Disk Accretion and Mass Loss From Young Stars
Hartigan, P. (Rice), Edwards, S. (Smith College), and Ghandour, L. (UMass)
In this paper we investigate how mass outflows from young
stars relate to circumstellar disks by analyzing spectroscopic
data of 42 T Tauri stars that span a broad range of
infrared excesses. We measure the optical excess continuum
(veiling) in each star from high-resolution 4-m spectra, and
extract forbidden line profiles uncontaminated by terrestrial
night sky emission , photospheric absorption lines, and
telluric absorption lines.
The veiling fluxes combined with existing infrared photometry
allow us to estimate reddenings and stellar luminosities
for the first time for heavily veiled stars. The new
estimates of the stellar luminosities of these objects
indicate that stars with the highest accretion rates are the
youngest in the sample. There is a one-to-one correspondence
between forbidden line emission, veiling, and near-infrared
color excesses among the stars in our sample; all disks
around young stars which are both optically thick and extend
inward to within a few stellar radii of the stellar surface
are, in fact, accretion disks, and have forbidden line emission.
Residual forbidden line profiles of [O I]6300, [O I]5577,
[S II]6731, and [N II]6583 represent a range of critical
densities from 10^4 to 10^8 cm^{-3}. We have determined
luminosities and line ratios for the two distinct velocity
components in each of these forbidden lines. The high
velocity component resembles a dense stellar jet, but
requires more than a single shock to account for the observed
line ratios. Luminosities of the high velocity component
indicate mass loss rates of ~ 10^{-8} to 10^{-10} Msun/yr
for most stars, and disk accretion rates derived from the
veiling fluxes are ~ 10^{-6} to 10^{-8} Msun/yr. The mass
outflow and mass accretion rates are correlated. The ratio
of the mass outflow rate to the mass accretion rate depends
on how the forbidden lines luminosities are interpreted, but
is probably ~ 0.01 for most classical T Tauri stars.
The low velocity component originates in a region of higher
density than the high velocity component and is characterized
by a small negative radial velocity (~ -5 km/s), possibly
associated with a disk wind or magnetic accretion columns.
The velocity shifts are largest for forbidden lines with the
lowest critical densities, suggesting that the low velocity
component accelerates away from the surface of the disk. If
the low velocity component arises on the surface of a disk
in Keplerian rotation, then the observed profiles imply a
disk surface brightness which decreases as ~ r^{-2.2}.
(Journal of publication: Ap J 452, 736, 1995)