Constraining the Role of Collisions in the beta Pictoris Debris Disk

Beta Pictoris is an edge-on debris disk that contains two planets (Lagrange et al. 2009, 2010, 2020; Nowak et al. 2020), and has been studied in detail from UV to millimeter wavelengths. With the improved calibration data made available from the Spitzer IRS team, we re-examine the Spitzer IRS data of the beta Pic debris disk with optimal extraction (Lebouteiller et al. 2010). The optimal extraction constructs a super-sampled point spread function (PSF) in the sub-pixel locations on the detector for the low-resolution spectral modes. The algorithm then weighs the extracted data by the SNR of each pixel and therefore produces a higher SNR spectrum compared to the typical extractions that weigh every pixel equally. We have extracted a spatially-unresolved low-resolution spectrum with emission from the star and the disk combined with optimal extraction and then subtracted the stellar photosphere from it. We have also extracted two spatially-resolved spectra for comparison: one collocated with the CO gas clump in the southwest side of the disk (Dent et al. 2014), and another one at a similar stellocentric distance but on the opposite side of the disk. Our optimal extraction shows a new and prominent 18.5-micron emission feature in addition to the existing 10, 23, 28, and 33-micron emission features (Chen et al. 2007). Our preliminary results show that the emission features in the entire IRS spectral range (5 to 35 microns) cannot be simultaneously well-fitted if we assume that the distribution of silicates does not change as a function of radius. This indicates that the silicates’(both crystalline and amorphous) distribution does indeed vary as a function of radius. In particular, the prominent 18.5-micron feature is shortward of the well-measured 19-micron forsterite feature, indicating the necessity to explore silicate stoichiometry, such as the Mg/Fe ratio that could shift the peak location of the feature. In addition, preliminary analyses have shown that the peak of the 10 and 18.5-micron features are different on one side of the disk compared to the other, indicating a difference in either composition or grain sizes at two sides of the disk.