VERA and ALMA Observations of the H2O Supermaser Burst in Orion KL

The 22 GHz H2O maser in Orion KL has shown extraordinary burst events in 1979-1985 and 1998-1999, sometimes called supermaser. We have conducted monitoring observations of the supermaser in Orion KL using VERA, VLBI Exploration of Radio Astrometry, in the current third burst since 2011 March. Three flux maxima are detected in 2011 and 2012 with rising and falling timescales of 2-7 months. Time variations of the supermaser seem symmetric for all of the active phases. The maximum total flux density of 135000 Jy is observed in 2012 June while it is still one order of magnitude lower than those in previous bursts. The supermaser consists of two spatially different components at different velocities. They are elongated along a northwest-southeast direction perpendicular to the low-velocity outflow driven by Source I. Proper motions of the supermaser features with respect to Source I are measured toward west and southwest directions, almost parallel to the low-velocity outflow. The flux density and linewidth show an anti-correlation as expected for an unsaturated maser emission. The supermaser is located close to the methylformate (HCOOCH3) line and continuum emission peaks in the Orion Compact Ridge detected by ALMA. The broader velocity range of the weak HCOOCH3 emission at the supermaser position would be an evidence of a shock front. On the other hand, the 321 GHz H2O line is not detected at the position of the supermaser. It can be explained qualitatively by one of the theoretical H2O excitation models without extraordinary conditions. Our results support a scenario that the supermaser is excited in the dense gas interacting with the low-velocity outflow in the Compact Ridge. The extremely high flux density and its symmetric time variation for rising and falling phases could be explained by a beaming effect during the amplification process rather than changes in physical conditions.

Identification of Bursting Water Maser Features in Orion KL

In February 2011, a burst event of the H$_{2}$O maser in Orion KL (Kleinmann-Low object) has started after 13-year silence. This is the third time to detect such phenomena in Orion KL, followed by those in 1979-1985 and 1998. We have carried out astrometric observations of the bursting H$_{2}$O maser features in Orion KL with VERA (VLBI Exploration of Radio Astrometry), a Japanese VLBI network dedicated for astrometry. The total flux of the bursting feature at the LSR velocity of 7.58 km s$^{-1}$ reaches 4.4$times10^{4}$ Jy in March 2011. The intensity of the bursting feature is three orders of magnitudes larger than that of the same velocity feature in the quiescent phase in 2006. Two months later, another new feature appears at the LSR velocity of 6.95 km s$^{-1}$ in May 2011, separated by 12 mas north of the 7.58 km s$^{-1}$ feature. Thus, the current burst occurs at two spatially different features. The bursting masers are elongated along the northwest-southeast direction as reported in the previous burst in 1998. We determine the absolute positions of the bursting features for the first time ever with a submilli-arcsecond (mas) accuracy. Their positions are coincident with the shocked molecular gas called the Orion Compact Ridge. We tentatively detect the absolute proper motions of the bursting features toward southwest direction. It is most likely that the outflow from the radio source I or another young stellar object interacting with the Compact Ridge is a possible origin of the H$_{2}$O maser burst.

Astrometry of H$_{2}$O Masers in Nearby Star-Forming Regions with VERA. III. IRAS 22198+6336 in L1204G

We present results of multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H$_{2}$O masers associated with a young stellar object (YSO) IRAS 22198+6336 in a dark cloud L1204G. Based on the phase-referencing VLBI astrometry, we derive an annual parallax of IRAS 22198+6336 to be 1.309$pm$0.047 mas, corresponding to the distance of 764$pm$27 pc from the Sun. Although the most principal error source of our astrometry is attributed to the internal structure of the maser spots, we successfully reduce the errors in the derived annual parallax by employing the position measurements for all of the 26 detected maser spots. Based on this result, we reanalyze the spectral energy distribution (SED) of IRAS 22198+6336 and find that the bolometric luminosity and total mass of IRAS 22198+6336 are 450$L_{odot}$ and 7$M_{odot}$, respectively. These values are consistent with an intermediate-mass YSO deeply embedded in the dense dust core, which has been proposed to be an intermediate-mass counterpart of a low-mass Class 0 source. In addition, we obtain absolute proper motions of the H$_{2}$O masers for the most blue-shifted components. We propose that the collimated jets aligned along the east-west direction are the most plausible explanation for the origin of the detected maser features.

Astrometry of Water Maser Sources in Nearby Molecular Clouds with VERA - II. SVS 13 in NGC 1333

We report on the results of multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H2O masers associated with the young stellar object SVS 13 in the NGC 1333 region. We have carried out phase-referencing VLBI astrometry and measured an annual parallax of the maser features in SVS 13 of 4.25+/-0.32 mas, corresponding to the distance of 235+/-18 pc from the Sun. Our result is consistent with a photometric distance of 220 pc previously reported. Even though the maser features were detectable only for 6 months, the present results provide the distance to NGC 1333 with much higher accuracy than photometric methods. The absolute positions and proper motions have been derived, revealing that the H2O masers with the LSR (local standard of rest) velocities of 7-8 km s-1 are most likely associated with VLA4A, which is a radio counterpart of SVS 13. The origin of the observed proper motions of the maser features are currently difficult to attribute to either the jet or the rotating circumstellar disk associated with VLA4A, which should be investigated through future high-resolution astrometric observations of VLA4A and other radio sources in NGC 1333.

Distance to Orion KL Measured with VERA

We present the initial results of multi-epoch VLBI observations of the 22 GHz H2O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry). With the VERA dual-beam receiving system, we have carried out phase-referencing VLBI astrometry and successfully detected an annual parallax of Orion KL to be 2.29+/-0.10 mas, corresponding to the distance of 437+/-19 pc from the Sun. The distance to Orion KL is determined for the first time with the annual parallax method in these observations. Although this value is consistent with that of the previously reported, 480+/-80 pc, which is estimated from the statistical parallax method using proper motions and radial velocities of the H2O maser features, our new results provide the much more accurate value with an uncertainty of only 4%. In addition to the annual parallax, we have detected an absolute proper motion of the maser feature, suggesting an outflow motion powered by the radio source I along with the systematic motion of source I itself.