Diffuse interstellar clouds  Detectability distribution of diameters and column densities
Abstract
Consideration is given to the detection probabilities, and diameter and column density distributions of diffuse interstellar clouds detected by means of the color excesses of A and F stars. The probability of detecting a set of clouds actually observed at distances less than 150 pc, which requires observations to be made of at least five stars, is calculated as a function of cloud diameter and distance using the spatial stellar density and Poissonian statistics of stellar distribution. It is noted that for diameters less than about 6 pc, the distance of maximum detection probability increases with diameter, and that the calculated detection probability matches the observed diameterdistance distribution. The probability of detecting a cloud of a given diameter in the volume sounded is found to peak at a diameter of 5 pc. The true diameter distribution for clouds of diameter greater than 1.5 pc is estimated on the basis of comparison of the volume out to the cloud distance with the volume in which the cloud would have been detected, and of an estimate of the detection probability, and the two estimations are found to agree with a powerlaw distribution of index 2.6. Statistical considerations indicate that about four stars per low excess cloud are sufficient to establish the reality of the cloud excess. The distribution of cloud color excesses is found to follow a 5/2 powerlaw distribution for color excesses greater than 0.02 magnitudes. The observed deficiency of smalldiameter clouds is noted as a real effect, attributed to the evaporation of smalldiameter clouds.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 May 1981
 Bibcode:
 1981A&A....98...74K
 Keywords:

 Astronomical Photometry;
 Density Distribution;
 Gas Density;
 Interstellar Gas;
 Molecular Clouds;
 Statistical Analysis;
 Colorimetry;
 Data Acquisition;
 Data Sampling;
 F Stars;
 Size Determination;
 Astronomy