Bosco H. K. Yung

HIGH VELOCITY PRECESSING JETS FROM THE WATER FOUNTAIN IRAS 18286-0959 REVEALED BY VERY LONG BASELINE ARRAY OBSERVATIONS

We report the results of multi-epoch Very Long Baseline Array observations of the 22.2?GHz H2O maser emission associated with the water fountain IRAS?18286?0959. We suggest that this object is the second example of a highly collimated bipolar precessing outflow traced by H2O maser emission, the other is W?43A. The detected H2O emission peaks are distributed over a velocity range from ?50?km?s?1 to 150?km?s?1. The spatial distribution of over 70% of the identified maser features is found to be highly collimated along a spiral jet (jet 1) extended southeast to northwest; the remaining features appear to trace another spiral jet (jet 2) with a different orientation. The two jets form a double-helix pattern which lies across ~200 mas. The maser distribution is reasonably fit by a model consisting of two bipolar precessing jets. The three-dimensional velocities of jet 1 and jet 2 are derived to be 138?km?s?1 and 99?km?s?1, respectively. The precession period of jet 1 is about 56 years. For jet 2, three possible models are tested and they give different values for the kinematic parameters. We propose that the appearance of two jets is the result of a single driving source with significant proper motion.

WATER MASER SURVEY ON AKARI AND IRAS SOURCES: A SEARCH FOR “LOW-VELOCITY” WATER FOUNTAINS

We present the results of a 22 GHz H2O maser survey toward a new sample of asymptotic giant branch (AGB) and post-AGB star candidates. Most of the objects are selected for the first time based on the AKARI data, which have high flux sensitivity in the mid-infrared ranges. We aim at finding H2O maser sources in the transient phase between the AGB and post-AGB stages of evolution, where the envelopes start to develop large deviations from spherical symmetry. The observations were carried out with the Effelsberg 100 m radio telescope. Among 204 observed objects, 63 detections (36 new) were obtained. We found four objects that may be “water fountain” sources (IRAS 15193+3132, IRAS 18056−1514, OH 16.3−3.0, and IRAS 18455+0448). They possess an H2O maser velocity coverage much smaller than those in other known water fountains. However, the coverage is still larger than that of the 1612 MHz OH maser. It implies that there is an outflow with a higher velocity than the envelope expansion velocity (typically 25 km s −1 ), meeting the criterion of the water fountain class. We suggest that these candidates are possibly oxygen-rich late AGB or early post-AGB stars in a stage of evolution immediately after the

CO Structure of the 21 μm Source IRAS 22272+5435: A Sign of a Jet Launch?

We report the results of radio interferometric observations of the 21 μm source IRAS 22272+5435 in the CO J = 2-1 line. 21 μm sources are carbon-rich objects in the post-asymptotic-giant-branch phase of evolution, which show an unidentified emission feature at 21 μm. Since 21 μm sources usually also have circumstellar molecular envelopes, the mapping of CO emission from the envelope will be useful in tracing the nebular structure. From observations made with the Combined Array for Research in Millimeter-wave Astronomy, we find that a torus and spherical wind model can explain only part of the CO structure. An additional axisymmetric region created by the interaction between an invisible jet and ambient material is suggested.

Maser Observations of Westerlund 1 and Comprehensive Considerations on Maser Properties of Red Supergiants Associated with Massive Clusters

We report the results of Australia Telescope Compact Array observations of the Westerlund 1 (Wd1) region in the SiO v = 1, J = 1-0, and H2O 616-523 maser lines, and we also report the analysis of maser properties of red supergiants (RSGs) associated with six massive clusters including Wd1. The primary purpose of this research is to explore possibilities of using maser emission for investigating the nature of massive clusters and associated RSGs. The SiO v = 1, J = 1-0, and H2O 616-523 maser lines are detected toward two of four known RSGs in Wd1. The large velocity ranges of maser emission are consistent with the RSG status. RSGs with maser emission tend to exhibit redder log (F 21/F 12) and [K-12.13] colors compared to RSGs with no maser emission. The mass-loss rates derived from dust radiative transfer modeling suggest that RSGs with maser emission tend to exhibit larger mass-loss rates compared to RSGs with no maser emission. In an extended sample of 57 RSGs in six massive clusters, detections in the SiO line tend to homogeneously distribute in absolute luminosity L, whereas those in the H2O line tend to distribute in a region with large L values.

EXPLORATION OF A RELIC CIRCUMSTELLAR ENVELOPE IN THE "WATER FOUNTAIN" SOURCE IRAS 18286−0959

The water fountain source IRAS 18286−0959 (I18286) was reported as an object exhibiting highly collimated, double-helix stellar jets traced by the H2O maser emission. Using the European VLBI Network, we measured the absolute coordinates of the 1612 MHz OH maser emission in I18286, which is very likely associated with a relic of a circumstellar envelope (CSE) developed in the asymptotic giant branch stage of the central star. The location of the OH maser is near the center of the H2O maser feature cluster, where one of the originating points of the two jets is located. We also mapped 22.2 GHz H2O maser emission in this object using the Very Long Baseline Array in the seasons of 2006–2007 as well as 2008–2009. In the first three epochs in 2006–2007, we detected at least 90 maser features per epoch. In six epochs in 2008–2009, when the observation data had already been published and analyzed to identify the double-helix jets, we newly identified 14 proper motions of H2O maser features which could not be measured in the previous analysis due to their short lifetimes. In this paper, together with the OH maser emission, we focus on “outlier” H2O maser features, which exhibit slow expansion velocities (Vexp 30 km s −1 ) and are likely associated with either the relic CSE or an equatorial flow of I18286. They were marginally distinguishable from the maser features associated with the jets in I18286.

Methanol observation of IRAS 19312+1950: A possible new type of class I methanol maser

We report the result of a systematic methanol observation toward IRAS 19312+1950. The properties of the SiO, H2O and OH masers of this object are consistent with those of mass-losing evolved stars, but some other properties are difficult to explain in the standard scheme of stellar evolution in its late stage. Interestingly, a tentative detection of radio methanol lines was suggested toward this object by a previous observation. To date, there are no confirmed detections of methanol emission towards evolved stars, so investigation of this possible detection is important to better understand the circumstellar physical/chemical environment of IRAS 19312+1950. In this study, we systematically observed multiple methanol lines of IRAS 19312+1950 in the lambda=3mm, 7mm, and 13mm bands, and detected 6 lines including 4 thermal lines and 2 class I maser lines. We derived basic physical parameters including kinetic temperature and relative abundances by fitting a radiative transfer model. According to the derived excitation temperature and line profiles, a spherically expanding outflow lying at the center of the nebulosity is excluded from the possibilities for methanol emission regions. The detection of class I methanol maser emission suggests that a shock region is involved in the system of IRAS 19312+1950. If the central star of IRAS 19312+1950 is an evolved star as suggested in the past, the class I maser detected in the present observation is the first case detected in an interaction region between an evolved star outflow and ambient molecular gas.

Morphokinematic properties of the 21 micron source IRAS 22272+5435

We obtained a high-resolution CO map of IRAS 22272+5435 in the CO J = 2–1 line using CARMA. The target exhibits a second biggest angular size of the circumstellar molecular envelope among known 21 μm sources. In the preliminary results, we found that the CO properties of IRAS 22272+5435 is clearly different from those of IRAS 07134+1005, which is another well-investigated 21 μm source. For example, elongations seen in the mid-infrared and CO images are extended in mutually perpendicular directions, although in case of IRAS 07134+1005 the CO feature coincides well with the mid-infrared structure.

Maser kinematics and infrared properties of the water fountain IRAS 18286–0959

A “water fountain” is a transitional object between an AGB star and a PN. The VLBA observations of 22.2 GHz water maser emission reveal a “double-helix” outflow pattern from one of the water fountains, IRAS 18286−0959. The pattern is reasonably fit by a model consisting of two precessing jets. We propose that the two jets observed are a result of a single driving source with a significant proper motion. Using data from the AKARI catalogs, we also found that water fountains might have their own IR colors which are affected by the 9.7 μm silicate feature and the optical thickness of stellar envelopes. The colors could serve as new criteria for searching this type of rare objects.