VLBI observations of two 43-GHz SiO masers in R Cas Jiyune Yi KVN Korea VLBI Network ( KVN ) group Korea Astronomy and Space Science Institute In collaboration with R. Booth 1,2 and J. Conway 1 1. Onsala Space Observatory, Sweden 2. Hartebeesthoek Radio Astronomy Observatory 8 th EVN Symposium 2006

M5 Asymptotic Giant Branch AGB star C/O core He burning inner shell H burning outer shell

Stellar masers & Evolved stars SiO, H 2 O and OH masers form in the extended stellar atmosphere & circumstellar envelope of evolved star (AGB stars)  High resolution studies of SiO masers ☞ unique tool to study extended stellar atmosphere of AGB stars ☞ unique tool to study extended stellar atmosphere of AGB stars

SiO maser in AGB star adopted by J. Hron, original idea by T. Le Bertre

Scientific goals VLBA observations of SiO masers To find significant constraints on SiO maser modellings  evidence of stellar phase dependence To provide highly plausible inputs for new models To extend our understanding on the physical and dynamical properties of CSEs  positions of individual maser clumps measured down to sub- milliarcsecond accuracy To put confidence in non-standard VLBI techniques, (both observations and calibrations)

6 Technical challenge To track the delay across the 301 MHz frequency gap between the v=1 and v=2 transitions  Simultaneous observations of the two maser transitions required To determine the relative position of the masers in the two transitions  Imaging the two maser maps relative to each other using cross-phase referencing

4 epochs of VLBA observations : R Cas R Cas Light curve (courtesy,AAVSO)

Epoch I (  ~ 0.25) 10 mas ~ 1.07 AU

9 Epoch II (  ~ 0.68)

10 Epoch III (  ~ 0.95) 10 mas ~ 1.07 AU

11 Epoch IV (  ~ 0.23)

R Cas image at 671 nm (Weigelt et al. 1996)

R Cas photospheric size measured by optical/IR Weigelt et al mas(700 nm), 49 mas(714) Hofmann et al mas(671), 37 mas(700) 49 mas(714), 30 mas(1045) Mennesson et al mas (  ~ 0.09) at 2.16  m mas (  ~ 0.17) at 3.79  m  Weigelt et al  700 nm R Cas  714 nm R Cas

Stellar photospheric size versus SiO maser shell size of R Cas Angular diameter < 30 mas, at near IR continuum Angular diameter > 30 mas, at visible  Comparison with 3.8  & 2.2  m radii, R(3.8) & R(2.2) at  0.17 & 0.09, respectively (obs. by Mennesson et al. 2002)  0.17  0.09 Epoch I (  0.25) 1.01 x R(3.8)1.3 x R(2.2) Epoch II (  0.68) 1.4 x R(3.8)1.8 x R(2.2) Epoch III & IV (  0.95 & 0.23) 1.6 x R(3.8)2.0 x R(2.2)

Summary : R Cas Instead of ring disruption at near maser minimum (Epoch II ) both masers formed circular rings. Both maser rings expanded and contracted depending on the stellar phase. At maser maximum, both masers showed many coincident masers. Outward-extending flare-like structure of emission survived over 2 epochs (~ 0.3 stellar phase). SiO maser shell diameters estimated around 1 ~ 2 stellar diameter. Asymmetry found at Epoch I, asymmetric ejection of material directed away from us ? Models which are predominantly collisionally pumped are in good agreement with our results. Missing flux (typically more than 50 %) density found, estimated a lower limit of the structure, 3~4 mas

16 TX Cam maps at 4 epochs (Jiyune Yi et al. 2005) 10 mas ~3.8 AU

17 Analysis of MASER ring radius

18 Comparison with models Epoch III Epoch IV v = 1 v = 2 v = 1 v = 2

19 Comparison with models Ring shape  Disruption of the ring structure at maser minimum development of the ring afterwards Ring radius  Expansion and contraction along the stellar cycle Ratio of the ring radius, v=2/v=1  96 % ( III ) vs 94 % (M) ; 91 % ( IV ) vs 92 %(M) relatively smaller R at IV  v=2, contracting while v=1, constant Ring thickness (25~75% percentiles), v=1 vs v=2  15.6 % vs 14.9 % ( III ) ; 19.5 % vs 18.6 % ( IV ) of the ring radius  twice thicker in the v=1 ring (M)

20

3600 K 1800 K Constraint on SiO maser models To excite the lowest vibrational state,v=1  Required temperature >1800 K  Unable to have spatial coincidence of masers in various v- states by radiative pumping

Velocity field of the masers V=1V=2 Epoch III

Spoke-like features Epoch IV V=1V=2

Radial Spokes Rectangles  spokes of gas flowing outward at different angle Thick rect.  the brightest spokes which we observe All spokes have the same velocity field, decelerating with radius. LOS1  LOS through a spoke in the sky plane, having a velocity coherent path equal to the spoke width LOS2  LOS through a spoke, having a maximum velocity coherent path length LOS3  LOS, velocity coherent path length decreasing because of the large velocity gradient along the path

Models of SiO masers in M-Miras (Humphreys et al. 2002: Gray & Humphreys 2000) 86 GHz v=1,J=1-0 v=2,J=1-0

Comparison with other observations Desmurs et al Diamond & Kemball 2003

GHz SiO maser & NIR observations in S Orionis  SiO maser at ~ 2 photospheric radii (Boboltz & Wittkowski 2005)