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Register a new userInvestigation of dust properties of the proto-planetary nebula IRAS 18276-1431
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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.
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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.
We present medium-resolution 3 um spectroscopy of the carbon-rich proto-planetary nebula IRAS 22272+5435. Spectroscopy with the Subaru Telescope adaptive optics system revealed a spatial variation of hydrocarbon molecules and dust surrounding the star. The ro-vibrational bands of acetylene (C2H2) and hydrogen cyanide (HCN) at 3.0 um are evident in the central star spectra. The molecules are concentrated in the compact region near the center. The 3.3 and 3.4 um emission of aromatic and aliphatic hydrocarbons is detected at 600--1300 AU from the central star. The separation of spatial distribution between gas and dust suggests that the small hydrocarbon molecules are indeed the source of solid material, and that the gas leftover from the grain formation is being observed near the central star. The intensity of aliphatic hydrocarbon emission relative to the aromatic hydrocarbon emission decreases with distance from the central star. The spectral variation is well matched to that of a laboratory analog thermally annealed with different temperatures. We suggest that either the thermal process after the formation of a grain or the variation in the temperature in the dust-forming region over time determines the chemical composition of the hydrocarbon dust around the proto-planetary nebula.
We review the properties of dust in protoplanetary disks around optically visible pre-main sequence stars obtained with a variety of observational techniques, from measurements of scattered light at visual and infrared wavelengths to mid-infrared spectroscopy and millimeter interferometry. A general result is that grains in disks are on average much larger than in the diffuse interstellar medium (ISM). In many disks, there is evidence that a large mass of dust is in grains with millimeter and centimeter sizes, more similar to sand and pebbles than to grains. Smaller grains (with micron-sizes) exist closer to the disk surface, which also contains much smaller particles, e.g., polycyclic aromatic hydrocarbons. There is some evidence of a vertical stratification, with smaller grains closer to the surface. Another difference with ISM is the higher fraction of crystalline relative to amorphous silicates found in disk surfaces. There is a large scatter in dust properties among different sources, but no evidence of correlation with the stellar properties, for samples that include objects from intermediate to solar mass stars and brown dwarfs. There is also no apparent correlation with the age of the central object, over a range roughly between 1 and 10 Myr. This suggests a scenario where significant grain processing may occur very early in the disk evolution, possibly when it is accreting matter from the parental molecular core. Further evolution may occur, but not necessarily rapidly, since we have evidence that large amounts of grains, from micron to centimeter size, can survive for periods as long as 10 Myr.
We present high-angular-resolution {it Hubble Space Telescope (HST)} optical and near-infrared imaging of the compact planetary nebula (PN) IRAS 21282+5050. Optical images of this object reveal several complex morphological structures including three pairs of bipolar lobes and an elliptical shell lying close to the plane of the sky. From near-infrared observations, we found a dust torus oriented nearly perpendicular to the major axis of elliptical shell. The results suggest that IRAS 21282+5050 is a multipolar PN, and these structures developed early during the post asymptotic-giant-branch (AGB) evolution. From a three-dimensional (3-D) model, we derived the physical dimensions of these apparent structures. When the 3-D model is viewed from different orientations, IRAS 21282+5050 shows similar apparent structures as other multipolar PNs. Analysis of the spectral energy distribution and optical spectroscopic observations of the nebula suggests the presence of a cool companion to the hot central star responsible for the ionization of the nebula. Whether the binary nature of the central star has any relations with the multipolar structure of the nebula needs to be further investigated.
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