IRAS Explanatory Supplement
VII. Analysis of Processing
F. Asteroids and Comets

1. Number Present in Catalog (Asteroid Source Density)

F. 1 Number Present in Catalog (Asteroid Source Density)

To determine the impact of these chance encounters on the reliability of catalog sources and their stated fluxes, the selection criteria and flux averaging methods discussed in Section V.H.1 must be applied. Seventy-four sources associated with known asteroids meet all the selection criteria and appear in the IRAS catalog. In the case of 17 of these IRAS failed to detect the asteroid. Of those detected, 13 had no effect on the cataloged quantities. The remaining 44 sources all have contaminated fluxes. These sources are identified in Table VII.F.1 and have the following characteristics:

1. Two-thirds of these sources had at least one high quality flux contaminated, although in no case did an asteroid association change the value of the flux quality flag.
2. In the absence of the asteroid, 80% of these sources would have been detected at only a single wavelength, with equal probability for each of the four bands. Of the remainder, 10% had both 12 and 25 µm fluxes and 10% had both 60 and 100 µm fluxes.
3. The flux discrepancy flag is only a weak indicator of the impact of the asteroid. All the correct flags are set in only 15% of the cases, but in 58% of the cases at least one flag was set correctly. In 37% of the cases no flag was set.
4. Seventy-five percent of the affected flux densitites are less than 1 Jy. The frequency of flux contamination is highest in the 25 µm band, however, and is a function of HCON as shown in Table VII.F.2.
5. The ecliptic latitude distribution of the contaminated sources follows the density distribution of the numbered asteroids. All the contaminated sources lie within 25" of the ecliptic with 90% of them within 15°.

The true population of asteroids in the WSDB is underestimated by the numbered asteroids. A plot of log N vs. log f_v, where N is the number of known asteroids detected with 25 µm flux density equal to or greater than f_v, shows a steep rise (N = kf_v^-3/2) to N = 100, a flatter region (N = kf_v^-1) from N = 100 to 1000, and a gradual roll-off to N = 3266. If the gradual roll-off is due to incompleteness of the numbered asteroids, extrapolation of the flat portion of the curve to the 25 µm flux limit estimates that the true population of single HCON asteroids in the WSDB is about 10,000. Since the frequency of chance encounters is proportional to the number of asteroids available, these numbers suggest that about 135 catalog sources could be affected. In general these may be expected to have characteristics similar to those discussed above for the numbered asteroids.

If the extrapolation of the asteroid population is valid, then there should be about 0.2 × 135 = 27 sources within 25° of the ecliptic plane observed only at 25 and 100 µm, resulting from the detection of infrared cirrus plus a faint asteroid. Of the 500 IRAS sources with measurements only at 25 and 100 µm, 34 of these clearly qualify as asteroid-cirrus combinations. The IRAS names of these objects are given in Table VII.F.3. Considering the small size of the sample, the agreement between the observed and predicted numbers is satisfactory and gives confidence in the estimated influence of asteroids.

While non-inertial sources of infrared radiation have no place in a catalog of fixed point sources, they are of great interest to studies of the Solar system. As a part of the normal data processing all detections of sources with colors appropriate to solar system objects ( 30 K < T < 400 K ) were written to off-line data files for further processing which attempts associations with a larger set of known asteroids.