Disk Accretion and Mass Loss From Young Stars

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Session 48 -- Young Stars
Display presentation, Tuesday, 10, 1995, 9:20am - 6:30pm

[48.15] Disk Accretion and Mass Loss From Young Stars

P. Hartigan (Rice), S. Edwards (Smith College), L. Ghandour (U Mass)

We have analyzed high resolution spectra of 42 T Tauri stars to study the relationship between disk accretion and energetic outflows from young stars. We use echelle spectra from the KPNO 4m to measure the optical excess continuum (veiling) flux and to extract residual forbidden line profiles free of contamination from underlying photospheric features and from telluric emission and 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 stars indicate that the 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 excess 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.

Forbidden lines in T~Tauri stars have two distinct components. There is a high velocity component that resembles a dense stellar jet, and requires more than a single shock to account for the observed line ratios. Luminosities of the high velocity component indicate the mass loss rates, which are $\sim$ $10^{-9} - 10^{-10}$ M$_{\odot}$ yr$^{-1}$ for most stars, and disk accretion rates derived from the veiling fluxes are $\sim$ $10^{-6} - 10^{-8}$ M$_{\odot}$ yr$^{-1}$. Hence, ${\dot M}_{wind}$/${\dot M}_{acc}$ $\sim$ 0.01 to 0.001 for most classical T~Tauri stars.

The low velocity component originates in a region of high density, and is characterized by small outflow velocities ($\sim 5$ km$\,$s$^{-1}$ ), possibly associated with a disk wind or magnetic accretion columns. The velocity shifts are largest for forbidden lines with the lowest critical density, suggesting that the wind 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$^{-2.2}$.

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