PRE-MAIN-SEQUENCE EVOLUTION IN THE TAURUS-AURIGA MOLECULAR CLOUD

This paper analyzes optical and infrared photometry of pre-main-sequen ce stars in the Taurus-Auriga molecular cloud. More than half of the s tars in our sample have excess near-infrared emission. The near-infrar ed excesses correlate very well with other measures of activity, such as Hot emission, ultraviolet excess emission, millimeter continuum emi ssion, and the presence of reflection nebulae and molecular outflows. The infrared colors and the ratio of far-infrared to bolometric lumino sity display a smooth progression from the most deeply embedded protos tars to optically visible T Tauri stars. Infalling envelope models acc ount for the colors of protostars; simple disk models similarly reprod uce the colors of many T Tauri stars. Both the stellar birth line and a 10(5) yr isochrone provide a reasonable upper envelope to the lumino sity distribution of optically visible stars in the H-R diagram. Only a few stars in the cloud have apparent ages exceeding 2-3 x 10(6) yr, as derived from detailed stellar evolution calculations. The distribut ion of stars in the H-R diagram indicates that the cloud has formed st ars at a roughly constant rate for the past 1-2 x 10(6) yr. Analyses o f the J- and K-luminosity functions support this conclusion. Within th e uncertainties, the observed mass distribution for optically visible stars agrees with a Miller-Scale initial mass function. Source statist ics imply a lifetime of 1-2 X 10(5) yr for the typical protostar in Ta urus-Auriga. There is no evidence, however, that these sources lie on the stellar birth line. Indeed, the protostellar luminosity function i s essentially identical to the luminosity function derived for optical ly visible T Tauri stars in the cloud. These results provide some supp ort for the evolutionary sequence-embedded protostar --> T Tauri star with a circumstellar disk --> T Tauri star without a circumstellar dis k-currently envisioned in standard models of low-mass star formation. Source statistics and infrared color-color diagrams demonstrate that p re-main-sequence stars develop bluer colors and display less evidence for circumstellar material with time. The data show little evidence, h owever, for the luminosity evolution expected along the proposed evolu tionary sequence. Time-dependent accretion during the infall phase may account for the low luminosity of Taurus-Auriga protostars; this hypo thesis requires more tests.