Studies on Evolved Stars Using KVN
The 12th East Asian VLBI Workshop
September 23-27, 2019 , Ibaraki University, Mito, Ibaraki, Japan
Se-Hyung Cho1,2, Youngjoo Yun1, Dong-Hwan Yoon1, Haneul Yang1,2, Jaeheon Kim3, Dong-Jin Kim4, Richard Dodson5, Maria
Rioja5 and Hiroshi Imai6
1KASI, 2Seoul Nat. Univ., 3Chungbuk Nat. Univ., 4MPI, 5ICRAR, 6Kagoshima Univ.

Abstract
At the commissioning phase of KVN from 2009 to 2013, single dish survey performed toward about 1000 evolved stars and monitoring.
observations
also toward about 60 relatively strong SiO and H2O maser sources. Based on these single dish results and VLBI feasibility test
observations at K/Q/W/D bands in 2014, KVN Key Science Project (KSP) centered on VLBI observations has started from 2015 and will becompleted in 2019 as phase I. Here we report the overview of studies on evolved stars using KVN. In KSP phase I, we have focused on nine
KSP sources which show a successful astrometrically registered maps of SiO and H2O masers using the Source Frequency Phase
Referencing (SFPR) method. We aim at investigating the spatial structure and dynamical effect from 43/42/86/129 GHz SiO to 22 GHz H2O
maser regions associated with a stellar pulsation and development of asymmetry in circumstellar envelopes.

Introduction

Observational Results■ The maser emission plays an important role for investigating the atmosphereand envelope of the central stars according to their different chemical andexcitation conditions. It is important to perform simultaneous observations ofboth SiO and H2O masers because their spatial distributions may be connectedfrom the innermost region to the radially accelerating region of the circumstellar
envelope.
■ The Korean VLBI Network (KVN) equipping a simultaneous observation systemof 22/43/86/129 GHz bands enables us to combined studies of SiO and H2O masers.
► We have performed single dish survey at commissioning phase of KVN and theKey Science Project of evolved stars since 2015A (SH Cho et al. 2018 ).

■ Successful SFPR maps of H2O and SiO masers were obtained from KSP 9 stars :658d583d
517dWX Psc.
VX Sgr, VY CMa, IK Tau, R Crt, W Hya, V1111 Oph, V627 Cas, V5102 Sgr,Supergiant Star VX Sgr (PhD thesis of DH Yoon 2019)

DH Yoon et al. 2018 Nature Comm. 9:2534■ As a science goal, (1) we aim at investigating the spatial
structure and dynamical effect from SiO to 22 GHz H2O
maser regions and the development of asymmetry via dust
formation layer according to stellar pulsation.
(2) Combined models of SiO and H2O masers will bestudied.
(3) The correlation and difference of SiO maser properties
among J=1-0, J=2-1, and J=3-2 masers are investigatedfor constraining SiO pumping models.

517d43 GHz SiO maps of wholemonitoring: 2014-2019A

Observations and Data Reduction
Target Sources for KVN Single Dish and VLBI Obs.

VX Sgr: (Left) NE-SW transformations appeared from 2017 Feb. which shows the shortest period, 517
days (green box in the light curve). ◄ probably caused by the variations of pulsation motion (Right)
Variation of SiO ring radius with optical light curve. ● Ring radius of 129 GHz maser increases near
optical max. ► Radiative pumping is dominant.

4 Band SFPR Maps (Movie) of H2O and SiO Masers toward VY CMa
Q band(43 GHz)
W band(86 GHz)

Number of monitoring obs. & obs. success rate

KVN Single Dish and VLBI Monitoring Obs.
■ Single dish survey toward ~1000 stars from 2009 : SRs, Miras, SGs, OH/IR stars(252), Post-AGB stars (182), Symbiotic stars■ Single dish monitoring toward about 10-15 strong SiO and H2O maser sources
together with VLBI every 1- 3 months regularly■ VLBI monitoring focused on 10 successful SFPR objects■ Obs. lines : H2O 616-523 , SiO v=1, 2, J=1-0, v=1, J=2-1, & v=1, J=3-2● KVN home page ► http://radio.kasi.re.kr/kvn/kvn.php

SH Cho et al. in prep., Movies by YJ Yun
VY CMa: (Left) Four band integrated intensity maps of H2O
and SiO masers. The SiO v=1, J=3-2 maser location isdifferent from those of other SiO masers. (Right) The H2O
and SiO maser movies for 8 epochs. The SiO movies show anelliptical pulsation motions (NE-SW pulsation motion).

Semiregular b-Type Variable R Crt : P~160 days
R Crt(2016 Jan. 7)◈ Source Frequency Phase Referencing (SFPR) technique (Dodson et al. 2014)
and SFPR pipeline (2019 YJ Yun) were adopted.

Observational Results

Ring size of SiO v=1, J=2-1maser in R Crt is smaller than that ofv=1, J=1-0 different from known results (DJ Kim et al. 2018).

Summary of KVN Single Dish Observations

Mira Variable
IK Tau

OH/IR star: WX Psc, V1111 Oph ►YJ Yun’s presentation■ Both SiO and H2O masers are detected from a large number of AGB stars(SR, Mira, SG: 188/401, OH/IR: 50/252) at one-epoch obs. ◄ Both H2O & SiO masers aredetected from known SiO-only and H2O-only detected sources (Kim et al. 2010, 2013,Cho et al. 2012).

Symbiotic star: V627 Cas ► HN Yang’s pres.

HN Yang et al. ApJL in Prep.

■ Intensity variations of SiO and H2O masers show similar patterns for several stars (Ref: H2O
maser maximum are scattered in the range of phase, 0.7~1.5; Shintani et al. 2008)(Kim et al. 2014).
Yoon et al. 2014

■ SiO maser intensities are stronger than H2O maser in mostevolved stars and at most optical phases. However, peakintensities of H2O are stronger than those of SiO aroundmaximum phases in several stars (Kim et al. 2014).
■ FWZP of SiO maser shows a similar value in both Miravariables and OH/IR stars, while FWZP of H2O maser is large
in OH/IR stars compared with that in Mira variables(Kim et al. 2012, 2014, Cho et al. 2017).
■ One-way, double peaks of H2O masers, and SiO v=2-only masers appear at late stage of AGB
evolution and at post-AGB stage (Yoon et al. 2014, Cho et al. 2017).
■ Different detection rates between SiO and H2O masers at post-AGB stage (Yoon et al. 2014).

We investigated that the variations of H2O & SiO masers are associated with the orbital motion of hot companion.

Discussion and Future Works◆ What is the development process for changing from ring (elliptical)-likestructure of SiO masers to highly asymmetric structure of H2O maser via dust
formation layer including the transformations of SiO ring-like structure ?
◆ Differences of pumping mechanism among SiO masers. ◆ Completion of movies
and polarization obs. ◆ Hydrodynamical model combining SiO and H2O maserpropagation ◄ Gray et al. 2009

ReferencesCho et al. 2018, Proceedings IAU Sym. 336
Dodson et al. 2014, AJ, 148, 97

Gray et al. 2009, MNRAS, 394, 51
Yoon et al. 2014, ApJS, 211, 15