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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
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
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
Contents
• Introduction
• Observations & Analysis
• Discussion
– H2 excitation mechanism
– Shaping of the knot
Observations
• Target: a knot in the Helix Nebula
– 219 pc (Harris et al. 2007)
Target knot
K1
AO guide star
• 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)
Integral field spectrograph
SINFONI
• Image + spectrum at
•
each pixel
Spectral variation
within a knot
2.12 m image
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
Rotational temperature
Vibrational temperature
• Uniform excitation temperature within the knot
H2 excitation temperature
• Level population
•
•
diagram
LTE
Excitation
temperature of 1800K
Temperature gradient
• 1800 K at knot in the
1040 K
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
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 Shock
Model
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
2.154 m v=2-1 S(2)
3
2.224 m v=1-0 S(0)
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
100
22
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)
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?