Investigation of dust properties of the proto-planetary nebula IRAS 18276-1431

We investigate the circumstellar dust properties of the oxygen-rich bipolar proto-planetary nebula IRAS 18276-1431 by means of two-dimensional radiative transfer simulations of the circumstellar dust shell. The model geometry is assumed to have a torus and an envelope. The parameters of the dust and the dust shell are constrained by comparing the SED and NIR intensity and polarisation data with the models. The polarisation in the envelope reaches 50 -- 60 % and is nearly constant in the H and K_S bands in the observations. This weak wavelength dependence of the polarisation can be reproduced with a grain size distribution function for the torus: 0.05 micron <= a with n(a)=a^{-(p=5.5)}exp(-a/{a_c=0.3 micron}). The power index p is significantly steeper than that for interstellar dust. Similar results have also been found in some other PPNs and suggest that mechanisms that grind down large particles may also have acted when the dust particles formed. The spectral opacity index beta is found to be 0.6+/-0.5 from the millimeter fluxes. This low value indicates the presence of large dust grains in the torus. We discuss two possible dust models for the torus. One has a size distribution function of 1.0 micron <= a <= a_max=5,000.0 micron with n(a)=a^{-(p=2.5)} and the other is 1.0 micron <= a <= a_max=10,000.0 micron with n(a)=a^{-(p=3.5)}. The former has beta of 0.633, but we are not able to find reasonable geometry parameters to fit the SED in the infrared. The latter has beta of 1.12, but reproduces the SED better over a wide wavelength range. With this dust model, the geometric parameters are estimated as follows: the inner and outer radii are 30 AU and 1000 AU and the torus mass is 3.0 M_sun. Assuming an expansion velocity of 15 kms^{-1}, the torus formation time and mass-loss rate are found to be sim300 yrs and sim10^{-2}M_sun yr^{-1} respectively.

A Massive Bipolar Outflow and a Dusty Torus with Large Grains in the Pre-Planetary Nebula IRAS 22036+5306

We report high angular-resolution (~1) CO J=3--2 interferometric mapping, using the Submillimeter Array (SMA), of IRAS22036+5306 (I22036), a bipolar pre-planetary nebula (PPN) with knotty jets discovered in our HST SNAPshot survey of young PPNs. In addition, we have obtained supporting lower-resolution (~10) CO and 13CO J=1-0 observations with the Owens Valley Radio Observatory (OVRO) interferometer, as well as optical long-slit echelle spectra at the Palomar Observatory. The CO J=3-2 observations show the presence of a very fast (~220 km/s), highly collimated, massive (0.03 Msun) bipolar outflow with a very large scalar momentum (about 10^{39} g cm s^{-1}), and the characteristic spatio-kinematic structure of bow-shocks at the tips of this outflow. The Halpha line shows an absorption feature blue-shifted from the systemic velocity by ~100 km/s, which most likely arises in neutral interface material between the fast outflow and the dense walls of the bipolar lobes at low latitudes. The fast outflow in I22036, as in most PPNs, cannot be driven by radiation pressure. We find an unresolved source of submillimeter (and millimeter-wave) continuum emission in I22036, implying a very substantial mass (0.02-0.04 Msun) of large (radius >~1 mm), cold (< ~50 K) dust grains associated with I22036s toroidal waist. We also find that the 13C/12C ratio in I22036 is very high (0.16), close to the maximum value achieved in equilibrium CNO-nucleosynthesis (0.33). The combination of the high circumstellar mass (i.e., in the extended dust shell and the torus) and the high 13C/12C ratio in I22036 provides strong support for this object having evolved from a massive (>~4 Msun) progenitor in which hot-bottom-burning has occurred.

The binary central star of the bipolar pre-planetary nebula IRAS 08005-2356 (V510 Pup)

Current models predict that binary interactions are a major ingredient for the formation of bipolar planetary nebulae (PNe) and pre-planetary nebulae (PPNe). Despite years of radial velocity (RV) monitoring, the paucity of known binaries amongst the latter systems is insufficient to examine this relationship in detail. In this paper, we report on the discovery of a long period (P=2654$pm$124 d) binary at the centre of the Galactic bipolar PPN, IRAS 08005-2356 (V510 Pup) determined from long-term spectroscopic and near-infrared time series data. The spectroscopic orbit is fit with an eccentricity of 0.36$pm$0.05 that is similar to other long period post-AGB binaries. Time resolved H$alpha$ profiles reveal high-velocity outflows (jets) with de-projected velocities up to 231$_{-27}^{+31}$ km s$^{-1}$ seen at phases when the luminous primary is behind the jet. The outflow traced by H$alpha$ is likely produced via accretion onto a main sequence companion for which we calculate a mass of 0.63$pm$0.13 M$_odot$. This discovery is one of the first cases of a confirmed binary PPN and demonstrates the importance of high-resolution spectroscopic monitoring surveys on large telescopes in revealing binarity among these systems.

ALMA Observations of the Water Fountain Pre-Planetary Nebula IRAS 16342-3814: High-velocity bipolar jets and an Expanding Torus

We have mapped 12CO J=3-2 and other molecular lines from the water-fountain bipolar pre-planetary nebula (PPN) IRAS 16342-3814 with ~0.35 resolution using ALMA. We find (i) two very high-speed knotty, jet-like molecular outflows, (ii) a central high-density (> few x 10^6 cm^{-3}), expanding torus of diameter 1300 AU, and (iii) the circumstellar envelope of the progenitor AGB, generated by a sudden, very large increase in the mass-loss rate to >3.5 x 10^{-4} Msun/yr in the past ~455 yr. Strong continuum emission at 0.89 mm from a central source (690 mJy), if due to thermally-emitting dust, implies a substantial mass (0.017 Msun) of very large (~mm-sized) grains. The measured expansion ages of the above structural components imply that the torus (age~160 yr) and the younger high-velocity outflow (age~110 yr) were formed soon after the sharp increase in the AGB mass-loss rate. Assuming a binary model for the jets in IRAS 16342, the high momentum rate for the dominant jet-outflow in IRAS 16342 implies a high minimum accretion rate, ruling out standard Bondi-Hoyle-Lyttleton wind accretion and wind Roche lobe overflow (RLOF) models with white-dwarf or main-sequence companions. Most likely, enhanced RLOF from the primary or accretion modes operating within common envelope evolution are needed.

ALMA imaging of the nascent planetary nebula IRAS 15103-5754

We present continuum and molecular line (CO, C$^{18}$O, HCO$^+$) observations carried out with the Atacama Large Millimeter/submillimeter Array toward the water fountain star IRAS 15103-5754, an object that could be the youngest PN known. We detect two continuum sources, separated by $0.39pm 0.03$ arcsec. The emission from the brighter source seems to arise mainly from ionized gas, thus confirming the PN nature of the object. The molecular line emission is dominated by a circumstellar torus with a diameter of $simeq 0.6$ arcsec (2000 au) and expanding at $simeq 23$ km s$^{-1}$. We see at least two gas outflows. The highest-velocity outflow (deprojected velocities up to 250 km s$^{-1}$), traced by the CO lines, shows a biconical morphology, whose axis is misaligned $simeq 14^circ$ with respect to the symmetry axis of the torus, and with a different central velocity (by $simeq 8$ km s$^{-1}$). An additional high-density outflow (traced by HCO$^+$) is oriented nearly perpendicular to the torus. We speculate that IRAS 15103-5754 was a triple stellar system that went through a common envelope phase, and one of the components was ejected in this process. A subsequent low-collimation wind from the remaining binary stripped out gas from the torus, creating the conical outflow. The high velocity of the outflow suggests that the momentum transfer from the wind was extremely efficient, or that we are witnessing a very energetic mass-loss event.