Report on the activity of the KaVA Evolved Stars
Sub-Working Group in 2012-2013
KaVA of Evolved Stars Sub-Working Group
Hiroshi Imai*1, Miyako Oyadomari1, Se-Hyung Cho*2, Youngjoo Yun2,Jaeheon Kim2, Maria J. Rioja2,7, Richard Dodson2,7, Yoshiharu Asaki3,Kozue Kusuno3, Soon-Wook Kim4, Jeong-Sook Kim4, Taehyun Jung4,
Naoko Matsumoto5, Cheul-Hong Min5, Yoon-Kyong Choi6*co-chair of sub-WG; 1Kagoshima Univ.; 2KVN/Yonsei Univ.; 3ISAS/JAXA;KVN/KASI; 5NAOJ Mizusawa; 6MPIfR Bonn; 7ICRAR/Univ. Western Australia

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Main scientific interest

Stellar and interstellar astrophysics infinal stage of stellar evolution probedby H2O and SiO (43 GHz) masers
mainly O-rich, intermediate andhigh mass stars with pulsation
periods > 200 days
0. excitation mechanisms of masers
1. periodic behaviers of circumstellar
envelopes linked to stellar pulsation
2. asymmetric stellar mass loss as
seen in water fountain sources

Major issues in planning of a
KaVA Large Projects

Understanding observed properties ofstellar masers and actual specificationsof KaVA operation (see Figure 1.)
Figure 1 Timeiine of the Evolved Stars sub-WG activity, including planning theLarge Project(ESTEMA and Full) and test observations for commissioning.

Demonstrated currently
a. distributions of spot sizes andlifetimes of stellar maser spots
b. flux density variation of masers
associated with target stars
c. capability of snapshot imaging(mainly presented in this poster)
Demonstrated in this season
d. capability of multi-band imaging

d.1. multi-frequency phase-referencing
d.2. multi-band imaging in single session

e. astrometry for image registration andtrigonometric parallax distances
WX Piscius

WX Piscius

S Per H2O masers

Figure 2 KaVA imaging results for stellar masers. Left and middle panels: the SiO

v=2 and 1 J=1→0 masers, respectively, in WX Piscius (by Y. Yun and S.-H. Cho).
The top panels shows the velocity-integrated intensity map, while the bottom ones
show the plots of root-mean-squares noise against integration time in the specifictime baseline in the observation session. A dashed line indicates a power-law fittedone (index ~-0.5) to the longer integration data points. The deviation of the shortest
integration data point from the power-law line indicates the exisitence of intense
sidelobes on the maser image cube. Right: Same as SiO but for the H2O masers inS Persi (by K. Kusuno and Y. Asaki).

Demonstration results

A. Snapshot imaging for integration time of >15 min (H2O masers)
and >30 min (SiO) is feasible. (see Fig. 2)
B. Fringe-rate mapping can determine absolute coordinates of H2O
masers in snapshot imaging (for W Hya at δ~-29°).
C. Sub-array mode of VERA in KaVA (i.e. 3 KVN + 2 VERA
telescopes might not be feasible for imaging SiO masers(appearing dummy peaks of maser emission)(under reconfirmation).

Planning the initial KaVA
Large ProjectESTEMA(Expanded Study on Stellar Masers)● within 200 hours in 2013--2014● snapshot maser imaging for ~100
evolved stars hosting H2O (~0.5hour) and/or SiO masers (~1 hour)● statistic of stellar masers
i. maser spot sizes
ii. distributions of H2O masers
w.r.t. locations of SiO masersiii. correlation with physicalparameters of circumstellar
envelopes and central stars● yielding a larger sample ofevolved star as targets of future
VLBI observations● selecting evolved stars for future
full Large Project (intensive VLBI
monitoring, every 0.5-3 months
for 3-10 years)
For proposing and running ESTEMA,
we have to .....
a) Decide which mode is more feasible(sub-array or moderately fast bandswithing of VERA in KaVA operation).
b) Test feasibility of phase-referencing
astrometry with KaVA, indispensable
for accurate maser map registration.
c) Demonstrate multi-frequency phasereferencing for masers with KVN(by-product in ESTEMA but anotherlarge project in KVN).