THE PHOTOIONIZATION OF A STAR-FORMING CORE IN THE TRIFID NEBULA

摘要

We have carried out a comprehensive multiwavelength study of the bright-rimmed globule TC2 in the Trifid Nebula, using the IRAM 30 m telescope, the VLA centimeter array, and the Infrared Space Observatory (ISO). TC2 is one of the very few globules to exhibit signs of active ongoing star formation while being photoevaporated by the Lyman continuum flux of the exciting star of the nebula (～10~(10) cm~(-2) s~(-1)). The globule consists of a cold dense core of mass 27 solar mass surrounded by a lower density envelope of molecular gas. The impinging Lyman continuum photons induce the propagation of an ionization front into the globule. The evaporation of the ionized gas forms a thin layer of density n_e = (1-2) x 10~3 cm~(-3) around the globule, which could be mapped with the VLA. The globule is illuminated mainly on its rear side, by a far-ultraviolet field of intensity G_0 approx = 1000. It creates a photon-dominated region (PDR) below the surface, which was mapped and characterized with the ISOCAM circular variable filter and the Short Wavelength Spectrometer (SWS) on board ISO. The physical conditions derived from the analysis of the far-infrared lines [O Ⅰ] 63, 145μm and [C Ⅱ] 158 μm and the continuum emission are in good agreement with some recent PDR models. The emission of the polycyclic aromatic hydrocarbon band at 6.2, 7.7, 8.6, and 11.3 μm is detected over the whole globule. The relative intensity variations observed across the globule, in the PDR and the photoionized envelope, are consistent with the changes in the ionization fraction. In the head of TC2, we find a second kinematic component, which is the signature of the radiatively driven collapse undergone by the globule. This component indicates that the PDR propagates at low velocity inside the body of TC2. The molecular emission suggests that the star formation process was probably initiated a few times 10~5 years ago, in the large burst that led to the formation of the nebula. The globule has already evaporated half the mass of its envelope. However, the ionization timescale of the globule is long enough (～2 Myr) to let the protostellar objects reach smoothly the ultimate stages of protostellar evolution. The impact of photoionization on the star formation process appears limited.