From Stardust to Planetesimals: Contributed Papers - NASA Lunar ...

EXAMPLES OF COMET-LIKE SPECTRA A ONPIC-LIKE STARSH. M. BUTNER DTM Carnegie Inst. of Washington, Washington, DC, USAH. J. WALKER CLRC Rutherford Appleton Laboratory, Didcot, UKD. H. WOODEN NASA Ames Research Center, Moffett Field, CA, USAF. C. WITTEBORN NASA Ames Research Center, Moffett Field, CA, USAABSTRACT. 0 Pic is a nearby main-sequence star with an extended dust disk,large infrared excess, and mid-infrared emission similar to that seen in some cometaryspectra. We selected a sample of stars whose infrared properties resemble a Picand obtained 8 to 13 ym spectra (resolution 200). We find a variety of silicateemission features among the stars in our sample, ranging from classic interstellardust features to spectra similar to that of comet P/Halley and and the laboratoryspectra of Interplanetary Dust Particles (IDPs).1. INTRODUCTIONIn 1984, a new class of stars was identified by IRAS. These stars, named Vega-likestars after their prototype (Vega), were first classified as main-sequence stars withsignificant excess infrared emission. The infrared emission was consistent with thatexpected from thermal emission from dust grains surrounding the stars. The lack ofoptical extinction and large infrared excesses suggested that the emission might arisefrom dust arranged in a disk-like geometry as opposed to a spherical distribution.Visual images of P Pic (Smith and Terrile 1984) dramatically confirmed the diskhypothesis, revealing a disk radius in excess of 200 AU. More recently, 0 Pic has beenimaged at a variety of wavelengths ranging from the optical (Burrows et al. 1996) to10 ym (Lagage and Pantin (1994).Other Vega-like stars have been identified in the IRAS database (Aumann 1985,Walker and Wolstencroft 1988). The traditional method is to examine a sample ofstarssuch as all A-stars within 25 parsecs) for evidence of excess emission in theIRAS ands. Most of the stars identified have turned out to have very small excessescompared to the proto-types, making them difficult to study. See Backman andParesce (1993) for a review of these objects. Among the many areas of study is theactual age of individual Vega-like candidates. For example, 0 Pic might be as youngas 10 million years old (Jura et al. 1993), though the age estimates are far fromcertain. Even if these disk systems turn out to be younger than initially thought,they represent a unique opportunity to study actual disks around nearby stars.13