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Stellar Evolution as seen through the eyes of VLBI and masers

Stellar Evolution as seen through the eyes of VLBI and masers. P.J.Diamond. Jodrell Bank Observatory University of Manchester. Castel San Pietro Terme 19 September 2001. Horizontal Branch: Core suddenly collapses. L decreases abruptly, star is still He-burning with L~50 L o.

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Stellar Evolution as seen through the eyes of VLBI and masers

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

  2. Horizontal Branch: Core suddenly collapses. L decreases abruptly, star is still He-burning with L~50Lo. Timescale ~ 130 M yr Asymptotic Giant Branch: New C/O + deg. elec. core. Thin He layer, H mantle. At 2200Lo , 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. ~0.1-0.2 Myr. Post-AGB: Fossil shell (last of mass loss). Core exposed. => PPN => PNe Red Giant Branch: Build core of He and degenerate electrons, and mantle of H. Increase in L  2500Lo Decrease in temp  2500K Timescale ~ 1 G yr Main Sequence: ~5 Gyrs Stellar Evolution in a flash

  3. Some definitions • AGB: Asymptotic Giant Branch • LPV: Long Period Variable (P > 50-100 days) • Mira variables: M-type stars with DV > 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

  4. 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.

  5. Variables with C-rich CSE Variable, very thick CSE PNe with cool CSE Variable with young CSE Variable, thick CSE O-rich, non-variable, no CSE Variable, more evolved CSE Colour-colour diagrams (van der Veen & Habing, A&A, 1988, 194, 125

  6. The ‘Lewis’ Chronology. Miras OH/IR PPN PNe

  7. 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 H2O maser studies.

  8. Compact central envelope, 2nd mass loss episode Diameter ~0.5 ly, ring has been expanding for 6800 yrs IRAM image of shell around Carbon star TT CygOlofsson et al 1999 : CO(1-0) @ 115GHz

  9. 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

  10. 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)

  11. Diamond et al (1994, ApJ, 430, L61) U Her TX Cam VX Sgr: 43 GHZ Greenhill et al (1995,ApJ,449,365) VX Sgr: 86 GHz Doeleman et al (1998, ApJ, 494, 400)

  12. Components in N, S, NE & W Component in E SiO: proper motions Predominant motion is outflow Strong evidence for shocks dominating the kinematics

  13. SiO: Polarization studies Tangential vectors confined to narrow inner edge of ring. Strong evidence of effects of shocks. Remarkable circular magnetic field structure. Origin unknown

  14. B~280mG (Vlemmings, Diamond & van Langevelde, 2001) S Per: EVN+MERLIN MERLIN proper motion measurements of H2O in NML Cyg shows bipolar flow NML Cyg 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

  15. 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= -17.050.85, -9.480.73 mas/yr. Parallax detected: VLBA= 4.2  1.2 mas.

  16. OH Shells: thick CSEs

  17. MERLIN observations of OH127.8-0.0 Bright blue-shifted spots show compact structure. observations ~10 years apart reveal surprisingly small changes 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)

  18. OH: Polarization 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 VX Sgr: 1612 MHz Szymczak et al, in prep

  19. 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

  20. Getting older • IRAS 18455+0448: a dying maser. Lewis et al, 2001 1665 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.

  21. 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

  22. dV/dx ~ 10 km/s/arcsec OH231.8: OH1667MHz Vtorus~35 km/s 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.

  23. Roberts 22: Dyer, Goss & Kemball V_torus ~ 20km/s dV/dx ~ 5 km/s/arcsec Modelling supports PPN hypothesis

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

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