Planetary nebulae mark the endpoint of stellar evolution for stars with
masses between ~ 0.8 and 8
M
(Iben & Renzini, 1983;
Dopita et al. 1997).
The expanding shell (or shells) of a planetary nebula is
ejected during the ascent of the asymptotic giant branch. The shell is
ionized by the ultraviolet radiation from the newly exposed, hot
helium-burning core. A large fraction of the radiation from the core,
which has a luminosity equal to a red giant, is absorbed via ionization
of hydrogen and helium, and converted into emission lines from the most
abundant elements. The temperature of the nebula is set by equilibrium
between heating from ionization and cooling from permitted and forbidden
emission lines. Radiation from [OIII]
4959/5007 is one of
the most efficient cooling lines; the monochromatic luminosity in
[OIII] 5007 is
comparable to the V-band luminosity of the
K-giants in old stellar populations. Consequently, PNe can be
efficiently identified by taking pairs of on-band and off-band
[OIII] 5007
images. When using ground-based telescopes, PNe in galaxies
beyond the Magellanic Clouds will be unresolved, absent in the off-band
image, and brighter in [OIII]
5007 than
H
. Detailed descriptions
of PNe survey techniques and issues that must be considered when
selecting and buying interference filters can be found in
Ford et al. (1989) and
Jacoby et al. (1992).
Telescope and camera combinations that are particularly suitable for PNe
surveys are summarized in Table 1. Column five gives
the product of the telescope's aperture times the field of view, A
× 
,
normalized to the Subaru telescope's "Suprime" camera. All else being
equal (seeing and camera throughput), A
× is a figure of
merit for a camera/telescope's survey capability. The last column gives
the distance at which a PN one magnitude fainter than the brightest PN will
have a signal-to-noise ratio of 10 in the sum of four 2250 second exposures.
We assumed that the seeing is 0.75" FWHM, the net
atmosphere/telescope/camera throughput is 0.5, the readnoise is 6
e- RMS per pixel, and the sky brightness
and isophotal galaxy brightness are V = 22 mag /
". For the MMT we assumed
that the sky and galaxy surface brightnesses are 21.8, and that the CCDs
are binned 3 × 3 before readout.
| Telescope | Camera | Pixel Size | FOV | A
× |
Dist |
| Format | (") | (arc-min) | (Mpc) | ||
| WHT 4.2-m/PNS | 2k × 2k | 0.33 × 0.30 | 11.3 × 10.3 | 0.045 | 20 |
| ESO 2.2-m | 8k × 8k | 0.24 | 34 × 33 | 0.12 | 14 |
| CTIO 4-m | 8k × 8k | 0.27 | 36 × 36 | 0.45 | 20 |
| MMT 6.5-m | 18k × 18k | 0.08 | 24 × 24 | 0.66 | 25 |
| CFHT 3.6-m | 20k × 18k | 0.18 | 62 × 56 | 0.97 | 18 |
| Subaru 8.2-m | 10k × 18k | 0.20 | 30 × 24 | 1 | 29 |
The Planetary Nebula Spectrograph (PNS)
(Arnaboldi et al. 2001;
Douglas, 2001)
on the William Herschel Telescope is a slitless spectrograph
preceded by a narrow band [OIII]
5007 filter. The
spectrograph is rotated
180° between exposures. When the images are compared, PNe will
be pairs of stellar sources whose separations are proportional to their
radial velocities, whereas stars will be dispersed into short
spectra. The PNS will be very effective for measuring the radial
velocities of PNe for two reasons. When the seeing is very good, as
frequently is the case on the WHT
(Wilson et al. 1999),
the effective slit will be the seeing FWHM. And, a second observing run
is not required to measure the radial velocities.
The Hubble Advanced Camera for Surveys will be able to detect ICPNe one
magnitude fainter than the brightest PN at a distance of ~ 200 Mpc when
the sky is as faint or fainter than 23.2 V-mag /
".