Jaeheon Kim

Simultaneous Observations of SiO and H2O Masers Toward Known Stellar SiO and H2O Maser Sources. I.

We present the results of simultaneous observations of both SiO and H2O masers toward 166 known SiO and H2O maser sources using the Korean VLBI Network Yonsei 21 m radio telescope during 2009 June. Both SiO and H2O maser emission were detected from 112 sources giving a detection rate of 67% at one epoch observation. SiO-only maser emission was detected from 42 sources, while H2O-only maser emission was detected from four sources. Most of the SiO masers appear around the stellar velocity, while H2O masers show different characteristics compared with SiO masers. There are more than 20 sources that show a one-way peak or double peaks with respect to the stellar velocity and SiO maser peak velocity. The H2O maser peak and integrated intensity ratios with respect to those of SiO (v = 1) show increasingly larger values from Mira variables, to OH/IR stars, to semi-regular variables. In addition, the IRAS two-color diagram of SiO and H2O maser observational results is discussed.

SiO AND H2O MASER SURVEY TOWARD POST-ASYMPTOTIC GIANT BRANCH AND ASYMPTOTIC GIANT BRANCH STARS

We performed simultaneous observations of SiO v = 1, 2, 29 SiO v = 0,J = 1–0 and H2 O6 16–523 maser lines toward 143 AGB and 164 post-asymptotic giant branch (AGB) stars in order to investigate how evolutionary characteristics from AGB to post-AGB stars appear in both SiO and H2O maser emissions. The observations were carried out from 2011 February to 2012 March using the Korean VLBI Network single-dish telescopes. We have detected SiO and/or H2O maser emission from 21 sources out of 164 post-AGB stars including 12 new detections. Of 143 AGB stars, we detected SiO and/or H2O maser emission from 44 stars including 24 new detections. SiO v = 2, J = 1–0 maser emission without a SiO v = 1 maser was detected from 7 sources among 14 SiO-detected post-AGB stars, and the intensity of the SiO v = 2,J = 1–0 maser tends to be much stronger than that of SiO v = 1, which is different from those of AGB stars. This may be related to the development of hot dust shells according to the evolutionary processes of post-AGB stars. We also found that both SiO and H2O masers were detected in the blue group (LI, or Left of IRAS), while only the H2O maser was detected in the red group (RI, or Right of IRAS) for post-AGB stars. These different detection rates between SiO and H2O masers may originate from the different abundances of masing molecules in the circumstellar envelope according to the different mass and expansion velocity between LI and RI regions together with their evolutionary stages.

THE FIRST VERY LONG BASELINE INTERFEROMETRY IMAGE OF A 44 GHz METHANOL MASER WITH THE KVN AND VERA ARRAY (KaVA)

We have carried out the first very long baseline interferometry (VLBI) imaging of a 44 GHz classI methanol maser (70‐61A + ) associated with a millimeter core MM2 in a massive star-forming region IRAS 18151−1208 with KaVA (KVN and VERA Array), which is a newly combined array of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We have succeeded in imaging compact maser features with a synthesized beam sizeof2.7milliarcseconds ×1.5milliarcseconds(mas).Thesefeaturesaredetectedatalimitednumber ofbaselines within the length of shorter than ≈ 650 km corresponding to 100 Mλ in the uv-coverage. The central velocity and the velocity width of the 44 GHz methanol maser are consistent with those of the quiescent gas rather than the outflow traced by the SiO thermal line. The minimum component size among the maser features is ∼5mas×2mas, which corresponds to the linear size of ∼15 AU × 6 AU assuming a distance of 3 kpc. The brightness temperatures of these features range from ∼3.5 × 10 8 to 1.0 × 10 10 K, which are higher than the estimated lower limit from a previous Very Large Array observation with the highest spatial resolution of ∼50 mas. The 44 GHz classI methanol maser in IRAS 18151−1208 is found to be associated with the MM2 core, which is thought to be less evolved than another millimeter core MM1 associated with the 6.7 GHz classII methanol maser.

SIMULTANEOUS OBSERVATIONS OF SiO AND H2O MASERS TOWARD SYMBIOTIC STARS

We present the results of simultaneous observations of SiO v = 1, 2, J = 1-0, 29SiO v = 0, J = 1-0, and H2O 616-523 maser lines performed with the KVN Yonsei 21 m radio telescope from 2009 November to 2010 January. We searched for these masers in 47 symbiotic stars and detected maser emission from 21 stars, giving the first time detection from 19 stars. Both SiO and H2O masers were detected from seven stars of which six were D-type symbiotic stars and one was an S-type star, WRAY 15-1470. In the SiO maser emission, the 28SiO v = 1 maser was detected from 10 stars, while the v = 2 maser was detected from 15 stars. In particular, the 28SiO v = 2 maser emission without the v = 1 maser detection was detected from nine stars with a detection rate of 60%, which is much higher than that of isolated Miras/red giants. The 29SiO v = 0 maser emission was also detected from two stars, H 2-38 and BF Cyg, together with the 28SiO v = 2 maser. We conclude that these different observational results between isolated Miras/red giants and symbiotic stars may be related with the presence of hot companions in a symbiotic binary system.

FIRST DETECTION OF 22 GHZ H2O MASERS IN TX CAMELOPARDALIS

Simultaneous time monitoring observations of H₂O 6 16 -5 23 , SiO J = 1-0, 2-1, 3-2, and 29 SiO v = 0, J = 1-0 lines are carried out in the direction of the Mira variable star TX Cam with the Korean VLBI Network single dish radio telescopes. For the first time, the H₂O maser emission from TX Cam is detected near the stellar velocity at five epochs from April 10, 2013 (o = 3.13) to June 4, 2014 (o = 3.89) including minimum optical phases. The intensities of H₂O masers are very weak compared to SiO masers. The variation of peak antenna temperature ratios among SiO v = 1, J = 1-0, J = 2-1, and J = 3-2 masers is investigated according to their phases. The shift of peak velocities of H₂O and SiO masers with respect to the stellar velocity is also investigated according to observed optical phases. The H₂O maser emission occurs around the stellar velocity during our monitoring interval. On the other hand, the peak velocities of SiO masers show a spread compared to the stellar velocity. The peak velocities of SiO J = 2-1, and J = 3-2 masers show a smaller spread with respect to the stellar velocity than those of SiO J = 1-0 masers. These simultaneous observations of multi-frequencies will provide a good constraint for maser pumping models and a good probe for investigating the stellar atmosphere and envelope according to their different excitation conditions.

Simultaneous VLBI Astrometry of H2O and SiO Masers toward the Semiregular Variable R Crateris

We obtained, for the first time, astrometrically registered maps of the 22.2 GHz H2O and 42.8, 43.1, and 86.2 GHz SiO maser emission toward the semiregular b-type variable (SRb) R Crateris, at three epochs (2015 May 21, and 2016 January 7 and 26) using the Korean Very-long-baseline Interferometry Network. The SiO masers show a ring-like spatial structure, while the H2O maser shows a very asymmetric one-side outflow structure, which is located at the southern part of the ring-like SiO maser feature. We also found that the 86.2 GHz SiO maser spots are distributed in an inner region, compared to those of the 43.1 GHz SiO maser, which is different from all previously known distributions of the 86.2 GHz SiO masers in variable stars. The different distribution of the 86.2 GHz SiO maser seems to be related to the complex dynamics caused by the overtone pulsation mode of the SRb R Crateris. Furthermore, we estimated the position of the central star based on the ring fitting of the SiO masers, which is essential for interpreting the morphology and kinematics of a circumstellar envelope. The estimated stellar coordinate corresponds well to the position measured by Gaia.

Astrometrically registered maps of H2O and SiO masers toward VX Sagittarii

The supergiant VX Sagittarii is a strong emitter of both H2O and SiO masers. However, previous VLBI observations have been performed separately, which makes it difficult to spatially trace the outward transfer of the material consecutively. Here we present the astrometrically registered, simultaneous maps of 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers toward VX Sagittarii. The H2O masers detected above the dust-forming layers have an asymmetric distribution. The multi-transition SiO masers are nearly circular ring, suggesting spherically symmetric wind within a few stellar radii. These results provide the clear evidence that the asymmetry in the outflow is enhanced after the smaller molecular gas clump transform into the inhomogeneous dust layers. The 129.3 GHz maser arises from the outermost region compared to that of 43.1/42.8/86.2 GHz SiO masers. The ring size of the 129.3 GHz maser is maximized around the optical maximum, suggesting that radiative pumping is dominant.The red supergiant VX Sagittarii is a strong emitter of H2O and SiO masers, however its mass loss dynamics are still poorly understood. Here, the authors present astrometrically registered, simultaneous maps of SiO and H2O maser regions, and provide observational evidence for a break in spherical symmetry between the SiO and H2O maser zone.

MONITORING OBSERVATIONS OF H₂O AND SiO MASERS TOWARD POST-AGB STARS

We present the results of simultaneous monitoring observations of H₂O 6 1,6 –5 2,3 (22GHz) and SiO J=1–0, 2–1, 3–2 maser lines (43, 86, 129GHz) toward five post-AGB (candidate) stars, using the 21-m single-dish telescopes of the Korean VLBI Network. Depending on the target objects, 7 – 11 epochs of data were obtained. We detected both H₂O and SiO maser lines from four sources: OH16.1−0.3, OH38.10−0.13, OH65.5+1.3, and IRAS 19312+1950. We could not detect H₂O maser emission toward OH13.1+5.1 between the late OH/IR and post-AGB stage. The detected H₂O masers show typical double-peaked line profiles. The SiO masers from four sources, except IRAS 19312+1950, show the peaks around the stellar velocity as a single peak, whereas the SiO masers from IRAS 19312+1950 occur above the red peak of the H₂O maser. We analyzed the properties of detected maser lines, and investigated their evolutionary state through comparison with the full widths at zero power. The distribution of observed target sources was also investigated in the IRAS two-color diagram in relation with the evolutionary stage of post-AGB stars. From our analyses, the evolutionary sequence of observed sources is suggested as OH65.5+1.3 → OH13.1+5.1→ OH16.1−0.3 → OH38.10−0.13, except for IRAS 19312+1950. In addition, OH13.1+5.1 from which the H₂O maser has not been detected is suggested to be on the gateway toward the post-AGB stage. With respect to the enigmatic object, IRAS 19312+1950, we could not clearly figure out its nature. To properly explain the unusual phenomena of SiO and H₂O masers, it is essential to establish the relative locations and spatial distributions of two masers using VLBI technique. We also include the 1.2 – 160 μm spectral energy distribution using photometric data from the following surveys: 2MASS, WISE, MSX, IRAS, and AKARI (IRC and FIS). In addition, from the IRAS LRS spectra, we found that the depth of silicate absorption features shows significant variations depending on the evolutionary sequence, associated with the termination of AGB phase mass-loss.

KaVA ESTEMA project

The ESTEMA (Expanded Study on Stellar Masers) project is one of three Large Programs of the KaVA (the combined array of the Korean VLBI Network and Japanese VLBI Exploration of Radio Astrometry), and conducted in 2015-2016. It aims to publish a database of the largest sample of VLBI images of circumstellar water (H2O) and silicon-monoxide (SiO) maser sources towards circumstellar envelopes (CSEs) of 80 evolved stars in late AGB to early post-AGB phase. Here we present the specifications of the ESTEMA observations and the planned scientific goals in order to share the basic information of the ESTEMA with astronomical community and encourage future collaborations with the ESTEMA and future follow-up observations for the targeted stars.

SIMULTANEOUS OBSERVATIONS OF H 2 O AND SIO MASERS TOWARD KNOWN EXTRAGALACTIC WATER MASER SOURCES

We observe ten known 22GHz H₂O maser galaxies during February 19–22, 2011 using the 21 m Tamna telescope of the Korean VLBI Network and a new wide-band digital spectrometer. Simultaneously we searched for 43GHz SiO v = 1, 2, J = 1–0 maser emission. We detect H₂O maser emission towards five sources (M 33, NGC 1052, NGC 1068, NGC 4258, M 82), with non-detections towards the remaining sources (UGC 3193, UGC 3789, Antennae H₂O-West, M 51, NGC 6323) likely due to sensitivity. Our 22GHz spectra are consistent with earlier findings. Our simultaneous 43GHz SiO maser search produced non-detections, yielding – for the first time – upper limits on the 43GHz SiO maser emission in these sources at a 3 σ sensitivity level of 0.018K–0.033K (0.24 Jy–0.44 Jy) in a 1.75 km s −1 velocity resolution. Our findings suggest that any 43GHz SiO masers in these sources (some having starburst-associated H₂O kilomasers) must be faint compared to the 22GHz H₂O maser emission.