Comprehensive study of nearby stellar masers: W Hydrae
Hiroshi Imai (Department of Physics and Astronomy, Kagoshima University, hiroimai@sci.kagoshima-u.ac.jp)
Abstract

In this poster, we propose a comprehensive study of one of nearby long-period variable star, W Hya, on the basis of systematic VLBI monitoring observations of SiO
v=1, 2, 3 (J=1-0) and H2O maser emission with, especially VERA and KVN using unique capabilities of these arrays. This research plan will enable us to produce“stellar maser movies” that should fascinate astronomers interested in stellar mass loss, pulsation, shock wave propagation into a circumstellar envelope, and maser
excitation as well as public people who have watched only movies of the Sun and unmoved sky. VLBI observaitons of W Hya are scientifically and technically feasible
only by comnbining VERA and KVN. The expected quality of maser source images may be comparable to or slightly better than images taken with ALMA and EVLA,
enabling to elucidate the transportation of stellar gas and dust from the stellar survace to the outer region of the circumstellar envelope.
TX Cam SiO v=1 J=1-0 (VLBA)

Figure 1. Only one “stellar maser movie”
to date, obtained from 75-epochs VLBA
observations (Diamond & Kemball 2003).

Motivation○ Unique science through SiO maser movie: material
ejection and contraction (including acceleration andinhomogeneity) around the surface of long-periodvariable stars such as Mira variables. (Figure 1)○ From SiO to H2O maser regions: tracing material
floating, dust formationm and accelration incircumstellar envelope (CSE). (Figure 2)○ Dust formation should enhance pulsation-driven
shock waves that accelerates the outward flow in H2O
maser regions in a CSE (Figure 3). However, only atentative confirmation was reported (Figure 4).
Figure 4. Possible detection of the passage of a pulsation○ SiO maser excitation mechanism is a key issue todriven shock wave, which was traced by acceleration in H2O
elucidate the dynamics of the inner CSE. It is difficult tomaser motion in RT Virginis (Imai et al. 2003)
unambiguously identify the pumping scheme if only v=1
and v=2 masers are mapped (Miyoshi et al. 1994, Yi et al.
Collisional pumping
Radiative pumping2005). But if v=3 maser is mapped as well, it is possible(Figure 5). The v=3 maser can be mapped with VLBI (Figure 6).

Observation plan with VERA+KVN(+JVN/EAVN)RadioSiO
H 2O

Figure 2. Superposition of VLA radio maps toinvestigate spatial correlation from the stellar
surface to the circumstellar envelope(Reid & Menten (2007).

○ W Hya: Nearest (80 pc) SiO and H2O maser star. VERA
alone can measure annual parallaxes of AGB stars at > 100 pc(Nagagawa et al. 2008), but NOT for W Hya. These masers arewell spatially resolved but suitable for VERA+KVN observations(Figure 7).
○ Monthly monitoring through the whole pulsation period(~400d) in order to detect a few pulsation-driven shock waves
travelling into a CSE through H2O maser acceleration (Figure 8).
○ Simultaneous mapping of SiO v=1, v=2, v=3 and H2O
masers, which may be possible only the combination ofKJJVC and VERA+KVN with new wide-band backend system.
(OCTAVIA/OCTADISK, Mark 5C).

Figure 5. Two different theoretical models of SiO
pumping mechanism based on different schemes;
maser photon number against gas density (higherdensity region is closer to the stellar surface)(Locket & Elizur 1992).

2009 February 27-28(VERA)

Stellar diameter

ALMA band 10 beam

Figure 3. Theoretical model of dust-induced
pulsation-driven shock waves moving
outwards in to a circumstellar envelope(Hofner et al. 1995).

2009 April-12 (VERA+NiCT+NRO)
Figure 6. Superposition of SiO J=1-0 maser maps obtained with VERA and JVN (Imai et al. 2010). (Left panel) The maser emission
regions of v=1 and v=2 are well correlated, while the v=3 emission has offsets from the v=1 and v=2 regions. (Right) one month
later, the v=3 maser regions well coincide with the v=2 region. These indicate a possible switch of the maser pumping schemes.

2005 March 3(VERA)

Figure 7. Various VLBI mapping of the W Hya H2O masers. The difference in maser spot extension size between
1985 and 1993 should be intrinsic variation due to stellar pulsation, while that between VLA/KNIFE and VERA
observations should be attributed to the difference in degrees of spatial resolution.

H 2O
opticalFigure 8. Periodic variation of H2O flux in W Hydrae withcorrelation with the optcal light curve plus phase lag,indicating maser excitation by stellar pulsation andshock waves (Shintani et al. 2008)

Refereces
Diamond, P.J. & Kemball A.J., 2003, ApJ, 599, 1372 Locket, P., & Elitur, M. 1992, ApJ, 399, 704 Imai, H. et al. 2010, PASJ, 62, 431 Imai, H. et al. 2003, ApJ, 590, 460
Imai, H. et al. 1997, A&A, 317, L67
Kamohara R., et al., A&A, 510, 69
Matsumoto, N. et al. 2008, PASJ, 60, 1039
Miyoshi M. et al. 1994, Nature. 371, 395
Nakagawa et al. 2010, PASJ, 62, in press
Nakagawa et al. 2008, PASJ, 60, 1013
Reid, M.J., Menten, K.M., 2007, ApJ, 671, 2068
Shintani, M. Imai, H., et al. 2008, PASJ, 60, 1077
Soria-Ruiz, R., et al. 2004, A&A, 426, 131Yi, J., et al. 2005, A&A, 432, 531