THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 101:117-171, 1995 November

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

                       SCOTT J. KENYON AND LEE HARTMANN
 Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138


                                   ABSTRACT

        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 Halpha 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 10^5 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 x 10^6 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 x 10^6 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 x 10^5 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.

     Subject headings: infrared: stars -- ISM: clouds -- stars: evolution
                       -- stars: luminosity function, mass function --
                       stars: pre-main-sequence