This proposal is a fishing trip to detect faint emission lines from exotic
ionic
species, such as Xe III, Br II, I IV, etc which provide evidence of s (and even
r)
processed elements in the central stars of two planetary nebulae. Both targets are
extended and bright. 18 wavelength settings have been chosen to target expected
faint ionic lines. The exposure time is mostly the same for all the OB's except
for a longer one at 1394.7 which has high atmospheric absorption, and a shorter
one at 2156.4, which includes the bright H Brackett gamma line. The S/N values quoted
in the OB's are estimates only (since these lines have never before been detected).
In all cases we will bin the signal along the slit (nebula extent along the
slit is up to 20arcsec) in order to detect the very faint lines. The requested slit
width is 0.4arcsec so the nebula will peep around the slit jaws providing visual
confirmation of the pointing. The aimed slit position is in the centre of
both nebulae, and in the case of NGC 2440 will bisect both blobs, as shown in
the Finding Charts. Both PN are taken with a specfiic slit orientation (SKY mode)
in order to sample the same part of the nebulae for each wavelength setting.
For the OB's with wavelength below 2000nm, Th-Ar lamps have been attached in order
to determine good wavelengths. For the other settings, we will use what is available
for wavelength calibration - sky emission lines and atmospheric absorption bands.
Of
course the PN are rich in lines, particularly from He I and He II in the whole of
the spectral range covered, so can provide their own wavelength calibration.
Special points:
All OB's for NGC 2440 and NGC 3918 have an associated telluric standard.
These are taken with the slit in ELEV mode since the absolute slit orientation is
not important (point sources). The telluric standard is used as the SVGS.
Following first observations, which were noted by the support astronomer as
having very faint or no line detection in NGC 2440, the OB's for this object
were placed on hold. After trial observation of NGC 3918, it was decided
to continue with NGC 3918 only. The NGC 2440 OB's therefore remain on hold
and the NGC 3918 OB's were increased in length to take up the available time.
A repeat OB on the L2156 spectral range for NGC 3918 was also added.
A waiver was granted to use a more distant but brighter SVGS for NGC 2440
(080.D-0767A.01.WAV).