Introduction: Infrared cirrus clouds were first discovered by IRAS (Low, et al 1984) as extended sources of 100m radiation at high galactic latitudes. These clouds are thought to be in the solar neighborhood at distances of around 100 pc. These clouds differ from those in the direction of the disk of the galaxy in that the correlation of the 100 m emmission with the H I(21 cm) column density has high residuals. A lot of effort has gone into determining the gas to 100m emission ratio in these clouds and it is thought that these clouds tend to have a similar gas to dust ratio as those found in the disk. The difference is that these clouds seem to be more molecular in nature. The paradoxical side of this argument is that the ratio of H I to CO in these clouds indicates that there is a weaker ISRF(Interstellar Radiation Field) in this region which would lead to a smaller gas to 100m emission ratio(Heithausen and Mebold, 1989)
Another interesting relationship which has been examined in these clouds is the relationship between observed intensity at 100 and the extinction of the cloud. For these high galactic lattitude cirrus clouds this quantity is believed to be somewhere around 18 whereas the corresponding clouds in the disk are around 5 . As of yet this puzzle remains unsolved.
Computing the Flux at 450 : The emission in the FIR from dust grains is well understood to be thermal in nature. Thus, the emission at 450 can be estimated by assuming a source function for the cloud and by assuming an emissivity law. In this case, we can take the source function to be:
The emissivity function will vary depending on the types of grains present. We assume an for graphite and for silicates.
The first step in finding sources for SPARO is to sort candidates by declination, preferring to look below -50.
F.X. Desert, et. al. have conducted an all-sky search for cirrus clouds by performing a correlation between the IRAS 100m observations and HI observations. Any regions where the residuals from this correslation were high, they flagged as IREC(Infrared excess clouds.) By writing a small c program, I converted the galactic coordinates to celestial coordinates and pared the list down from 516 candidates, eliminating all but those with declination less than -50(1950).
In an article by McGee, et.al., Large, very structured cirrus clouds have been detected in the direction of the LMC. These clouds are thought to be local to the sun and have been studied with HI observations as well. This particular paper studied about 30 points in this comples using HI observations and then listed the associated 60m and 100m IRAS fluxes(background subtracted).
To calculate an expected integration time, I wrote a program called detect which given a desired signal to noise, tau, and grain emissivity law (In the far infrared/submillimeter), will calculate an expected integration time for a particular cloud. The outline of this program is as follows:
First the color temperature is calculated implementing the IRAS 60 and 100m
observations. To do this, an emissivity law is assumed:
In order to calculate color temperature, we assume n=1.5 for that part of the spectrum and take the ratio of the intensities at each wavelength.
This equation can be solved numerically to give a color temperature.
Once the color temperature is derived, the 450m intensity can be calculated:
From this calculated intensity, and given that the NEFD's add in quadrature where:
Table 1: n=1, =10, =1
Table 2: n=2, =10, =1