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Register a new userSpatially Resolved Molecular Hydrogen Emission in the Inner 200AU Environments of Classical T Tauri Stars
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Tracy L. Beck
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We present 2.0-2.4micron integral field spectroscopy at adaptive optics spatial resolution (~0.1) obtained with the Near-infrared Integral Field Spectrograph (NIFS) at Gemini North Observatory of six Classical T Tauri stars: T Tau, DG Tau, XZ Tau, HL Tau, RW Aur and HV Tau C. In all cases, the v=1-0 S(1) (2.12 micron) emission is detected at spatially extended distances from the central stars. The bulk of the H_2 emission is typically not spatially coincident with the location of continuum flux. Multiple transitions detected in the K-band spectra show that H_2 level populations are typical of gas in thermal equilibrium with excitation temperatures in the 1800K-2300 K range. Three of the stars have H_2 velocity profiles that are centered at the stellar radial velocity, and three show velocity shifts with respect to the system. Each of the stars studied here show observed excitation temperatures, spatial extents, and kinematics of the H_2 that are most consistent with shock excited emission from the inner regions of the known Herbig-Haro energy flows or from wide-angle winds encompassing the outflows rather than predominantly from UV or X-ray stimulated emission from the central stars. The data presented in this study highlights the sensitivity of adaptive optics-fed integral field spectroscopy for spatially resolving emission line structures in the environments of bright young stars.
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Core-accretion planet formation begins in protoplanetary disks with the growth of small, ISM dust grains into larger particles. The progress of grain growth, which can be quantified using 10 micron silicate spectroscopy, has broad implications for the final products of planet formation. Previous studies have attempted to correlate stellar and disk properties with the 10 micron silicate feature in an effort to determine which stars are efficient at grain growth. Thus far there does not appear to be a dominant correlated parameter. In this paper, we use spatially resolved adaptive optics spectroscopy of 9 T Tauri binaries as tight as 0.25 to determine if basic properties shared between binary stars, such as age, composition, and formation history, have an effect on dust grain evolution. We find with 90-95% confidence that the silicate feature equivalent widths of binaries are more similar than those of randomly paired single stars, implying that shared properties do play an important role in dust grain evolution. At lower statistical significance, we find with 82% confidence that the secondary has a more prominent silicate emission feature (i.e., smaller grains) than the primary. If confirmed by larger surveys, this would imply that spectral type and/or binarity are important factors in dust grain evolution.
We present results from a survey searching for spatially resolved near-infrared line emission from molecular hydrogen gas in the circumstellar environments of nine young stars: AA Tau, AB Aur, DoAr 21, GG Tau, GM Aur, LkCa 15, LkH$alpha$ 264, UY Aur, and V773 Tau. Prior high-resolution spectra of these stars showed the presence of ro-vibrational H$_2$ line emission at 2.12$mu$m with characteristics more typical of gas located in proto-planetary disks rather than outflows. In this study, we spatially resolve the H$_2$ emission in the eight stars where it is detected. LkCa 15 is the only target that exhibits no appreciable H$_2$ despite a prior detection. We find an anti-correlation between H$_2$ and X-ray luminosities, likely indicating that the X-ray ionization process is not the dominant H$_2$ excitation mechanism in these systems. AA Tau, UY Aur, and V773 Tau show discrete knots of H$_2$, as typically associated with shocks in outflowing gas. UY Aur and V773 Tau exhibit spatially resolved velocity structures, while the other systems have spectrally unresolved emission consistent with systemic velocities. V773 Tau exhibits a complex line morphology indicating the presence of multiple excitation mechanisms, including red and blue-shifted bipolar knots of shock-excited outflowing gas. AB Aur, GM Aur, and LkH$alpha$ 264 have centralized, yet spatially resolved H$_2$ emission consistent with a disk origin. The H$_2$ images of AB Aur reveal spiral structures within the disk, matching those observed in ALMA CO maps. This survey reveals new insights into the structure and excitation of warm gas in the circumstellar environments of these young stars.
We present radiative transfer models of the circumstellar environment of classical T Tauri stars, concentrating on the formation of the H-alpha emission. The wide variety of line profiles seen in observations are indicative of both inflow and outflow, and we therefore employ a circumstellar structure that includes both magnetospheric accretion and a disc wind. We perform systematic investigations of the model parameters for the wind and the magnetosphere to search for possible geometrical and physical conditions which lead to the types of profiles seen in observations. We find that the hybrid models can reproduce the wide range of profile types seen in observations, and that the most common profile types observed occupy a large volume of parameter space. Conversely, the most infrequently observed profile morphologies require a very specific set of models parameters. We find our model profiles are consistent with the canonical value of the mass-loss rate to mass-accretion rate ratio (mu=0.1) found in earlier magneto-hydrodynamic calculations and observations, but the models with 0.05<mu<0.2 are still in accord with observed H-alpha profiles. We investigate the wind contribution to the line profile as a function of model parameters, and examine the reliability of H-alpha as a mass accretion diagnostic. Finally, we examine the H-alpha spectroscopic classification used by Reipurth et. al, and discuss the basic physical conditions that are required to reproduce the profiles in each classified type.
Magnetospheric accretion has been thoroughly studied in young stellar systems with full non-evolved accretion disks, but it is poorly documented for transition disk objects with large inner cavities. We aim at characterizing the star-disk interaction and the accretion process onto the central star of LkCa 15, a transition disk system with an inner dust cavity. We obtained quasi-simultaneous photometric and spectropolarimetric observations of the system over several rotational periods. We analyzed the system light curve, as well as changes in spectral continuum and line profile to derive the properties of the accretion flow from the edge of the inner disk to the central star. We also derived magnetic field measurements at the stellar surface. We find that the system exhibits magnetic, photometric, and spectroscopic variability with a period of about 5.70 days. The light curve reveals a periodic dip, which suggests the presence of an inner disk warp that is located at the corotation radius at about 0.06 au from the star. Line profile variations and veiling variability are consistent with a magnetospheric accretion model where the funnel flows reach the star at high latitudes. This leads to the development of an accretion shock close to the magnetic poles. All diagnostics point to a highly inclined inner disk that interacts with the stellar magnetosphere. The spectroscopic and photometric variability of LkCa 15 is remarkably similar to that of AA Tau, the prototype of periodic dippers. We therefore suggest that the origin of the variability is a rotating disk warp that is located at the inner edge of a highly inclined disk close to the star. This contrasts with the moderate inclination of the outer transition disk seen on the large scale and thus provides evidence for a significant misalignment between the inner and outer disks of this planet-forming transition disk system.
Classical T Tauri stars (CTTS) are young, late-type objects, that still accrete matter from a circumstellar disk. Analytical treatments and numerical simulations predict instabilities of the accretion shock on the stellar surface. We search for variability on timescales below a few minutes in the CTTS TW Hya and AA Tau. TW Hya was observed with SALTICAM on the Southern African Large Telescope (SALT) in narrow-band filters around the Balmer jump. The observations were performed in slit mode, which provides a time resolution of about 0.1 s. For AA Tau we obtained observations with OPTIMA, a single photon-counting device with even better time resolution. Small-scale variability typically lasts a few seconds, however, no significant periodicity is detected. We place a 99 % confidence upper limit on the pulsed fraction of the lightcurves. The relative amplitude is below 0.001 for TW Hya in the frequency range 0.02-3 Hz in the 340 nm filter and 0.1-3 Hz in the 380 nm filter. The corresponding value for AA Tau is an amplitude of 0.005 for 0.02-50 Hz. The relevant timescales indicate that shock instabilites should not be seen directly in our optical and UV observations, but the predicted oscialltions would induce observable variations in the reddening. We discuss how the magnetic field could stabilise the accretion shock.
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