NGC 2440

In Puppis

See the spectrum.

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 2440 NGC 2440
"Very bright" (Curtis). His drawing (on the left), a composite from a series of photographic exposures, nicely matches the Hubble view on the right if you rotate the latter about 20 degrees to the right. Like NGC 7027, the visual appearance of this planetary nebula seems to be affected by local circumnebular dust. The distance is not well known. If at an estimated distance of 4500 light years, the maximum angular diameter of 74 seconds of arc gives a physical diameter of 1.6 or so light years.

Again like NGC 7027, NGC 2440 is one of the most highly excited nebulae known. It is so bright from the exceedingly hot (219,000 Kelvin, a near record) central star that the star becomes very hard to see against the nebular background. Even though radiating with a power of more than 500 Suns, most of the light comes out in the ultraviolet. Indeed, Curtis states "Has no central star," though in the Hubble view it pops out nicely. The star has probably passed its maximum temperature and has begun the cooling and dimming phase that will take it into the realm of the white dwarfs. The nebula is enriched in helium, nitrogen, and carbon, which it is donating back to interstellar space. NGC 2440 also exhibits twin opposing flows of gas from the center that are seen in the radio radiation given off by carbon monoxide and that lie nicely along the long axis as drawn by Curtis.

Left: Image and quotes by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: NASA and the Hubble Heritage Team (AURA/STScI).

THE REMARKABLE HIGH-EXCITATION SPECTRUM OF NGC 2440

The three panels below show the entire optical and near-ultraviolet emission line spectrum down to the atmospheric cutoff near 3100 Angstroms. The emission lines flanking the nebular spectra are used to establish the wavelength scale, which is in Angstroms (1 A is a hundred-millionth of a centimeter). They appear above the nebular spectra in Panels 2 and 3. Square brackets indicate forbidden lines caused by collisions between electrons and atoms or ions. They are obviously not really "forbidden," just hard to produce in the laboratory. Those without brackets are recombination lines created by capture of free electrons by ions. See NGC 7009 for further discussion. The spectra of IC 418, NGC 7009, NGC 6720, and NGC 2440 span nearly a full range of nebular excitations and central star temperatures. See also the Orion Nebula.

These historic slit spectrograms were all taken with the 120-inch telescope of Lick Observatory by Lawrence Aller, the second and third at the coude focus around 1964, the first at the prime focus some ten years later. Note the classic H-Beta and "Nebulium" [O III] lines at far left in Panel 1 and far right in Panel 2, the great strength of the He II line at 4686 Angstroms, the strong forbidden [N II] lines of ionized nitrogen at far right in Panel 1 (flanking H-Alpha), and the forbidden [O II] line of ionized oxygen near the center of Panel 3, plus the lines of [Fe V], [K V], etc., which together reveal a huge range of excitation and ionization within the nebula. Note also the strong Balmer continuum to the left of center in Panel 3.

NGC 2440
Panel 1. See full resolution. The two strong forbidden ionized lines of [N II] are almost blended with the H- Alpha line at 6563 Angstroms at far right. The strength of the low-excitation nitrogen lines contrast sharply with the existence of the {K V] line of four times ionized potassium. The even-lower excitation neutral lines of [O I] are notable as well, as is the recombination line of He I at 5876 Angstroms near center. At far right are a pair of forbidden ionized sulfur lines at 6717 and 6731 Angstroms that are used to find the gas density, as are the forbidden doubly ionized [O III] chlorine lines left of center at 5517 and 5537 Angstroms. The spectrogram was one of the first in which Doppler shifts within the lines were used to establish a planetary nebula's structure. At far left are the two [O III] "Nebulium" lines also seen, and further described, in Panel 2, with H-Beta to the left of them. The ratio of the intensity of H-Alpha to that of H-Beta is used to derive the degree of interstellar dust absorption.

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Panel 2. See full resolution . The spectrum is dominated at far right by the forbidden [O III] "Nebulium" lines" at 5007 and 4959 Angstroms, which in 1928 were found to be forbidden lines of doubly ionized oxygen, [O III], first discovered in the spectrum of NGC 6543 by Sir William Huggins in 1864 with his visual spectroscope. H-Beta, at 4861 Angstroms, again stands just to the left of them. The 4686 Angstrom recombination line of ionized helium is almost as strong as H-Beta. Its strength reflecting central star temperature, it's not seen at all in the spectrum of low-excitation IC 418 and is modest in that of NGC 7009. Another [O III] forbidden line at 4363 Angstroms just to the right of H-Gamma at 4340 Angstroms is used with the bright [O III] lines at 4959 and 5007 Angstroms to get the gas temperature. The very long lines seen here and in Panel 3 are from mercury vapor street lights in the valley below.

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Panel 3. See full resolution. The powerful blended low-excitation [O II] lines at 3727 Angstroms (separated in the spectrum of NGC 7009) contrast sharply with very high excitation [K V] and [Fe V] lines in Panel 2. The ratio of the [O II] lines is also used to derive gas density. The confluence of the hydrogen Balmer lines culminates in the strong Balmer continuum that begins at 3646 Angstroms and is caused by the capture of free electrons. The O III recombination lines of doubly ionized oxygen in the ultravioilet at far left are produced by a fluorescent mechanism, and are also present in the spectrum of NGC 7009.

First panel from J. B. Kaler and L. H. Aller, Publications of the Astronomical Society. of the Pacific , vol. 86, p. 635, 1974; panels 2 and 3 from L. H. Aller, S. J. Czyzak, and J. B. Kaler, Astrophysical Journal, vol. 151, p. 187, 1968.