Tauri Star TW Hydrae Research Articles

A measure of the size of the magnetospheric accretion region in TW Hydrae.

Stars form by accreting material from their surrounding disks. There is a consensus that matter flowing through the disk is channelled onto the stellar surface by the stellar magnetic field. This is thought to be strong enough to truncate the disk close to the corotation radius, at which the disk rotates at the same rate as the star. Spectro-interferometric studies in young stellar objects show that hydrogen emission (a well known tracer of accretion activity) mostly comes from a region a few milliarcseconds across, usually located within the dust sublimation radius1-3. The origin of the hydrogen emission could be the stellar magnetosphere, a rotating wind or a disk. In the case of intermediate-mass Herbig AeBe stars, the fact that Brackett γ (Brγ) emission is spatially resolved rules out the possibility that most of the emission comes from the magnetosphere4-6 because the weak magnetic fields (some tenths of a gauss) detected in these sources7,8 result in very compact magnetospheres. In the case of T Tauri sources, their larger magnetospheres should make them easier to resolve. The small angular size of the magnetosphere (a few tenths of a milliarcsecond), however, along with the presence of winds9,10 make the interpretation of the observations challenging. Here we report optical long-baseline interferometric observations that spatially resolve the inner disk of the T Tauri star TW Hydrae. We find that the near-infrared hydrogen emission comes from a region approximately 3.5 stellar radii across. This region is within the continuum dusty disk emitting region (7 stellar radii across) and also within the corotation radius, which is twice as big. This indicates that the hydrogen emission originates in the accretion columns (funnel flows of matter accreting onto the star), as expected in magnetospheric accretion models, rather than in a wind emitted at much larger distance (more than one astronomical unit).

Magnetic accretion onto young stars

Young T Tauri stars exhibit strong solar-type magnetic activity, with extremely high temperature coronae and energetic flares. In a few systems discovered with Chandra and XMM-Newton there is also evidence for X-ray emission produced by shocks associated with magnetically channeled accretion. A recent 489 ksec Chandra HETG/ACIS-S observation of the classical T Tauri star TW Hydrae has provided a wealth of spectroscopic diagnostics not available in lower signal-to-noise ratio observations. Using line ratios for electron temperature, electron density, and column density we have found that the shock produced by the accelerating material in the accretion stream behaves as predicted by standard theory. However, the properties of the post-shock plasma differ substantially from the predictions of standard 1D shock models (Brickhouse et al. in Astrophys. J. 710:1835, 2010). The accretion process apparently heats the stellar atmosphere up to soft X-ray emitting temperatures, providing hot ions to populate the magnetic corona, in loops, stellar wind, and/or jets. This gas is highly turbulent, as evidenced by non-thermal line broadening. The observed properties of the accretion-fed corona should constrain theoretical models of an accretion-driven dynamo.

AN ABSENCE OF X-RAY ACCRETION SHOCK INSTABILITY SIGNATURES IN TW HYDRAE

Gas accreting onto T Tauri stars should form shocks that are susceptible to the classical radiative shock instability. The instability should give rise to strong periodic modulation in the X-ray emission from the shock-heated plasma. Time series analysis of soft X-rays thought to arise predominantly in an accretion shock on the classical T Tauri star TW Hydrae reveals no periodic variations and a 99% confidence pulsed fraction limit of 5% over the frequency range 0.0001-6.81 Hz. We find no clear explanation for the absence of X-ray instability signatures, but suggest that existing one-dimensional models are too simple to explain the three-dimensional shock structure, or that preheating and deceleration of the accretion stream by the damping of magnetohydrodynamic waves excited either by the shock itself, or more deeply in the stellar envelope, could ameliorate the instability in the likely case of a sub-Alfvenic shock.

Resolving the Chemistry in the Disk of TW Hydrae. I. Deuterated Species

We present Submillimeter Array observations of several deuterated species in the disk around the classical T Tauri star TW Hydrae at arcsecond scales, including detections of the DCN J = 3–2 and DCO+ J = 3–2 lines and upper limits to the HDO 31,2–22,1, ortho-H2D+ 11,0–11,1, and para-D2H+ 11,0–10,1 transitions. We also present observations of the HCN J = 3–2, HCO+ J = 3–2, and H13CO+ J = 4–3 lines for comparison with their deuterated isotopologues. We constrain the radial and vertical distributions of various species in the disk by fitting the data using a model where the molecular emission from an irradiated accretion disk is sampled with a two-dimensional Monte Carlo radiative transfer code. We find that the distribution of DCO+ differs markedly from that of HCO+. The D/H ratios inferred change by at least 1 order of magnitude (0.01-0.1) for radii <30 to ≥70 AU, and there is a rapid falloff of the abundance of DCO+ at radii larger than 90 AU. Using a simple analytical chemical model, we constrain the degree of ionization, x(e−) = n(e−)/n(H2), to be ~10^−7 in the disk layer(s) where these molecules are present. Provided the distribution of DCN follows that of HCN, the ratio of DCN to HCN is determined to be (1.7 ± 0.5) × 10^−2; however, this ratio is very sensitive to the poorly constrained vertical distribution of HCN. The resolved radial distribution of DCO+ indicates that in situ deuterium fractionation remains active within the TW Hydrae disk and must be considered in the molecular evolution of circumstellar accretion disks.

Spectropolarimetry of the Classical T Tauri Star TW Hydrae

We present high-resolution (R ≈ 60,000) circular spectropolarimetry of the classical T Tauri star TW Hydrae. We analyze 12 photospheric absorption lines and measure the net longitudinal magnetic field for six consecutive nights. While no net polarization is detected the first five nights, a significant photospheric field of Bz = 149 ± 33 G is found on the sixth night. To rule out spurious instrumental polarization, we apply the same analysis technique to several nonmagnetic telluric lines, detecting no significant polarization. We further demonstrate the reality of this field detection by showing that the splitting between right and left polarized components in these 12 photospheric lines shows a linear trend with Lande g-factor times wavelength squared, as predicted by the Zeeman effect. However, this longitudinal field detection is still much lower than that which would result if a pure dipole magnetic geometry is responsible for the mean magnetic field strength of 2.6 kG previously reported for TW Hya. We also detect strong circular polarization in the He I λ5876 and Ca II λ8498 emission lines, indicating a strong field in the line formation region of these features. The polarization of the Ca II line is substantially weaker than that of the He I line, which we interpret as being due to a larger contribution to the Ca II line from chromospheric emission in which the polarization signals cancel. However, the presence of polarization in the Ca II line indicates that accretion shocks on classical T Tauri stars do produce narrow emission features in the infrared triplet lines of calcium.

Measuring the Magnetic Field of the Classical T Tauri Star TW Hydrae

We present infrared (IR) and optical echelle spectra of the classical T Tauri star TW Hydrae. Using the optical data, we perform detailed spectrum synthesis to fit atomic and molecular absorption lines and determine key stellar parameters: Teff = 4126 ± 24 K, log g = 4.84 ± 0.16, [M/H] = -0.10 ± 0.12, and log g = 5.8 ± 0.6 km s-1. The IR spectrum is used to look for Zeeman broadening of photospheric absorption lines. We fit four Zeeman-sensitive Ti I lines near 2.2 μm and find that the average value of the magnetic field over the entire surface is B = 2.61 ± 0.23 kG. In addition, several nearby magnetically insensitive CO lines show no excess broadening above that produced by stellar rotation and instrumental broadening, reinforcing the magnetic interpretation for the width of the Ti I lines. We carry out extensive tests to quantify systematic errors in our analysis technique that may result from inaccurate knowledge of the effective temperature or gravity, finding that reasonable errors in these quantities produce a ~10% uncertainty in the mean field measurement.

A Hot Wind from the Classical T Tauri Stars: TW Hydrae and T Tauri

Spectroscopy of the infrared He I (10830 A) line with NIRSPEC on Keck and CSHELL at the IRTF, and of the ultraviolet C III (977 A) and O VI (1032 A) emission with FUSE, reveals that the classical T Tauri star TW Hydrae exhibits P Cygni profiles, line asymmetries, and absorption indicative of a continuous, fast (~400 km s-1), hot (~300,000 K) accelerating outflow with a mass-loss rate ~10-11 to 10-12 M☉ yr-1 or larger. Spectra of T Tau N appear consistent with such a wind. The source of the emission and outflow seems restricted to the stars themselves. Although the mass accretion rate is an order of magnitude less for TW Hya than for T Tau, the outflow reaches higher velocities at chromospheric temperatures in TW Hya. Winds from young stellar objects may be substantially hotter and faster than previously thought.

Evidence for Accretion: High‐Resolution X‐Ray Spectroscopy of the Classical T Tauri Star TW Hydrae

We present high-resolution X-ray spectra of the X-ray-bright classical T Tauri star, TW Hydrae, covering the wavelength range of 1.5-25 A. The differential emission measure derived from fluxes of temperature-sensitive emission lines shows a plasma with a sharply peaked temperature distribution, peaking at log T = 6.5. Abundance anomalies are apparent, with iron very deficient relative to oxygen, while neon is enhanced relative to oxygen. Density-sensitive line ratios of Ne IX and O VII indicate densities near log ne = 13. A flare with rapid (~1 ks) rise time was detected during our 48 ks observation; however, based on analysis of the emission-line spectrum during quiescent and flaring states, the derived plasma parameters do not seem strongly time-dependent. The inferred plasma temperature distribution and densities are consistent with a model in which the bulk of the X-ray emission from TW Hya is generated via mass accretion from its circumstellar disk. Assuming that accretion powers the X-ray emission, our results for log ne suggest an accretion rate of ~10-8 M☉ yr-1.

Near-Infrared Coronagraphic Imaging of the Circumstellar Disk around TW Hydrae

We present ground-based near-infrared (H-band) imaging of the circumstellar disk around the nearby classical T Tauri star TW Hydrae. The scattered-light image shows a face-on disk with a radius of 4'' (corresponding to 225 AU) and a morphology that agrees with recent images from the Hubble Space Telescope and the Very Large Array. The best-fit power-law for the disk's radial surface brightness profile obeys the law r-3.3±0.3. We use our image and published continuum flux densities to derive properties of the disk with a simple model of emission from a flat disk. The best-fit values for disk mass and inner radius are 0.03 M☉ and 0.3 AU, respectively; the best-fit values for the temperature, density, and grain opacity power-law exponents (q, p, and β) are 0.7, 1.3, and 0.9, respectively. These properties are similar to those of disks around classical T Tauri stars located in more distant molecular clouds. Because of TW Hydrae's nearby location and pole-on orientation, it is a uniquely favorable object for future studies of radial disk structure at the classical T Tauri stage.

WFPC2 Images of a Face‐on Disk Surrounding TW Hydrae

Hubble Space Telescope observations of the isolated T Tauri star TW Hydrae reveal the presence of a compact circumstellar nebula. After subtraction of a reference point-spread function (PSF), a smooth, symmetrical, circular halo can be seen in both R- and I-band WFPC2 images. Its intensity declines with radius until reaching an outer sensitivity limit at 35 (≈200 AU). Numerical experiments show that PSF subtraction artifacts cannot account for the halo's brightness distribution. Instead, the most likely explanation is that the halo is a face-on circumstellar disk. The radial brightness profile of the halo is complex and can be described with multiple, contiguous zones with individual power-law intensity relations. The halo appears slightly blue relative to the star, especially in the outer zones. We compare the TW Hya halo to single-scattering models of face-on disks with multiple radial zones. While optically thin disk models with vertical optical depth τv ≈ 10-2 can reproduce the relative brightness of the nebula and star, we find that such models have large midplane optical depths and are therefore not self-consistent. We present an optically thick disk model that matches the radial brightness profile self-consistently and has a dust mass close to that implied by submillimeter continuum measurements. The zonal structure found in the disk could arise from radial variations in the dust properties that determine the local equilibrium temperature or perhaps via dynamical effects of unseen companions.