In accreting young stars, one of the prominent spectral features in the near-infrared is the Paschen and Brackett series in emission. We examine hydrogen line ratios for 16 classical T Tauri stars ...from SpeX spectra and assess the trends with veiling and accretion. The observed line ratios are compared with two theoretical models for line formation: (1) Baker & Menzel's Case B for radiative ionization and recombination and (2) a set of local line excitation calculations designed to replicate the conditions in T Tauri winds and magnetic accretion columns (KF). While the comparison between Case B and observed line ratios implies a wide range in electron density and temperature among the hydrogen line formation regions in T Tauri stars, the predictions of the local line excitation models give consistent results across multiple diagnostics. Under the assumptions of the local line excitation calculations, we find that n sub(H) in the hydrogen line formation region is constrained to 2 x 10 super(10)-2 x 10 super(11) cm super(-3), where stars with higher accretion rates have densities at the higher end of this range. Because of uncertainties in extinction, temperature is not well delineated, but falls within the range expected for collisional excitation to produce the line photons. We introduce new diagnostics for assessing extinction based on near-infrared hydrogen line ratios from the local line excitation calculations.
Interstellar reddening corrections are necessary to reconstruct the intrinsic spectral energy distributions (SEDs) of accreting protostellar systems. The stellar SED determines the heating and ...chemical processes that can occur in circumstellar disks. Measurement of neutral hydrogen absorption against broad Ly alpha emission profiles in young stars can be used to obtain the total H I column density (N(H I)) along the line of sight. We measure N(H I) with new and archival ultraviolet observations from the Hubble Space Telescope (HST) of 31 classical T Tauri and Herbig Ae/Be stars. The H I column densities range from log sub(10)(N(H I)) approximately 19.6-21.1, with corresponding visual extinctions of A sub(V) = 0.02-0.72 mag, assuming an R sub(V) of 3.1. We find that the majority of the H I absorption along the line of sight likely comes from interstellar rather than circumstellar material. Extinctions derived from new HST blue-optical spectral analyses, previous IR and optical measurements, and new X-ray column densities on average overestimate the interstellar extinction toward young stars compared to the N(H I) values by ~0.6 mag. We discuss possible explanations for this discrepancy in the context of a protoplanetary disk geometry.
Abstract
The Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Director’s Discretionary Program of low-mass pre-main-sequence stars, coupled with forthcoming data from Atacama ...Large Millimeter/submillimeter Array and James Webb Space Telescope, will provide the foundation to revolutionize our understanding of the relationship between young stars and their protoplanetary disks. A comprehensive evaluation of the physics of disk evolution and planet formation requires understanding the intricate relationships between mass accretion, mass outflow, and disk structure. Here we describe the Outflows and Disks around Young Stars: Synergies for the Exploration of ULLYSES Spectra (ODYSSEUS) Survey and present initial results of the classical T Tauri Star CVSO 109 in Orion OB1b as a demonstration of the science that will result from the survey. ODYSSEUS will analyze the ULLYSES spectral database, ensuring a uniform and systematic approach in order to (1) measure how the accretion flow depends on the accretion rate and magnetic structures, (2) determine where winds and jets are launched and how mass-loss rates compare with accretion, and (3) establish the influence of FUV radiation on the chemistry of the warm inner regions of planet-forming disks. ODYSSEUS will also acquire and provide contemporaneous observations at X-ray, optical, near-IR, and millimeter wavelengths to enhance the impact of the ULLYSES data. Our goal is to provide a consistent framework to accurately measure the level and evolution of mass accretion in protoplanetary disks, the properties and magnitudes of inner-disk mass loss, and the influence of UV radiation fields that determine ionization levels and drive disk chemistry.
HOT GAS LINES IN T TAURI STARS Ardila, David R; Herczeg, Gregory J; Gregory, Scott G ...
The Astrophysical journal. Supplement series,
07/2013, Volume:
207, Issue:
1
Journal Article
Peer reviewed
Open access
For Classical T Tauri Stars (CTTSs), the resonance doublets of Nv, Si IV, and C IV, as well as the He II 1640 A line, trace hot gas flows and act as diagnostics of the accretion process. In this ...paper we assemble a large high-resolution, high-sensitivity data set of these lines in CTTSs and Weak T Tauri Stars (WTTSs). The sample comprises 35 stars: 1 Herbig Ae star, 28 CTTSs, and 6 WTTSs. For the overall sample, the Si IV and Nv 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, while CV Cha, DX Cha, RU Lup, and RW Aur may be silicon-rich.
Abstract
Accretion plays a central role in the physics that governs the evolution and dispersal of protoplanetary disks. The primary goal of this paper is to analyze the stability over time of the ...mass accretion rate onto TW Hya, the nearest accreting solar-mass young star. We measure veiling across the optical spectrum in 1169 archival high-resolution spectra of TW Hya, obtained from 1998–2022. The veiling is then converted to accretion rate using 26 flux-calibrated spectra that cover the Balmer jump. The accretion rate measured from the excess continuum has an average of 2.51 × 10
−9
M
⊙
yr
−1
and a Gaussian distribution with an FWHM of 0.22 dex. This accretion rate may be underestimated by a factor of up to 1.5 because of uncertainty in the bolometric correction and another factor of 1.7 because of excluding the fraction of accretion energy that escapes in lines, especially Ly
α
. The accretion luminosities are well correlated with He line luminosities but poorly correlated with H
α
and H
β
luminosity. The accretion rate is always flickering over hours but on longer timescales has been stable over 25 years. This level of variability is consistent with previous measurements for most, but not all, accreting young stars.
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 H Delta *a and He I Delta *l10830 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 X 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 have observed five classical T Tauri stars known to have strong forbidden-line emission with STIS in slitless mode on the Hubble Space Telescope. This technique makes it possible to image jets ...within a few tens of AU of their exciting sources, a region of great interest for models of accretion disks and jets. Slitless images generate emission-line images at all wavelengths, including those where no narrowband filters exist. Images of the forbidden-line regions around each object, constructed by subtracting the stellar continuum and combining observations taken at different orientations, show O I jets from CW Tau, HN Tau, UZ Tau E, DF Tau, and the primary of DD Tau. Jets exist on both sides of the close binary DF Tau, either as a jet and its counterjet or as separate jets from the primary and secondary. Several emission lines not previously seen in jets close to the star exist in the HN Tau jet; the F IIE lambda7155/lambda8617 ratio is particularly useful because it measures the electron density in the densest regions of stellar jets, where log N sub(e) > similar to 6. Electron densities in the inner 30 AU of the HN Tau jet range from log N sub(e) = 6.2 to 6.9. We construct diagnostic diagrams for the density, temperature, and ionization fraction in jets close to their stars, using various emission lines of O I and O II. The red auroral O II lines are bright close to HN Tau, indicating that the emitting regions of the inner 35 AU of the jet have a substantial ionized component-20% if the emission comes from a shock and 50% for an isothermal flow. We discuss mass-loss rates and filling factors for these two cases. The intensity of the HN Tau jet in O I lambda6300 declines exponentially with distance beyond approx15 AU. The superior continuum subtraction with slitless data, as compared with narrowband images, makes it possible to resolve the widths of jets at distances as close as approx15 AU from the star. The two best examples, HN Tau and UZ Tau E, have jets that expand with distance. When projected back to the source, the width of the jet in HN Tau is a few AU at the 3 capital sigma level, while the jet in UZ Tau E is spatially unresolved. The new images of CW Tau reveal proper motions in this jet, which has ejected at least two knots since 1980. There is no indication that CW Tau brightened when it ejected the largest of these knots, but the photometric record of this star over the last two decades is fragmentary.
The role of the star-disk interaction region in launching the high velocity component of accretion-driven outflows is examined. Spectroscopic indicators of high velocity inner winds have been ...recognized in T Tauri stars for decades, but identifying the wind launch site and the accompanying mass loss rates has remained elusive. A promising new diagnostic is He I λ10830, whose metastable lower level results in a powerful probe of the geometry of the outflowing gas in the interaction region. This, together with other atomic and molecular spectral diagnostics covering a wide range of excitation and ionization states, suggests that more than one launch site of the innermost wind is operational in most accreting stars.
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