CFHT, Current Image of the Week


May 8th, 2000

Deuterium in Orion

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Deuterium in Orion

Credit: Image courtesy of G. Hébrard

Institut d'Astrophysique, Paris, France

(for more details see: Hébrard, Péquignot, Vidal-Madjar, Walsh, Ferlet, 2000, Astronomy & Astrophysics Letters, Vol. 354, p. L79-L82.)

Hydrogen is the most abundant element in the Universe, by far! It is the lightest and simplest element, with only one light negatively charged particle, an electron, orbiting one heavy positively charged particle, a proton. Sometimes an extra neutron is tied to the proton, in that case there are two particles in the nucleus and Hydrogen is then called Deuterium. It is rare relative to normal Hydrogen.

Current scenarios for the synthesis of atomic elements, the nucleosynthesis, suggest that Deuterium was entirely created during the Big Bang and that it's abundance has been steadily decreasing since then. Deuterium is destroyed by astration - the production by stars of heavier atomic elements (Helium, Carbon, Nitrogen, Oxygen, etc...) from lighter ones (Hydrogen or Deuterium)

Various measurements of the Deuterium abundance exist. Intergalactic clouds are old and contain the signature of Deuterium as it was 15 billion years ago, very early in the history of the universe. Giant planets (Jupiter, Saturn) in our solar system trace Deuterium as it was at the epoch of formation of our planetary system, 4.5 billion years ago. The nearby interstellar medium contains traces of deuterium as it remains today. These measurements show that the abundance of Deuterium decreased by a factor 2 or 3 over 15 billion years. However, although the general trends in its evolution are understood, there is considerable variations in the measurements and any new and reliable way of estimating Deuterium abundance is welcome.

This week's image, or spectra rather!, is the result of a recent and innovative attempt to measure Deuterium in the nearby interstellar medium. The group of astronomers pointed the Canada-France-Hawaii Telescope to the great Nebula in Orion, just South of the Trapezium. Because it contains many bright stars, the surrounding gas is ionised and the emission lines of Hydrogen in particular are very intense. Two lines of the Hydrogen Balmer series are partly visible in this week's spectra. They are H-alpha and H-beta (labeled in the image). They correspond to transition from the N = 3->2 and N=4->2 electronic levels. The lines are so bright that they do not fit on the image, they had to be truncated. On the scale showed on the left of the image, H-alpha has an intensity of 7250 and H-beta 2500! What is interesting are the two smaller lines identified to the left of the normal Hydrogen ones. The team identifed these lines as the corresponding lines of Deuterium. Hence their names: D-alpha and D-beta.

This is the first time Deuterium is identified in a Nebula by optical spectroscopy and further research will allow to derive an estimation of its abundance.

Technical description:

The spectra presented this week were obtained with Gecko, CFHT's coudé echelle spectrograph. The high surface brightness of the Orion nebula helped to secure these high quality, high signal-to-noise observations. Integration times were 4.5 hours for the H-alpha/D-alpha spectrum and 1.5 hours for H-beta/D-beta spectrum. Deuterium was detected at three different position in the Orion Nebula. The position of one of the spectrograph entrance slit, south of the Trapezium is presented for orientation. It is not to scale. The actual slit was 40" long and about 3" wide, slightly different for H-alpha and H-beta.

next week: Exploring the Bar of NGC 2903

editors: François Ménard & Jean-Charles Cuillandre
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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.