No abstract

No abstract

We have examined one of the key predictions of the magnetospheric infall model for classical T Tauri stars (CTTS), namely, the formation of a shock on the stellar surface. We Ðnd that accretion column emission can successfully reproduce the main observational properties of the excess continuum that veils the absorption features in CTTS. This success adds further support to the magnetospheric infall model for disk accretion. We have calculated the structure and the spectral energy distribution of the emergent continuum emission from a shock at the base of the magnetospheric accretion column. We Ðnd that the spectral shape of the excess can be understood as optically thick emission from the heated photosphere below the shock, appearing mostly on the Paschen and Brackett continua, and optically thin emission from the preshock and attenuated postshock regions, dominating at wavelengths shorter than the Balmer threshold. The accretion luminosity and rate depend on two parameters : the energy Ñux of the accretion Ñow, F, and the surface coverage of the column, f, with typical values log F D 10.5È11.5, of the order or up to a factor of 10 higher than the intrinsic stellar Ñux, and f D 0.1%È1% of the surface area. The so-called continuum stars have accretion columns with similar energy Ñuxes than the less veiled, typical CTTS but with much larger surface emitting areas of f º 10%. At near-infrared wavelengths, the predicted veiling from the accretion column for typical CTTS parameters is nearly constant and ¹0.1. Only for the "" continuum stars ÏÏ are signiÐcant amounts of near-infrared veiling from the accretion column expected. For accretion columns with low values of F, the Paschen continuum emission has spectral features, rendering the usual deveiling procedures uncertain. Magnetospheric Ñow Balmer emission may also contribute to the excess for stars with low-F columns. Finally, accretion column emission can successfully reproduce the observed correlation between the excess luminosity at the U band and the accretion luminosity. This correlation does not depend on the characteristics (mass, spectral type) of the underlying star, at least in the range M3ÈK5.

We compare accretion-shock models with optical and ultraviolet spectra of preÈmain-sequence stars to (1) make the Ðrst determinations of accretion rates in intermediate-mass T Tauri stars from continuum emission and (2) derive improved estimates of accretion rates and extinctions for continuum T Tauri stars. Our method extends the shock models developed by Calvet & Gullbring to enable comparisons with optical and archival International Ultraviolet Explorer ultraviolet spectra. We Ðnd good agreement between the observations and the model predictions, supporting the basic model of magnetospheric accretion shocks as well as previous determinations of accretion rates and interstellar reddening for the low-mass T Tauri stars. The accretion rates determined for the intermediate-mass T Tauri stars agree well with values obtained through the other methods of Muzerolle and coworkers that use near-infrared hydrogen line strengths.

No abstract