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Astron. Astrophys. 342, 823-830 (1999)

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6. Summary and conclusions

This paper is motivated by our interest in understanding the origin of the H2 emission in PNe. Recently, Natta & Hollenbach (1998) have computed the evolution with time of the H2 emission expected in PNe of different core mass and shell (or torus) properties. Their models compute the emission originating in the photodissociation region (PDR) created by the UV radiation of the central core incident on the inner edge of the neutral shell, as well as the emission in the shock associated to the expansion of the torus inside the precursor red-giant wind. NH98 show that both regions can produce intense H2 emission and that detailed studies of individual PNe are necessary.

NGC 2346 is a good candidate. It is a bright, young PN, with a typical butterfly morphology characterised by well developed bipolar lobes of emission and a bright torus around the central star. The same morphology is seen in H[FORMULA], CO and in the H2 1-0S(1) line. There is a large amount of information in the literature, concerning the properties of the ionized region and the gas kinematic that can be used in our analysis.

We have collected near infrared observations of NGC 2346. The data include broad K band image, an image in the H2 vibrationally excited 1-0S(1) line and slit spectra in the K band in three positions in the nebula. The images confirm the well-known NGC 2346 morphology, with a central, bright torus, surrounded by weaker emission with a typical butterfly shape. The K band spectra show 11 H2 lines with excitation energies from 6150 to 12552 K. Profiles of the lines intensity along the slit show evidence of secondary condensations outside the torus midplane.

A comparison of our observations with the NH98 models shows that PDR emission can account for the H2 observations. This requires a low-density shell ([FORMULA] cm-3), in agreement with the low density measured in the ionized region. Note that steady-state models or models where the soft X-ray radiation from the star is ignored predict a 1-0S(1) intensity one order of magnitude lower than models where both these effects are included. PDR models of the H2 emission needs a central star significantly more luminous than estimated in the literature (250 L[FORMULA] against 17-90 L[FORMULA]). However, we think that this is consistent with all the available data, including our own Br[FORMULA] observations.

It is unlikely that the H2 emission originates in the shock between the expanding shell and the precursor red giant wind. Shock models require a much larger momentum input to the torus than possible from the central star. We estimate the discrepancy to be at least a factor 600 for the current luminosity of the central star (taken to be 250 L[FORMULA]) and a factor 12 if we consider the maximum luminosity ever reached by the star in its previous evolution.

In conclusion, we have proved that the PDR origin for the H2 emission in NGC 2346 is likely and that the models, even if very simple, can account for a number of the observed properties. We suggest some additional observations of lines from higher excitation vibrational levels and in the 0-0 band, which may help in determining the physical conditions in the shell with higher accuracy.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999