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We developed a grid of stellar rotation models for low-mass and solar-type classical T Tauri stars (CTTS; 0.3 M ⊙ < M * < 1.2 M ⊙). These models incorporate the star–disk interaction and magnetospheric ejections to investigate the evolution of the stellar rotation rate as a function of the mass of the star M *, the magnetic field (B *), and stellar wind ( M ̇ wind ). We compiled and determined stellar parameters for 208 CTTS, such as projected rotational velocity v sin (i) , mass accretion rate M ̇ acc , stellar mass M *, ages, and estimated rotational periods using Transiting Exoplanet Survey Satellite (TESS) data. We also estimated a representative value of the mass-loss rate for our sample using the [O i ] λ 6300 spectral line. Our results confirm that v sin (i) measurements in CTTS agree with the rotation rates provided by our spin models in the accretion-powered stellar winds picture. In addition, we used the approximate Bayesian computation technique to explore the connection between the model parameters and the observational properties of CTTS. We find that the evolution of v sin (i) with age might be regulated by variations in (1) the intensity of B * and (2) the fraction of the accretion flow ejected in magnetic winds, removing angular momentum from these systems. The youngest stars in our sample (∼1 Myr) show a median branching ratio M ̇ wind / M ̇ acc ∼ 0.16 and median B * ∼ 2000 G, in contrast to ∼0.01 and 1000 G, respectively, for stars with ages ≳3 Myr. [ABSTRACT FROM AUTHOR]

Here we report the detection of dust depletion in a misaligned inner disk around UX Tau A using JWST MIRI spectra. Mid-infrared (MIR) continuum “seesaw” variability was detected in this disk by Spitzer and attributed to variable shadows cast on the outer disk by the inner disk. The JWST MIRI spectrum of UX Tau A also shows seesaw variability but with a significant decrease of emission shortward of 10 μ m to nearly photospheric levels. We argue that UX Tau A’s MIR continuum variability is due to depletion of dust in a misaligned inner disk. We speculate that this dust depletion occurs because the inner disk is misaligned from the outer disk, which can disrupt the replenishment of the inner disk from the outer disk. Using contemporaneous measurements of the mass accretion rate of UX Tau A and estimating the amount of dust necessary to produce the MIR excess in the Spitzer observations, we estimate a minimum dust depletion timescale of ∼0.1 yr. These observations show that we can indirectly detect the signatures of misaligned inner disks through MIR continuum variability and that in some cases the inner disk may be significantly depleted of dust and become optically thin.

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. [ABSTRACT FROM AUTHOR]

Classical T Tauri stars (TTS) are low-mass pre-main sequence stars with an active circumstellar environment. In this work, we present the identification and study of 260 classical TTS using LAMOST Data Release 6, among which 104 stars are newly identified. We distinguish classical TTS from giants and main-sequence dwarfs based on the log g values, and the presence of H α emission line and infrared excess that arises from the circumstellar accretion disk. We estimated the mass and age of 210 stars using the Gaia color–magnitude diagram. The age is from 0.1 to 20 Myr, where 90% of the stars have age <10 Myr and the mass ranges between 0.11 and 1.9 M ⊙ . From the measured H α equivalent widths, we homogeneously estimated the mass accretion rates for 172 stars, with most values ranging from 10 - 7 to 10 - 10 M ⊙ yr - 1 . The mass accretion rates are found to follow a power law distribution with the mass of the star, having a relation of the form M ˙ acc ∝ M ∗ 1.43 ± 0.26 , in agreement with previous studies. [ABSTRACT FROM AUTHOR]

Investigating the angular momentum evolution of pre-main-sequence (PMS) stars provides important insight into the interactions between Sun-like stars and their protoplanetary disks, and the timescales that govern disk dissipation and planet formation. We present projected rotational velocities ( v sin i values) of 254 T Tauri stars (TTSs) in the ∼3 Myr old open cluster NGC 2264, measured using high-dispersion spectra from the WIYN 3.5 m telescope’s Hydra instrument. We combine these with literature values of temperature, rotation period, luminosity, disk classification, and binarity. We find some evidence that weak-lined TTSs may rotate faster than their classical TTS counterparts and that stars in binary systems may rotate faster than single stars. We also combine our v sin i measurements with rotation period to estimate the projected stellar radii of our sample stars, and then use a maximum likelihood modeling technique to compare our radii estimates to predicted values from stellar evolution models. We find that starspot-free models tend to underestimate the radii of the PMS stars at the age of the cluster, while models that incorporate starspots are more successful. We also observe a mass dependence in the degree of radius inflation, which may be a result of differences in the birth-line location on the HR diagram. Our study of NGC 2264 serves as a pilot study for analysis methods to be applied to four other clusters ranging in age from 1−14 Myr, which is the timescale over which protoplanetary disks dissipate and planetary systems begin to form.

Classical T Tauri stars (CTTSs) are young, low-mass stars that accrete material from their surrounding protoplanetary disk. To better understand accretion variability, we conducted a multiepoch, multiwavelength photometric monitoring campaign of four CTTSs, TW Hya, RU Lup, BP Tau, and GM Aur, in 2021 and 2022, contemporaneous with Hubble Space Telescope UV and optical spectra. We find that all four targets display significant variability in their light curves, generally on days-long timescales (but in some cases year-to-year), often due to periodicity associated with stellar rotation and to stochastic accretion variability. There is a strong connection between mass accretion and photometric variability in all bands, but the relationship varies per target and epoch. Thus, photometry should be used with caution as a direct measure of accretion in CTTSs.

We present MIRI Medium-resolution Spectrograph observations of the large, multi-gapped protoplanetary disk around the T Tauri star AS 209. The observations reveal hundreds of water vapor lines from 4.9–25.5 μ m toward the inner ∼1 au in the disk, including the first detection of rovibrational water emission in this disk. The spectrum is dominated by hot (∼800 K) water vapor and OH gas, with only marginal detections of CO _2 , HCN, and a possible colder water vapor component. Using slab models with a detailed treatment of opacities and line overlap, we retrieve the column density, emitting area, and excitation temperature of water vapor and OH, and provide upper limits for the observable mass of other molecules. Compared to MIRI spectra of other T Tauri disks, the inner disk of AS 209 does not appear to be atypically depleted in CO _2 nor HCN. Based on Spitzer Infrared Spectrograph observations, we further find evidence for molecular emission variability over a 10 yr baseline. Water, OH, and CO _2 line luminosities have decreased by factors of 2–4 in the new MIRI epoch, yet there are minimal continuum emission variations. The origin of this variability is yet to be understood.

Theoretical models indicate that photoevaporative and magnetothermal winds play a crucial role in the evolution and dispersal of protoplanetary disks and affect the formation of planetary systems. However, it is still unclear what wind-driving mechanism is dominant or if both are at work, perhaps at different stages of disk evolution. Recent spatially resolved observations by Fang et al. of the [O i ] 6300 Å spectral line, a common disk wind tracer in TW Hya, revealed that about 80% of the emission is confined to the inner few astronomical units of the disk. In this work, we show that state-of-the-art X-ray-driven photoevaporation models can reproduce the compact emission and the line profile of the [O i ] 6300 Å line. Furthermore, we show that the models also simultaneously reproduce the observed line luminosities and detailed spectral profiles of both the [O i ] 6300 Å and the [Ne ii ] 12.8 μ m lines. While MHD wind models can also reproduce the compact radial emission of the [O i ] 6300 Å line, they fail to match the observed spectral profile of the [O i ] 6300 Å line and underestimate the luminosity of the [Ne ii ] 12.8 μ m line by a factor of 3. We conclude that, while we cannot exclude the presence of an MHD wind component, the bulk of the wind structure of TW Hya is predominantly shaped by a photoevaporative flow.

Previous analyses of mid-infrared water spectra from young protoplanetary disks observed with the Spitzer-IRS found an anticorrelation between water luminosity and the millimeter dust disk radius observed with ALMA. This trend was suggested to be evidence for a fundamental process of inner disk water enrichment proposed decades ago to explain some properties of the solar system, in which icy pebbles drift inward from the outer disk and sublimate after crossing the snow line. Previous analyses of IRS water spectra, however, were uncertain due to the low spectral resolution that blended lines together. We present new JWST-MIRI spectra of four disks, two compact and two large with multiple radial gaps, selected to test the scenario that water vapor inside the snow line is regulated by pebble drift. The higher spectral resolving power of MIRI-MRS now yields water spectra that separate individual lines, tracing upper level energies from 900 to 10,000 K. These spectra clearly reveal excess emission in the low-energy lines in compact disks compared to large disks, demonstrating an enhanced cool component with T ≈ 170–400 K and equivalent emitting radius R _eq ≈ 1–10 au. We interpret the cool water emission as ice sublimation and vapor diffusion near the snow line, suggesting that there is indeed a higher inward mass flux of icy pebbles in compact disks. Observation of this process opens up multiple exciting prospects to study planet formation chemistry in inner disks with JWST.

This work presents ground-based spectrally resolved water emission at R = 30,000–100,000 over infrared wavelengths covered by the JWST (2.9–12.8 μ m). Two new surveys with iSHELL and the VISIR are combined with previous spectra from the CRIRES to cover parts of multiple rovibrational and rotational bands observable within telluric transmission bands, for a total of ≈160 spectra and 85 disks (30 of which are JWST targets in Cycle 1). The general expectation of a range of regions and excitation conditions traced by infrared water spectra is for the first time supported by the combined kinematics and excitation as spectrally resolved at multiple wavelengths. The main findings from this analysis are: (1) water lines are progressively narrower from the rovibrational bands at 2–9 μ m to the rotational lines at 12 μ m, and partly match broad and narrow emission components, respectively, as extracted from rovibrational CO spectra; (2) rotation diagrams of resolved water lines from upper-level energies of 4000–9500 K show vertical spread and curvatures indicative of optically thick emission (≈10 ^18 cm ^−2 ) from a range of excitation temperatures (≈800–1100 K); and (3) the new 5 μ m spectra demonstrate that slab model fits to the rotational lines at >10 μ m strongly overpredict the rovibrational emission bands at

Vera C. Rubin Observatory, through the Legacy Survey of Space and Time (LSST), will allow us to derive a panchromatic view of variability in young stellar objects (YSOs) across all relevant timescales. Indeed, both short-term variability (on timescales of hours to days) and long-term variability (months to years), predominantly driven by the dynamics of accretion processes in disk-hosting YSOs, can be explored by taking advantage of the multiband filters option available in Rubin LSST, in particular the u , g , r , i filters that enable us to discriminate between photospheric stellar properties and accretion signatures. The homogeneity and depth of sky coverage that will be achieved with LSST will provide us with a unique opportunity to characterize the time evolution of disk accretion as a function of age and varying environmental conditions (e.g., field crowdedness, massive neighbors, metallicity) by targeting different star-forming regions. In this contribution to the Rubin LSST Survey Strategy Optimization Focus Issue, we discuss how implementing a dense observing cadence to explore short-term variability in YSOs represents a key complementary effort to the Wide–Fast–Deep observing mode that will be used to survey the sky over the full duration of the main survey (≈10 yr). The combination of these two modes will be vital to investigate the connection between the inner-disk dynamics and longer-term eruptive variability behaviors, such as those observed on EX Lupi–type objects.

T Tauri stars produce broad Ly α emission lines that contribute ∼88% of the total UV flux incident on the inner circumstellar disks. Ly α photons are generated at the accretion shocks and in the protostellar chromospheres and must travel through accretion flows, winds, and jets, the protoplanetary disks, and the interstellar medium before reaching the observer. This trajectory produces asymmetric, double-peaked features that carry kinematic and opacity signatures of the disk environments. To understand the link between the evolution of Ly α emission lines and the disks themselves, we model HST-COS spectra from targets included in Data Release 3 of the Hubble UV Legacy Library of Young Stars as Essential Standards program. We find that resonant scattering in a simple spherical expanding shell is able to reproduce the high-velocity emission line wings, providing estimates of the average velocities within the bulk intervening H i . The model velocities are significantly correlated with the K -band veiling, indicating a turnover from Ly α profiles absorbed by outflowing winds to emission lines suppressed by accretion flows as the hot inner disk is depleted. Just 30% of targets in our sample have profiles with redshifted absorption from accretion flows, many of which have resolved dust gaps. At this stage, Ly α photons may no longer intersect with disk winds along the path to the observer. Our results point to a significant evolution of Ly α irradiation within the gas disks over time, which may lead to chemical differences that are observable with ALMA and JWST.

PDS 70 is so far the only young disk where multiple planets have been detected by direct imaging. The disk has a large cavity when seen at submillimeter and near-infrared wavelengths, which hosts two massive planets. This makes PDS 70 the ideal target to study the physical conditions in a strongly depleted inner disk shaped by two giant planets, and in particular to test whether disk winds can play a significant role in its evolution. Using X-Shooter and HARPS spectra, we detected for the first time the wind-tracing [O i ] 6300 Å line, and confirm the low-moderate value of mass-accretion rate in the literature. The [O i ] line luminosity is high with respect to the accretion luminosity when compared to a large sample of disks with cavities in nearby star-forming regions. The FWHM and blueshifted peak of the [O i ] line suggest an emission in a region very close to the star, favoring a magnetically driven wind as the origin. We also detect wind emission and high variability in the He i 10830 Å line, which is unusual for low accretors. We discuss that, although the cavity of PDS 70 was clearly carved out by the giant planets, the substantial inner-disk wind could also have had a significant contribution to clearing the inner disk.

EX Lup is the archetype for the class of young stars that undergoes repeated accretion outbursts of ∼5 mag at optical wavelengths that last for months. Despite extensive monitoring that dates back 130 yr, the accretion history of EX Lup remains mostly qualitative and has large uncertainties. We assess historical accretion rates of EX Lup by applying correlations between optical brightness and accretion, developed on multi-band magnitude photometry of the ∼2 mag optical burst in 2022. Two distinct classes of bursts occur: major outbursts (Δ V ∼ 5 mag) have year-long durations, are rare, reach accretion rates of ${\dot{M}}_{\mathrm{acc}}\sim {10}^{-7}$ M _⊙ yr ^−1 at peak, and have a total accreted mass of around 0.1 Earth mass. The characteristic bursts (Δ V ∼ 2 mag) have durations of ∼2–3 months, are more common, reach accretion rates of ${\dot{M}}_{\mathrm{acc}}\sim {10}^{-8}$ M _⊙ yr ^−1 at peak, and have a total accreted mass of around 10 ^−3 Earth masses. The distribution of total accreted mass in the full set of bursts is poorly described by a power law, which suggests different driving causes behind the major outburst and characteristic bursts. The total mass accreted during two classes of bursts is around 2 times the masses accreted during quiescence. Our analysis of the light curves reveals a color-dependent time lag in the 2022 post-burst light curve, attributed to the presence of both hot and cool spots on the stellar surface.

We present the analysis of 16 classical T Tauri stars (CTTSs) using LAMOST and TESS data, investigating spectral properties, photometric variations, and mass accretion rates. All 16 stars exhibit emissions in H α lines, from which the average mass accretion rate of 1.76 × 10 ^−9 M _⊙ yr ^−1 is derived. Two of the stars, DL Tau and Haro 6-13, show mass accretion bursts simultaneously in TESS, ASAS-SN, and/or the ZTF survey. Based on these observations, we find that the mass accretion rates of DL Tau and Haro 6-13 reach their maxima of 2.5 × 10 ^−8 M _⊙ yr ^−1 and 2 × 10 ^−10 M _⊙ yr ^−1 , respectively, during the TESS observation. We detect 13 flares among these stars. The flare frequency distribution shows that the CTTSs’ flare activity is not only dominated by strong flares with high energy but also much more active than those of solar-type and young low-mass stars. By comparing the variability classes reported in the literature, we find that the transition timescale between different classes of variability in CTTSs, such as from stochastic (S) to bursting (B) or from quasi-periodic symmetric to quasi-periodic dipping, may range from 1.6 to 4 yr. We observe no significant correlation between inclination and mass accretion rates derived from the emission indicators. This suggests that inner disk properties may be more important than those of outer disks. Finally, we find a relatively significant positive correlation between the asymmetric metric M and the cold disk inclination compared to the literature. A weak negative correlation between the periodicity metric Q value and inclination has also been found.

Accretion rates ( $\dot{M}$ ) of young stars show a strong correlation with object mass ( M ); however, extension of the $\dot{M}\mbox{--}M$ relation into the substellar regime is less certain. Here, we present the Comprehensive Archive of Substellar and Planetary Accretion Rates (CASPAR), the largest compilation to date of substellar accretion diagnostics. CASPAR includes: 658 stars, 130 brown dwarfs, and 10 bound planetary mass companions. In this work, we investigate the contribution of methodological systematics to scatter in the $\dot{M}\mbox{--}M$ relation and compare brown dwarfs to stars. In our analysis, we rederive all quantities using self-consistent models, distances, and empirical line flux to accretion luminosity scaling relations to reduce methodological systematics. This treatment decreases the original 1 σ scatter in the $\mathrm{log}\dot{M}\mbox{--}\mathrm{log}M$ relation by ∼17%, suggesting that it makes only a small contribution to the dispersion. The CASPAR rederived values are best fit by $\dot{M}\propto {M}^{2.02\pm 0.06}$ from 10 M _J to 2 M _⊙ , confirming previous results. However, we argue that the brown-dwarf and stellar populations are better described separately and by accounting for both mass and age. Therefore, we derive separate age-dependent $\dot{M}\mbox{--}M$ relations for these regions and find a steepening in the brown-dwarf $\dot{M}\mbox{--}M$ slope with age. Within this mass regime, the scatter decreases from 1.36 dex to 0.94 dex, a change of ∼44%. This result highlights the significant role that evolution plays in the overall spread of accretion rates, and suggests that brown dwarfs evolve faster than stars, potentially as a result of different accretion mechanisms.

Very young ( t ≲ 10 Myr) stars possess strong magnetic fields that channel ionized gas from the interiors of their circumstellar disks to the surface of the star. Upon impacting the stellar surface, the shocked gas recombines and emits hydrogen spectral lines. To characterize the density and temperature of the gas within these accretion streams, we measure equivalent widths of Brackett (Br) 11–20 emission lines detected in 1101 APOGEE spectra of 326 likely pre-main-sequence accretors. For sources with multiple observations, we measure median epoch-to-epoch line strength variations of 10% in Br11 and 20% in Br20. We also fit the measured line ratios to predictions of radiative transfer models by Kwan & Fischer. We find characteristic best-fit electron densities of n _e = 10 ^11 –10 ^12 cm ^−3 , and excitation temperatures that are inversely correlated with electron density (from T ∼ 5000 K for n _e ∼ 10 ^12 cm ^−3 to T ∼ 12,500 K at n _e ∼ 10 ^11 cm ^−3 ). These physical parameters are in good agreement with predictions from modeling of accretion streams that account for the hydrodynamics and radiative transfer within the accretion stream. We also present a supplementary catalog of line measurements from 9733 spectra of 4255 Brackett emission-line sources in the APOGEE Data Release 17 data set.