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?