Mikako Matsuura National Astronomical Observatory of Japan University College of London

1. Infrared integral field spectroscopic observations of globules (cometary knots) in the Helix Nebula (NGC 7293) Mikako Matsuura National Astronomical Observatory of Japan University College of London A.K. Speck, M.D. Smith, A.A. Zijlstra, K.T.E. Lowe, S. Viti, M. Redman, C.J. Wareing, E. Lagadec

2. Contents • Introduction • Observations & Analysis • Discussion • H2 excitation mechanism • Shaping of the knot

3. Introduction • Globules or (cometary) knots • Smallest scale structures observed in PNe (1-2 arcsec at ~219pc in the Helix) • ~20,000 knots in the Helix (Meixner et al. 2005) • Commonly found in nearby PNe • Brightest parts of PNe; understanding physics in knots might help to understand physics in PNe • Formation mechanisms of knots • Radiation: sunny side at the tip + tail (e.g. Speck et al. 2002) • Instability of winds (e.g. Dyson et al. 2006) • H2 excitations • Photon dominated region (PDR) • Shocks

4. Contents • Introduction • Observations & Analysis • Discussion • H2 excitation mechanism • Shaping of the knot

5. Target knot K1 AO guide star Observations • Target: a knot in the Helix Nebula • 219 pc (Harris et al. 2007) • Observations • 8.2-meter Very Large Telescope (VLT) • Spectrograph for INtegral Field Observations (SINFONI) • Adaptive Optics (AO) guided by a nearby star • 125x250 mas2 (pixel size limited spatial resolution): re-sampled to 125x125 mas2 • 50x100 mas2 : re-sampled to 50x50 mas2 • K-band grating (R~4490)

6. 2.12 m image Integral field spectrograph SINFONI • Image + spectrum at each pixel • Spectral variation within a knot

7. Shape of the knot • Tadpole shape • Narrower tail than the head Narrower tail Matsuura et al. Submitted to MNRAS

8. Spectra • Up to 12 H2 lines (9 in this figure) • No Br Spectra at brightest point of the knot

9. H2 excitation temperature • Uniform excitation temperature within the knot Rotational temperature Vibrational temperature

10. H2 excitation temperature • Level population diagram • LTE • Excitation temperature of 1800K

11. Temperature gradient 1040 K • 1800 K at knot in the inner ring (2.5 arcmin from the central star) • 900-1000 K at outer ring (Cox et al. 1995; O’dell et a. 2007) • Temperature gradient 900K 1800 K 1080 K

12. Contents • Introduction • Observations & Analysis • Discussion • H2 excitation mechanism • Shaping of the knot

13. C-type shock Relatively well reproduced line ratio at wind velocity 27 km s-1 (Kaufman & Neufeld 1996) Observed velocity is ~10 km s-1 PDR model 72 Solar luminosity at 219 pc UV strength G0=8 Only 100 K Observed 1800 K Shock H2 excitation at the knot K1? H2 excitation mechanism

14. Density Shaping • Among existing models, wind instability models by Pittard et al (2005) & Dyson et al. (2006) can reproduce the shape well • Wind + grain • Wind velocity of 22 km s-1 required (faster than observed velocities; Meaburn et al. 2005; 10 km s-1) Wind instability model (J-type shock; Pittard et al. 2005) Density Dyson et al. (2006)

15. Conclusions • Among existing models, shock can produce the shape and H2 line ratio of the knot K1 well. • Stellar wind is important at the inner ring of the Helix? • Wind velocity at knot K1 is 20-30 km s-1?