The Structure and Emission of the Accretion Shock in T Tauri Stars

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.

Ionization and heating by X-rays and cosmic rays

“…This velocity is an upper limit as the material starts only at the inner border of the circumstellar disk, probably following curved magnetic field lines down toward the star; a more realistic terminal speed is v m ≈ 0.8v ff (Calvet & Gullbring 1998). Upon braking at the stellar surface, a shock develops which, according to the strong-shock theory, reaches a temperature of T s = 3 16k m p μv 2 m ≈ 3.5 × 10 6 M M R R −1…”

“…Such electron temperatures should therefore produce soft Xray radiation. The bulk of the ensuing X-rays are probably absorbed in the shock, contributing to its heating, although part of the X-rays may escape and heat the pre-shock gas (Calvet & Gullbring 1998); suggestive evidence for accretion-related X-ray production is available in the very soft X-ray range (e.g., based on flux excess in the O vii and Ne ix line triplets or indications of very high densities; Kastner et al 2002).…”

A Constraint on the Formation Timescale of the Young Open Cluster NGC 2264: Lithium Abundance of Pre-Main Sequence Stars

“…The equivalent widths of spectral lines can be underestimated by excess continuum emission due to mass accretion activity (Calvet & Gullbring 1998;Lim et al 2014a ) were measured by integrating the observed emission lines. In the case of the He I line, we computed the equivalent width of the line core within a narrow wavelength range of 0.7 Å to minimize the contribution of the error in the continuum level.…”

“…It is confirmed that a total of 27 stars have either a broad Hα emission line or a He I emission line, or both. If the spectral lines are veiled by accretion activities, l W 6708 (Li) may (Calvet & Gullbring 1998), and the lifetime of hot spots created by the accreted materials may also be different for a different accretion rate. The intensive time series observations of some PMS stars in NGC 2264 show aperiodic or periodic brightening events in their light curves on a timescale of several hours to 30 days (Stauffer et al 2014(Stauffer et al , 2016.…”

The Gas Disk: Evolution and Chemistry

“…This velocity is an upper limit as the material starts only at the inner border of the disk; a more realistic terminal speed is v m ≈ 0.8v ff (Calvet and Gullbring 1998). On the stellar surface, a shock develops with a temperature of…”