Stellar Evolution as seen through the eyes of VLBI and masers P.J.DiamondP.J.Diamond Jodrell Bank Observatory University of Manchester Jodrell Bank Observatory University of Manchester Castel San Pietro Terme 19 September 2001 Castel San Pietro Terme 19 September 2001

Stellar Evolution in a flash Main Sequence: ~5 Gyrs Red Giant Branch: Build core of He and degenerate electrons, and mantle of H. Increase in L  2500L o Decrease in temp  2500K Timescale ~ 1 G yr Asymptotic Giant Branch: New C/O + deg. elec. core. Thin He layer, H mantle. At 2200L o, T~3000K star enters TP-AGB.H burns into He at inner edge of mantle => He burns into C & O => temp. increase in L. Called ‘thermal pulse’. Mass loss occurs, continues until mantle gone. ~ Myr. Horizontal Branch: Core suddenly collapses. L decreases abruptly, star is still He-burning with L~50L o. Timescale ~ 130 M yr Post-AGB: Fossil shell (last of mass loss). Core exposed. => PPN => PNe

Some definitions AGB: Asymptotic Giant Branch LPV: Long Period Variable (P > days) Mira variables: M-type stars with  V > 2.5 m (P~ d) 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 H 2 O formed in lower envelope. Masers from H 2 O. UV photons disassociate H 2 O => OH, H. OH masers form. Gas at terminal velocity.

Variable, very thick CSE Variable, thick CSE Variable, more evolved CSE Variable with young CSE O-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. StageKey ChangeSiOH2O1665/7 OH1612 OH 1SiO masersX 2Add H 2 OXX 3Add 1665/7XXX 4Add 1612XXXX 51665/7 weakenXXXX 6Lose 1665/7XXX 7Lose H 2 OXX 8Lose SiOX 9Add 1665/7XX /7 strongXX 11PN stage Miras OH/IR PPN PNe

Interferometry VLBA 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 H 2 O maser studies. MERLIN is instrument of choice for OH and larger-scale H 2 O maser studies.

IRAM image of shell around Carbon star TT Cyg Olofsson et al 1999 : 115GHz Diameter ~0.5 ly, ring has been expanding for 6800 yrs Compact central envelope, 2 nd 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 GHZ Greenhill et al (1995,ApJ,449,365) TX Cam U Her Diamond et al (1994, ApJ, 430, L61) VX Sgr: 86 GHz Doeleman et al (1998, ApJ, 494, 400)

SiO: proper motions Components in N, S, NE & W Component 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 confined to narrow inner edge of ring. Strong evidence of effects of shocks. Remarkable circular magnetic field structure. Origin unknown SiO: Polarization studies

H2OH2O Proper motions of masers around stars relatively ‘undeveloped’ subject until the advent of the VLBA. Marvel (Ph.D, 1996) studied the proper motion of H 2 O masers around several stars. Demonstrated that masers were expanding as expected, some peculiarities showing departures from smooth, symmetric outflows S Per: EVN+MERLIN NML Cyg MERLIN proper motion measurements of H 2 O in NML Cyg shows bipolar flow B~280mG (Vlemmings, Diamond & van Langevelde, 2001) B~280mG (Vlemmings, Diamond & van Langevelde, 2001)

OH shells: thin CSEs & Miras Multiple epoch VLBA observations of the ‘thin’ shell source U Her (van Langevelde et al, 2000) Designed to measure parallax and study fine structure.  VLBA =  0.85,  0.73 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 observations of OH Bright 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 : a dying maser. Lewis et al, remains largely unaffected. Maybe witnessing the early stage of expansion of fossil shell prior to it becoming a planetary nebula. Mass loss  0, UV photodissociating OH.

OH : 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: 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. V V torus ~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

MERLIN The Big Picture NML Cyg MERLIN VLBA Progenitor to Planetary Nebulae? Is axisymmetry controlled by magnetic field? Progenitor to Planetary Nebulae? Is axisymmetry controlled by magnetic field?