Presentation #319.07 in the session Exoplanet Formation of Planets and Protoplanetary Disks I.
While β Pictoris is known to host silicates in ring-like structures, whether the properties of these silicates vary with stellocentric distance remain a question. We re-analyze the Spitzer Infrared Spectrograph (IRS) β Pic debris disk data and a new IRTF/SpeX spectrum to investigate trends in Fe/Mg ratio, shape, and crystallinity in grains as a function of wavelength, a proxy for stellocentric distance. By analyzing a re-calibrated and re-extracted spectrum, we identify a new 18 μm forsterite emission band and recover a 23 μm forsterite emission band with a substantially larger line-to-continuum ratio than previously reported. We find that these prominent spectral features are primarily produced by small μm-sized grains, which are continuously generated and replenished from planetesimal collisions in the disk and can elucidate their parent bodies’ composition. We discover three trends about these small grains: as stellocentric distance increases, (1) small silicate grains become more crystalline (less amorphous), (2) they become more irregular in shape, and (3) for crystalline silicate grains, the Fe/Mg ratio decreases. Applying these trends to β Pic’s planetary architecture, we find that the dust population exterior to the orbits of β Pic b and c differs substantially in crystallinity and shape. We also find a tentative 3-5 μm dust excess due to spatially unresolved hot dust emission close to the star. From our findings, we infer that the surfaces of large planetesimals are more Fe-rich and collisionally-processed closer to the star but more Fe-poor and primordial farther from the star.