BOLOMETRIC TEMPERATURES OF YOUNG STELLAR OBJECTS

We propose the '' bolometric temperature '' T(bol) as a measure of the circumstellar obscuration and evolutionary development of a young stellar object (YSO). T(bol) is the temperature of a blackbody having the same mean frequency as the observed continuum spectrum. A log-log plot of bolometric luminosity L(bol) versus T(bol) has the same main sequence as the Hertzsprung-Russell diagram, but for YSOs T(bol) can have much lower values (approximately 30 K) than can the photospheric temperature T(eff) (approximately 2800 K). We present three indications that a YSO evolves toward the main sequence from low to high T(bol) as a YSO clears its natal circumstellar dust: (1) For 129 YSOs in Taurus-Auriga, T(bol) ranges continuously from 60 to 5250 K, from '' protostars '' to '' classical '' T Tauri stars (CTTs) to '' weak-line '' T Tauri stars (WTTs), and a plot of L(bol) versus T(bol) terminates abruptly at the main sequence. (2) In T(eff) CTTs and WTTs are indistinguishable, with T(eff) approximately 4200 K, but in T(bol) WTTs are distinctly hotter (3600 K) than CTTs (2100 K). These temperatures indicate that circumstellar matter intercepts a larger fraction of the stellar luminosity for CTTs (0.5) than for WTTs (0.2). (3) In stellar groups, YSOs with low T(bol) are fewer and more concentrated, while YSOs with high T(bol) are more numerous and widespread. As T(bol) increases, an increasing fraction of YSOs lie outside a fiducial contour of (CO)-C-13 line emission: more than half the YSOs are excluded when T(bol) > 2500 K. Thus colder YSOs are probably younger, and hotter YSOs older, than the dispersal time for gas traced by the (CO)-C-13 line, estimated to be 1-3 Myr.