1. eSMA workshop Leiden, 1-2 Feb Evolved Stars eSMA science case for evolved stars (AGB, post-AGB, proto-PNe and PNe) Wouter Vlemmings With various levels of input by: Albert Zijlstra (Manchester) Anita Richards (Manchester) Rebeca Soria-Ruiz (JIVE) Dinh Van Trung (ASIAA) Hans Olofsson (Onsala)

2. eSMA workshop Leiden, 1-2 Feb Evolved Stars AGB Mass-loss The mass-loss determines: –AGB lifetime –Luminosity –Gas/dust return to the ISM –Chemical composition of the returned gas Unfortunately, the process is not understood yet –What determined the mass-loss rate Pulsations Radiation pressure Both unable to reproduce observations (Woitke 2006) –Possible need for magnetic field (Vidotto & Jatenco-Pereira 2006) –Need to explain sometimes episodic nature and high rate near the end of the AGB IRC +10216, post-AGB object showing episodic mass ejections

3. eSMA workshop Leiden, 1-2 Feb Evolved Stars Asymmetry of the CSE AGB stars and their envelopes are nicely spherical ?? –CO (Castro-Carrigo et al.; Olofsson et al. 2000) –Masers PNe formed by fast wind overtaking initial slow wind. <20% of the PNe are spherical ! Several indications of asymmetry already during the AGB phase IR interferometry (IOTA) reveals asymmetries in large sample at ~10 mas level (Ragland et al. 2006) mid-IR shows asymmetric dust distribution around IK Tau (Weiner et al. 2006) NIR speckle interferometry (Wittkowski et al. 1998) And CO & maser observations But origin and onset of asymmetries unknown Companions / magnetic field / ‘star’spots

4. eSMA workshop Leiden, 1-2 Feb Evolved Stars Evolved star: dust Polarization AGB/PPNe observations find strong magnetic fields might be responsible for driving and shaping mass-loss –Understanding essential in studies of ISM enrichment and PNe shaping Submm dust polarization observations are needed to confirm suspected magnetic shaping –Polarization originates from asymmetric dust grain distribution aligned with magnetic field –PNe NGC 6537: Timescale for dust alignment t  B -2, which for 1 mG fields is ~10 6 yr However, nebula timescale is ~10 4 yr –Alignment occurs closer to the star and is maintained in the outflow  magnetic shaping of the outflow (Sabin et al. 2007, Greaves 2002) NGC 6537 850  m SCUBA polarization

5. eSMA workshop Leiden, 1-2 Feb Evolved Stars Evolved star: dust Polarization The values: –Linear polarization: 5-15% –Envelope extent: 1-10 arcsec Larger for PNe (~20 arcsec) –Flux (at 1 kpc) varies wildly Most sources of interest have fluxes of several hundred mJy –For significant detection of polarization we need rms of ~10mJy Current observational problems: –Only high angular resolution observations (subarcsec) can probe inner region of the envelope where dust alignment occurs without suffering badly from beam depolarization –In the inner part of PNe, besided beam depolarization, linear polarization is reduced due to ionization. Weaker linear polarization down to ~1% needs to be accessible –Shorter wavelengths emission is stronger but in single dish the resolution insufficient  eSMA combination of polarization sensitivity and angular resolution essential CRL 2688 NGC 7027

6. eSMA workshop Leiden, 1-2 Feb Evolved Stars Submm dust polarization PNe NGC 6537 (~1.5 kpc; Matsuura et al. 2005) 3  detection limit of 1% linear polarization in 1 eSMA observing track Cold Dust (50 K)

7. eSMA workshop Leiden, 1-2 Feb Evolved Stars Weighing the dust Recent paper by Peretto et al. (2007) find massive dust torus in a PNe using JCMT and SMA observations –Need to determine how widespread this phenomenon is Including its relation to magnetic field and binaries –Many sources need higher angular resolution NGC 6302 only barely resolved CO observations at high angular resolution can resolve and weigh the dust torus –Using 12 CO and 13 CO ratio at same line transition (J=3-2, 330 and 345 GHZ) SMA CO map (top) and CO ratio (bottom) NGC 6302

8. eSMA workshop Leiden, 1-2 Feb Evolved Stars CO observations CO observations at high angular resolution can resolve circumstellar asymmetries while not being limited to only the most nearby objects CO observations are fairly straightforward probes of the envelope kinematics –And thus good comparison for the widespread maser observations Spatial variation in CO ratio can illuminate mass-loss history –i.e. Thermal pulses Possibility for CO polarization studies up to now only done at lower transitions in star-forming regions Olofsson, PdB at 115 GHz Castro-Carrizo, PdB

9. eSMA workshop Leiden, 1-2 Feb Evolved StarsConclusions Main project would consist of continuum dust observations of evolved stars. –Mapping the dust distribution of PPNe and AGB stars to find disks/outflows etc. is well suited for eSMA as: Optimal frequency around 350 GHz Sources are compact  high angular resolution Sources are weak  high sensitivity –Linear polarization of the circumstellar dust around evolved stars, in particular (P-)PNe Essential to determine the driving forces behind the AGB mass-loss Current observations limited by low resolution, beam depolarization and sensitivity (Sabin et al. 2007, Greaves et al. 2002). eSMA can uniquely map the distribution of 12 CO and 13 CO (J=3-2), resolve massive dusty disks and determine mass- loss history

10. eSMA workshop Leiden, 1-2 Feb Evolved Stars Summary of AGB stars Asymptotic Giant Branch (AGB) stars: –Stars of low and intermediate mass (~0.8 - 8 M  ) –High luminosity (~10 4 L  ) –Cool (T eff ~3000 K) –bloated (core radius ~several 100 R  ) –very extended molecular envelope Pulsations –P~300 (Miras) to >1000 days Lifetime ~10 5 years Mass loss –10 -8 - 10 -4 M  /yr –V exp ~ 10 - 20 km/s –Creates thick circumstellar envelope (CSE) Evolves into (Proto-)Planetary Nebulae

11. eSMA workshop Leiden, 1-2 Feb Evolved Stars Circumstellar Envelopes Schematic view of an AGB star

12. eSMA workshop Leiden, 1-2 Feb Evolved Stars Evolved star CSE Magnetic Fields Maser Magnetic Fields: SiO at ~2 stellar radii Typical magnetic field strength B~3.5 G (Herpin et al. 2006) up to several tens of Gauss Ordered (radial) magnetic field (Kemball & Diamond 1997) H 2 O at ~50-500 AU Magnetic fields of B~0.1-2 G (Vlemmings et al. 2002, 2005) Supergiant VX Sgr shows dipole field (Vlemmings et al. 2005) OH at ~250-10.000 AU Field strengths of B~1-10 mG (e.g. Reid et al. 1979) Indication of alignment with circumstellar envelope (e.g. Etoka & Diamond 2004) Dipole Solar-type