arXiv subjects

Toshiya Ueta

Publications and source records attributed to Toshiya Ueta.

At least 37 records · Page 2Linked to original sources

arXiv2010

Differential Proper-Motion Study of the Circumstellar Dust Shell of the Enigmatic Object, HD 179821

HD179821 is an enigmatic evolved star that possesses characteristics of both a post-asymptotic giant branch star and a yellow hyper-giant, and there has been no evidence that unambiguously defines its nature. These two hypotheses are products of an indeterminate distance, presumed to be 1 kpc or 6 kpc. We have obtained the two-epoch Hubble Space Telescope WFPC2 data of its circumstellar shell, which shows multiple concentric arcs extending out to about 8 arcsec. We have performed differential proper-motion measurements on distinct structures within the circumstellar shell of this mysterious star in hopes of determining the distance to the object, and thereby distinguishing the nature of this enigmatic stellar source. Upon investigation, rather than azimuthal radially symmetric expansion, we discovered a bulk motion of the circumstellar shell of (2.41+-0.43, 2.97+-0.32) mas/yr. This corresponded to a translational ISM flow of (1.28+-0.95, 7.27+-0.75) mas/yr local to the star. This finding implies that the distance to HD 179821 should be rather small in order for its circumstellar shell to preserve its highly intact spherical structure in the presence of the distorting ISM flow, therefore favoring the proposition that HD 179821 is a post-AGB object.

astro-ph.SR↗

arXiv2009

Mass Loss History of the AGB star, R Cas

We report here on the discovery of an extended far-infrared shell around the AGB star, R Cassiopeia, made by AKARI and Spitzer. The extended, cold circumstellar shell of R Cas spans nearly 3 arcmin and is probably shaped by interaction with the interstellar medium. This report is one of several studies of well-resolved mass loss histories of AGB stars under AKARI and Spitzer observing programs labeled "Excavating Mass Loss History in Extended Dust Shells of Evolved Stars (MLHES)".

astro-ph.GA↗

arXiv2009

3-D Dynamics of Interactions between Stellar Winds and the Interstellar Medium as Seen by AKARI and Spitzer

Recent far-infrared mapping of mass-losing stars by the AKARI Infrared Astronomy Satellite and Spitzer Space Telescope have suggested that far-infrared bow shock structures are probably ubiquitous around these mass-losing stars, especially when these stars have high proper motion. Higher spatial resolution data of such far-infrared bow shocks now allow detailed fitting to yield the orientation of the bow shock cone with respect to the heliocentric space motion vector of the central star, using the analytical solution for these bow shocks under the assumption of momentum conservation across a physically thin interface between the stellar winds and interstellar medium (ISM). This fitting analysis of the observed bow shock structure would enable determination of the ambient ISM flow vector, founding a new technique to probe the 3-D ISM dynamics that are local to these interacting systems. In this review, we will demonstrate this new technique for three particular cases, Betelgeuse, R Hydrae, and R Cassiopeiae.

astro-ph.GA↗

arXiv2008

Cometary Astropause of Mira Revealed in the Far-Infrared

Evolved mass-losing stars such as Mira enrich the interstellar medium (ISM) significantly by their dust-rich molecular wind. When these stars move fast enough relative to the ISM, the interaction between the wind and ISM generates the structure known as the astropause (a stellar analog of the heliopause), which is a cometary stellar wind cavity bounded by the contact discontinuity surface between the wind and ISM. Far-infrared observations of Mira spatially resolve the structure of its astropause for the first time, distinguishing the contact surface between Mira's wind and the ISM and the termination shock due to Mira's wind colliding with the ISM. The physical size of the astropause and the estimated speed of the termination shock suggest the age of the astropause to be about 40,000 yr, confirming a theoretical prediction of the shock re-establishment time after Mira has entered the Local Bubble.

astro-ph↗

arXiv2008

AKARI/FIS Mapping of the ISM-Wind Bow Shock around Alpha Ori

We present 10' x 50' scan maps around an M supergiant Alpha Ori at 65, 90, 140 and 160 microns obtained with the AKARI Infrared Astronomy Satellite. Higher spatial resolution data with the exact analytic solution permit us to fit the de-projected shape of the stellar wind bow shock around Alpha Ori to have the stand-off distance of 4.8', position angle of 55 degrees and inclination angle of 56 degrees. The shape of the bow shock suggests that the peculiar velocity of Alpha Ori with respect to the local medium is v_* = 40 (n_H)^(-1/2), where n_H is the hydrogen nucleus density at Alpha Ori. We find that the local medium is of n_H = 1.5 to 1.9 cm^(-3) and the velocity of the local flow is at 11 km s^(-1) by using the most recent astrometric solutions for Alpha Ori under the assumption that the local medium is moving away from the Orion OB 1 association. AKARI images may also reveal a vortex ring due to instabilities on the surface of the bow shock as demonstrated by numerical models. This research exemplifies the potential of AKARI All-Sky data as well as follow-up observations with Herschel Space Telescope and Stratospheric Observatory for Infrared Astronomy for this avenue of research in revealing the nature of interaction between the stellar wind and interstellar medium.

astro-ph↗

arXiv2007

HST/NICMOS Imaging Polarimetry of Proto-Planetary Nebulae II: Macro-morphology of the Dust Shell Structure via Polarized Light

The structure of the dusty circumstellar envelopes (CSEs) of proto-planetary nebulae (PPNs) reveals the mass-loss history of these sources and how such histories may differ for elliptical (SOLE) and bipolar (DUPLEX) PPNs. To study the PPN structures via dust-scattered linearly polarized starlight, we have compiled the imaging-polarimetric data for all 18 evolved stars that have been obtained to date with NICMOS on-board the Hubble Space Telescope (HST). This alternative imaging technique provides a unique way to probe the distribution of dust grains that scatter light around evolved stars. The new perspective gained from the imaging-polarimetric data has revealed several new aspects to the structures of PPNs. Point-symmetry is a prevalent imaging-polarimetric characteristic resulting from the azimuthal density gradient in the CSEs. Among these point-symmetric nebulae, three detailed morphological types can be differentiated by their polarized intensity, I_pol, and polarization strength, P. While the azimuthal density gradient is reversed above and below the equatorial plane in optically thicker bipolar nebulae, there is no gradient reversal in optically thinner elliptical nebulae. The equatorial plane of the system defined by the integrated angle of polarization is not necessarily orthogonal to the axis of the apparent bipolar structure in the total intensity data.

astro-ph↗

arXiv2006

Detached shells as tracers of AGB-ISM bow shocks

New Spitzer imaging observations have revealed the structure around the Mira variable star R Hya to be a one-sided parabolic arc 100 arcsec to the West stretching from North to South. We successfully model R Hya and its surroundings in terms of an interaction of the stellar wind from an asymptotic giant branch (AGB) star with the interstellar medium (ISM) the star moves through. Our three-dimensional hydrodynamic simulation reproduces the structure as a bow shock into the oncoming ISM. We propose this as another explanation of detached shells around such stars which should be considered alongside current theories of internal origin. The simulation predicts the existence of a tail of ram-pressure-stripped AGB material stretching downstream. Indications for such a tail behind R Hya are seen in IRAS maps.

astro-ph↗

arXiv2006

Spitzer/MIPS Imaging of NGC 650: Probing the History of Mass Loss on the Asymptotic Giant Branch

We present the far-infrared (IR) maps of a bipolar planetary nebula (PN), NGC 650, at 24, 70, and 160 micron taken with the Multiband Imaging Photometer for Spitzer (MIPS) on-board the Spitzer Space Telescope. While the two-peak emission structure seen in all MIPS bands suggests the presence of a near edge-on dusty torus, the distinct emission structure between the 24 micron map and the 70/160 micron maps indicates the presence of two distinct emission components in the central torus. Based on the spatial correlation of these two far-IR emission components with respect to various optical line emission, we conclude that the 24 micron emission is largely due to the [O IV] line at 25.9 micron arising from highly ionized regions behind the ionization front, whereas the 70 and 160 micron emission is due to dust continuum arising from low-temperature dust in the remnant asymptotic giant branch (AGB) wind shell. The far-IR nebula structure also suggests that the enhancement of mass loss at the end of the AGB phase has occurred isotropically, but has ensued only in the equatorial directions while ceasing in the polar directions. The present data also show evidence for the prolate spheroidal distribution of matter in this bipolar PN. The AGB mass loss history reconstructed in this PN is thus consistent with what has been previously proposed based on the past optical and mid-IR imaging surveys of the post-AGB shells.

astro-ph↗

arXiv2006

Spitzer Survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE) I: Overview and Initial Results

We are performing a uniform and unbiased, ~7x7 degrees imaging survey of the Large Magellanic Cloud (LMC), using the IRAC and MIPS instruments on board the Spitzer Space Telescope in order to survey the agents of a galaxy's evolution (SAGE), the interstellar medium (ISM) and stars in the LMC. The detection of diffuse ISM with column densities >1.2x10^21 H cm^-2 permits detailed studies of dust processes in the ISM. SAGE's point source sensitivity enables a complete census of newly formed stars with masses >3 solar masses that will determine the current star formation rate in the LMC. SAGE's detection of evolved stars with mass loss rates >1x10^-8 solar masses per year will quantify the rate at which evolved stars inject mass into the ISM of the LMC. The observing strategy includes two epochs in 2005, separated by three months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are non-proprietary. The data processing includes IRAC and MIPS pipelines and a database for mining the point source catalogs, which will be released to the community in support of Spitzer proposal cycles 4 and 5. We present initial results on the epoch 1 data with a special focus on the N79 and N83 region. The SAGE epoch 1 point source catalog has ~4 million sources. The point source counts are highest for the IRAC 3.6 microns band and decrease dramatically towards longer wavelengths consistent with the fact that stars dominate the point source catalogs and that the dusty objects, e.g. young stellar objects and dusty evolved stars that detected at the longer wavelengths, are rare in comparison. We outline a strategy for identifying foreground MW stars, that may comprise as much as 18% of the source list, and background galaxies, that may comprise ~12% of the source list.

astro-ph↗

arXiv2005

Proper-Motion Measurements of the Cygnus Egg Nebula

We present the results of proper-motion measurements of the dust shell structure in the Egg Nebula (AFGL 2688, CRL 2688, V1610 Cyg), based on the archived two-epoch imaging-polarimetry data in the 2 micron band taken with NICMOS on-board the Hubble Space Telescope. We measured the amount of motions of local structures in the nebula by determining their relative shifts over an interval of 5.5 years. The dynamical age of the nebula is found to be roughly 350 years based on the overall motion of the nebula that exhibits a Hubble-law-esque linear relation between the measured proper motion and the projected radial distance from the origin of the expansion. By adopting the de-projected velocity of 45 km/s at the tips of the bipolar lobes, our proper-motion measurements indicate that the distance to the Egg Nebula is about 420 pc and that the lobes are inclined at 7.7 degrees with respect to the plane of the sky. The refined distance estimate yields the luminosity of the central star of 3.3 x 10^3 L_sun, the total shell mass of 1.2 M_sun, and the mass loss rate (the upper limit) of 3.6 x 10^(-3) M_sun/yr. Assuming 0.6 M_sun central post-AGB stellar mass, the initial mass of the Egg is 1.8 M_sun. Upon analysis, we also discovered that the central star of the Egg Nebula has proper motion of its own at the rate of (14, -10) mas/yr.

astro-ph↗

arXiv2004

HST/NICMOS Imaging Polarimetry of Proto-Planetary Nebulae: Probing of the Dust Shell Structure via Polarized Light

Using NICMOS on HST, we have performed imaging polarimetry of proto-planetary nebulae. Our objective is to study the structure of optically thin circumstellar shells of post-asymptotic giant branch stars by separating dust-scattered, linearly polarized star light from unpolarized direct star light. This unique technique allows us to probe faint reflection nebulae around the bright central star, which can be buried under the point-spread-function of the central star in conventional imaging. Our observations and archival search have yielded polarimetric images for five sources: IRAS 07134+1005 (HD 56126), IRAS 06530-0213, IRAS 04296+3429, IRAS (Z)02229+6208, and IRAS 16594-4656. These images have revealed the circumstellar dust distribution in an unprecedented detail via polarized intensity maps, providing a basis to understand the 3-D structure of these dust shells. We have observationally confirmed the presence of the inner cavity caused by the cessation of AGB mass loss and the internal shell structures which is strongly tied to the progenitor star's mass loss history on the AGB. We have also found that equatorial enhancement in these circumstellar shells comes with various degrees of contrast, suggesting a range of optical depths in these optically thin shells. Our data support the interpretation that the dichotomy of PPN morphologies is due primarily to differences in optical depth and secondary to the inclination effect. The polarization maps reveal a range of inclination angles for these optically thin reflection nebulae, dispelling the notion that elliptical nebulae are pole-on bipolar nebulae.

astro-ph↗

arXiv2003

HD 161796 (IRAS 17436+5003)

Past imaging observations of a post-AGB star, HD 161796, have probed different parts of its circumstellar shell which correspond to different epochs of the star's AGB mass loss history. While the overall structure of the shell can be described by an axisymmetric model consisting of three layers of characteristic structure, the mass distribution in the shell appears to rotate its axis of symmetry in a continuous manner. It is thus interesting to observe the halo region of the shell using from far-IR to sub-mm wavelengths and increase the ``time resolution'' during this critical epoch of the star's mass loss history.

astro-ph↗

arXiv2003

HST/NICMOS Imaging Polarimetry of PPNs

We present the preliminary results from our HST/NICMOS imaging polarimetry on optically thin PPNs. The data show the dust distribution in an unprecedented detail. The structure of these faint PPNs is revealed via polarization properties of the dust-scattered light without much interference from the bright central star, proving the unique advantages of the technique over simple imaging.

astro-ph↗

arXiv2003

Near-Infrared Photometric Survey of Proto-Planetary Nebula Candidates

We present JHK' photometric measurements of 78 objects mostly consisting of proto-planetary nebula candidates. Photometric magnitudes are determined by means of imaging and aperture photometry. Unlike the observations with a photometer with a fixed-sized beam, the method of imaging photometry permits accurate derivation of photometric values because the target sources can be correctly identified and confusion with neighboring sources can be easily avoided. Of the 78 sources observed, we report 10 cases in which the source seems to have been misidentified or confused by nearby bright sources. We also present nearly two dozen cases in which the source seems to have indicated a variability which prompts a follow-up monitoring. There are also a few sources that show previously unreported extendedness. In addition, we present H band finding charts of the target sources.

astro-ph↗

arXiv2002

2-Dust : a Dust Radiative Transfer Code for an Axisymmetric System

We have developed a general purpose dust radiative transfer code for an axisymmetric system, 2-Dust, motivated by the recent increasing availability of high-resolution images of circumstellar dust shells at various wavelengths. This code solves the equation of radiative transfer following the principle of long characteristic in a 2-D polar grid while considering a 3-D radiation field at each grid point. A solution is sought through an iterative scheme in which self-consistency of the solution is achieved by requiring a global luminosity constancy throughout the shell. The dust opacities are calculated through Mie theory from the given size distribution and optical properties of the dust grains. The main focus of the code is to obtain insights on (1) the global energetics of dust grains in the shell (2) the 2-D projected morphologies that are strongly dependent on the mixed effects of the axisymmetric dust distribution and inclination angle of the shell. Here, test models are presented with discussion of the results. The code can be supplied with a user-defined density distribution function, and thus, is applicable to a variety of dusty astronomical objects possessing the axisymmetric geometry.

astro-ph↗

arXiv2001

Westbrook's Molecular Gun: Discovery of Near-IR Micro-Structures in AFGL 618

We present high-sensitivity near-IR images of a carbon-rich proto-planetary nebula, AFGL 618, obtained with the Subaru Telescope. These images have revealed ``bullets'' and ``horns'' extending farther out from the edges of the previously known bipolar lobes. The spatial coincidence between these near-IR micro-structures and the optical collimated outflow structure, together with the detection of shock-excited, forbidden IR lines of atomic species, strongly suggests that these bullets and horns represent the locations from which [\ion{Fe}{2}] IR lines arise. We have also discovered CO clumps moving at $> 200$ km s$^{-1}$ at the positions of the near-IR bullets by re-analyzing the existing $^{12}$CO $J=1-0$ interferometry data. These findings indicate that the near-IR micro-structures represent the positions of shocked surfaces at which fast-moving molecular clumps interface with the ambient circumstellar shell.

astro-ph↗

arXiv2000

Spatial Distributions of Multiple Dust Components in the PPN/PN Dust Shells

We investigate spatial distributions of specific dust components in the circumstellar shells of a proto-planetary nebula candidate, HD 179821, and a planetary nebula, BD$+30^{\circ}$3639, by means of spectral imaging. With high-resolution ground-based images and ISO spectra in the mid-infrared, we can derive ``dust feature only'' maps by subtracting synthesized continuum maps from the observed images at the feature wavelength. Such spatially detailed information will help to develop models for the evolution of dust grains around evolved stars.

astro-ph↗

arXiv2000

Morphological Properties of PPNs: Mid-IR and HST Imaging Surveys

We will review our mid-infrared and HST imaging surveys of the circumstellar dust shells of proto-planetary nebulae. While optical imaging indirectly probes the dust distribution via dust-scattered starlight, mid-IR imaging directly maps the distribution of warm dust grains. Both imaging surveys revealed preferencially axisymmetric nature of PPN dust shells, suggesting that axisymmetry in planetary nebulae sets in by the end of the asymptotic giant branch phase, most likely by axisymmetric superwind mass loss. Moreover, both surveys yielded two morphological classes which have one-to-one correspondence between the two surveys, indicating that the optical depth of circumstellar dust shells plays an equally important role as the inclination angle in determining the morphology of the PPN shells.

astro-ph↗