Next Contents Previous


Planetary nebulae mark the endpoint of stellar evolution for stars with masses between ~ 0.8 and 8 Modot (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] lambda4959/5007 is one of the most efficient cooling lines; the monochromatic luminosity in [OIII] lambda5007 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] lambda5007 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] lambda5007 than Halpha. 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 × Omega, normalized to the Subaru telescope's "Suprime" camera. All else being equal (seeing and camera throughput), A × Omega 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 / box". 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.

Table 1. Telescopes and Cameras That Are Suitable for PNe Surveys

Telescope Camera Pixel Size FOV A × Omega 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] lambda5007 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 / box".

Next Contents Previous