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Stellar Evolution as seen through the eyes Stellar Evolution as seen through the eyes of VLBI and masersof VLBI and masers
Stellar Evolution as seen through the eyes Stellar Evolution as seen through the eyes of VLBI and masersof VLBI and masers
P.J.DiamondP.J.DiamondP.J.DiamondP.J.Diamond
Jodrell Bank ObservatoryJodrell Bank Observatory
University of ManchesterUniversity of Manchester
Jodrell Bank ObservatoryJodrell Bank Observatory
University of ManchesterUniversity of Manchester
Castel San Pietro Terme19 September 2001
Castel San Pietro Terme19 September 2001
Stellar Evolution in a flash
Main Sequence:~5 Gyrs
Red Giant Branch:Build core of He and degenerateelectrons, and mantle of H.
Increase in L 2500Lo
Decrease in temp 2500KTimescale ~ 1 G yr
Asymptotic Giant Branch:New C/O + deg. elec. core.Thin He layer, H mantle.
At 2200Lo , T~3000K star entersTP-AGB.H burns into He at inneredge of mantle => He burns intoC & O => temp. increase in L.Called ‘thermal pulse’. Mass lossoccurs, continues until mantle gone.~0.1-0.2 Myr.
Horizontal Branch:Core suddenly collapses.L decreases abruptly, star is still He-burning with L~50Lo.
Timescale ~ 130 M yr
Post-AGB:Fossil shell (last of massloss). Core exposed.=> PPN => PNe
Some definitions
• AGB: Asymptotic Giant Branch
• LPV: Long Period Variable (P > 50-100 days)
• Mira variables: M-type stars with V > 2.5m (P~100-500d)
• OH/IR star, LPVs with P ~500 – 3000 days. Rarely optically detected, bright in the IR – thick dust shells.
• PNe/PPN: Planetary Nebulae, Pre-Planetary Nebulae
• EVN: European VLBI Network
• VLBA: Very Long Baseline Array
• MERLIN: Multi Element Radio Linked Interferometer Network
The ‘Standard Model’
• Goldreich & Scoville (1976)
• Red giant, spherically symmetric gas outflow. SiO exists, mases.
• At r, dust forms, absorb light re-emit as IR, momentum coupled to gas, radiation pressure => gas accelerating and heating.
• Cools by H2O formed in lower envelope. Masers from H2O.
• UV photons disassociate H2O => OH, H. OH masers form. Gas at terminal velocity.
Variable, very thick CSE
Variable, thick CSEVariable, more evolved CSE
Variable with young CSEO-rich, non-variable, no CSE
Colour-colour diagrams (van der Veen & Habing, A&A, 1988, 194, 125
PNe with cool CSE
Variables with C-rich CSE
The ‘Lewis’ Chronology.
Stage Key Change SiO H2O 1665/7 OH 1612 OH
1 SiO masers XX
2 Add H2O XX XX
3 Add 1665/7 XX XX XX
4 Add 1612 XX XX XX XX
5 1665/7 weaken XX XX XX XX
6 Lose 1665/7 XX XX XX
7 Lose H2O XX XX
8 Lose SiO XX
9 Add 1665/7 XX XX
10 1665/7 strong XX XX
11 PN stage
Miras
OH/IR
PPN
PNe
InterferometryVLBA provides frequency flexibility and
ability to observe up to 43 GHz.
EVN, with big dishes, provides sensitivity
at 18cm and ability to observe at 6 GHz
MERLIN is instrument of choice for
OH and larger-scale H2O maser studies.
MERLIN is instrument of choice for
OH and larger-scale H2O maser studies.
IRAM image of shell around Carbon star TT CygOlofsson et al 1999 : CO(1-0) @ 115GHz
Diameter
~0.5 ly,
ring has
been
expanding
for 6800
yrs
Compact
central
envelope,
2nd mass
loss episode
• Not all circumstellar envelopes are smooth and symmetric
• Monnier et al (ApJ, 512, 351, 1999) observed the circumstellar dust around the supergiant VY Cma using adaptive optics and aperture masking techniques
SiO Masers
• VLBA has enabled routine observations of SiO masers• Early attempts by Moran et al (1979, ApJ, 231, L67), Lane
(1982, Ph.D), McIntosh (1987, Ph.D) demonstrated the difficulty of 7mm VLBI and showed that, with the instruments available at that time, the SiO emission was clustered in regions similar in size to that of the stars.
• Colomer et al (1992, A&A, 254, L17) showed that modern equipment could detect compact structure in SiO masers
• Diamond et al (1994, ApJ, 430, L61) produced the first image of SiO masers around the stars TX Cam and U Her– ring-like structures, ordered not random, masers are tangentially
beamed
• Confirmed by Miyoshi et al (1994, Nature, 371, 395) and Greenhill et al (1995, ApJ, 449, 365)
VX Sgr: 43 GHZVX Sgr: 43 GHZG
reen
hill
et a
l
(199
5,A
pJ,4
49,3
65)
TX CamTX Cam U HerU Her
Diamond et al (1994, ApJ, 430, L61)
VX Sgr: 86 GHzVX Sgr: 86 GHz
Doeleman et al (1998, ApJ, 494, 400)
SiO: proper motions
Components in N, S, NE & WComponent in E
Predominant motion is outflow
Strong evidence for shocks dominating
the kinematics
Predominant motion is outflow
Strong evidence for shocks dominating
the kinematics
Tangential vectors confinedto narrow inner edge of ring.Strong evidence of effects ofshocks.
Remarkable circular magneticfield structure. Origin unknown
SiO: Polarization studiesSiO: Polarization studies
H2O
• Proper motions of masers around stars relatively ‘undeveloped’ subject until the advent of the VLBA.
• Marvel (Ph.D, 1996) studied the proper motion of H2O masers around several stars.
• Demonstrated that masers were expanding as expected, some peculiarities showing departures from smooth, symmetric outflows
S Per: EVN+MERLINS Per: EVN+MERLIN
NML CygNML CygNML CygNML Cyg
MERLIN proper motion
measurements of H2O in
NML Cyg shows bipolar flow
B~280mGB~280mG(Vlemmings, Diamond(Vlemmings, Diamond
& van Langevelde, 2001)& van Langevelde, 2001)
B~280mGB~280mG(Vlemmings, Diamond(Vlemmings, Diamond
& van Langevelde, 2001)& van Langevelde, 2001)
OH shells: thin CSEs & MirasMultiple epoch VLBA observations of the ‘thin’ shell source U Her (van Langevelde et al, 2000)Designed to measure parallax and study fine structure.
VLBA= -17.050.85, -9.480.73 mas/yr.
Parallax detected: VLBA= 4.2 1.2 mas.
OH Shells: thick CSEs
OH Shells
• MERLIN observations were the first to show the existence of shells of masing gas around AGB stars (Booth et al, 1981, Nature, 290, 382)
MERLIN observationsMERLIN observations
of OH127.8-0.0of OH127.8-0.0Bright blue-shifted spots show compact structure.observations ~10 years apart reveal surprisingly small changes
OH: Polarization
VX Sgr: 1612 MHz
Szymczak et al, in prep
Polzn vectors tangential
to circumstellar envelope.
Linear polzn ~ 10-20%
Structure favours a radial
field – maybe we are viewing
a dipole field end-on.
Circular polarization =>
B ~ 1.1 mG
OH: polarization
• MERLIN observations of NML Cyg (Diamond & Etoka, in prep)
• In this case:– 1612 MHz field lines are
predominantly parallel to each other
– 1665 MHz field lines suggest a tangential structure
Getting older• IRAS 18455+0448: a dying maser. Lewis et al, 2001
1665 remains largely unaffected.Maybe witnessing the early stageof expansion of fossil shell priorto it becoming a planetary nebula.Mass loss 0, UV photodissociatingOH.
OH009.1-0.4: Sevenster & Chapman (2001)
• Presence of 1720 MHz OH in CSE
• Suggest is early post-AGB object
• Suggest 1720 MHz collisionally excited in region where interaction of remnant AGB wind and hotter, fast post-AGB wind cause shocks
OH231.8: OH1667MHzOH231.8: OH1667MHzOH231.8: OH1667MHzOH231.8: OH1667MHz
Pre-planetary nebulae
• Zijlstra et al (in press) investigate development of bipolar outflows. Observed 10 OH/IR stars with irregular OH spectra and unusually large expansion velocities.
VVtorus~35 km/s
V/x ~ 10 km/s/arcsec
Roberts 22: Dyer, Goss & Kemball
V_torus ~ 20km/s
V/x ~ 5 km/s/arcsec
Modelling supports
PPN hypothesis
MERLINMERLIN
The Big Picture
NML CygNML CygNML CygNML Cyg
MERLINMERLIN
VLBAVLBA
Progenitor to Planetary Nebulae? Is axisymmetry controlled
by magnetic field?
Progenitor to Planetary Nebulae? Is axisymmetry controlled
by magnetic field?