EVOLUTION OF THE BOLOMETRIC TEMPERATURE AND LUMINOSITY OF YOUNG STELLAR OBJECTS

We model the broadband emission from a star-disk-envelope system to ob tain expressions for the bolometric temperature T-bol and luminosity L -bol as functions of time, from the youngest class 0 protostars to sta rs on the zero-age main sequence. The model predicts evolution, driven by infall and contraction luminosity, in terms of position on the log T-bol-log L-bol diagram, a close analog of the H-R diagram. The evolu tionary tracks depend on the envelope initial conditions, the main-seq uence mass of the star, and the envelope dissipation timescale. The mo del L-bol rises due to infall and then falls due to contraction, while T-bol increases steadily toward the main sequence due to central heat ing and envelope dissipation. In order to smoothly join the protostell ar and pre-main-sequence phases it is necessary to model the terminati on of infall as gradual rather than sudden. This change reduces the pe ak infall luminosity for the collapse of a singular isothermal sphere by a factor 4, bringing predicted infall luminosities into better agre ement with observations. For stars of main-sequence mass 0.5 M., the m odel decrease in L-bol from its peak value of similar to 3 L. at T-bol similar to 250 K (class I) to similar to 0.4 L. at T-bol similar to 3 000 K (class II/III) closely matches the observed decrease in median L -bol for young stellar objects in Chamaeleon, Corona Australis, Lupus, Ophiuchus, and Taurus. The model should be useful for estimating the distributions of mass and age, and for describing the birth history, o f stars younger than 1 Myr in well-studied complexes.