Figure 4.11 Halo of X-ray emitting stars in the Chamaeleon region. The solid contours trace 100 |m continuum emission, delineating the dust and gas. Dotted lines and curves indicate right ascension and declination.

Chamaeleon. In this region, close to the South celestial pole, the molecular gas is confined to three well-defined, coherent structures, as labeled in Figure 4.11. Of these, Chamaeleon I has both the highest visual extinction and the largest population of young stars, while the more irregular Chamaeleon III appears to lack any star formation activity. Since the Chamaeleon constellation is 15° from the Galactic plane, all three clouds are relatively free of background stars.

The HR diagram for Chamaeleon I includes some 80 association members. About half were first identified optically, either from Ha emission surveys or else by virtue of their photometric variability, another characteristic property of classical T Tauri stars. The remainder are weak-lined stars discovered initially through their X-ray emission by the ROSAT satellite. It is clear from the diagram in Figure 4.9c that the classical and weak-lined stars are thoroughly intermingled in both mass and age, with many close to the birthline. Further evidence of ongoing star formation is the presence of infrared sources with no optical counterparts; some of these are exciting Herbig-Haro objects and molecular outflows.

X-ray observations have revealed an additional population of stars in the Chamaeleon region. Some lie inside the molecular cloud boundaries, but most do not, and are distributed over tens of parsecs (Figure 4.11). Similar halos surround the Taurus-Auriga, Orion, and Lupus regions. A relatively young age is indicated for some of these objects by the presence of surface lithium, which is gradually destroyed in the course of stellar evolution (Chapter 16). This subset consists largely of post-T Tauri stars, i. e., contracting objects intermediate in their properties between classical and weak-lined stars on the one hand, and those already settled onto the main sequence (Chapter 17). The rest are even older, with ages perhaps as great as 108 yr. In any case,

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