Infrared integral field spectroscopic observations of

globules (cometary knots) in the Helix Nebula (NGC 7293)

Mikako MatsuuraNational 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

Contents

• Introduction• Observations & Analysis• Discussion

– H2 excitation mechanism– Shaping of the knot

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 hel

p 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

Contents

• Introduction• Observations & Analysis• Discussion

– H2 excitation mechanism– Shaping of the knot

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 t

o 125x125 mas2

– 50x100 mas2 : re-sampled to 50x50 mas2

– K-band grating (R~4490)

• Image + spectrum at each pixel

• Spectral variation within a knot

2.12 m image

Integral field spectrograph SINFONI

Shape of the knot

• Tadpole shape– Narrower tail

than the head

Narrower tail

Matsuura et al.Submitted to MNRAS

Spectra

• Up to 12 H2 lines (9 in this figure)

• No Br

Spectra at brightest point of the knot

H2 excitation temperature

• Uniform excitation temperature within the knot

Rotational temperature Vibrational temperature

H2 excitation temperature

• Level population diagram

• LTE• Excitation temperatur

e of 1800K

Temperature gradient

• 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

1800 K

1080 K

900K

1040 K

Contents

• Introduction• Observations & Analysis• Discussion

– H2 excitation mechanism– Shaping of the knot

H2 excitation mechanism

• 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 line Line Ratio

Obs ShockModel

1.958 m v=1-0 S(3) 220 91

2.034 m v=1-0 S(2) 36 36

2.073 m v=2-1 S(3) 4

2.128 m v=1-0 S(1) 100 100

2.154 m v=2-1 S(2) 3

2.224 m v=1-0 S(0) 22 22

2.248 m v=2-1 S(1) 9

2.408 m v=1-0 Q(1) 99 75

2.413 m v=1-0 Q(2) 29 24

2.424 m v=1-0 Q(3) 99 70

2.438 m v=1-0 Q(4) 30 20

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)

DensityDensity

Wind instability modelWind instability model(J-type shock; Pittard et al. 2005)(J-type shock; Pittard et al. 2005)

Dyson et al. (2006)Dyson et al. (2006)

DensityDensity

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?