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Session 32 - General ISM.
Oral session, Wednesday, January 07

[32.01] Gas and Dust in the Pleiades Reflection Nebula

S. J. Gibson (U. of Calgary)

For my dissertation, I have conducted a multiwavelength investigation of reflection nebulosity around the Pleiades cluster. Scattered light photometry obtained in the vacuum ultraviolet with the Wide-field Imaging Survey Polarimeter and in the visible with a Burrell Schmidt deep Bj-band mosaic agree with previous measurements taken near 23 Tauri, but show the larger nebula is fainter than expected in the UV, with a much flatter spectrum than the illuminating stars. A forward-scattering geometry is implied by the greater concentration of visible and UV light than thermal radiation observed in IRAS and DIRBE surveys. A similar relation between the UV and visible components indicates the dust grains are more forward-throwing at 2200 than 4400 Angstroms. The flatness of the nebular spectrum is best explained by reddening of starlight incident on the grains, despite the nebula's optical thinness along most observed sightlines.

I have also tested the interposed-sheet scattering model for the Pleiades by studying the HI 21cm line emission and Na-I D-line interstellar absorption associated with the nebula. A 64-pointing VLA D-array mosaic combined with short spacing data from a Green Bank 140-foot survey demonstrates excellent agreement between dust filaments and HI structure over a limited velocity range at angular scales as small as 1'. Coude Feed echelle observations show neutral sodium absorption features in this same range toward cluster stars but absent in nearer sightlines. The identity of gas and dust filaments at a radial velocity 10 km/s greater than the cluster's conclusively demonstrates the nebula is the result of an interstellar collision. The fine-scale HI structure is the smallest yet observed (0.04 pc), and may have bearing on the UV flux problem if dust inhabits clumpy structures. These narrow HI filaments are characteristic of the entire nebula, and perhaps of the diffuse ISM in general.

This work was completed under the supervision of Kenneth Nordsieck and in collaboration with Mark Holdaway. Financial support was provided by NASA grant NAG5-647 under contract with the University of Wisconsin.

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