VII.F. Asteroids and Comets

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

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  1. Number Present in Catalog (Asteroid Source Density)

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

The WSDB contained 326 hours-confirmed sources associated with known asteroids and comets; among these, 108 sources have two or more hours-confirmations. Close examination of the sources with two or more HCONs reveals that they are all chance positional coincidences with inertially fixed objects. There is no case of any known asteroid or comet passing the weeks- or months-confirmed on tests by self-confirmation.

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 fv, where N is the number of known asteroids detected with 25 µm flux density equal to or greater than fv, shows a steep rise (N = kfv-3/2) to N = 100, a flatter region (N = kfv-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.

IRAS Names of Sources Contaminated by Numbered Asteroids
Table VII.F.1
00260-1016 02449-0249 03341+1224 03301+1820 04369+0449*
05536+1944 06000+1644 07093+2522 07153+1128 07294+2521
08140+3940 07575+1756 08070+2503 08090+1239 08534+0850
09285+0847 10253+1101 15113-1310 15170-2739 15301-3259
15427-2604 16137-2047 16475-0930 16378-3133 16528-0808
16515-1634 17013-2451 17082-2903 17239-0259 17311-2013
17424-2331 17451-2102 17457-3623 17486-1701 17586-1724
18236-2320 18317-1646 18477-0940 18559-2045 18478+0215
19171-2047 19420-1304 19509-1925 21303-1858 23586-0116
*Source deleted by high source density processor.

Frequency of Flux Contamination
Frequency (%)
Table VII.F.2
# HCONs 12 µm 25 µm 60 µm 100 µm
2 8 97 81 16
3 60 53 27 7
4 25 50 25 0

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.

Cirrus Sources Possibly Contaminated by Asteroids
Table VII.F.3
02428+0748 02313+2729 03105+0744 03100+1049 03408+0101
03085+3945 03484+1744 04070+1128 04063+1220 04012+2044
04055+1859 04438+1643 04376+2922 04362+3347 05063+2015
05341+3420 05369+2350 05533+3147 07113+1225 08230+2535
14200-1907 15049-2606 15246-2503 15455-2931 16447-1644
16478-3129 17052-2835 17343-0358 17397-1317 17538-0740
18285-2217 18371-3148 19160-1606 21531-1009  

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