AMMONIA IMAGES NEAR OBJECTS WITH CLASS-0 SPECTRAL ENERGY-DISTRIBUTIONS

A shroud of cold dust shapes the 'Class 0' spectral distribution of th e youngest stars. We have imaged a number of these objects in the inve rsion lines of ammonia to investigate the molecular circumstellar envi ronment on thousand AU scales. Although the dust spectra of these obje cts resemble one another, a variety of physical characteristics occur in the sample. in some circumstellar environments, NGC1333IRAS4A and S 106FIR, ammonia bearing gas clearly traces outflowing warm gas and a d ense warm elongated structure immediately surrounding the young star. More commonly ammonia bearing gas shows only a weak association with a ny structure in immediate contact with the dusty near-stellar environm ent (NGC1333IRAS4B, IRAS4C, IRAS16293-2422 A and B) and in some cases no evidence can be found that ammonia occurs in the near-stellar envir onment in association with the dust at all (VLA1623, SM-2). The ammoni a abundance in all objects must lie two orders of magnitude or so belo w its value in more extended cloud regions. The high molecular excitat ion conditions which mark the spectra of other molecules, such as form aldehyde, are invisible in ammonia spectra taken with beamsizes of pro jected diameters of similar to 1000 AU.