An Optical Spectroscopic Study of T Tauri Stars. I. Photospheric
  Properties
  Measurements of masses and ages of young stars from their location in the HR
diagram are limited by not only the typical observational uncertainties that
apply to field stars, but also by large systematic uncertainties related to
circumstellar phenomena. In this paper, we analyze flux calibrated optical
spectra to measure accurate spectral types and extinctions of 283 nearby T
Tauri stars. The primary advances in this paper are (1) the incorporation of a
simplistic accretion continuum in optical spectral type and extinction
measurements calculated over the full optical wavelength range and (2) the
uniform analysis of a large sample of stars. Comparisons between the
non-accreting TTS photospheric templates and stellar photosphere models are
used to derive conversions from spectral type to temperature. Differences
between spectral types can be subtle and difficult to discern, especially when
accounting for accretion and extinction. The spectral types measured here are
mostly consistent with spectral types measured over the past decade. However,
our new spectral types are 1-2 subclasses later than literature spectral types
for the original members of the TWA and are discrepant with literature values
for some well known Taurus CTTSs. Our extinction measurements are consistent
with other optical extinction measurements but are typically 1 mag lower than
nIR measurements, likely the result of methodological differences and the
presence of nIR excesses in most CTTSs. As an illustration of the impact of
accretion, SpT, and extinction uncertainties on the HR diagrams of young
clusters, we find that the resulting luminosity spread of stars in the TWA is
15-30%. The luminosity spread in the TWA and previously measured for binary
stars in Taurus suggests that for a majority of stars, protostellar accretion
rates are not large enough to significantly alter the subsequent evolution.
