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تسجيل مستخدم جديدExtremely Strong ^{13}CO J=3-2 Line in the Water Fountain IRAS 16342-3814: Evidence for the Hot-Bottom Burning
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تأليف
Hiroshi Imai
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We observed four water fountain sources in the CO J=3-2 line emission with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope in 2010-2011. The water fountain sources are evolved stars that form high-velocity collimated jets traced by water maser emission. The CO line was detected only from IRAS 16342-3814. The present work confirmed that the ^{12}CO to ^{13}CO line intensity ratio is ~1.5 at the systemic velocity. We discuss the origins of the very low ^{12}CO to ^{13}CO intensity ratio, as possible evidence for the hot-bottom burning in an oxygen-rich star, and the CO intensity variation in IRAS 16342-3814.
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We observed CO J=3-2 emission from the water fountain sources, which exhibit high-velocity collimated stellar jets traced by water maser emission, with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope. We detected the CO emission
from two sources, IRAS 16342-3814 and IRAS 18286-0959. The IRAS 16342-3814 CO emission exhibits a spectrum that is well fit to a Gaussian profile, rather than to a parabolic profile, with a velocity width (FWHM) of 158+/-6 km/s and an intensity peak at VLSR = 50+/-2 km/s. The mass loss rate of the star is estimated to be ~2.9x10^-5 M_sun/yr. Our morpho-kinematic models suggest that the CO emission is optically thin and associated with a bipolar outflow rather than with a (cold and relatively small) torus. The IRAS 18286-0959 CO emission has a velocity width (FWHM) of 3.0+/-0.2 km/s, smaller than typically seen in AGB envelopes. The narrow velocity width of the CO emission suggests that it originates from either an interstellar molecular cloud or a slowly-rotating circumstellar envelope that harbors the water maser source.
We investigate the circumstellar dust shell of the water fountain source IRAS 16342-3814. We performed two-dimensional radiative transfer modeling of the dust shell, taking into account previously observed spectral energy distributions (SEDs) and our
new $J$-band imaging and $H$- and $K_S$-band imaging polarimetry obtained using the VLT/NACO instrument. Previous observations expect an optically thick torus in the equatorial plane because of a striking bipolar appearance and a large viewing angle of 30 - 40$degr$. However, models with such a torus as well as a bipolar lobe and an AGB shell cannot fit the SED and the images simultaneously. We find that an additional optically and geometrically thick disk located inside a massive torus solves this problem. The masses of the disk and the torus are estimated to be 0.01 $M_{sun}$ at the $a_mathrm{max}=100 mu$m dust and 1 $M_{sun}$ at $a_mathrm{max}=10 mu$m dust, respectively. We discuss a possible formation scenario for the disk and torus based on a similar mechanism to the equatorial back flow. IRAS 16342-3814 is expected to undergo mass loss at a high rate. The radiation from the central star is shielded by the dust that was ejected in the subsequent mass loss event. As a result, the radiation pressure on dust particles cannot govern the motion of the particles anymore. The mass loss flow can be concentrated in the equatorial plane by help of an interaction, which might be the gravitational attraction by the companion, if it exists in IRAS 16342-3814. A fraction of the ejecta is captured in a circum-companion or circum-binary disk and the remains are escaping from the central star(s) and form the massive torus.
In this work we aimed to describe the three-dimensional morphology and kinematics of the molecular gas of the water-fountain nebula IRAS 16342-3814. In order to do this, we retrieved data from the ALMA archive to analyse it using a simple spatio-kine
matical model. We used the software SHAPE to construct a three-dimensional spatio-kinematical model of the molecular gas in IRAS 16342-3814. By reproducing the intensity distribution and position-velocity diagram of the CO emission from the ALMA observations we derived the morphology and velocity field of the gas. We used CO(1-0) data to support the physical interpretation of the model. 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 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 ~70-100 years. An oscillating pattern was found associated to the high-velocity collimated outflow. The oscillation period of the pattern is T~60-90 years and its opening angle is ~2 degrees. The CO (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 to the driving jet. It is likely that the jet in those objects has already disappeared since it is expected to last only for a couple of hundred years.
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.
We present Expanded Very Large Array (EVLA) water maser observations at 22 GHz toward the source IRAS 18113-2503. Maser components span over a very high velocity range of ~500 km/s, the second largest found in a Galactic maser, only surpassed by the
high-mass star forming region W49N. Maser components are grouped into a blue and a redshifted cluster, separated by 0.12. Further mid-IR and radio data suggest that IRAS 18113-2503 is a post-AGB star, thus a new bona fide member of the rare class of water fountains. It is the evolved object with the largest total velocity spread in its water masers, and with the highest velocity dispersion within its red- and blue-shifted lobes (~170 km/s). The large total velocity range of emission probably indicates that IRAS 18113-2503 has the fastest jet among the known water fountain stars. On the other hand, the remarkably high velocity dispersion within each lobe may be interpreted in terms of shocks produced by an episode of mass ejection whose velocity increased up to very high values or, alternatively, by projection effects in a jet with a large opening angle and/or precessing motions.
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