DOI: 10.25549/usctheses-c127-637591
title: Warm debris and multiple-belts around main sequence stars via Spitzer space telescope infrared observations — probing the underlying planetary systems

description: Using the Spitzer Space Telescope IRS spectrometer low-resolution modules covering wavelengths from 5 to 35 μm, we observed 52 main-sequence A and late B type stars previously seen using Spitzer/MIPS photometry to have excess infrared emission at 24 μm above that expected from the stellar photosphere. The mid-IR excess is confirmed in all cases but two. While prominent spectral features are not evident in any of the spectra, we observed a striking diversity in the overall shape of the spectral energy distributions (SEDs). Most of the IRS excess spectra are consistent with single-temperature blackbody emission, suggestive of dust located at a single orbital radius—a narrow ring. Assuming the excess emission originates from a population of large blackbody grains, dust temperatures range from 70 to 324 Kelvin, with median of 190 K corresponding to a distance of 10 AU. Thirteen stars however, have dust emission that follows a powerlaw distribution, Fᵥ = F₀λᵅ, with exponent α ranging from 1.0 to 2.9. The warm dust in these systems may span a greater range of orbital locations—an extended disk. All of the stars have also been observed with Spitzer/MIPS at 70 μm, with 27 of the 50 excess sources detected (S/N &gt; 3). Most 70 μm fluxes are suggestive of a cooler, Kuiper belt-like component that may be completely independent of the asteroid belt-like warm emission detected at the IRS wavelengths. 14 of 37 sources with blackbody-like fits are detected at 70 μm. The thirteen objects with IRS excess emission fit by a power-law disk model, however, are all detected at 70 μm (four above, three on, and six below the extrapolated power-law), suggesting that the mid-IR IRS emission and far-IR 70 μm emission may be related for these sources. Overall, the observed blackbody and powerlaw thermal profiles reveal debris distributed in a wide variety of radial structures that do not appear to be correlated with spectral type or stellar age. An additional 43 fainter A and late B type stars without 70 μm photometry were also observed with Spitzer/IRS; results are summarized in appendix A. ❧ Subsequently, we compare the properties of warm dust emission from the sample of main-sequence A-type stars (B8–A7) to those of dust around solar-type stars (F5–K0) with similar Spitzer IRS/MIPS data and similar ages. Both samples include stars with sources with infrared SEDs that show evidence of multiple components. Over the range of stellar types considered, we obtain nearly the same characteristic dust temperatures (~190 K & ~60 K for the inner & outer dust components respectively)—slightly above the ice evaporation temperature for the inner belts. The warm inner dust temperature is readily explained if populations of small grains are being released by sublimation of ice from icy planetesimals. Evaporation of low-eccentricity icy bodies at ~150 K can deposit particles into an inner/warm belt, where the small grains are heated to Tdust ~190 K. Alternatively, enhanced collisional processing of an asteroid belt like system of parent planetesimals just interior to the snow line may account for the observed uniformity in dust temperature. The similarity in temperature of the warmer dust across our B8-K0 stellar sample strongly suggests that dust-producing planetesimals are not found at similar radial locations around all stars, but that dust production is favored at a characteristic temperature horizon. ❧ Finally, we adopt real grain optical properties to model the spatially resolved cold/outer dust belt around HD 107146 (G2V) to attempt to break the grain size-distance degeneracy that exists in the modeling of spatially unresolved disks. We consider homogeneous particles of astronomical silicates, and inhomogeneous icy particles of two different internal structures: a) inclusions-matrix particles (IMPs), which we take to be AstroSil aggregates embedded in an icy matrix (of water or ‘dirty’ ice), and b) core-mantle particles (CMPs), where an AstroSil core is coated with an icy layer of water or ‘dirty’ ice. Although the shape of the SED varies for IMPs versus CMPs, we find no significant difference in the estimate of the dust’s radial location. Interestingly, a single population of inhomogeneous icy grains can fit the cold thermal emission around HD 107146, provided the grain size distribution is bounded by 1–6× the blowout limit at the lower end. Modeling the SEDs of resolved debris systems in this manner allows for a better understanding of disk architecture, grain composition and structure, and serves as reference for unresolved systems. Robust estimates of disk radial locations will greatly enhance future exoplanet searches.