Deep Emission Line Spectroscopy of IC418 in the N window

Coordinator: J. R. Walsh (ST-ECF, ESO), Y. Tsamis, D. Pequignot (IAP, Paris)

The high surface brightness medium excitation planetary nebulae IC418 will be observed with the VISIR low resolution grating to obtain a first deep census of the emission line spectrum in the 8-13 micron region.
Ionized nebulae have been milestones in the interpretation of a wide variety of astronomical objects. Measurement of their spectra provide diagnostics of physical conditions and abundances. The energy levels of the emitting ions allow different regions of nebulae to be studied depending on their ionization and physical conditions. This is the reason why it is important to include spectra from many parts of the electromagnetic spectrum. A good example is the difference in electron density and temperature derived from collisionally excited lines in the optical and mid-IR, e.g. [O III] 88/52 micron.
A scientifically motivated proposal and one that also tests the instrument capabilities, is to obtain deep spectra of planetary nebulae in the 8-13 micron window. The whole window would be observed with the Low resolution mode then as much as possible of the the whole range observed at medium resolution to detect faint lines. To date only a few bright lines - e.g. [Si IV], [ArIII], [Ne II] have been detected in the N band. The scientific aim would be to compare the 8-13 micron spectra with optical and UV spectra to determine diagnostic ratios. The date would be a fiducial for emission line studies in this wavelength range and would, in addition, provide good cross comparisons between the two modes.
The target IC 418 is suggested. It is bright, has been well observed in the optical, UV and large aperture fluxes in the IR have been measured with ISO (e.g. Liu et al. 2001, MNRAS, 323, 343). The measured IR line fluxes will be compared with the optical both through H lines detected in the N band and by comparison with long slit ground based spectra. The N-band emission line spectrum will then reach a similar fidelity to optical and UV spectra.
IC418 has very high surface brightness and a diameter of about 12arcsec encompassed by the long slit. Direct images in the emission lines of [Ar III] 8.99 micron and [Ne II] 12.8 micron will be obtained for comparison with optical emission line imaging and determining the exact position of the slit. Spectra in the four Low Resolution modes will be obtained to enable a full 8-13 micron spectrum to be measured. Since line fluxes are to be measured, spectrophotometric calibration is required and assumed to be provided.

Program is available and data products can be downloaded

Target list
NameRA(2000)DEC(2000)ModeObs.Time (h)
IC41805 27 28.2-12 41 50.2IMG/127ma18
IC41805 27 28.2-12 41 50.2LR 8.819
IC41805 27 28.2-12 41 50.2LR 9.819
IC41805 27 28.2-12 41 50.2LR 11.419
IC41805 27 28.2-12 41 50.2LR 12.419