CFHT, Current Image of the Week


September 4th, 2000

H2 filaments in NGC 2023

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H2 filaments in NGC 2023

Credit: Image courtesy of D. Field1 & J.L. Lemaire2
1 Institute of Physics and Astronomy, University of Aarhus, Denmark
2 Observatoire de Paris-Meudon, Meudon, France

(for more details see: D. Field, J.L. Lemaire, G. Pineau des Forets, M. Gerin, S.Leach, F. Rostas & D. Rouan, 1998, A&A 333, 280.)

Stars are born in various sizes, from the smallest to the heaviest. When a star is massive, it lives a bright, luminous, but short life. In that case the internal temperature is large enough that huge supplies of hydrogen are burned at once, very rapidly, and almost as soon as the stars is born, it burns out, in a matter of a few millions years only. By comparison to our Sun that is 5 Billion years old, in its mid-life, these massive stars die at an early age. If our Sun could be assimilated to a 40 year old human, in its mid-life, these stars would die before they are 6 months old on the same scale!

During this brief life, these stars shine like no other. Their light illuminates the gas and dust enveloppes surrounding them, creating extended nebulae. The strong UV radiation emanating from these hot stars strongly excite nearby gas molecules and atoms which in turn reemit light at specific wavelengths, producing emission lines. In particular, one of the main constituent of the interstellar medium, the hydrogen molecule (H2), possesses a few strong lines in the near-infrared domain. By shining strongly on them, massive stars excite the hydrogen molecules in their environment and thereby allow a detailed study of the material from which stars are born.

This week's image features NGC 2023, a reflection nebula surrounding a massive B star located at a distance of 500 pc from the Sun. The image shows pure emission from the H2 molecule, excited by the strong UV field of the central B star (luminous knot at the center of the map, best located in the corresponding continuum image). The map reveals large filaments of molecular gas which show structure down to a milliparsec scale, the smallest scales observable. The brightness of the emission detected also allows to estimate the density of the particles forming the filaments. The densities derived of 10^5 particles per cubic centimeter are 5 orders of magnitude larger than the average density between stars in our Galaxy. Stars form in a very dense and highly structured medium !

Technical description:

NGC 2023 was observed with the REDEYE. near infrared camera attached to the f/8 CVassegrain focus of the Canada-France-Hawaii Telescope. the observations were performed on December 1st & 2nd, 1993.

Two filters were used to produce the image presented this week. One centered on a strong molecular hydrogen line at 2.121 microns (H2 S(1)(1-0) and an adjacent continuum filter centered at 2.179 microns. The continuum image has been subtracted to the line image to produce the map of pure H2 emission presented here. The resolution of the images, limited by the seeing, is about 0.8 arcsec and the field of view is 128x128 arcsec.

next week: First images of Titan's surface at 1.3microns

editors: François Ménard, Jean-Charles Cuillandre & Catherine Dougados
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