Literature Review #3: Characterizing dust

c2d SPITZER IRS SPECTRA OF DISKS AROUND T TAURI STARS. I.
SILICAT EMISSION AND GRAIN GROWTH

As I’ve written about previously, this term I will be working on a model of the circumstellar environment around RR Tau. Both Hugh and Bernadette suggested looking at Spitzer data as a way to inform my work. Spitzer is a space telescope that is run by NASA that observes in the infrared. Apparently infrared is where you want to look when you want to characterize dust.

This is the first paper that I read specifically for my tutorial. A lot of it was a bit over my head and I think that I’ll have to keep coming back to it as I learn more. Overall, though, it had some really great information that I think will help my project greatly.

It’s the first paper in a series of data presenting the data from 40 solar mass T Tauri stars and 7 intermiediate mass Herbig Ae stars in the ~5-35 micron region. And, yes, RR Tau just happens to be one of the stars included in their set. The team was able to resolve both the 10 an 20 micron features. The main goal of the paper is to characterize the dust grains in circumstellar disks and determine properties of the disks from the dust grains. This is important because changes in grain size and composition have been linked to disk properties and planet formation but the rates of grain growth and processing in disks is not understood completely. Not to mention the underlying mechanism instigating all of that.

The authors of the paper linked grain features to disk features by comparing the spectroscopic results with synthetic results given by models and doing a statistical analysis over the whole set. Most of their observational focus seems to be on silicates. It’s not clear to me why silicates are so important other than maybe that just happens to be what circumstellar disks are made out of (?). Since it’s possible to determine grain size, shape, and composition from the strength, peak, and shape of an emission line the authors modeled the opacity of the sample grains various, shapes, sizes, and compositions. They then compared the grain opacities to the observed emission without the influence of continuum.

The authors go on to interpret the changes in the line shape and strengths as source-to-source variations in grain size. This is apparently a unique way of doing this but it seems sensible to me. What (I think) this means is that they cannot find a general rule for the type of grain in certain stars. Anything like, “In this type of star you’ll find this type of dust grain”. They attribute this to fast grain growth and turnover (processing). Basically, whatever the process that makes silicates grow and go from amorphous to crystalline has to be relatively quick in order for there to be such disparate source-to-source variations. Some conclusive things they did find is that the equivalent width of H-alpha is not correlated with disk age. On this they say, “This suggests the importance of turbulence and regeneration of small (micron-sized) grans on the disk surface.” and that M stars show much flatter silicate features, which means larger grain sizes, than A/B stars. They tentatively attribute this to the differences in disk temperature which affects where in the radius the emission line comes from. This suggests that the grain size differs as you move through the radius.

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