www.eso.org/sci/publications/messenger/archive/no.132-jun08/messenger-no132-37-40.pdfObservations of Comet-shaped Knots in the Planetary Nebula NGC 7293 (the Helix Nebula)
Mikako Matsuura1, 2 Angela Speck3Michael Smith4Albert Zijlstra5Krispian Lowe6Serena Viti2Matt Redman7Christopher Wareing8Eric Lagadec51National Astronomical Observatory of Japan, Tokyo, Japan 2Department of Physics and Astronomy, University College London, United King-dom3Physics and Astronomy, University of Missouri, Columbia, USA4Centre for Astrophysics and Planetary Science, School of Physical Sciences, The University of Kent, Canterbury, United Kingdom5School of Physics and Astronomy, Uni-versity of Manchester, United Kingdom6Science and Technology Research Cen-tre, University of Hertfordshire, Hat eld, United Kingdom7Department of Physics, National University of Ireland Galway, Republic of Ire-land8School of Mathematics, University of Leeds, United Kingdom
One of the most interesting characteristics of the planetary nebula NGC 7293 (the Helix Nebula) is its comet-shaped knots. We have observed one of the knots using the SINFONI imaging spec-trometer on the VLT with adaptive optics. The spectra are analyzed to obtain the spatial variation of molecular hydrogen line intensities within the knot. The images clearly show the detailed structure, which resembles a tadpole in shape, suggesting that hydrodynamic flows around the knot core create a pressure gradient behind the core. The three-dimensional spectra reveal that the excitation temperature is uniform at approximately 1800 K within the knot. The SINFONI observations help to determine the H2 excitation mechanism in planetary nebulae, as well as the importance of hydrodynamics in shaping the knots.
Cometary knots in the planetary nebula NGC 7293The Helix Nebula (NGC 7293; Figure 1), located at 219 pc (Harris et al., 2007), is one of the closest planetary nebulae (PNe). The diameter of the entire nebula is approximately a quarter of the size of the full moon. This proximity enables a study of the detailed structure inside the nebula. The most interesting feature in the Helix Nebula is its knots, which have a typical size of 0.5ñ3 arcsec at their heads and are accompanied by tails up to 15 arcsec long (OíDell and Handron 1996; Figure 1). All of the knots have bright tips facing the central star and the tails extend in the opposite direction away from the central star. The knots were first discovered by Walter Baade in about 1940, according to Vorontsov-Velyaminov (1968). The formation mechanism of these tails is not well understood: the two most likely explanations are mass injection from cores followed by hydrodynamic processes creating a tail, or photo-ionisation of an optically thick globule creating a shadow. Furthermore, ionised lines, hydrogen recombination lines, and molecular emission (H2 and CO) have been detected from these knots. However, the excitation mechanisms of the molecular lines are not yet known. In particular, the H2 line excitation mechanism of PNe in general is still a subject of dispute and UV excitation and shock excitation have both been proposed.
Planetary nebulae are the late stages of the stellar evolution for low- and intermediate-mass stars (1ñ8 M on the main sequence). The Sun is expected to reach this stage in about 5 billion years. The circumstellar shell was produced by mass-loss processes during the previous evolutionary stage, the asymptotic giant branch (AGB). As the central star evolves from the AGB towards the PN phase, the effective temperature increases, and the circumstellar material in the nebula is photo-ionised. PNe are regarded as a typical case of a photon-dominant region. Moreover, a fast but low mass-loss rate stellar wind from the PN central star catches up with the slow but dense AGB wind, and so the circumstellar envelopes Figure 1. The optical image of the Helix Nebula (top) formed from images in a 502 nm lter ([OIII]) and 658 nm lter (HA and [NII]) is shown. The square region is enlarged in the lower panel to show the knots in the inner region of the Helix Nebula. Adopted from images taken with HST and NOAO. (Credit: NASA, NOAO, ESA, M. Meixner and T. A. Rector)
38The Messenger 132 ñ June 2008Astronomical Scienceof PNe are a good test case for hydrodynamic simulations. The power of SINFONI Integral field spectrometers allow the simultaneous measurements of an image and many spectra on an individual pixel basis, and so these instruments are ideal for investigating spatial variation of line intensities, and thus line excitation mechanisms within nebulae (e.g. Tsamis et al., 2008). SINFONI is a near-infrared integral field spectrometer, covering the wave-length range which contains molecular hydrogen rotational-vibrational lines around 2 microns. SINFONI can be supported by an adaptive optics (AO) system, which allows observations to be taken with high spatial resolution. Thus it is an ideal instrument to investigate the structure of small-scale (a few arcsec) objects, such as knots in planetary nebu-lae.We used SINFONI to obtain 2 micron integral field spectra of a knot within the Helix Nebula (Matsuura et al., 2007). There are many field stars inside this nebula, and we chose a knot close to one of the field stars, which was used as a guide star for the AO system. The images obtained with AO assistance achieve the highest angular resolution of any images of the Helix Nebula, up to about 70 milliarcsec.
Figure 2 shows the SINFONI image of the knot in the Helix Nebula. The knot was clearly resolved into an elongated head and two narrow tails. The overall shape resembles that of a tadpole. The brightest emission is found in a crescent within the head. The diameter of the head is approximately 2.5 arc-sec, while that of the tails is about 2.9 arc-sec. We found that the tails are brighter in H2 than in optical H-alpha+[NII] emission (Figure 3).
In the optical, almost only the crescent at the head is found and the tail is barely seen. As an integral field spectrometer, SIN-FONI can provide a spectrum at every single spatial pixel over the field. Figure 4 shows representative spectra within the knot obtained by the SINFONI instrument. Integral field spectroscopy enables the spatial variation of the intensities of H2 lines to be studied. This is the first time that spatially resolved H2 spectra have been obtained within a knot. The SIN-FONI data also provide line ratio maps, which gives a measure of the H2 excitation temperature.
The line ratio maps ap-Figure 2.
(a) Optical image showing the location of the target and a nearby field star which was used for adaptive optics.
(b) The SINFONI image of the knot at 2.12 micron covering the H2 v = 1ñ0 S(1) line;
(c) close-up of the crescent taken with SINFONI at a scale of 100 milliarcsec per pixel but resampled to 50 milliarcsec pixels. The image size is 4.2 arcsec by 3.9 arcsec.
Figure 3. A comparison of SINFONI H2and optical images. The tails are much more obvious in the H2 line (a) than in the 658 nm image (b). The knot is found to be negative in 500.7 nm [OIII] line (c), because the dust in the knot absorbs the background emission, while the surrounding area has diffuse [OIII] emission unaffected by dust. Matsuura M. et al., SINFONI Observations of Comet-shaped Knots in NGC 7293AO Guide Star(a)(b)(c)(a) SINFONI 2.12 micron H2 v = 1ñ 0 S(1) (b) HST F658N[NII] 658.3 nm + H 656.3 nm(c) HST F502N [OIII] 500.7 nm
39The Messenger 132 ñ June 2008 appear to be uniform within the knot (Fig-ure 5). Using all of the H2 lines detected at 2 microns (up to 12 lines), we derived an excitation temperature of 1800 K, and all of the intensities follow local thermodynamic equilibrium (LTE).
Figure 4 (above). SINFONI spectra of H2 lines, with identi cation of transitions in the right bottom panel, at different locations around the knot shown in the central image.
Figure 5 (below). Line ratio maps, showing (a) the rotational temperature and (b) the vibrational temper-ature of H2 molecules. The excitation temperatures appear to be uniform within the knot, outlined by a white line. Shaping of the knotsThe mechanism responsible for the for-mation and shaping of the knots has been the subject of a long-term debate. The long tails of the tadpole-like knots indicate the involvement of hydrodynam-ics. The bright crescent is a bow shock and the stellar wind from the central star blows the material off, or alternatively radiation from the central star heats the core evaporating the gas which forms 2.02.12.22.32.4Wavelength (μm)02 ◊ 1084 ◊ 1086 ◊ 1088 ◊ 108I ( J y s rñ 1)2.02.12.22.32.4Wavelength (μm)02 ◊ 1084 ◊ 1086 ◊ 1088 ◊ 108I ( J y s rñ 1)2.02.12.22.32.4Wavelength (μm)02 ◊ 1084 ◊ 1086 ◊ 1088 ◊ 108I ( J y s rñ 1)v = 1 ñ 0 S ( 3 )v = 1 ñ 0 S ( 2 )v = 1 ñ 0 S ( 1 )v = 1 ñ 0 S ( 0 )v = 2 ñ 1 S ( 2 )v = 2 ñ 1 S ( 1 )v = 1 ñ 0 Q ( 1 )v = 1 ñ 0 Q ( 2 )v = 1 ñ 0 Q ( 3 )2.02.12.22.32.4Wavelength (μm)02 ◊ 1084 ◊ 1086 ◊ 1088 ◊ 108I ( J y s rñ 1)0123450123450.00.20.40.60.8I2.03 micron v = 1ñ0 S(2)/I2.12 micron v = 1ñ0 S(1)R a t i o2 ◊ 10ñ 82 ◊ 10ñ 8Distance (arcsec)Distance (arcsec)0123450123450.00.20I2.24 micron v = 2ñ1 S(1)/I2.12 micron v = 1ñ0 S(1)R a t i o2 ◊ 10ñ 82 ◊ 10ñ 80.150.100.05Distance (arcsec)Distance (arcsec)(a)(b)
40The Messenger 132 ñ June 2008Astronomical ScienceMatsuura M. et al., SINFONI Observations of Comet-shaped Knots in NGC 7293a pony-tail pointing away from the central star.
The tail is narrower than the head, probably because of the pressure difference behind the head caused by the stellar wind (Figure 6). One of the best models to reproduce the tadpole shape has been developed by Pitterd, Dyson, and their colleagues (e.g. Dyson et al., 2006). According to their model, the tad-pole shape can be produced if material is constantly removed from the core of a knot by the stellar wind, and if the velocity of the wind is about supersonic. Their models show that the velocity of the ambient gas is between 26 and 38 km/s in the case of the Helix knots.
Such a high expansion velocity component has been detected in the outer part of the Nebula (Walsh and Meaburn, 1987), but not yet in the inner part; in the inner part, only slower (10 km/s) winds have been detected so far.
The detection of high ve-locity components from the knotty region would be of further interest.
The excitation mechanism of molecular hydrogen
The excitation mechanism of molecular hydrogen in PNe has been a long-term controversy. From the knots, CO mole-cular emission has been detected but their upper level energy is only 6 K, much lower than that for H2 (typically from 6000 to 8000 K for the rotational-vibration lines at 2 microns). In order to reach the energy of such an upper level and to acquire an excitation temperature of thousands of K, a line excitation mechanism should be present. There are two major mechan-isms proposed to excite rotational-vibration lines of molecular hydrogen at 2 microns: collisional and fluorescent excitation.
For shock excitation, kinetic energy is absorbed in increasing the internal energy of the molecules, and the H2 line intensities follow a thermal population distribution. However UV radiation in photon-dominant regions increases the population of rotation-vibrationally excited states, and vibrationally excited molecules then decay by emitting photons in the near-infrared. The population of UV excited states are often detected in non-LTE in rotational-vibration lines, but in an intense UV field, and depending on the gas density, fluorescence can create a thermal population distribution. Within the choice of existing models and their parameter ranges, C-type (continuous) shock models with an ambient gas velocity of 27 km/s (Burton et al., 1992) can reproduce the measured H2 intensities in the Helix Nebula.
In contrast, UV radiation models can fit the observations, only if the UV flux is increased by a factor of 250 above that expected from the central star. Within the current choice, our observations favour shock excitation of H2 lines. The non-detection of a shocked region in the SINFONI image still remains a mystery: if the H2 lines are excited by shocks, the excitation temperature should be highest at the tip of the tadpole, i.e., on the crescent. However, our SINFONI ob-servations did not detect such a shocked region (Figure 5). This could be because the scale of the shocked region is still too small to be resolved even with SINFONI.
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Figure 6. A schematic view of the formation of a knot in a planetary nebula by the effects of the stellar wind.Gas flowPressure from the stellar windStellar windCore (not real size)Molecules removed from the coreorMolecules could be formed by collisionat the surface of the coreStellar windorPhotons
