Trends in silicates in the β Pic disk

C.X. Lu, (Johns Hopkins), C.H. Chen (STScI, Johns Hopkins), B.A. Sargent (Johns Hopkins, STScI), D.M. Watson (Univ. Rochester), C.M. Lisse (Applied Physics Lab.), J.D. Green (STScI), M.L. Sitko (Univ. of Cincinnati, Space Sci. Inst.), T. Mittal (MIT), V. Lebouteiller (Univ. Paris-Saclay), G.C. Sloan (STScI), I. Rebollido (STScI), D.C. Hines (STScI), J.H. Girard (STScI), M.W. Werner (JPL), K.R. Stapelfeldt (JPL), W. Wu (Johns Hopkins, Univ. of Michigan), K. Worthen (Johns Hopkins)

2022, ApJ, 933, 54

Full manuscript available locally (PDF) or from the arXiv (2205.09138).

While β Pic is known to host silicates in ring-like structures, whether the properties of these silicate dust vary with stellocentric distance remains an open question. We re-analyze the β Pictoris debris disk spectrum from the Spitzer Infrared Spectrograph (IRS) and a new Infrared Telescope Facility Spectrograph and Imager 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 feature and recover a 23 µm forsterite emission feature with a substantially larger line-to-continuum ratio than previously reported. We find that these prominent spectral features are primarily produced by small submicron-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.

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