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Fourteen models are calculated with the shock velocity ranging from 200 to 330 km s$^{-1}$ and pre-shock hydrogen nucleon density ranging from $2.5\times 10^{12}$ to $4\times 10^{13}$ cm$^{-3}$. Among them the summed emergent flux of all spectral lines accounts for about 0.1-0.3 of the total veiling flux. The hydrogen Balmer continuum accounts for 0.17-0.1, while a nearly constant fraction close to 0.5 comes from emission produced by the stellar atmosphere. The main results derived from the veiling continuum energy distributions are two strong correlations: 1) the Balmer jump (BJ) increases as $F_K$, the shock kinetic energy flux, decreases; 2) at a fixed fraction of surface coverage by accretion shocks $r_{\lambda}$, the ratio of veiling to photospheric continuum flux at wavelength $\lambda$, decreases as $F_K$ decreases. Using the BJ - $F_K$ and $r_{4500}$ - $F_K$ relations, the observed excess continua of 10 T Tauri stars are modelled. For BP Tau and 3 Orion stars our accretion luminosities are higher than published values by a factor of a few. For the 6 Chamaeleon I stars our observed accretion luminosities are about 27 - 78\% higher than corresponding published values. Comparison of model results on the HeI $\lambda$5876$~$flux with observed data indicates that, while those stars with dominant $\lambda$5876$~$narrow components can be readily accounted for by the calculated models, those with much stronger broad components cannot, and suggests that for the latter objects the bulk of their excess continua at 5876$~$do not originate from accretion shocks., Comment: 46 pages, 13 figures, 4 tables

In accreting young stars one of the prominent spectral features in the near infrared is the Paschen and Brackett series in emission. We examine hydrogen line ratios for 16 classical T Tauri stars from SpeX spectra and and assess the trends with veiling and accretion. The observed line ratios are compared to two theoretical models for line formation: (1) Baker and Menzel's (1938) Case B for radiative ionization and recombination and (2) a set of local line excitation calculations designed to replicate the conditions in T Tauri winds and magnetic accretion columns (Kwan & Fischer 2011). While the comparison between Case B and observed line ratios implies a wide range in electron density and temperature among the hydrogen line formation regions in T Tauri stars, the predictions of the local line excitation models give consistent results across multiple diagnostics. Under the assumptions of the local line excitation calculations, we find that n_H in the hydrogen line formation region is constrained to 2x10^10 -- 2x10^11 cm^-3, where stars with higher accretion rates have densities at the higher end of this range. Because of uncertainties in extinction, temperature is not well delineated but falls within the range expected for collisional excitation to produce the line photons. We introduce new diagnostics for assessing extinction based on near infrared hydrogen line ratios from the local line excitation calculations., Comment: Accepted by ApJ, 17 pages, 13 figures; v2 has some minor copyediting

We present the first characterization of the excess continuum emission of accreting T Tauri stars between optical and near-infrared wavelengths. With nearly simultaneous spectra from 0.48 to 2.4 microns acquired with HIRES and NIRSPEC on Keck and SpeX on the IRTF, we find significant excess continuum emission throughout this region, including the I, Y, and J bands, which are usually thought to diagnose primarily photospheric emission. The IYJ excess correlates with the excess in the V band, attributed to accretion shocks in the photosphere, and the excess in the K band, attributed to dust in the inner disk near the dust sublimation radius, but it is too large to be an extension of the excess from these sources. The spectrum of the excess emission is broad and featureless, suggestive of blackbody radiation with a temperature between 2200 and 5000 K. The luminosity of the IYJ excess is comparable to the accretion luminosity inferred from modeling the blue and ultraviolet excess emission and may require reassessment of disk accretion rates. The source of the IYJ excess is unclear. In stars of low accretion rate, the size of the emitting region is consistent with cooler material surrounding small hot accretion spots in the photosphere. However, for stars with high accretion rates, the projected area is comparable to or exceeds that of the stellar surface. We suggest that at least some of the IYJ excess emission arises in the dust-free gas inside the dust sublimation radius in the disk., Comment: Accepted to ApJ, 31 pages, 21 figures

We perform local excitation calculations to obtain line opacities and emissivity ratios and compare them with observed properties of H, He I, O I, Ca II, and Na I lines to determine the density, temperature, and photon ionization rate. We find that UV photoionization is the most probable excitation mechanism for generating the He I 10830 opacities that produce all the associated absorption features. We also calculate the specific line flux at an observed velocity of v_obs = +/- 150 km/s for both radial wind and infall models. All the model results, together with observed correlations between absorption and emission features and between narrow and broad emission components, are used to deduce the origins of the strong H, He I, and Ca II broad line emission. We conclude that the first two arise primarily in a radial outflow that is highly clumpy. The bulk of the wind volume is filled by gas at a density ~10^9 cm^-3 and optically thick to He I 10830 and H alpha, but optically thin to He I 5876, Pa gamma, and the Ca II infrared triplet. The optically thick He I 5876 emission occurs mostly in regions of density greater than or equal to 10^11 cm^-3 and temperature greater than or equal to 1.5x10^4 K, while the optically thick H alpha and Pa gamma emission occur mostly in regions of density around 10^11 cm^-3 and temperature between 8750 and 1.25x10^4 K. In producing the observed line fluxes at a given v_obs, the covering factor of these emission clumps is sufficiently small not to incur significant absorption of the stellar and veiling continua in either He I or H lines. The strong Ca II broad line emission likely arises in both the magnetospheric accretion flow and the disk boundary layer, where the gases dissipate part of their rotational energies before infalling along magnetic field lines. The needed density and temperature are ~10^12 cm^-3 and less than or equal to 7500 K., Comment: Accepted to MNRAS, 88 pages, 24 figures

We probe the geometry of magnetospheric accretion in classical T Tauri stars by modeling red absorption at He I 10830 via scattering of the stellar and veiling continua. Under the assumptions that the accretion flow is an azimuthally symmetric dipole and helium is sufficiently optically thick that all incident 1-micron radiation is scattered, we illustrate the sensitivity of He I 10830 red absorption to both the size of the magnetosphere and the filling factor of the hot accretion shock. We compare model profiles to those observed in 21 CTTS with subcontinuum redshifted absorption at He I 10830 and find that about half of the stars have red absorptions and 1-micron veilings that are consistent with dipole flows of moderate width with accretion shock filling factors matching the size of the magnetospheric footpoints. However, the remaining 50% of the profiles, with a combination of broad, deep absorption and low 1-micron veiling, require very wide flows where magnetic footpoints are distributed over 10-20% of the stellar surface but accretion shock filling factors are < 1%. We model these profiles by invoking large magnetospheres dilutely filled with accreting gas, leaving the disk over a range of radii in many narrow "streamlets" that fill only a small fraction of the entire infall region. In some cases accreting streamlets need to originate in the disk between several stellar radii and at least the corotation radius. A few stars have such deep absorption at velocities greater than half the stellar escape velocity that flows near the star with less curvature than a dipole trajectory seem to be required., Comment: 26 pages, emulateapj format, Accepted by ApJ, to appear 2008 November 10

The high opacity of He I 10830 makes it an exceptionally sensitive probe of the inner wind geometry of accreting T Tauri stars. In this line blueshifted absorption below the continuum results from simple scattering of stellar photons, a situation which is readily modeled without definite knowledge of the physical conditions and recourse to multi-level radiative transfer. We present theoretical line profiles for scattering in two possible wind geometries, a disk wind and a wind emerging radially from the star, and compare them to observed He I 10830 profiles from a survey of classical T Tauri stars. The comparison indicates that subcontinuum blueshifted absorption is characteristic of disk winds in ~30% of the stars and of stellar winds in ~40%. We further conclude that for many stars the emission profile of helium likely arises in stellar winds, increasing the fraction of accreting stars inferred to have accretion-powered stellar winds to ~60%. Stars with the highest disk accretion rates are more likely to have stellar wind than disk wind signatures and less likely to have redshifted absorption from magnetospheric funnel flows. This suggests the possibility that when accretion rates are high, disks can extend closer to the star, magnetospheric accretion zones can be reduced in size and conditions arise that favor radially outflowing stellar winds., Comment: 41 pages, 11 figures. Accepted by Astrophysical Journal

We have developed a self-consistent physical model for super stellar cluster winds based on combining a 1-D steady-state adiabatic wind solution and a non-equilibrium ionization calculation. Comparing with the case of collisional ionization equilibrium, we find that the non-equilibrium ionization effect is significant in the regime of a high ratio of energy to mass input rate and manifests in a stronger soft X-ray flux in the inner region of the star cluster. Implementing the model in X-ray data analysis softwares (e.g., XSPEC) directly facilitates comparisons with X-ray observations. Physical quantities such as the mass and energy input rates of stellar winds can be estimated by fitting observed X-ray spectra. The fitted parameters may then be compared with independent measurements from other wavelengths. Applying our model to the star cluster NGC 3603, we find that the wind accounts for no more than 50% of the total "diffuse" emission, and the derived mass input rate and terminal velocity are comparable to other empirical estimates. The remaining emission most likely originate from numerous low-mass pre-main-sequence stellar objects., Comment: 29 pages, 17 figures. accepted by MNRAS

In a high dispersion 1 micron survey of 39 classical T Tauri stars veiling is detected in 80%, and He I 10830 and Pgamma line emission in 97% of the stars. On average, the 1 micron veiling exceeds the level expected from previously identified sources of excess emission, suggesting the presence of an additional contributor to accretion luminosity in the star-disk interface region. Strengths of both lines correlate with veiling, and at Pgamma there is a systematic progression in profile morphology with veiling. He I 10830 has an unprecedented sensitivity to inner winds, showing blueshifted absorption below the continuum in 71% of the CTTS compared to 0% at pgamma. This line is also sensitive to magnetospheric accretion flows, with redshifted absorption below the continuum found in 47% of the CTTS compared to 24% at Pgamma. The blueshifted absorption at 10830 shows considerable diversity in its breadth and penetration depth into the continuum, indicating that a range of inner wind conditions exist in accreting stars. We interpret the broadest and deepest blue absorptions as formed from scattering of the 1 micron continuum by outflowing gas whose full acceleration region envelopes the star, suggesting radial outflow from the star. In contrast, narrow blue absorption with a range of radial velocities more likely arises via scattering of the 1 micron continuum by a wind emerging from the inner disk. Both stellar and disk winds are accretion powered since neither is seen in non-accreting WTTS and among the CTTS helium strength correlates with veiling., Comment: 55 pages, including 16 figures. Astrophysical Journal, accepted

One of the puzzles in understanding the spectra of active galactic nuclei (AGN) is the origin of the FeII emission. FeI emission, if present, will help reveal the physical conditions of the emitting gas. In an attempt to verify the presence of FeI lines, high S/N spectra of two FeII-strong quasars, IRAS 07598-6508 and PHL 1092, were obtained at the Multiple Mirror Telescope and the Steward 2.3 m Telescope. We have identified emission lines of FeI and TiII. The source of energy for FeII, FeI and TiII emission is probably not from ionization by the photon continuum, but heat. The high rate of energy generation and the presence of both high and low velocity gas indicate that the heat is generated not over a large area, but a narrow band in accretion disk, in which the rotational speed decreases rapidly., Comment: 17 pages, Latex, figures available upon request, to appear in ApJ