Spitzer/IRS Survey

I've been working for Dr. Chas Beichman and his group at JPL and MSC/NExScI/IPAC (mainly Dr. Geoff Bryden, Dr. Angelle Tanner, and Dr. Beth Holmes) since the summer of 2002. During that time I've worked on several projects, all focusing on debris disks around nearby solar-type stars. Several papers have resulted from this work, including my first first-author paper which was just submitted to the Astrophysical Journal!

Spitzer/IRS Spectra of Debris Disks Around Solar-Type Stars

We looked at 152 mature FGK and early M stars within about 50 pc to see what percentage have debris disks that are visible with the IRS instrument on Spitzer, which registers wavelengths between 5 and 35 um. We were hoping to find more stars like HD 69830, which has a very bright excess at all IRS wavelengths, complete with spectral features, allowing the composition of the dust to be identified. We didn't find any stars like that.

Filled bars are stars with excesses. Figures from Lawler et al. 2008, submitted.

What we did find was 16 stars with strong IRS excesses. Combining that with results from previous surveys, we find that 11.8% +/- 2.5% of nearby solar type stars have detectable excesses in the longer IRS wavelengths (30-34 um). Only about 1% of these stars have excesses in the shorter IRS wavelengths (8.5-12 um). We found that within our sample there was no dependence on spectral type, age, or metallicity (which is in sharp contrast to the strong dependence of the presence of an RV-detected planet on the host star's metallicity).

Model and measured spectra for 10um dust grains. Figure from Lawler et al. 2008, submitted.

And out of all of these excesses, only 2 showed possible spectral features, and these features were very weak. Because of the lack of features, we assumed fairly large (~10 um) dust grains, and were able to model where the dust is in these systems. We combined the IRS data with MIPS 70 um data (when available) for better dust modeling.

Modeled dust temperatures and distances. "I" means an IRS excess, "M" means a MIPS 70 um excess, and "w" means the excess is weak. Figure from Lawler et al. 2008, submitted.

We found that all of the dust was between 1-35 AU, corresponding to temperatures of 50-450 K. Out of the 73 stars we had MIPS 70 um data for, 19 have an excess. 16 of those 19 have IRS excesses, leaving only 3 stars with a MIPS 70 um excess and no IRS excess. Overall, we can say that very cold (<50 K) and very warm (> 500 K) dust around these stars is very rare.

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