A NEAR-INFRARED STUDY OF THE PLANETARY NEBULA NGC 2346
B. Vicini(1), A. Natta(2), A. Marconi(2), L. Testi(3), D. Hollenbach(4)
and B.T. Draine(5)
1 - Dipartimento di Astronomia e Scienza dello Spazio, Universita'
degli Studi di Firenze, Largo E.Fermi 5, I-50125 Firenze, Italy
2 - Osservatorio Astrofisico di Arcetri, Largo E.Fermi 5,
I-50125 Firenze, Italy
3 - Division of Physics, Mathematics and Astronomy, Caltech,
MS 105-24, Pasadena, CA 91125, USA
4 - NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA
5 - Princeton University Observatory, Peyton Hall, Princeton, NJ 08544, USA
This paper presents new near-infrared
observations of the planetary nebula NGC 2346.
The data include
a broad K-band image, an image in the H2 vibrationally excited
1-0S(1) line and K band slit spectra
at three positions in the nebula.
In the H2 1-0S(1) line, the nebula is characterized by a central,
bright torus, surrounded by weaker emission with a typical
butterfly shape, as seen in Halpha and CO lines.
The K band spectra show 11 H2 lines with excitation energies from
6150 to 12552 K.
The H2 data have been compared to the predictions of models
which follow the evolution with time of the H2 emission in PNe
of different core mass and
shell properties (Natta & Hollenbach 1998). These
models compute the emission originating
in the photodissociation region (PDR) created at the inner edge of
the neutral shell by the UV radiation of the
central core, as well as
the emission in the shock associated to the expansion of the shell
inside the precursor red-giant wind.
In NGC 2346, a PDR origin of the H2 emission in a low-density
molecular shell (n<~10^4 cm^-3) is indicated. At these low
H2 chemistry and X-ray heating of the neutral gas enhance
the predicted PDR H2 line intensity by large factors.