This paper analyzes optical and infrared photometry of pre-main-sequence stars in the Taurus-Auriga molecular cloud. More than half of the stars in our sample have excess near-infrared emission. The near-infrared excesses correlate very well with other measures of activity, such as Ho emission, ultraviolet excess emission, millimeter continuum emission, and the presence of reflection nebulae and molecular outflows. The infrared colors and the ratio of far-infrared to bolometric luminosity display a smooth progression from the most deeply embedded protostars to optically visible T Tauri stars. Infalling envelope models account for the colors of protostars; simple disk models similarly reproduce the colors of many T Tauri stars.

Both the stellar birth line and a l0⁵ yr isochrone provide a reasonable upper envelope to the luminosity distribution of optically visible stars in the H-R diagram. Only a few stars in the cloud have apparent ages exceeding 2- 3 × 10⁶ yr, as derived from detailed stellar evolution calculations. The distribution of stars in the H-R diagram indicates that the cloud has formed stars at a roughly constant rate for the past 1-2 × 10⁶ yr. Analyses of the J and K-luminosity functions support this conclusion. Within the uncertainties, the observed mass distribution for optically visible stars agrees with a Miller-Scalo initial mass function. Source statistics imply a lifetime of 1-2 × l0⁵ yr for the typical protostar in Taurus-Auriga. There is no evidence, however, that these sources lie on the stellar birth line. Indeed, the protostellar luminosity function is essentially identical to the luminosity function derived for optically visible T Tauri stars in the cloud.

These results provide some support for the evolutionary sequence-embedded protostar → T Tauri star with a circumstellar disk → T Tauri star without a circumstellar disk-currently envisioned in standard models of low-mass star formation. Source statistics and infrared color-color diagrams demonstrate that pre-main-sequence stars develop bluer colors and display less evidence for circumstellar material with time. The data show little evidence, however, for the luminosity evolution expected along the proposed evolutionary sequence. Time-dependent accretion during the infall phase may account for the low luminosity of Taurus-Auriga protostars; this hypothesis requires more tests.

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