ISOPHOT Serendipity Survey observations of interstellar clouds I. Detection of the Coldest Cores in Chamaeleon

ISOPHOT Serendipity Survey (ISOSS) observations at 170 \um\ reveal cold dust clouds in the nearby star forming region Chamaeleon. The distribution of the ISOSS/IRAS based T(I₁₇₀/I₁₀₀) colour temperature have been studied in a nearby low-mass star forming region. T(I₁₇₀/I₁₀₀) separates the obscured and intercloud regions as seen from a comparison to optical extinction data. The interstellar medium in Chamaeleon-Musca appears to be cold with dust colour temperatures of ~ 16.3 K for intercloud regions, and <= 14.5 K for obscured clouds. The cold clouds account for 3 % of the area. ISOSS found 9 very cold cores with colour temperatures as low as T_dust <~ 12.5 K in an 11°ee x 8°ee sized region. All \CeiO\ cores of the Chamaeleon main clouds which were crossed by ISOSS slews were also detected. Of these 11 (73 %) are cold with T_dust <= 15 K, 6 (40 %) are associated with very cold cores. The very cold cores have high gas column densities, N(Htwo )>10²¹ cm^-2, and 7 out of 9 have low gas kinetic temperatures as indicated by T_ex(CeiO ) 8 K. The physical parameters of the very cold cores agree with the results of radiative transfer calculations of a spherical model cloud heated from outside by one third of the the solar neighbourhood ISRF intensity. A check of the photometric calibration of ISOSS against ISOPHOT AOT PHT22 maps and DIRBE interpolated 170 \um\ values showed an agreement of +/- 10 % and +/- 20 %, respectively. We expect to discover in total more than 100 very cold cores in the Galaxy by ISOSS.