Infrared studies of pre-main-sequence intermediate-mass stars - LkH-Alpha 198

The paper presents FIR scans of LkH-Alpha 198 at 50 and 100 microns along several directions; the observations reveal an extended, roughly spherical source diameter of 33 arcsec, or 20,000 AU, at 100 microns and not more than 10 arcsec, or 15,000 AU, at 50 microns. A luminosity of 250 solar luminosities at a distance of 600 pc is estimated on the basis of a comparison of the observed properties at optical, IR, and millimeter wavelengths with the predictions of radiation transfer models. The optical depth at 100 microns is 0.004-0.006, implying a mass of about 2 solar masses with a radius of 10,000 AU. It is argued that the inner radius of the dusty envelope is rather small, less than about 300 AU (0.5 arcsec), and it may be as small as the dust destruction front: in LkH-Alpha 198 there is no evidence of a large region devoid of grains.