Winds and Accretion in Young Stars

Establishing the origin of accretion powered winds from forming stars is critical for understanding angular momentum evolution in the star‐disk interaction region. Here, the high velocity component of accretion powered winds is launched and accreting stars are spun down, in defiance of the expected spin‐up during magnetospheric accretion. T Tauri stars in the final stage of disk accretion offer a unique opportunity to study the connection between accretion and winds and their relation to stellar spindown. Although spectroscopic indicators of high velocity T Tauri winds have been known for decades, the line of He I 10830 offers a promising new diagnostic to probe the magnetically controlled star‐disk interaction and wind‐launching region. The high opacity and resonance scattering properties of this line offer a powerful probe of the geometry of both the funnel flow and the inner wind that, together with other atomic and molecular spectral lines covering a wide range of excitation and ionization states, suggests that the magnetic interaction between the star and disk, and the subsequent launching of the inner high velocity wind, is sensitive to the disk accretion rate

Redshifted Absorption at He I 10830 as a Probe of the Accretion Geometry of T Tauri Stars

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 1

The properties and environment of primitive solar nebulae as deduced from observations of solar-type pre-main sequence stars

The following topics were discussed: (1) current observation evidence for the presence of circumstellar disks associated with solar type pre-main sequence (PMS) stars; (2) the properties of such disks; and (3) the disk environment

The evolution of young stellar object disks and their environment

The main efforts were directed towards determining the frequency of disk occurrence and the timescales for disk evolution for solar-type and intermediate mass stars. The results of the investigation showed that optically thick disks are accretion disks. The projected accomplishments are also discussed

Modeling T Tauri Winds from He I λ10830 Profiles

The high opacity of He I λ10830 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 that is readily modeled without definite knowledge of the physical conditions and recourse to multilevel 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 λ10830 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 can arise that favor radially outflowing stellar winds

Helium Emission from Classical T Tauri Stars: Dual Origin in Magnetospheric Infall and Hot Wind

High-resolution emission-line profiles of He I and He II in 31 classical T Tauri stars are analyzed with the aim of probing the environs of the star-disk interface in accreting low-mass young stars. The diagnostic power of the helium lines lies in their high-excitation potentials, which restrict their formation to a region either of high temperature or close proximity to a source of ionizing radiation. The He I profiles are decomposed into kinematic components that support the paradigm of magnetically controlled accretion from the disk onto the stellar surface but also require a significant contribution from a hot wind. A narrow component, seen in 28/31 stars, is characterized by relatively uniform line widths and centroid velocities among all the helium lines. Our analysis supports previous conclusions that this feature is consistent with formation in the decelerating postshock gas at the magnetosphere footpoint. A broad component, seen in 22/31 stars, displays a diversity of kinematic properties. Our analysis suggests that in many stars the He I broad component is itself composite. At one extreme are stars where the broad component is redshifted in excess of 8 km s-1, as would occur if helium emission arises primarily from polar angles less than 547 in the funnel flow. At the other extreme are stars where the broad component is blueshifted in excess of -30 km s-1, requiring an origin in outflowing gas. The additional occurrence of maximum blue wing velocities exceeding -200 km s-1 in 14 stars leads us to argue that hot winds are present in about half of our sample. The relation between the narrow component and the optical veiling differs between the stars with or without a hot helium wind, suggesting that when the hot wind is present the luminosity and temperature of the accretion shock are reduced. A comparison of broad component helium emission with standard outflow indicators leads us to suggest that there are two sources of inner wind in T Tauri accretion disk systems: one a hot polar/coronal wind that prevails in stars with high veiling, and the other a more widespread cool disk wind that is likely launched at the magnetosphere/disk boundary

Permitted Iron Emission Lines in the Classical T Tauri Star DR Tauri

We present a study of permitted emission lines of Fe I and Fe II in the spectrum of the high-accretion rate classical T Tauri star DR Tau. Echelle spectra collected at the 4 m Mayall telescope at Kitt Peak National Observatory between 1988 and 1992 include four epochs with red spectral coverage (~5000-6800 Å) and three with blue spectral coverage (~4000-4950 Å). A total of 62 unblended Fe I and Fe II lines are identified, their profiles are examined, and ratios of line pairs that are sensitive to column density or temperature are analyzed. The unblended Fe profiles exhibit a systematic behavior, with FWHM increasing from 20 to between 70 and 90 km s-1 as the equivalent width increases from 0.05 to several Å. Two-component fits to the profiles suggest a composite origin, comprising a narrow component (NC), with FWHM ~ 20 km s-1 and a radial velocity at rest with respect to the photosphere, and a broad component (BC), with FWHM ~ 100 km s-1 and a tendency to be blueshifted by ≤10 km s-1. These two kinematic components are present in differing proportions among lines of differing intensity, thereby accounting for the systematic behavior of the profiles with line strength. Estimates of opacities and column densities are obtained by comparing observed intensity ratios of lines from a common upper level with values expected from a local escape probability calculation. We find that (1) opacities in the NC exceed those in the BC by factors of 2-4 and (2), for the BC, NFe I 1017-1018 cm-2 and NFe II 1018-1019 cm-2 for kinetic temperatures in the range 4000-10,000 K. Ratios of NC-to-BC emission from a pair of Fe I lines that are insensitive to opacity suggest that the kinetic temperature in the NC exceeds that in the BC by several thousand degrees. The centroid velocity and width of the NC in Fe I and Fe II are comparable to those from photospheric lines, suggesting a thermal or turbulent origin further broadened by stellar rotation. In the context of a magnetospheric accretion model, the NC is consistent with an origin in the postshock gas close to the stellar surface. In contrast, the BC is likely to be broadened by bulk motion, such as infalling gas in the accretion funnel or rotation in the region coupling the inner disk to the stellar magnetic field

Modeling T Tauri Winds from He I 10830 Profiles

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 Journa

F, G, K, M Spectral Standards in the Y Band (0.95-1.11 um)

We take advantage of good atmospheric transparency and the availability of high quality instrumentation in the 1 um near-infrared atmospheric window to present a grid of F, G, K, and M spectral standards observed at high spectral resolution (R ~ 25,000). In addition to a spectral atlas, we present a catalog of atomic line absorption features in the 0.95-1.11 um range. The catalog includes a wide range of line excitation potentials, from 0-13 eV, arising from neutral and singly ionized species, most frequently those of Fe I and Ti I at low excitation, Cr I, Fe I, and Si I at moderate excitation, and C I, S I, and Si I having relatively high excitation. The spectra also include several prominent molecular bands from CN and FeH. For the atomic species, we analyze trends in the excitation potential, line depth, and equivalent width across the grid of spectroscopic standards to identify temperature and surface gravity diagnostics near 1 um. We identify the line ratios that appear especially useful for spectral typing as those involving Ti I and C I or S I, which are temperature sensitive in opposite directions, and Sr II, which is gravity sensitive at all spectral types. Ascii versions of all spectra are available to download with the electronic version of the journal.Comment: 40 pages, 21 figures, 4 tables, spectra available for download with source file, updated to mirror published versio