Multi-epoch VERA observations of SiO masers toward R Aquarii
Cheulhong Min , Mareki Honmaand VERA project team(2)(1)(2),(1)(1)(1)(2)(1)(2)Tomoya Hirota, Katsunori M. Shibata

The Graduate University for Advanced Studies (SOKENDAI)(2) National Astronomical Observatory of Japan (NAOJ)

Abstract
We carried out multi-epoch phase-referencing VERA (VLBI Exploration of Radio Astrometry) observations of v=1,2 J=1-0 SiO masers toward R Aquarii (R Aqr)
covering about three stellar pulsation periods from Oct. 2011 to Jun. 2014. Overall distributions of the SiO masers show clumpy and partial ring-like structures
dominant in NE (2012) and SE (2013, 14) regions. The v=2 J=1-0 SiO maser rings is located closer than v=1 J=1-0 SiO maser consistent with previous VLBIobservations. The SiO maser rings appear to be contracting for the v=1 transition but expanding after contraction for v=2 transition due to shock propagations.

• SiO masers in AGB stars

Introduction- Formed mostly in Oxygen-rich ([O]/[C] >1) AGB stars- Perfect target for VLBI observations with a high spatial resolution- Useful for astrometry : distance (parallax) and proper motions- Tracer of inner most region of circumstellar envelope (CSE)- Formed between photosphere and dust formation region- Important probe for studying structures and dynamics of the CSE- Many vibrational/rotational transitions- Appears as a ring-like structure- Tangential amplification• AGB (Asymptotic Giant Branch) stars- Late-stage of a few solar mass stellar evolution- Pulsation with Large mass-loss rate (~10−6 𝑀𝑆𝑈𝑁 𝑦𝑟 −1 )– Main source for chemical enrichment of universe– Driving mechanism not well understood• R Aquarii (R Aqr)
- The closest symbiotic star (Mira + WD binary system)- Pulsation period : 387 days
- Hour-glass like inner & outer nebulae
Adam Block/Mt. Lemmon SkyCenter
Univ. of Arizona- Bipolar jet feature - powered by accretion disk on the WD- Circumstellar SiO maser source associated with Mira variable

Fig 1. Schematic structure of a typical AGB
circumstellar envelope

Observations••••

Multi-epoch phase-referencing VERA observations (4 stations)
Period : From Oct. 2011 to 2014 Jun. – total 21 epochs
Frequency : 28SiO masers of v=1,2 J=1-0 (43.112, 42.820 GHz)
Simultaneous observations for the target (R Aqr) and reference source(J2348-16) – separation ~ 1.6 degree
Targets

R.A. (J2000)

Dec.(J2000)

R Aquarii

23h 43m 49.4616s-15° 17’ 04.202”

J2348-1631

23h 48m 02.6085s-16° 31’ 12.022”••••

Record : DIR2000 16IF x 16MHz
Correlation : Mitaka FX correlator
Velocity resolution : 0.21 km s −1
Data reduction : NRAO AIPS software- Phase-referencing & Imaging done: 16 epochs- Synthesis beam : 1.00 mas × 0.43 mas
P.A. −27.22 deg
Fig 2. VERA arrayResults
 SiO masers distributions (Fig 3)

 Maser ring variations (Fig 4)- Clumpy and Partial ring-like distributions• Asymmetric distributions – dominant in NE(2012) and SE(2013,2014) regions• Mean maser radii : 𝑅𝑣=2 ~14.68 mas < 𝑅𝑣=1 ~15.39 mas(𝑅𝑣=2 ~ 3.20 AU < 𝑅𝑣=1 ~ 3.35 AU @ 218 pc[1])• v=2 SiO maser is closed to the star than v=1 SiO maser• SiO masing region 𝑅𝑆𝑖𝑂 = 2.79 ~2.93 𝑅𝑀𝐼𝑅𝐴 (𝑅𝑀𝐼𝑅𝐴 = 1.14 AU[2])- Spike-like features• East region in 2012 / South-West region in 2014

- Contraction and Expansion• Ballistic contraction of the v=1 SiO maser region (gravitational effect)- Mean acceleration : 𝑔𝑆𝑖𝑂 ~ 3.9 × 10−7 km s−2(𝑔𝑆𝑖𝑂 ~ 1.5 × 10−7 km s−2 for TX Cam[3])(𝑔𝑆𝑖𝑂 ~ 3.3 × 10−7 km s−2 for R Cas[4])• Contraction → Expansion of the v=2 SiO maser region (shock effect)- Contraction : Before encountering the propagated shock- Expansion : After shock encountering – following shock propagation

 SiO masers flux variations (Fig 4,5)•••

Following the pulsation period of the Mira variable- correlation with optical light curve (phase lag ~0.1)
Slightly coincidence maximum radio flux with opticalmaximum between 2012 and 2013
Correlation SiO maser radii and flux variations- smaller radius (closed to the star) being more intense Future Works
Compare with SiO maser models and dynamic
atmospheric models of the oxygen-rich Mira variables- SiO maser pumping mechanism- confine SiO maser emitting region- correlation masing radius with its intensity- non-uniform shock propagation• Linked to other observational results for CES in AGBstars•Fig 3. VLBI map of SiO masers toward R Aqr(Left) Velocity integrated contour map of SiO masers(Right) Distributions of SiO maser spots in LSR velocity field

References

Fig 4. (Top) Optical light curve of R Aqr (AAVSO)(Middle) Variations of the SiO maser ring radii(Bottom) Integrated XC spectra of SiO masers

Fig 5. Integrated SiO maser spectra plottedwith respect to the ring radii[1] Min et al., 2014, PASJ 66, 38[3] Gonidakis et al., 2010, MNRAS 406, 395[2] van Leeuwen et al., 1997, MNRAS 287, 955 [4] Assaf et al., 2011, MNRAS 415, 1083