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VLT/near-infrared integral field spectrometer observations of molecular hydrogen lines in the knots of the planetary nebula NGC 7293 (the Helix Nebula)
Knots are commonly found in nearby planetary nebulae (PNe) andstar-forming regions. Within PNe, knots are often found to be associatedwith the brightest parts of the nebulae and understanding the physicsinvolved in knots may reveal the processes dominating in PNe. As one ofthe closest PNe, the Helix Nebula (NGC 7293) is an ideal target to studysuch small-scale (~300au) structures. We have obtained infrared integralspectroscopy of a comet-shaped knot in the Helix Nebula using theSpectrograph for INtegral Field Observations in the Near Infrared(SINFONI) on the Very Large Telescope at high spatial resolution (50-125mas). With spatially resolved 2-?m spectra, we find that theH2 rotational temperature within the cometary knots isuniform. The rotational-vibrational temperature of the cometary knot(situated in the innermost region of the nebula, 2.5 arcmin away fromthe central star) is 1800 K, higher than the temperature seen in theouter regions (5-6arcmin from the central star) of the nebula (900 K),suggesting that the excitation temperature varies across the nebula. Theobtained intensities are reasonably well fitted with 27 kms-1C-type shock model. This ambient gas velocity is slightly higher thanthe observed [HeII] wind velocity of 13 kms-1. The gasexcitation can also be reproduced with a photon-dominant region (PDR)model, but this requires an order of magnitude higher ultravioletradiation. Both models have limitations, highlighting the need formodels that treat both hydrodynamical physics and the PDR.Based on observations with the European Southern Observatory, Very LargeTelescope with an instrument, SINFONI (the proposal numbers:076.D-0807).E-mail: mikako@optik.mtk.nao.ac.jp
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