GAS AND DUST IN THE PLEIADES REFLECTION NEBULA

Steven John Gibson

Under the supervision of Professor Kenneth H. Nordsieck
at the University of Wisconsin-Madison

Abstract

In the following work, I present results of a multiwavelength investigation of reflection nebulosity around the Pleiades cluster. Dust in the region is studied photometrically at ultraviolet, optical, and far-infrared wavelengths to analyze nebular colors, grain properties, and scattering geometry. Constraints on this geometry are provided by spectroscopy of radio emission lines and optical absorption lines of the interstellar gas, which also reveal a significant amount of fine-scale filamentary structure.

Scattered light in the nebula has been observed in the ultraviolet with the WISP rocket payload telescope and by myself with a mosaic of Burrell Schmidt deep Bj-band CCD exposures. These data agree with previous photometry taken near Merope, but show the general nebulosity is fainter than expected in the UV, with a much flatter spectrum than the illuminating stars. Thermal radiation by the grains is measured from IRAS and DIRBE sky survey data and used with an assumed albedo to estimate the stellar flux incident on various parts of the nebula.

Neutral atomic gas in the region is examined in H I 21cm emission with the Green Bank 140-ft. telescope and the Very Large Array, and in Na I D-line interstellar absorption with the Coudé feed echelle spectrograph. I find excellent agreement between dust filaments and H I structure down to angular scales of 1' at the same velocities exhibiting strong Na I absorption toward Pleiades stars, indicating a predominantly forward-scattering geometry. The concentration of scattered light relative to thermal radiation in the cluster also supports this conclusion. A similar gradient in UV-to-optical color indicates the grains are more forward-throwing at 2200 than 4400 Å.

A simple analytic model applied to the nebular photometry shows the scattering situation to be complex, with the light in most regions provided by many stars. In addition, some combination of more extreme forward-throwing, lower grain albedo, and significant reddening of incident light is required to explain the faint UV flux. Selective extinction may arise from clumpy dust structures; filaments 0.04 pc in width are resolved in H I, and may be typical of the diffuse interstellar medium.


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