VERA Users Meeting 2014

KaVA Large Programs ofCircumstellar Masers
Hiroshi Imai (Kagoshima University, Japan)
Se-Hyung Cho (KASI/KVN)
In behalf of the KaVA Evolved stars sub-WG

Draft proposal of
KaVA Large Programs of Stellar Masers! Scientific back ground of energetic mass loss in final

stellar evolution and circumstellar maser excitation! Scientific goals through intensive VLBI monitorings ofstellar H2O and SiO masers! Specifications of the 10 year Large Programs" Phase 1 (2015A&B, ~200 hours):

Large snapshot sampling of stellar masers" Phase 2 (2016A—2024, ~500 hours/year):

Full monitorings of stellar masers! Technical justification" Current progress of case studies

Basic structure ofa circumstellar
envelopeand masers

What actually
happen inmass loss
from dying stars?

Unknowns in basic
stellar mass loss
mechanism
Höfner(2011)! Radial acceleration" Type and amplitude of dust/gas acceleration

Radiative pressure on dust ……
! absorbing stellar radiation (amorphous dust in C-rich stars)! scattering stellar radiation (silicate dust in O-rich stars)?

"Efficiency of dust-driven wind! Correlation of mass loss

with stellar pulsation?

" Enhancement of mass loss bypropagation of pulsation-driven
shock wave?

Distance from star

Höfner et al. (1995)

Unknowns in circumstellar masers
as sensitive probes of stellar mass loss
! Maser excitation mechanism (SiO masers nearby stars)" Collisional pumping (by shocks)" Line overlapping pumping (mid-IR radiation from H2O)
! Origin of periodic and chaotic variation in masers" Not only in flux density!
" but maser spot distribution as well?
" By propagation of pulsation-driven shock waves?
" By just change in physical parameters (T) in envelope?
! What is actually traced by maser emission?
" Real Ballistic motions of material?
" Just movement of “ghosts”(propagation of physical conditions)?

Unknowns will
still remain unknown
without intensive
maser monitoring.
Propagation of physicalinformation(heating/shock)in SiO maser region?

We need new “colored”
movies rather than“monochromatic” ones.
SiO masers in TX Cam(Gonidakis et al. 2013)

We need H2O maser movies as well!
(Rapid) acceleration up to H2O maser region?
H2O masers in RT Vir (Imai et al. 2003)

Planning the Large Programs(KaVA Evolved Stars sub-WG)!

Large-sample, intensive study of stellar H2O and SiO masers forvisualizing shock propagation and asymmetry in CSEs
" AGB/post-AGB stars with long-period pulsations(100 d < P < 1600 d or 2.0 < log(P) < 3.2)
Stellar pulsation dynamics in cirumstellar envelopes" Sub-samples of “exotic sources”: e.g. “water fountains”
Evolution of asymmetric stellar mass loss

Phase 1 (2015): Large snapshot sampling (~80 sources)! Phase 2 (2016~2024): intensive monitoring for ~20 sources
9 years ~ 3300 days > 2.0 P max!

Possible KaVA stellar maser monitoring program

Phase 2 (2016~2024), 18 stars, for 10 years, ~500 hours/year
Annual schedule block

1 session ~ 4 hours of observation

Phase 1 Large Program: ESTEMA(Expanded Study on Stellar Masers)•

Snapshot imaging (~1.5 hours/source integration) of~80 stellar 22 GHz H2O and 43 GHz SiO masers~200 hours in total (K/Q band observations) in 2015•

Expand the number of stellar masers as VLBI targets•

List of ~50 new VLBI targets of stellar masers expectedwith ESTEMA (assuming ~75% fringe detection rate)
Source selection for new decadal study withKaVA (Phase 2), ALMA and VLTI••※including water fountains, symbiotic stars

Microscopic (maser feature structure ~0.1 AU) tomacroscopic (CSE diameter ~100 AU) view fromthe one-shot programESTEMA = maser imaging + astrometry
Finding the origin of asymmetric stellar mass loss
SiO masers pinpoint
stellar positions in H2O
maser regions in CSE.
! Dynamical timescale ofstellar outflow traced byH2O masers! Velocity field correlation
between SiO and H2O
maser regions! Asymmetry of stellar
mass loss!

VY CMa (Choi et al. 2008)

Multi frequency
phase-referencing(KVN) or/and
astrometry (VERA) arecrucial.

Long exposure (>4 hours)with KaVA

20 masSnapshot is also feasible.
S Per H2O (Kusuno et al.)WX Psc SiO v=2 & v=1 J=1#0(Yun et al.)First maser images
with KaVA data from
Daejeon Correlator(KJCC processor)~2.3 hour integration

BX Cam(Imai et al., 2014 Sep.)

First successful
multi frequency phase-referencing technique
R LMi (Dodson et al. 2014)

H2O masers

v=1 SiO masers
v=2 SiO masers

VLBIposition uncertainty

Central star(HIPPARCOS prediction)

Summary
We are being ready for the Large Programs (phase 1 at least).
! Technical and operation issues have been fixed towards

going ESTEMA (1st phase Large Program) including newDaejeon Correlator processing and user data processing inpipelines.

! Synergies with VLTI (imaging stars), ALMA (thermal/maserline and dust emission mapping) and Nano-JASMINE(Japanese space optical astrometry mission from 2015)
become realistic.

! Astronomers who are interested in subsections of the Large

Program science and further commissioning to(challenging) technical issues are always welcome to join
the KaVA Evolved Stars sub-Working Group.