Do Rocky Extrasolar Minor Planets Have a Composition Similar to Bulk Earth?
We propose to observe six externally-polluted white dwarfs with dust disks to measure accurate abundances of volatile elements, especially carbon and sulfur. These data will enable us to determine whether the minor planets now being accreted onto the target white dwarfs more resemble primitive meteorites such as CI chondrites or more resemble bulk Earth.
We have two different observing strategies. For four targets (GD 40, G241-6 G29-38 and GD 133), we propose to obtain spectra with the G130M grating in the FUV module and achieve a signal-to-noise ratio (SNR) of at least 20. Our main lines of interest are C I 1329 Å, Cl I 1334 Å, Cl I 1347 Å̊, N I 1167 Å, S I 1425 Å and Ge II 1237 Å. To estimate the number of orbits, we use the GALEX magnitude measured at 1528 Å and then extrapolate to shorter wavelengths with our model atmospheres. With these estimated fluxes, we then use the COS ETC to determine the required on-target observing time. We request a SNR of 20 since some of our target lines are likely to be weak. We include the overheads for target acquisition and instrumental set-up to determine the expected number of cycles.
For two targets (GD 362 and PG 1225-079), we propose to obtain COS spectra with the G185M grating in the NUV module and with a signal-to-noise of at least 10 with the goal of determining abundances from C I 1931 Å and S I 1820 Å . Following an analogous procedure for the other four targets, we use the COS ETC to determine the required on-target observing time. With a SNR of 10, we find from our simulations that we can determine C and S abundances with an uncertainty of 0.3 dex which will be adequate to distinguish between models where the elemental compositions differ by at least a factor of 8.
Our targets are sufficiently faint that COS safety is not a problem.
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