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Register a new userTime Variability of Emission Lines for Four Active T Tauri Stars (I): October--December in 2010
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We present optical spectrophotometric monitoring of four active T Tauri stars (DG Tau, RY Tau, XZ Tau, RW Aur A) at high spectral resolution ($R ga 1 times 10^4$), to investigate the correlation between time variable mass ejection seen in the jet/wind structure of the driving source and time variable mass accretion probed by optical emission lines. This may allow us to constrain the understanding of the jet/wind launching mechanism, the location of the launching region, and the physical link with magnetospheric mass accretion. In 2010, observations were made at six different epochs to investigate how daily and monthly variability might affect such a study. We perform comparisons between the line profiles we observed and those in the literature over a period of decades and confirm the presence of time variability separate from the daily and monthly variability during our observations. This is so far consistent with the idea that these line profiles have a long term variability (3-20 years) related to episodic mass ejection suggested by the structures in the extended flow components. We also investigate the correlations between equivalent widths and between luminosities for different lines. We find that these correlations are consistent with the present paradigm of steady magnetospheric mass accretion and emission line regions that are close to the star.
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For Classical T Tauri Stars (CTTSs), the resonance lines of N V, Si IV, and C IV, as well as the He II 1640 A line, act as diagnostics of the accretion process. Here we assemble a large high-resolution dataset of these lines in CTTSs and Weak T Tauri Stars (WTTSs). We present data for 35 stars: one Herbig Ae star, 28 CTTSs, and 6 WTTSs. We decompose the C IV and He II lines into broad and narrow Gaussian components (BC & NC). The most common (50 %) C IV line morphology in CTTSs is that of a low-velocity NC together with a redshifted BC. The velocity centroids of the BCs and NCs are such that V_BC > 4 * V_NC, consistent with the predictions of the accretion shock model, in at most 12 out of 22 CTTSs. We do not find evidence of the post-shock becoming buried in the stellar photosphere due to the pressure of the accretion flow. The He II CTTSs lines are generally symmetric and narrow, less redshifted than the CTTSs C IV lines, by ~10 km/sec. The flux in the BC of the He II line is small compared to that of the C IV line, consistent with models of the pre-shock column emission. The observations are consistent with the presence of multiple accretion columns with different densities or with accretion models that predict a slow-moving, low-density region in the periphery of the accretion column. For HN Tau A and RW Aur A, most of the C IV line is blueshifted suggesting that the C IV emission is produced by shocks within outflow jets. In our sample, the Herbig Ae star DX Cha is the only object for which we find a P-Cygni profile in the C IV line, which argues for the presence of a hot (10^5 K) wind. For the overall sample, the Si IV and N V line luminosities are correlated with the C IV line luminosities, although the relationship between Si IV and C IV shows large scatter about a linear relationship and suggests that TW Hya, V4046 Sgr, AA Tau, DF Tau, GM Aur, and V1190 Sco are silicon-poor.
The mechanism for jet formation in the disks of T Tauri stars is poorly understood. Observational benchmarks to launching models can be provided by tracing the physical properties of the kinematic components of the wind and jet in the inner 100 au of the disk surface. In the framework of the GHOsT (GIARPS High-resolution Observations of T Tauri stars) project, we aim to perform a multi-line analysis of the velocity components of the gas in the jet acceleration zone. We analyzed the GIARPS-TNG spectra of six objects in the Taurus-Auriga complex (RY Tau, DG Tau, DL Tau, HN Tau, DO Tau, RW Aur A). Thanks to the combined high-spectral resolution (R=50000-115000) and wide spectral coverage (~400-2400 nm) we observed several O, S+, N, N+, and Fe+ forbidden lines spanning a large range of excitation and ionization conditions. In four objects (DG Tau, HN Tau, DO Tau, RW Aur A), temperature (T_e), electron and total density (n_e, n_H), and fractional ionization (x_e) were derived as a function of velocity through an excitation and ionization model. The abundance of gaseous iron, X(Fe), a probe of the dust content in the jet, was derived in selected velocity channels. The physical parameters vary smoothly with velocity, suggesting a common origin for the different kinematic components. In DG Tau and HN Tau, T_e, x_e, and X(Fe) increase with velocity (roughly from 6000 K, 0.05, 10% X(Fe)_sun to 15000 K, 0.6, 90% X(Fe)_sun). This trend is in agreement with disk-wind models in which the jet is launched from regions of the disk at different radii. In DO Tau and RW Aur A, we infer x_e < 0.1, n_H ~10^6-7 cm^-3, and X(Fe) <~ X(Fe)_sun at all velocities. These findings are tentatively explained by the formation of these jets from dense regions inside the inner, gaseous disk, or as a consequence of their high degree of collimation.
We present the results of a variability study of accreting young stellar objects in the Chameleon I star-forming region which is based on ~300 high resolution optical spectra from the multi-object fibre spectrograph FLAMES/GIRAFFE at the ESO/VLT. Twenty five objects with spectral types from G2-M5.75 were observed 12 times over the course of 15 months. Using the emission lines Ha (6562.81 A) and Ca II (8662.1 A) as accretion indicators we found 10 accreting and 15 non-accreting objects. We derived accretion rates for all accretors in the sample using the Ha equivalent width, Ha 10% width and the CaII equivalent width. The mean amplitude of variations in derived accretion rate from Ha equivalent width was ~ 0.37 dex, from Ca II equivalent width ~0.83 dex and from Ha 10% width ~1.11 dex. Based on the large amplitude of variations in accretion rates derived from the Ha 10% width with respect to the other diagnostics, we do not consider it to be a reliable accretion rate estimator. Taking the variations in Ha equivalent width and CaII equivalent width accretion rates to be closer to the true value, they suggest that the spread which has been found around the accretion rate to stellar mass relation is not due to the variability of individual objects on time-scales of weeks to ~1 year. From these variations we can also infer that the accretion rates are stable within < 0.37 dex over time-scales of less than 15 months. A major portion of the accretion variability was found to occur on less than the shortest time-scales in our observations, 8-25 days, which is comparable with the rotation periods of these young stellar objects. This could be an indication that what we are probing is spatial structure in the accretion flows, and also suggests that observations on time-scales of ~a couple of weeks are sufficient to limit the total extent of accretion rate variations in typical young stars.
Aims. In this paper, we aim to measure the strength of the surface magnetic fields for a sample of five intermediate mass T Tauri stars and one low mass T Tauri star from late-F to mid-K spectral types. While magnetic fields of T Tauri stars at the low mass range have been extensively characterized, our work complements previous studies towards the intermediate mass range; this complementary study is key to evaluate how magnetic fields evolve during the transition from a convective to a radiative core. Methods. We studied the Zeeman broadening of magnetically sensitive spectral lines in the H-band spectra obtained with the CRIRES high-resolution near-infrared spectrometer. These data are modelled using magnetic spectral synthesis and model atmospheres. Additional constraints on non-magnetic line broadening mechanisms are obtained from modelling molecular lines in the K band or atomic lines in the optical wavelength region. Results. We detect and measure mean surface magnetic fields for five of the six stars in our sample: CHXR 28, COUP 107, V2062 Oph, V1149 Sco, and Par 2441. Magnetic field strengths inferred from the most magnetically sensitive diagnostic line range from 0.8 to 1.8 kG. We also estimate a magnetic field strength of 1.9 kG for COUP 107 from an alternative diagnostic. The magnetic field on YLW 19 is the weakest in our sample and is marginally detected, with a strength of 0.8 kG. Conclusions. We populate an uncharted area of the pre-main-sequence HR diagram with mean magnetic field measurements from high-resolution near-infrared spectra. Our sample of intermediate mass T Tauri stars in general exhibits weaker magnetic fields than their lower mass counterparts. Our measurements will be used in combination with other spectropolarimetric studies of intermediate mass and lower mass T Tauri stars to provide input into pre-main-sequence stellar evolutionary models.
The high-energy radiation fields of T Tauri stars (TTS) should affect the surrounding circumstellar disk, having implications for disk transport and heating. Yet, observational evidence of the effect of high-energy fields on disks is scarce. Here we investigate the connection between X-ray emission and the innermost gas disk by leveraging the variability of TTS. We obtained multiple epochs of coordinated data (taken either simultaneously or within a few hours) of accreting TTS with the Hubble Space Telescope, the Neil Gehrels Swift Observatory, and the Chandra X-ray Observatory. We measured the far-ultraviolet (FUV) H2 bump feature at 1600 A, which traces gas <1 AU from the star; the near-ultraviolet (NUV) emission, from which we extract the accretion luminosity; and also the X-ray luminosity. We do not find a correlation between the FUV H2 bump and X-ray luminosity. Therefore, an observable tracer of the effect of X-ray ionization in the innermost disk remains elusive. We report a correlation between the FUV H2 bump and accretion luminosity, linking this feature to the disk surface density. We also see a correlation between the X-ray luminosity and the accretion column density, implying that flaring activity may influence accretion. These results stress the importance of coordinated multiwavelength work to understand TTS.
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