VLBI observations of two 43-GHz SiO masers in R Cas

Jiyune YiKorea VLBI Network ( KVNKVN ) 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

8th EVN Symposium 2006

M5

Asymptotic Giant Branch

AGB star C/O core

He burning inner shellH burning outer shell

Stellar masers & Evolved stars

• SiO, H2O and OH masers form in the extended stellar atmosphere &

circumstellar envelope of evolved star (AGB stars)

High resolution studies of SiO masersHigh resolution studies of SiO masers

☞ ☞ unique tool to study extended stellar unique tool to study extended stellar atmosphere of AGB stars atmosphere of AGB stars

SiO maser in AGB star

adopted by J. Hron, original idea by T. Le Bertre

Scientific goalsVLBA 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)

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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

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Epoch II ( ~0.68)

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Epoch III ( ~ 0.95)

10 mas ~ 1.07 AU

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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. 1996 36 mas(700 nm), 49 mas(714)• Hofmann et al. 200044 mas(671), 37 mas(700)49 mas(714), 30 mas(1045)

• Mennesson et al. 200224.78 mas (~ 0.09) at 2.16 m31.09 mas ( ~ 0.17) at 3.79 m

Weigelt et al. 1996

700 nm R Cas

714 nm R Cas

cm13108~4

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& 2.2m radii, R(3.8) & R(2.2) at 0.17 & 0.09, respectively (obs. by Mennesson et al. 2002)

AUcm 3.5~7.2108~4 13

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

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52.0

TX Cam maps at 4 epochs (Jiyune Yi et al. 2005)

60.0 83.0 05.010 mas~3.8 AU

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Analysis of MASER ring radius

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Comparison with models Epoch III

Epoch IV

v = 1

v = 2

v = 1v = 2

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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)

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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=1 V=2Epoch III

Spoke-like featuresEpoch IV

V=1 V=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

)0(

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 observationsDesmurs et al. 2000Diamond & Kemball 2003

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43.1-GHz SiO maser

& NIR observations

in S Orionis

SiO maser at

~ 2 photospheric radii

(Boboltz & Wittkowski 2005)