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Classical T Tauri stars are low-mass objects, which are still accreting material from the surrounding circumstellar disk. The accretion process is essential in the formation of Sun-like stars and in setting the properties of the disk at the time when planet formation occurs. We constructed a complex dataset in order to examine the accretion process of VW Cha, a classical T Tauri multiple system with the aim of studying the physical origin of the photometric and spectroscopic variability of the system. The TESS Space Telescope observed VW Cha between 2019 April 22 and June 19, and we complemented these data with contemporaneous ground-based $I_CJHK$ band photometric measurements. In addition, we obtained high-resolution optical spectra with the VLT/ESPRESSO and the 2.2\,m/FEROS instruments. Analyzing these data, we found that the TESS light curve shows photometric variations on timescales from minutes to weeks with a peak-to-peak amplitude of $\sim$0.8 mag. The near-infrared light curves follow the shape of the optical measurements, however, the peak-to-peak amplitudes are slightly increasing towards the longer wavelengths. We took spectra in both fainter and brighter photometric states of the system, allowing us to examine the origin of a photometric brightening event. Our results show that this brightening event can be explained by increased accretion. In addition, our spectroscopic data also suggest that the primary component of VW Cha is a spectroscopic binary, as it was proposed in earlier works., Accepted for publication in ApJ. 18 pages, 10 figures

Light curves of young star systems show photometric variability due to different kinematic and physical processes. One of the main contributors to the photometric variability is the changing mass accretion rate, which regulates the interplay between the forming young star and the protoplanetary disk. We collected high-resolution spectroscopy in eight different epochs, as well as ground-based and space-borne multiepoch optical and infrared photometry of WX Cha, an M0 binary system, with an almost edge-on disk (i = 87°) in the Chamaeleon I star-forming region. Spectroscopic observations cover 72 days, the ground-based optical monitoring covers 42 days while space-borne TESS photometry extends for 56 days. The multiwavelength light curves exhibit quasi-periodic variability of 0.35–0.53 mag in the near-infrared, and of 1.3 mag in the g band. We studied the variability of selected emission lines that trace the accretion, computed the accretion luminosity and the mass accretion rate using empirical relations, and obtained values between L acc ∼ 1.6 L ⊙ − 3.2 L ⊙ and M ̇ acc ∼ 3.31 × 10 − 7 M ⊙ yr − 1 − 7.76 × 10 − 7 M ⊙ yr−1. Our results show that WX Cha is accreting at a rate larger than what is typical for T Tauri stars in the same star-forming region with the same stellar parameters. We theorize that this is due to the higher disk mass of WX Cha than what is usual for stars with similar stellar mass and to the binary nature of the system. Daily changes in the accretion luminosity and in the extinction can explain the photometric variability.

The goal of the present study is to construct, test, and validate a high-resolution synthetic spectral library using PHOENIX model atmospheres and develop a reliable tool to estimate stellar parameters from high-resolution optical and/or near-infrared spectra of M dwarfs. We report here the preliminary results of tests characterizing main sequence G–K stars from high-resolution spectra. We anchored the atomic line-list using the stellar standards Sun, ξ Boo A, and ϵ Eri to ensure the synthetic spectra computed with PHOENIX reproduce their observed counterparts. These stars were chosen because their parameters are very well characterized, and on which the absolute accuracy of our method depends on. We successfully estimated the stellar parameters with associated error bars for 17 stars. Using a pseudo Monte Carlo statistical analysis, we present overall improved uncertainties on the stellar parameters compared to those in the literature (on average 9 K, 0.014 dex, and 0.008 dex for the effective temperature, the surface gravity, and the metallicity, respectively). Our estimated stellar parameters are also in good agreement with values found in the literature.

We present surface magnetic field maps of the two accreting T Tauri stars, CV Cha and CR Cha. Our magnetic field maps show evidence for strong, complex multi-polar fields similar to those obtained on young rapidly rotating main sequence stars. Both CV Cha and CR Cha show magnetic field patterns that are more complex than those recovered for the lower mass, fully convective T Tauri star, BP Tau.By comparing our maps with previously published maps of classical T Tauri stars, we infer that magnetic field patterns on T Tauris, and their underlying magnetic field generation mechanisms evolve quickly as they develop radiative cores. This may have implications for the efficiency with which T Tauri stars can effectively lock onto their surrounding disks under the magnetospheric accretion model scenario. This, in turn, has implications for the angular momentum evolution of T Tauri stars as they evolve towards the main sequence.

Young stars surrounded by disks with very low mass accretion rates are likely in the final stages of inner disk evolution and therefore particularly interesting to study. We present ultraviolet (UV) observations of the ~5-9 Myr old stars RECX-1 and RECX-11, obtained with the Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph on the Hubble Space Telescope, as well as optical and near-infrared spectroscopic observations. The two stars have similar levels of near-UV emission, although spectroscopic evidence indicates that RECX-11 is accreting and RECX-1 is not. The line profiles of Hα and He I λ10830 in RECX-11 show both broad and narrow redshifted absorption components that vary with time, revealing the complexity of the accretion flows. We show that accretion indicators commonly used to measure mass accretion rates, e.g., U-band excess luminosity or the Ca II triplet line luminosity, are unreliable for low accretors, at least in the middle K spectral range. Using RECX-1 as a template for the intrinsic level of photospheric and chromospheric emission, we determine an upper limit of 3 × 10^(–10) M_☉ yr–1 for RECX-11. At this low accretion rate, recent photoevaporation models predict that an inner hole should have developed in the disk. However, the spectral energy distribution of RECX-11 shows fluxes comparable to the median of Taurus in the near-infrared, indicating that substantial dust remains. Fluorescent H2 emission lines formed in the innermost disk are observed in RECX-11, showing that gas is present in the inner disk, along with the dust.

We exploit the high sensitivity and moderate spectral resolution of the Hubble Space Telescope Cosmic Origins Spectrograph to detect far-ultraviolet (UV) spectral features of carbon monoxide (CO) present in the inner regions of protoplanetary disks for the first time. We present spectra of the classical T Tauri stars HN Tau, RECX-11, and V4046 Sgr, representative of a range of CO radiative processes. HN Tau shows CO bands in absorption against the accretion continuum. The CO absorption most likely arises in warm inner disk gas. We measure a CO column density and rotational excitation temperature of N(CO) = (2 ± 1) × 1017 cm−2 and T rot(CO) 500 ± 200 K for the absorbing gas. We also detect CO A–X band emission in RECX-11 and V4046 Sgr, excited by UV line photons, predominantly H i Lyα. All three objects show emission from CO bands at λ > 1560 Å, which may be excited by a combination of UV photons and collisions with non-thermal electrons. In previous observations these emission processes were not accounted for due to blending with emission from the accretion shock, collisionally excited H2, and photo-excited H2, all of which appeared as a “continuum” whose components could not be separated. The CO emission spectrum is strongly dependent upon the shape of the incident stellar Lyα emission profile. We find CO parameters in the range: N(CO) ∼ 1018–1019 cm−2, T rot(CO) ≳ 300 K for the Lyα-pumped emission. We combine these results with recent work on photo-excited and collisionally excited H2 emission, concluding that the observations of UV-emitting CO and H2 are consistent with a common spatial origin. We suggest that the CO/H2 ratio (≡ N(CO)/N(H2)) in the inner disk is ∼1, a transition between the much lower interstellar value and the higher value observed in solar system comets today, a result that will require future observational and theoretical study to confirm.