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In the spectra below, the data appear in blue, and a template for a spectrum, scaled at 10-15 µm, in purple. As can be seen, fainter targets are more likely to show a curvature in LL which leads to an apprent deficit in the 24 µm region.
K0 giants (all relatively bright)
Bright A dwarfs
Faint A dwarfs - all 5 show the 24 µm deficit
Note that these plots are in Rayleigh-Jeans units. A Rayleigh-Jeans tail would appear as a horizontal line. The top of each panel is 40% higher than the bottom.
I have measured the 24 µm deficit for the entire calsfx sample of A dwarfs and K giants by comparing the expected and actual flux, as averaged from 22 to 26 µm. The plots below show that the deficit grows significant for the fainter sources. A dwarfs are plotted with triangles, K giants with squares. Remember that we calibrated with the brighter ones. We could have calibrated with fainter sources, but then we'd have an excess for bright sources.
deficit vs. expected flux
deficit vs. log(expected flux)
I noted this problem with the S9.5 pipeline output when I published a paper on a debris disk around HD 46190, and I also developed a work-around using the two faint standards HR 5467 and HR 7018 (see Fig. 2 in the HD 46190 paper). Until we can address this problem in the pipeline itself, such a solution might be useful now, too. In order to implement it, though we need to reobserve HR 5467 and HR 7018, as the new faint standard, eta1 Dor, appears to deviate from its expected spectrum at longer wavelength (details).
For those who are interested, here are all of the K giants and
A dwarfs in the calsfx sample:
G9
K0
K1
K2
K3
K4
K5
faint K
bright A
intermediate A
faint A
debris disks
Last modified 9 May, 2006. © The IRS Team.