Jeffrey S. Bary

Detections of Rovibrational H2 Emission from the Disks of T Tauri Stars

We report the detection of quiescent H_2 emission in the v=1-0 S(1) line at 2.12183 um in the circumstellar environment of two classical T Tauri stars, GG Tau A and LkCa 15, in high-resolution (R 60,000) spectra, bringing to four, including TW Hya and the weak-lined T Tauri star DoAr 21, the number of T Tauri stars showing such emission. The equivalent widths of the H_2 emission line lie in the range 0.05-0.10 \AA and, in each case, the central velocity of the emission line is centered at the stars systemic velocity. The line widths range from 9 to 14 km s^{-1}, in agreement with those expected from gas in Keplerian orbits in circumstellar disks surrounding K-type stars at distances > 10 AU from the sources. We suggest that UV fluorescence and X-ray heating are likely candidates responsible for producing the observed emission. We present mass estimates from the measured line intensities and show that the estimated masses are consistent with those expected from the possible mechanisms responsible for stimulating the observed emission. The high temperatures and low densities required for significant emission in the v=1-0 S(1) line suggests that we have detected reservoirs of hot H_2 gas located in the low density, upper atmospheres of circumstellar disks of these stars.

DETECTION OF QUIESCENT MOLECULAR HYDROGEN GAS IN THE CIRCUMSTELLAR DISK OF A CLASSICAL T TAURI STAR

We have detected emission from the quiescent, molecular hydrogen gas in the disk around the X-ray bright, classical T Tauri star TW Hya in the 1¨0 S(1) line at 2.1218 km through high-resolution spectros- copy using CSHELL on NASAs Infrared Telescope Facility. The brightness of the observed emission line is consistent with that predicted from models for X-ray excitation of the 1¨0 S(1) line of This H 2 . result demonstrates that X-ray ionization is a plausible mechanism for excitation of the in the H 2 gaseous disks of T Tauri stars. In addition, from these high-resolution spectra we have measured the radial velocities of three T Tauri stars in the TW Hya association, CD (33i7795, HD 98800, and TW Hya. We —nd that the radial velocities of these three stars are very similar to each other and to those of at least three other stars presumed to be members of the TW Hya association, Hen 600A, Hen 600B, and CD (29i8887. This result, combined with the similar proper motions of all six of these stars, lends support to the hypothesis that all of these stars share a common origin in a now-dispersed molecular cloud. Subject headings: infrared: starsopen clusters and associations: individual (TW Hydrae, HD 98800, CD (33i7795) ¨ stars: premain-sequenceX-rays: stars

Quiescent H2 Emission From Pre-Main-Sequence Stars in Chamaeleon I

We report the discovery of quiescent emission from molecular hydrogen gas located in the circumstellar disks of six pre-main sequence stars, including two weak-line TTS, and one Herbig AeBe star, in the Chamaeleon I star forming region. For two of these stars, we also place upper limits on the 2 → 1 S(1)/1 → 0 S(1) line ratios of ∼ 0.4 and 0.5. Of the 11 pre-main sequence sources now known to be sources of quiescent near-infrared hydrogen emission, four possess transitional disks, which suggests that detectable levels of H2 emission and the presence of inner disk holes are correlated. These H2 detections demonstrate that these inner holes are not completely devoid of gas, in agreement with the presence of observable accretion signatures for all four of these stars and the recent detections of [Ne ii] emission from three of them. The overlap in [Ne ii] and H2 detections hints at a possible correlation between these two features and suggests a shared excitation mechanism of high energy photons. Our models, combined with the kinematic information from the H2 lines, locate the bulk of the emitting gas at a few tens of AU from the stars. We also find a correlation between H2 detections ··· ···

Physical Conditions of Accreting Gas in T Tauri Star Systems

We present results from a low-resolution ( -->R 300) near-infrared spectroscopic variability survey of actively accreting T Tauri stars (TTSs) in the Taurus-Auriga star-forming region. Paschen and Brackett series H I recombination lines were detected in 73 spectra of 15 classical T Tauri systems. The values of the Pa -->nup/Paβ, Br -->nup/Brγ, and Brγ/Pa -->nup H I line ratios for all observations exhibit a scatter of 20% about the weighted mean, not only from source to source, but also for epoch-to-epoch variations in the same source. A representative or global value was determined for each ratio in both the Paschen and Brackett series, as well as the Brγ/Pa -->nup line ratios. A comparison of observed line ratio values was made to those predicted by the temperature- and electron density-dependent models of case B hydrogen recombination line theory. The measured line ratios are statistically well fit by a tightly constrained range of temperatures ( -->T 2000 K) and electron densities ( -->109 cm −3 < ne 1010 cm−3). A comparison of the observed line ratio values to the values predicted by the optically thick and thin local thermodynamic equilibrium cases rules out these conditions for the emitting H I gas. Therefore, the emission is consistent with having an origin in a non-LTE recombining gas. While the range of electron densities is consistent with the gas densities predicted by existing magnetospheric accretion models, the temperature range constrained by the case B comparison is considerably lower than that expected for accreting gas. The cooler gas temperatures will require a nonthermal excitation process (e.g., coronal/accretion-related X-rays and UV photons) to power the observed line emission.

Possible planet formation in the young, low-mass, multiple stellar system GG Tau A.

The formation of planets around binary stars may be more difficult than around single stars. In a close binary star (with a separation of less than a hundred astronomical units), theory predicts the presence of circumstellar disks around each star, and an outer circumbinary disk surrounding a gravitationally cleared inner cavity around the stars. Given that the inner disks are depleted by accretion onto the stars on timescales of a few thousand years, any replenishing material must be transferred from the outer reservoir to fuel planet formation (which occurs on timescales of about one million years). Gas flowing through disk cavities has been detected in single star systems. A circumbinary disk was discovered around the young low-mass binary system GG Tau A (ref. 7), which has recently been shown to be a hierarchical triple system. It has one large inner disk around the single star, GG Tau Aa, and shows small amounts of shocked hydrogen gas residing within the central cavity, but other than a single weak detection, the distribution of cold gas in this cavity or in any other binary or multiple star system has not hitherto been determined. Here we report imaging of gas fragments emitting radiation characteristic of carbon monoxide within the GG Tau A cavity. From the kinematics we conclude that the flow appears capable of sustaining the inner disk (around GG Tau Aa) beyond the accretion lifetime, leaving time for planet formation to occur there. These results show the complexity of planet formation around multiple stars and confirm the general picture predicted by numerical simulations.

DETECTION OF MOLECULAR HYDROGEN ORBITING A "NAKED" T TAURI STAR

Astronomers have established that for a few million years, newborn stars possess disks of orbiting gas and dust. Such disks, which are likely sites of planet formation, appear to disappear once these stars reach ages of yr; yet, ≥10 7 yr is thought necessary for giant planet formation. If disks dissipate in less time than 6 (5–10) # 10 is needed for giant planet formation, such planets may be rare and those known around nearby stars would be anomalies. Here we report the discovery of H2 gas orbiting a weak-lined T Tauri star heretofore presumed nearly devoid of circumstellar material. We estimate that a significant amount of H 2 persists in the gas phase, but only a tiny fraction of this mass emits in the near-infrared. We propose that this star possesses an evolved disk that has escaped detection thus far because much of the dust has coagulated into planetesimals. This discovery suggests that the theory that disks are largely absent around such stars should be reconsidered. The widespread presence of such disks would indicate that planetesimals can form quickly and giant planet formation can proceed to completion before the gas in circumstellar disks disperses. Subject headings: circumstellar matter — infrared: stars — solar system: formation — stars: individual (DoAr 21) — stars: pre–main-sequence

Variations of the 10 μm Silicate Features in the Actively Accreting T Tauri Stars: DG Tau and XZ Tau

Using the Infrared Spectrograph aboard the Spitzer Space Telescope, we observed multiple epochs of 11 actively accreting T Tauri stars in the nearby Taurus-Auriga star-forming region. In total, 88 low-resolution mid-infrared spectra were collected over 1.5 years in Cycles 2 and 3. The results of this multi-epoch survey show that the 10 μm silicate complex in the spectra of two sources—DG Tau and XZ Tau—undergoes significant variations with the silicate feature growing both weaker and stronger over month- and year-long timescales. Shorter timescale variations on day- to week-long timescales were not detected within the measured flux errors. The time resolution coverage of this data set is inadequate for determining if the variations are periodic. Pure emission compositional models of the silicate complex in each epoch of the DG Tau and XZ Tau spectra provide poor fits to the observed silicate features. These results agree with those of previous groups that attempted to fit only single-epoch observations of these sources. Simple two-temperature, two-slab models with similar compositions successfully reproduce the observed variations in the silicate features. These models hint at a self-absorption origin of the diminution of the silicate complex instead of a compositional change in the population of emitting dust grains. We discuss several scenarios for producing such variability including disk shadowing, vertical mixing, variations in disk heating, and disk wind events associated with accretion outbursts.

GG Tauri : the fifth element

We aim at unveiling the observational imprint of physical mechanisms that govern planetary formation in young, multiple systems. In particular, we investigate the impact of tidal truncation on the inner circumstellar disks. We observed the emblematic system GG Tau at high-angular resolution: a hierarchical quadruple system composed of low-mass T Tauri binary stars surrounded by a well-studied, massive circumbinary disk in Keplerian rotation. We used the near-IR 4-telescope combiner PIONIER on the VLTI and sparse-aperture-masking techniques on VLT/NaCo to probe this proto-planetary system at sub-AU scales. We report the discovery of a significant closure-phase signal in H and Ks bands that can be reproduced with an additional low-mass companion orbiting GG Tau Ab, at a (projected) separation rho = 31.7 +/- 0.2mas (4.4 au) and PA = 219.6 +/- 0.3deg. This finding offers a simple explanation for several key questions in this system, including the missing-stellar-mass problem and the asymmetry of continuum emission from the inner dust disks observed at millimeter wavelengths. Composed of now five co-eval stars with 0.02 <= Mstar <= 0.7 Msun, the quintuple system GG Tau has become an ideal test case to constrain stellar evolution models at young ages (few 10^6yr).

ANOMALOUS ACCRETION ACTIVITY AND THE SPOTTED NATURE OF THE DQ TAU BINARY SYSTEM

We report the detection of an anomalous accretion flare in the tight eccentric pre-main-sequence binary system DQ Tau. In a multi-epoch survey consisting of randomly acquired low to moderate resolution near-infrared spectra obtained over a period of almost ten years, we detect a significant and simultaneous brightening of four standard accretion indicators (CaII infrared triplet, the Paschen and Brackett series HI lines, and HeI 1.083 um), on back-to-back nights (phase = 0.372, 0.433) with the flare increasing in strength as the system approached apastron (phase = 0.5). The mass accretion rate measured for the anomalous flare is nearly an order of magnitude stronger than the average quiescent rate. While previous observations established that frequent, periodic accretion flares phased with periastron passages occur in this system, these data provide evidence that orbitally-modulated accretion flares occur near apastron, when the stars make their closest approach to the circumbinary disk. The timing of the flare suggests that this outburst is due to interactions of the stellar cores (or the highly truncated circumstellar disks) with material in non-axisymmetric structures located at the inner edge of the circumbinary disk. We also explore the optical/infrared spectral type mismatch previously observed for T Tauri stars and successfully model the shape of the spectra from 0.8 to 1.0 um and the strengths of the TiO and FeH bands as manifestations of large cool spots on the surfaces of the stellar companions in DQ Tau. These findings illustrate that a complete model of near-infrared spectra of many T Tauri stars must include parameters for spot filling factors and temperatures.

First detection of H2S in a protoplanetary disk: The dense GG Tauri A ring

Studying molecular species in protoplanetary disks is very useful to characterize the properties of these objects, which are the site of planet formation. We attempt to constrain the chemistry of S-bearing molecules in the cold parts of circumstellar disk of GG Tau A. We searched for H