Fair-infrared photometry and mapping of Herbig As/Be stars with ISO

Seven Herbig Ae/Be stars were observed at mid-and far-infrared wavelengths with ISOPHOT, the photometer on-board the Infrared Space Observatory. At la mbda less than or equal to 25 mu m, where the emission mainly arises from a compact circumstellar region, the observed spectral energy distributions c an be described by power-law relationships between wavelength and flux dens ity (F(nu)proportional to nu(-n)). The exponent of the power-law changes co nsider ably among the stars, from n approximate to 0 for MWC 1080 to n appr oximate to 2.3 in the case of LkH alpha 234, with a typical value of around 1. Interpreting the observed power-law relationships in terms of circumste llar disks, in 5 out of 6 cases relatively shallow radial temperature distr ibutions have to be assumed (T proportional to r(-q) where 0.37 less than o r equal to q less than or equal to 0.53). At longer wavelengths the observe d emission is spatially extended, and in some cases significant discrepancy with IRAS was found due to beam size effects. The peak of the SEDs (in F-n u) is typically at 60-100 mu m, corresponding to temperatures of around 50 K. At lambda>100 mu m the emission observed by ISOPHOT is never dominated b y the Herbig Ae/Be stars. The most likely sources of the far-infrared radia tion are dust cores of about 1 arcminute in size. The dust cores are probab ly located in the vicinity of the stars, and may be related to the star for ming process.