arXiv subjects

Gregory J. Herczeg

Publications and source records attributed to Gregory J. Herczeg.

At least 145 records · Page 8Linked to original sources

arXiv2013

Probing the Inner Regions of Protoplanetary Disks with CO Absorption Line Spectroscopy

Carbon monoxide (CO) is the most commonly used tracer of molecular gas in the inner regions of protoplanetary disks. CO can be used to constrain the excitation and structure of the circumstellar environment. Absorption line spectroscopy provides an accurate assessment of a single line-of-sight through the protoplanetary disk system, giving more straightforward estimates of column densities and temperatures than CO and molecular hydrogen emission line studies. We analyze new observations of ultraviolet CO absorption from the Hubble Space Telescope along the sightlines to six classical T Tauri stars. Gas velocities consistent with the stellar velocities, combined with the moderate-to-high disk inclinations, argue against the absorbing CO gas originating in a fast-moving disk wind. We conclude that the far-ultraviolet observations provide a direct measure of the disk atmosphere or possibly a slow disk wind. The CO absorption lines are reproduced by model spectra with column densities in the range N(^{12}CO) ~ 10^{16} - 10^{18} cm^{-2} and N(^{13}CO) ~ 10^{15} - 10^{17} cm^{-2}, rotational temperatures T_{rot}(CO) ~ 300 - 700 K, and Doppler b-values, b ~ 0.5 - 1.5 km s^{-1}. We use these results to constrain the line-of-sight density of the warm molecular gas (n_{CO} ~ 70 - 4000 cm^{-3}) and put these observations in context with protoplanetary disk models.

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arXiv2013

Continuum Variability of Deeply Embedded Protostars as a Probe of Envelope Structure

Stars may be assembled in large growth spurts, however the evidence for this hypothesis is circumstantial. Directly studying the accretion at the earliest phases of stellar growth is challenging because young stars are deeply embedded in optically thick envelopes, which have spectral energy distributions that peak in the far-IR, where observations are difficult. In this paper, we consider the feasibility of detecting accretion outbursts from these younger stars by investigating the timescales for how the protostellar envelope responds to changes in the emission properties of the central source. The envelope heats up in response to an outburst, brightening at all wavelengths and with the emission peak moving to shorter wavelengths. The timescale for this change depends on the time for dust grains to heat and re-emit photons and the time required for the energy to escape the inner, optically-thick portion of the envelope. We find that the dust response time is much shorter than the photon propagation time and thus the timescale over which the emission varies is set by time delays imposed by geometry. These times are hours to days near the peak of the spectral energy distribution and weeks to months in the sub-mm. The ideal location to quickly detect continuum variability is therefore in the mid- to far-IR, near the peak of the spectral energy distribution, where the change in emission amplitude is largest. Searching for variability in sub-mm continuum emission is also feasible, though with a longer time separation and a weaker relationship between the amount of detected emission amplitude and change in central source luminosity. Such observations would constrain accretion histories of protostars and would help to trace the disk/envelope instabilities that lead to stellar growth.

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arXiv2012

A Hubble Space Telescope Survey of H2 Emission in the Circumstellar Environments of Young Stars

The formation timescale and final architecture of exoplanetary systems are closely related to the properties of the molecular disks from which they form. Observations of the spatial distribution and lifetime of the molecular gas at planet-forming radii (r < 10 AU) are important for understanding the formation and evolution of exoplanetary systems. Towards this end, we present the largest spectrally resolved survey of H2 emission around low-mass pre-main sequence stars compiled to date. We use a combination of new and archival far-ultraviolet spectra from the COS and STIS instruments on the Hubble Space Telescope to sample 34 T Tauri stars (27 actively accreting CTTSs and 7 non-accreting WTTSs) with ages ranging from roughly 1-10 Myr. We observe fluorescent H2 emission, excited by LyA photons, in 100 of the accreting sources, including all of the transitional disks in our sample (CS Cha, DM Tau, GM Aur, UX Tau A, LkCa15, HD 135344B and TW Hya). The spatial distribution of the emitting gas is inferred from spectrally resolved H2 line profiles. Some of the emitting gas is produced in outflowing material, but the majority of H2 emission appears to originate in a rotating disk. For the disk-dominated targets, the H2 emission originates predominately at r < 3 AU. The emission line-widths and inner molecular radii are found to be roughly consistent with those measured from mid-IR CO spectra.

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arXiv2012

A Far-ultraviolet Atlas of Low-resolution Hubble Space Telescope Spectra of T Tauri Stars

We present a far-ultraviolet (FUV) spectral atlas consisting of spectra of 91 pre-main sequence stars. Most stars in this sample were observed with the Space Telescope Imaging Spectrograph (STIS) and Advanced Camera for Surveys (ACS) on the \emph{Hubble Space Telescope} (\emph{HST}). We find strong correlations among the \ion{O}{1} $λ$1304 triplet, %\ion{C}{2} $λ$1335, the \ion{Si}{4} $λλ$1394/1403 doublet, the \ion{C}{4} $λ$1549 doublet, and the \ion{He}{2} $λ$1640 line luminosities. For classical T Tauri stars (CTTSs), we also find strong correlations between these lines and the accretion luminosity, suggesting that these lines form in processes related to accretion. These FUV line fluxes and X-ray luminosity correlate loosely with large scatters. The FUV emission also correlates well with H$α$, H$β$, and \ion{Ca}{2} K line luminosities. These correlations between FUV and optical diagostics can be used to obtain rough estimates of FUV line fluxes from optical observations. Molecular hydrogen (H$_{2}$) emission is generally present in the spectra of actively accreting CTTSs but not the weak-lined T Tauri stars (WTTSs) that are not accreting. The presence of H$_2$ emission in the spectrum of HD 98800 N suggests that the disk should be classified as actively accreting rather than a debris disk. The spectra in the atlas are available at http://archive.stsci.edu/prepds/ttauriatlas.

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arXiv2012

The warm gas atmosphere of the HD 100546 disk seen by Herschel (Evidence of a gas-rich, carbon-poor atmosphere?)

(Abridged) Context. With the Herschel Space Observatory, lines of simple molecules (C+, O, and CO) have been observed in the atmosphere of protoplanetary disks. When combined with ground-based [CI], all principle forms of carbon can be studied. The absence of neutral carbon [CI], which is predicted by models to be strong, can then be interpreted together with [CII] and carbon monoxide. Aims. We study the gas temperature, excitation, and chemical abundance of the simple carbon-bearing species by the method of chemical-physical modeling. We explore the sensitivity of the lines to the entering parameters and constrain the region from which the line radiation emerges. Methods. Numerical models of the radiative transfer are used together with a chemical network simulation and a calculation of the gas energetics to obtain the gas temperature. We present our new model, which is based on our previous models but includes several improvements. Results. A model of the disk around the Herbig Be star HD 100546 is able to reproduce the CO ladder together with the atomic fine-structure lines of [OI] and either [CI] or [CII]. We find that the high-J lines of CO can only be reproduced by a warm atmosphere with Tgas>>Tdust. The high-J CO observable with PACS are dominated from regions within some tens of AU. Conclusions. Only a warm atmosphere with Tgas>>Tdust can reproduce the CO ladder. The CO ladder together with [O I] and the upper limit to [CI] can be reproduced by models with a high gas/dust ratio and a low abundance of volatile carbon. These models however produce too small amounts of [CII]. Models with a low gas/dust ratio and more volatile carbon also reproduce CO and [OI], are in closer agreement with observations of [CII], but overproduce [CI]. Due to the uncertain origin of the [CII] emission, we prefer the high gas/dust ratio models, indicating a low abundance of volatile carbon.

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arXiv2012

Water in star-forming regions with Herschel: highly excited molecular emission from the NGC 1333 IRAS 4B outflow

During the embedded phase of pre-main sequence stellar evolution, a disk forms from the dense envelope while an accretion-driven outflow carves out a cavity within the envelope. Highly excited H2O emission in spatially unresolved Spitzer/IRS spectra of a low-mass Class 0 object, NGC 1333 IRAS 4B, has previously been attributed to the envelope-disk accretion shock but could instead be produced in an outflow. As part of the survey of low-mass sources in the Water in Star Forming Regions with Herschel (WISH-LM) program, we used Herschel/PACS to obtain a far-IR spectrum and several Nyquist-sampled spectral images with to determine the origin of excited H2O emission from NGC 1333 IRAS 4B. The spectrum has high signal-to-noise in a rich forest of H2O, CO, and OH lines, providing a near-complete census of far-IR molecular emission from a Class 0 protostar. The excitation diagrams for the three molecules all require fits with two excitation temperatures, indicating the presence of two physical components. The highly excited component of H2O emission is characterized by subthermal excitation of 1500 K gas with a density of 10^5 - 10^7 cm-3, conditions that also reproduce the mid-IR H2O emission detected by Spitzer. On the other hand, a high density, low temperature gas can reproduce the H2O spectrum observed by Spitzer but underpredicts the H2O lines seen by Herschel. Nyquist-sampled spectral maps of several lines show two spatial components of H2O emission, one centered at 1200 AU south of the central source at the position of the blueshifted outflow lobe and a second centered on-source. Both spatial components of the far-IR H2O emission are consistent with emission from the outflow. The gas cooling from the IRAS 4B envelope cavity walls is dominated by far-IR H2O emission, in contrast to stronger [O I] and CO cooling from more evolved protostars. [one sentence truncated]

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arXiv2011

A 30 AU radius CO gas hole in the disk around the Herbig Ae star Oph IRS 48

The physical processes leading to the disappearance of disks around young stars are not well understood. A subclass of transitional disks, the so-called cold disks with large inner dust holes, provide a crucial laboratory for studying disk dissipation processes. IRS 48 has a 30 AU radius hole previously measured from dust continuum imaging at 18.7 micron. Using new optical spectra, we determine that IRS 48 is a pre-main sequence A0 star. In order to characterize this disk's gas distribution, we obtained AO-assisted VLT CRIRES high resolution (R ~100,000) spectra of the CO fundamental rovibrational band at 4.7 micron. All CO emission, including that from isotopologues and vibrationally excited molecules, is off-source and peaks at 30 AU. The gas is thermally excited to a rotational temperature of 260 K and is also strongly UV pumped, showing a vibrational excitation temperature of ~5000 K. We model the kinematics and excitation of the gas and posit that the CO emission arises from the dust hole wall. Prior imaging of UV-excited PAH molecules, usually a gas tracer, within the hole makes the large CO hole even more unexpected.

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arXiv2011

CO and H2 Absorption in the AA Tauri Circumstellar Disk

The direct study of molecular gas in inner protoplanetary disks is complicated by uncertainties in the spatial distribution of the gas, the time-variability of the source, and the comparison of observations across a wide range of wavelengths. Some of these challenges can be mitigated with far-ultraviolet spectroscopy. Using new observations obtained with the HST-Cosmic Origins Spectrograph, we measure column densities and rovibrational temperatures for CO and H2 observed on the line-of-sight through the AA Tauri circumstellar disk. CO A-X absorption bands are observed against the far-UV continuum. The CO absorption is characterized by log(N(^{12}CO)) = 17.5 +/- 0.5 cm^{-2} and T_rot(CO) = 500$^{+500}_{-200} K, although this rotational temperature may underestimate the local kinetic temperature of the CO-bearing gas. We also detect ^{13}CO in absorption with an isotopic ratio of ~20. We do not observe H2 absorption against the continuum; however, hot H2 (v > 0) is detected in absorption against the LyA emission line. We measure the column densities in eight individual rovibrational states, determining a total log(N(H2)) = 17.9^{+0.6}_{-0.3} cm^{-2} with a thermal temperature of T(H2) = 2500^{+800}_{-700} K. The high-temperature of the molecules, the relatively small H2 column density, and the high-inclination of the AA Tauri disk suggest that the absorbing gas resides in an inner disk atmosphere. If the H2 and CO are co-spatial within a molecular layer ~ 0.6 AU thick, this region is characterized by ~ 10^{5} cm^{-3} with an observed ratio of ~ 0.4. We also find evidence for a departure from a purely thermal H2 distribution, suggesting that excitation by continuum photons and H2 formation may be altering the level populations in the molecular gas.

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arXiv2011

Disks and Outflows in CO Rovibrational Emission from Embedded, Low-Mass Young Stellar Objects

Young circumstellar disks that are still embedded in dense molecular envelopes may differ from their older counterparts, but are historically difficult to study because emission from a disk can be confused with envelope or outflow emission. CO fundamental emission is a potentially powerful probe of the disk/wind structure within a few AU of young protostars. In this paper, we present high spectral (R=90,000) and spatial (0.3") resolution VLT/CRIRES M-band spectra of 18 low-mass young stellar objects (YSOs) with dense envelopes in nearby star-froming regions to explore the utility of CO fundamental 4.6 micron emission as a probe of very young disks. CO fundamental emission is detected from 14 of the YSOs in our sample. The emission line profiles show a range of strengths and shapes, but can generally be classified into a broad, warm component and a narrow, cool component. The broad CO emission is detected more frequently from YSOs with bolometric luminosities of <15 Lsun than those with >15 Lsun, and as with CO emission from CTTSs is attributed to the warm (~1000 K) inner AU of the disk. The CO emission from objects with high bolometric luminosity is produced in cooler (~320 K), narrow lines in 12CO and in rarer isotopologues. From some objects, the narrow lines are blueshifted by up to ~10 km/s, indicating a slow wind origin. For other sources the lines are located at the systemic velocity of the star and likely arise in the disk. For a few YSOs, spatially-extended CO and H2 S(9) emission is detected up to 2" from the central source and is attributed to interactions between the wind and surrounding molecular material. Warm CO absorption is detected in the wind of six objects with velocities up to 100 km/s, often in discrete velocity components. That the wind is partially molecular where it is launched favors ejection in a disk wind rather than a coronal or chromospheric wind.

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arXiv2011

The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems II: CO Fourth Positive Emission and Absorption

We exploit the high sensitivity and moderate spectral resolution of the $HST$-Cosmic Origins Spectrograph to detect far-ultraviolet 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. We measure a CO column density and rotational excitation temperature of N(CO) = 2 +/- 1 $\times$ 10$^{17}$ 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 ultraviolet line photons, predominantly HI LyA. 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 H$_{2}$, 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 LyA emission profile. We find CO parameters in the range: N(CO) 10$^{18-19}$ cm$^{-2}$, T_{rot}(CO) > 300 K for the LyA-pumped emission. We combine these results with recent work on photo- and collisionally-excited H$_{2}$ emission, concluding that the observations of ultraviolet-emitting CO and H2 are consistent with a common spatial origin. We suggest that the CO/H2 ratio 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.

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arXiv2010

A Spitzer c2d Legacy Survey to Identify and Characterize Disks with Inner Dust Holes

Understanding how disks dissipate is essential to studies of planet formation. However, identifying exactly how dust and gas dissipates is complicated due to difficulty in finding objects clearly in the transition of losing their surrounding material. We use Spitzer IRS spectra to examine 35 photometrically-selected candidate cold disks (disks with large inner dust holes). The infrared spectra are supplemented with optical spectra to determine stellar and accretion properties and 1.3mm photometry to measure disk masses. Based on detailed SED modeling, we identify 15 new cold disks. The remaining 20 objects have IRS spectra that are consistent with disks without holes, disks that are observed close to edge-on, or stars with background emission. Based on these results, we determine reliable criteria for identifying disks with inner holes from Spitzer photometry and examine criteria already in the literature. Applying these criteria to the c2d surveyed star-forming regions gives a frequency of such objects of at least 4% and most likely of order 12% of the YSO population identified by Spitzer. We also examine the properties of these new cold disks in combination with cold disks from the literature. Hole sizes in this sample are generally smaller than for previously discovered disks and reflect a distribution in better agreement with exoplanet orbit radii. We find correlations between hole size and both disk and stellar masses. Silicate features, including crystalline features, are present in the overwhelming majority of the sample although 10 micron feature strength above the continuum declines for holes with radii larger than ~7 AU. In contrast, PAHs are only detected in 2 out of 15 sources. Only a quarter of the cold disk sample shows no signs of accretion, making it unlikely that photoevaporation is the dominant hole forming process in most cases.

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arXiv2010

The Enigmatic Young Low-Mass Variable TWA 30

TWA 30 is a remarkable young (7+/-3 Myr), low-mass (0.12+/-0.04 Msun), late-type star (M5+/-1) residing 42+/-2 pc away from the sun in the TW Hydrae Association. It shows strong outflow spectral signatures such as [S II], [O I], [O II], [O III], and Mg I], while exhibiting weak Halpha emission (-6.8+/-1.2 Angstroms). Emission lines of [S II] and [O I] are common to T Tauri stars still residing in their natal molecular clouds, while [O III] and Mg I] emission lines are incredibly rare in this same population; in the case of TWA 30, these latter lines may arise from new outflow material colliding into older outflow fronts. The weak Halpha emission and small radial velocity shifts of line emission relative to the stellar frame of rest (generally <=10 km/s) suggest that the disk is viewed close to edge-on and that the stellar axis may be inclined to the disk, similar to the AA Tau system, based on its temporal changes in emission/absorption line strengths/profiles and variable reddening (A_V=1.5-9.0). The strong Li absorption (0.61+/-0.13 Angstroms) and common kinematics with members of the TWA confirm its age and membership to the association. Given the properties of this system such as its proximity, low mass, remarkable outflow signatures, variability, and edge-on configuration, this system is a unique case study at a critical time in disk evolution and planet-building processes.

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arXiv2009

Variable Sodium Absorption in a Low-Extinction Type Ia Supernova

Recent observations have revealed that some Type Ia supernovae exhibit narrow, time-variable Na I D absorption features. The origin of the absorbing material is controversial, but it may suggest the presence of circumstellar gas in the progenitor system prior to the explosion, with significant implications for the nature of the supernova progenitors. We present the third detection of such variable absorption, based on six epochs of high-resolution spectroscopy of the Type Ia supernova SN 2007le from Keck and the HET. The data span ~3 months, from 5 days before maximum light to 90 days after maximum. We find that one component of the Na D absorption lines strengthened significantly with time, indicating a total column density increase of ~2.5 x 10^12 cm^-2. The changes are most prominent after maximum light rather than at earlier times when the UV flux from the SN peaks. As with SN 2006X, we detect no change in the Ca II H&K lines over the same time period, rendering line-of-sight effects improbable and suggesting a circumstellar origin for the absorbing material. Unlike the previous two SNe exhibiting variable absorption, SN 2007le is not highly reddened (E_B-V = 0.27 mag), also pointing toward circumstellar rather than interstellar absorption. Photoionization models show that the data are consistent with a dense (10^7 cm^-3) cloud or clouds of gas located ~0.1 pc from the explosion. These results broadly support the single-degenerate scenario previously proposed to explain the variable absorption, with mass loss from a nondegenerate companion star responsible for providing the circumstellar gas. We also present tentative evidence for narrow Halpha emission associated with the SN, which will require followup observations at late times to confirm. [abridged]

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arXiv2009

High Resolution Spectroscopy of [NeII] Emission from AA Tau and GM Aur

We present high resolution (R=80,000) spectroscopy of [NeII] emission from two young stars, GM Aur and AA Tau, which have moderate to high inclinations. The emission from both sources appears centered near the stellar velocity and is broader than the [NeII] emission measured previously for the face-on disk system TW Hya. These properties are consistent with a disk origin for the [NeII] emission we detect, with disk rotation (rather than photoevaporation or turbulence in a hot disk atmosphere) playing the dominant role in the origin of the line width. In the non-face-on systems, the [NeII] emission is narrower than the CO fundamental emission from the same sources. If the widths of both diagnostics are dominated by Keplerian rotation, this suggests that the [NeII] emission arises from larger disk radii on average than does the CO emission. The equivalent width of the [NeII] emission we detect is less than that of the spectrally unresolved [NeII] feature in the Spitzer spectra of the same sources. Variability in the [NeII] emission or the mid-infrared continuum, a spatially extended [NeII] component, or a very (spectrally) broad [NeII] component might account for the difference in the equivalent widths.

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arXiv2009

Measuring tiny mass accretion rates onto young brown dwarfs

We present low-resolution Keck I/LRIS spectra spanning from 3200-9000 A of nine young brown dwarfs and three low-mass stars in the TW Hya Association and in Upper Sco. The optical spectral types of the brown dwarfs range from M5.5-M8.75, though two have near-IR spectral types of early L-dwarfs. We report new accretion rates derived from excess Balmer continuum emission for the low-mass stars TW Hya and Hen 3-600A and the brown dwarfs 2MASS J12073347-3932540, UScoCTIO 128, SSSPM J1102-3431, UScoJ160606.29-233513.3, DENIS-P J160603.9-205644, and Oph J162225-240515B, and upper limits on accretion for the low-mass star Hen 3-600B and the brown dwarfs UScoCTIO 112, Oph J162225-240515A, and USco J160723.82-221102.0. For the six brown dwarfs in our sample that are faintest at short wavelengths, the accretion luminosity or upper limit is measurable only when the image is binned over large wavelength intervals. This method extends our sensivity to accretion rate down to ~1e-13 solar masses/year for brown dwarfs. Since the ability to measure an accretion rate from excess Balmer continuum emission depends on the contrast between excess continuum emission and the underlying photosphere, for objects with earlier spectral types the upper limit on accretion rate is much higher. Absolute uncertainties in our accretion rate measurements of ~3-5 include uncertainty in accretion models, brown dwarf masses, and distance. The accretion rate of 2e-12 solar masses/year onto 2MASS J12073347-3932540 is within 15% of two previous measurements, despite large changes in the H-alpha flux.

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arXiv2008

UV excess measures of accretion onto young very low-mass stars and brown dwarfs

Low-resolution spectra from 3000-9000 AA of young low-mass stars and brown dwarfs were obtained with LRIS on Keck I. The excess UV and optical emission arising in the Balmer and Paschen continua yields mass accretion rates ranging from 2e-12 to 1e-8 Mo/yr. These results are compared with {\it HST}/STIS spectra of roughly solar-mass accretors with accretion rates that range from 2e-10 to 5e-8 Mo/yr. The weak photospheric emission from M-dwarfs at <4000 A leads to a higher contrast between the accretion and photospheric emission relative to higher-mass counterparts. The mass accretion rates measured here are systematically 4-7 times larger than those from H-alpha emission line profiles, with a difference that is consistent with but unlikely to be explained by the uncertainty in both methods. The accretion luminosity correlates well with many line luminosities, including high Balmer and many He I lines. Correlations of the accretion rate with H-alpha 10% width and line fluxes show a large amount of scatter. Our results and previous accretion rate measurements suggest that accretion rate is proportional to M^(1.87+/-0.26) for accretors in the Taurus Molecular Cloud.

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arXiv2007

High-resolution Spectroscopy of [Ne II] Emission from TW Hya

We present high-resolution echelle spectra of [Ne II] 12.81 micron emission from the classical T Tauri star (CTTS) TW Hya obtained with MICHELLE on Gemini North. The line is centered at the stellar radial velocity and has an intrinsic FWHM of 21\pm 4 km/s. The line width is broader than other narrow emission lines typically associated with the disk around TW Hya. If formed in a disk, the line broadening could result from turbulence in a warm disk atmosphere, Keplerian rotation at an average distance of 0.1 AU from the star, or a photoevaporative flow from the optically-thin region of the disk. We place upper limits on the [Ne II] emission flux from the CTTSs DP Tau and BP Tau.

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arXiv2006

The Origins of Fluorescent H2 Emission From T Tauri~Stars

We survey fluorescent H2 emission in HST/STIS spectra of the classical T Tauri stars (CTTSs) TW Hya, DF Tau, RU Lupi, T Tau, and DG Tau, and the weak-lined T Tauri star (WTTS) V836 Tau. From each of those sources we detect between 41-209 narrow H2 emission lines, most of which are pumped by strong Ly-alpha emission. H2 emission is not detected from the WTTS V410 Tau. The fluorescent H2 emission appears to be common to circumstellar environments around all CTTSs, but high spectral and spatial resolution STIS observations reveal diverse phenomenon. Blueshifted H2 emission detected from RU Lupi, T Tau, and DG Tau is consistent with an origin in an outflow. The H2 emission from TW Hya, DF Tau, and V836 Tau is centered at the radial velocity of the star and is consistent with an origin in a warm disk surface. The H2 lines from RU Lupi, DF Tau, and T Tau also have excess blueshifted H2 emission that extends to as much as -100 km/s. The strength of this blueshifted component from DF Tau and T Tau depends on the upper level of the transition. In all cases, the small aperture and attenuation of H2 emission by stellar winds restricts the H2 emission to be formed close to the star. The Ly-alpha and the H2 emission blueshifted by 15 km/s relative to RU Lupi are extended to the SW by 0.07 arcsec, although the faster H2 gas that extends to about 100 km/s is not spatially extended. We also find a small reservoir of H2 emission from TW Hya and DF Tau consistent with an excitation temperature of about 2.5x10^4 K.

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