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A. J. Weinberger (Carnegie DTM), E. E. Becklin, B. Zuckerman (UCLA)
We present spatially resolved mid-infrared spectra of thermal emission from the disk around the 5 Myr old Herbig AeBe star HD 141569A. Observations were made with the Long Wavelength Spectrometer at the W. M. Keck Observatory in which a 0.''5 wide slit was placed along the major axis of the disk, i.e. a PA=-3 deg. The instrument provided a spectral range of 7.7 -- 12.3 \micron\ at resolution R~120. After calibration to remove Telluric features and subtract the stellar point spread function, spectra were extracted every 0.''3 (30 AU) along the disk. The disk was detected out to 90 AU, i.e. out almost to the maximum radius of 100 AU seen in previous mid-infrared imaging. The disk extends to more than 500 AU in scattered light imaging.
Lines from polycyclic aromatic hydrocarbons (PAHs) are detected at every radius, and their line-to-continuum ratios actually increase with radius. This is the opposite behavior expected if the number of PAHs remains constant or decreases with radius, because the exciting flux for the PAHs (i.e. stellar UV continuum) falls off with radius. We find a best fit for the PAH spectrum using small ionized PAHs. These are the ones most subject to radiation pressure ejection and photodestruction on short timescales. No silicate emission is observed. Qualitatively, the presence of PAHs within 100 AU and lack of crystalline silicates together favor the cold coagulation dust model of Li & Lunine (2003) for the dust production. However, the pileup of small grains at ~90 AU is difficult to explain without an enhanced production rate at that radius.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.