typically found in recent VLBI astrometric results. The angular distribution and three-dimensional velocity field of H2O maser features associated with the northern YSO indicate the existence of a bipolar outflow with a major axis along the northeast-southwest direction.Comment: 9 pages, 2 figures and 4 tables. Accepted for publication on PAS

Bow shocks in water fountain jets

We briefly introduce the VLBI maser astrometric analysis of IRAS 18043-2116 and IRAS 18113-2503, two remarkable and unusual water fountains with spectacular bipolar bow shocks in their high-speed collimated jet-driven outflows. The 22 GHz H2O maser structures and velocities clearly show that the jets are formed in very short-lived, episodic outbursts, which may indicate episodic accretion in an underlying binary system.Comment: To appear in the proceedings of the IAU Symposium 336: Astrophysical Masers: Unlocking the Mysteries of the Universe (4-8 September 2017, Cagliari, Italy) - IAU Proceedings Series, eds. A. Tarchi, M. J. Reid, and P. Castangi

Kinematics of the Outflow From The Young Star DG Tau B: Rotation in the vicinities of an optical jet

We present $^{12}$CO(2-1) line and 1300 $\mu$m continuum observations made with the Submillimeter Array (SMA) of the young star DG Tau B. We find, in the continuum observations, emission arising from the circumstellar disk surrounding DG Tau B. The $^{12}$CO(2-1) line observations, on the other hand, revealed emission associated with the disk and the asymmetric outflow related with this source. Velocity asymmetries about the flow axis are found over the entire length of the flow. The amplitude of the velocity differences is of the order of 1 -- 2 km s$^{-1}$ over distances of about 300 -- 400 AU. We interpret them as a result of outflow rotation. The sense of the outflow and disk rotation is the same. Infalling gas from a rotating molecular core cannot explain the observed velocity gradient within the flow. Magneto-centrifugal disk winds or photoevaporated disk winds can produce the observed rotational speeds if they are ejected from a keplerian disk at radii of several tens of AU. Nevertheless, these slow winds ejected from large radii are not very massive, and cannot account for the observed linear momentum and angular momentum rates of the molecular flow. Thus, the observed flow is probably entrained material from the parent cloud. DG Tau B is a good laboratory to model in detail the entrainment process and see if it can account for the observed angular momentum.Comment: Accepted to Ap

A synchrotron jet from a post-asymptotic giant branch star

The evolution of low- and intermediate-initial-mass stars beyond the asymptotic giant branch (AGB) remains poorly understood. High-velocity outflows launched shortly after the AGB phase are thought to be the primary shaping mechanism of bipolar and multipolar planetary nebulae. However, little is known about the launching and driving mechanism for these jets, whose momentum and energy often far exceed the energy that can be provided by radiation pressure alone. Here, we report on the direct evidence of a magnetically collimated jet shaping the bipolar morphology of the circumstellar envelope of a post-AGB star. We present radio continuum observations of the post-AGB star IRAS 15445-5449 (OH 326.5-0.4) which has water masers tracing a fast bipolar outflow. Our observations confirm the earlier observed steep negative spectral index of the spectral energy distribution (SED) above similar to 3 GHz, and resolve, for the first time, the emission to originate from a radio jet, proving the existence of such jets around a post-AGB star. The SED is consistent with a synchrotron jet embedded in a sheath of thermal electrons. We find a close correspondence between the extent and direction of the synchrotron jet and the bipolar shape of the object observed at other wavelengths, suggesting that the jet is responsible for the source morphology. The jet is collimated by a magnetic field of the order of mG at almost 7000 au from the central star. We recover observations from the Australia Telescope Compact Array archive that indicate that the emission measure of the thermal component has increased by a factor of 3 between 1998 and 2005 after which it has remained constant. The short time-scale evolution of the radio emission suggests a short lifetime for the jet. The observations of a synchrotron jet from a post-AGB star with characteristics similar to those from protostars and young stellar objects, for instance, suggest that magnetic launching and collimation is a common feature of astrophysical jets

First detection of methanol towards a post-AGB object, HD101584

The circumstellar environments of objects on the asymptotic giant branch and beyond are rich in molecular species. Nevertheless, methanol has never been detected in such an object, and is therefore often taken as a clear signpost for a young stellar object. However, we report the first detection of CH3OH in a post-AGB object, HD101584, using ALMA. Its emission, together with emissions from CO, SiO, SO, CS, and H2CO, comes from two extreme velocity spots on either side of the object where a high-velocity outflow appears to interact with the surrounding medium. We have derived molecular abundances, and propose that the detected molecular species are the effect of a post-shock chemistry where circumstellar grains play a role. We further provide evidence that HD101584 was a low-mass, M-type AGB star

A study of radial self-similar non-relativistic MHD outflow models: parameter space exploration and application to the water fountain W43A

Outflows, spanning a wide range of dynamical properties and spatial extensions, have now been associated with a variety of accreting astrophysical objects, from supermassive black holes at the core of active galaxies to young stellar objects. The role of such outflows is key to the evolution of the system that generates them, for they extract a fraction of the orbiting material and angular momentum from the region close to the central object and release them in the surroundings. The details of the launching mechanism and their impact on the environment are fundamental to understand the evolution of individual sources and the similarities between different types of outflow-launching systems. We solve semi-analytically the non-relativistic, ideal, magnetohydrodynamics equations describing outflows launched from a rotating disc threaded with magnetic fields using our new numerical scheme. We present here a parameter study of a large sample of new solutions. We study the different combinations of forces that lead to a successfully launched jet and discuss their global properties. We show how these solutions can be applied to the outflow of the water fountain W43A for which we have observational constraints on magnetic field, density and velocity of the flow at the location of two symmetrical water maser emitting regions

Spatio-kinematical model of the collimated molecular outflow in the water-fountain nebula IRAS 16342-3814

Context. Water-fountain nebulae are asymptotic giant branch (AGB) and post-AGB objects that exhibit high-velocity outflows traced by water-maser emission. Their study is important for understanding the interaction between collimated jets and the circumstellar material that leads to the formation of bipolar and/or multi-polar morphologies in evolved stars. Aims. The aim of this paper is to describe the three-dimensional morphology and kinematics of the molecular gas of the water-fountain nebula IRAS 16342-3814. Methods. Data was retrieved from the ALMA archive for analysis using a simple spatio-kinematical model. The software SHAPE was employed to construct a three-dimensional, spatio-kinematical model of the molecular gas in IRAS 16342-3814, and to then reproduce the intensity distribution and position-velocity diagram of the CO emission from the ALMA observations to derive the morphology and velocity field of the gas. Data from CO(J = 1 -> 0) supported the physical interpretation of the model. Results. A spatio-kinematical model that includes a high-velocity collimated outflow embedded within material expanding at relatively lower velocity reproduces the images and position-velocity diagrams from the observations. The derived morphology is in good agreement with previous results from IR and water-maser emission observations. The high-velocity collimated outflow exhibits deceleration across its length, while the velocity of the surrounding component increases with distance. The morphology of the emitting region, the velocity field, and the mass of the gas as function of velocity are in excellent agreement with the properties predicted for a molecular outflow driven by a jet. The timescale of the molecular outflow is estimated to be similar to 70-100 yr. The scalar momentum carried by the outflow is much larger than it can be provided by the radiation of the central star. An oscillating pattern was found associated with the high-velocity collimated outflow. The oscillation period of the pattern is T approximate to 60-90 yr and its opening angle is theta(op) approximate to 2 degrees. Conclusions. The CO (J= 3 -> 2) emission in IRAS 16342-3814 is interpreted in terms of a jet-driven molecular outflow expanding along an elongated region. The position-velocity diagram and the mass spectrum reveal a feature due to entrained material that is associated with the driving jet. This feature is not seen in other more evolved objects that exhibit more developed bipolar morphologies. It is likely that the jet in those objects has already disappeared since it is expected to last only for a couple hundred years. This strengthens the idea that water fountain nebulae are undergoing a very short transition during which they develop the collimated outflows that shape the circumstellar envelopes. The oscillating pattern seen in the CO high-velocity outflow is interpreted as due to precession with a relatively small opening angle. The precession period is compatible with the period of the corkscrew pattern seen at IR wavelengths. We propose that the high-velocity molecular outflow traces the underlying primary jet that produces such a pattern

Rapid angular expansion of the ionized core of CRL 618

Context. During the transition from the asymptotic giant branch (AGB) to the planetary nebulae phase the circumstellar envelopes of most low- and intermediate-mass stars experience a dramatic change in morphology. CRL 618 exhibits characteristics of both an AGB and post-AGB star. It also displays a spectacular array of bipolar lobes with a dense equatorial region, which makes it an excellent object for studying the development of asymmetries in evolved stars. In recent decades, an elliptical compact HII region located in the center of the nebula has been seen to be increasing in size and flux. This seems to be due to the ionization of the circumstellar envelope by the central star, and it probably indicates the beginning of the planetary nebula phase for CRL 618. Aims. We aim to determine the physical conditions under which the ionization of the circumstellar envelope of CRL 618 began to take place as well as the subsequent propagation of the ionization front. Methods. We analyzed interferometric radio continuum data at ~5 and 22 GHz from observations carried out at seven epochs with the VLA. We traced the flux increase of the ionized region over a period of ~26 years. We measured the dimensions of the HII region directly from the brightness distribution images to determine the increase of its size over time. For one of the epochs we analyzed observations at six frequencies from which we estimated the electron density distribution. We carried out model calculations of the spectral energy distribution at two different epochs to corroborate our observational results. Results. We found that the radio continuum flux and the size of the ionized region have been increasing monotonically in the past three decades. The size of the major axis of the HII region shows a dependance on frequency, which has been interpreted as a result of the gradient of the electron density in this direction. The growth of the HII region is due to the expansion of an ionized wind whose mass-loss rate increased continuously for a period of ~100 years until a few decades ago, when the mass-loss rate experienced a sudden decline. Our results indicate that the circumstellar envelope began to be ionized around 1971, which marks the start of the planetary nebula phase of CRL 618. © ESO, 2013