Centroid Velocity Research Articles

The Farthest Quasar Mini-Broad Absorption Line Outflow from Its Central Source: Very Large Telescope/UVES Observation of SDSS J0242+0049

Abstract We analyze Very Large Telescope/UVES observations of the quasar SDSS J024221.87+004912.6. We identify four absorption outflow systems: a C iv broad absorption line (BAL) at v ≈ −18,000 km s−1 and three narrower low-ionization systems with centroid velocities ranging from –1200 to –3500 km s−1. These outflows show similar physical attributes to the [O iii] outflows studied by Liu et al. (2013). We find that two of the systems are energetic enough to contribute to active galactic nucleus feedback, with one system reaching above 5% of the quasar’s Eddington luminosity. We also find that this system is at a distance of 67 kpc away from the quasar, the farthest detected mini-BAL absorption outflow from its central source to date. In addition, we examine the time-variability of the BAL and find that its velocity monotonically increases, while the trough itself becomes shallower over time.

Simultaneous inversion for surface wave phase velocity and earthquake centroid parameters: methodology and application

Simultaneous inversion for surface wave phase velocity and earthquake centroid parameters: methodology and application

Simultaneous inversion for surface wave phase velocity and earthquake centroid parameters: methodology and application

Simultaneous inversion for surface wave phase velocity and earthquake centroid parameters: methodology and application

Dynamics of a gas bubble penetrating through porous media

Porous media have been widely used for liquid-gas separation, benefiting from the strong capillary force generated from the micro/nanoscale pores. Understanding the flow characteristics in pore scale is significant for the design of porous structure. In this study, a numerical model was established to investigate the dynamics of a bubble penetrating through porous media at the pore scale. The two-phase interface was captured using the diffuse interface method. The influence of pore shape, width, and height on the bubble deformation, velocity, and critical pressure was investigated. For the same pore size, the largest bubble centroid velocity and the highest critical pressure exist in the quadrilateral pores compared rather than in the circular or triangular pores. As the pore width decreases, both the average velocity of the bubble centroid and the critical pressure increase. However, the critical pressure is independent of the pore height. As the pore height increases, the average velocity of the bubble centroid increases. A new correlation of the critical pressure for bubble penetration has been proposed, which is a function of the shape factor, the pore width, and the bubble diameter. The findings of this work can contribute to improving the design of porous media for two-phase separation.

The “Maggie” filament: Physical properties of a giant atomic cloud

Context. The atomic phase of the interstellar medium plays a key role in the formation process of molecular clouds. Due to the line-of-sight confusion in the Galactic plane that is associated with its ubiquity, atomic hydrogen emission has been challenging to study. Aims. We investigate the physical properties of the “Maggie” filament, a large-scale filament identified in H I emission at line-of-sight velocities, vLSR ~−54 km s−1. Methods. Employing the high-angular resolution data from The H I/OH Recombination line survey of the inner Milky Way (THOR), we have been able to study H I emission features at negative vLSR velocities without any line-of-sight confusion due to the kinematic distance ambiguity in the first Galactic quadrant. In order to investigate the kinematic structure, we decomposed the emission spectra using the automated Gaussian fitting algorithm GAUSSPY+. Results. We identify one of the largest, coherent, mostly atomic H I filaments in the Milky Way. The giant atomic filament Maggie, with a total length of 1.2 ± 0.1 kpc, is not detected in most other tracers, and it does not show signs of active star formation. At a kinematic distance of 17 kpc, Maggie is situated below (by ≈500 pc), but parallel to, the Galactic H I disk and is trailing the predicted location of the Outer Arm by 5−10 km s−1 in longitude-velocity space. The centroid velocity exhibits a smooth gradient of less than ±3 km s−1 (10 pc)−1 and a coherent structure to within ±6 km s−1. The line widths of ~10 km s−1 along the spine of the filament are dominated by nonthermal effects. After correcting for optical depth effects, the mass of Maggie’s dense spine is estimated to be 7.2−1.9+2.5 × 105 M⊙. The mean number density of the filament is ~4 cm−3, which is best explained by the filament being a mix of cold and warm neutral gas. In contrast to molecular filaments, the turbulent Mach number and velocity structure function suggest that Maggie is driven by transonic to moderately supersonic velocities that are likely associated with the Galactic potential rather than being subject to the effects of self-gravity or stellar feedback. The probability density function of the column density displays a log-normal shape around a mean of ⟨NH I⟩ = 4.8 × 1020 cm−2, thus reflecting the absence of dominating effects of gravitational contraction. Conclusions. While Maggie’s origin remains unclear, we hypothesize that Maggie could be the first in a class of atomic clouds that are the precursors of giant molecular filaments.

Amoeba: Automated Molecular Excitation Bayesian Line-fitting Algorithm

Abstract The hyperfine transitions of the ground-rotational state of the hydroxyl radical (OH) have emerged as a versatile tracer of the diffuse molecular interstellar medium. We present a novel automated Gaussian decomposition algorithm designed specifically for the analysis of the paired on-source and off-source optical depth and emission spectra of these OH transitions. In contrast to existing automated Gaussian decomposition algorithms, Amoeba (Automated Molecular Excitation Bayesian line-fitting Algorithm) employs a Bayesian approach to model selection, fitting all four optical-depth and four emission spectra simultaneously. Amoeba assumes that a given spectral feature can be described by a single centroid velocity and full width at half maximum, with peak values in the individual optical-depth and emission spectra then described uniquely by the column density in each of the four levels of the ground-rotational state, thus naturally including the real physical constraints on these parameters. Additionally, the Bayesian approach includes informed priors on individual parameters that the user can modify to suit different data sets. Here we describe Amoeba and establish its validity and reliability in identifying and fitting synthetic spectra with known (but hidden) parameters, finding that the code performs very well in a series of practical tests. Amoeba’s core algorithm could be adapted to the analysis of other species with multiple transitions interconnecting shared levels (e.g., the 700 MHz lines of the first excited rotational state of CH). Users are encouraged to adapt and modify Amoeba to suit their own use cases.

Multi-beam energy moments of measured compound ion velocity distributions

Compound ion distributions, fi(v), have been measured with high-time resolution by NASA's Magnetospheric Multi-Scale Mission (MMS) and have been found in reconnection simulations. A compound distribution, fi(v), consisting, for example, of essentially disjoint pieces will be called a multi-beam distribution and modeled as a sum of “beams,” fi(v) = f1(v) + ⋯ + fN(v). Velocity moments of fi(v) are taken beam by beam and summed. Such multi-beam moments of fi(v) have advantages over the customary standard velocity moments of fi(v), for which there is only one mean flow velocity. For example, the standard thermal energy moment of a pair of equal and opposite cold particle beams is non-zero even though each beam has zero thermal energy. We therefore call this thermal energy pseudothermal. By contrast, a multi-beam moment of two or more beams has no pseudothermal energy. We develop three different ways of approximating a compound ion velocity distribution, fi(v), as a sum of beams and finding multi-beam moments for both a compound fi(v) measured by MMS in the dayside magnetosphere during reconnection and a compound fi(v) found in a particle-in-cell simulation of magnetotail reconnection. The three methods are (i) a visual method in which the velocity centroid of each beam is estimated and the beam densities are determined self-consistently, (ii) a k-means method in which particles in a particle representation of fi(v) are sorted into a minimum energy configuration of N (= k) clusters, and (iii) a nonlinear least squares method based on a fit to a sum of N kappa functions. Multi-beam energy moments are calculated and compared with standard moments for the thermal energy density, pressure tensor, thermal energy flux (heat plus enthalpy fluxes), bulk kinetic energy density, ram pressure, and bulk kinetic energy flux. Applying this new formalism to real data demonstrates in detail how multi-beam techniques provide new insights into the energetics of observed space plasmas.

High-resolution LAsMA 12CO and 13CO observation of the G305 giant molecular cloud complex

Context. Understanding the effect of feedback, interaction of young massive stars with their parental giant molecular clouds, is of central importance for studies of the interstellar medium and star formation. Aims. We observed the G305 star-forming complex in the J = 3–2 lines of 12CO and 13CO to investigate how molecular gas surrounding the central stellar clusters is being impacted by feedback. Methods. The Atacama Pathfinder EXperiment (APEX) telescope’s Large APEX sub-Millimeter Array (LAsMA) multibeam receiver was used to observe the region. Excitation temperatures and column density maps were produced. Combining our data with data from the structure, excitation, and dynamics of the inner Galactic interstellar medium survey resulted in a 13CO J = 3−2∕2−1 excitation map. To verify whether feedback from stellar clusters is responsible for exciting the gas, the distribution of CO excitation was compared with that of 8 μm emission imaged with Spitzer, which is dominated by UV-excited emission from polycyclic aromatic hydrocarbons. Line centroid velocities, as well as stacked line profiles were examined to investigate the effect of feedback on the gas dynamics. Results. Line profiles along radially outward directions demonstrate that the excitation temperature and 13CO J = 3−2∕2−1 ratio increase steeply by factors of ~2–3 at the edge of the denser gas traced by 13CO that faces the hot stars at the center of the complex and steadily decreases away from it. The column density also increases at the leading edge, but it does not always decrease steadily outward. Regions with a higher 8 μm flux have higher median excitation temperatures, column densities, and 13CO J = 3−2∕2−1 ratio. The centroid velocity probability distribution function of the region shows exponential wings, indicative of turbulence driven by strong stellar winds. Stacked spectra in regions with stronger feedback have higher skewness and narrower peaks with pronounced wings compared to regions with weaker feedback. Conclusions. Feedback from the stellar cluster in G305 has demonstrable effects on the excitation as well as on the dynamics of the giant molecular cloud.

Identification of pre-stellar cores in high-mass star forming clumps via H2D+ observations with ALMA

Context. The different theoretical models concerning the formation of high-mass stars make distinct predictions regarding their progenitors, which are the high-mass pre-stellar cores. However, no conclusive observation of such objects has been made to date. Aims. We aim to study the very early stages of high-mass star formation in two infrared-dark massive clumps. Our goal is to identify the core population that they harbour and to investigate their physical and chemical properties at high spatial resolution. Methods. We obtained Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 6 observations of continuum emission at 0.8 mm and of the ortho-H2D+ transition at 372 GHz towards the two clumps. We used the SCIMES algorithm to identify substructures (i.e. cores) in the position-position-velocity space, finding 16 cores. We modelled their observed spectra using a Bayesian fitting approach in the approximation of local thermodynamic equilibrium. We derived the centroid velocity, the line width, and the molecular column density maps. We also studied the correlation between the continuum and molecular data, which in general do not present the same structure. Results. We report, for the first time, the detection of ortho-H2D+ in high-mass star-forming regions performed with an interferometer. The molecular emission shows narrow and subsonic lines, suggesting that locally, the temperature of the gas is below 10 K. From the continuum emission, we estimated the cores’ total masses and compare them with the respective virial masses. We also computed the volume density values, which are found to be higher than 106 cm−3. Conclusions. Our data confirm that ortho-H2D+ is an ideal tracer of cold and dense gas. Interestingly, almost all the H2D+-identified cores are less massive than ≈13 M⊙, with the exception of one core in AG354, which could be as massive as 39 M⊙ under the assumption of low dust temperature (5 K). Furthermore, most of them are sub-virial and larger than their Jeans masses. These results are hence difficult to explain in the context of the turbulent accretion models, which predict massive and virialised pre-stellar cores. However, we cannot exclude that the cores are still in the process of accreting mass and that magnetic fields are providing enough support for the virialisation. ALMA could also be seeing only the innermost parts of the cores, and hence the cores’ total masses could be higher than inferred in this work. Furthermore, we note that the total masses of the investigated clumps are below the average for typical high-mass clumps, and thus studies of more massive sources are needed.

Measuring Magnetization with Rotation Measures and Velocity Centroids in Supersonic MHD Turbulence

Abstract The interstellar turbulence is magnetized and thus anisotropic. The anisotropy of turbulent magnetic fields and velocities is imprinted in the related observables, rotation measures (RMs), and velocity centroids (VCs). This anisotropy provides valuable information on both the direction and strength of the magnetic field. However, its measurement is difficult, especially in highly supersonic turbulence in cold interstellar phases, due to the distortions by isotropic density fluctuations. By using 3D simulations of supersonic and sub-Alfvénic magnetohydrodynamic (MHD) turbulence, we find that the problem can be alleviated when we selectively sample the volume filling low-density regions in supersonic MHD turbulence. Our results show that in these low-density regions the anisotropy of RM and VC fluctuations depends on the Alfvénic Mach number as . This anisotropy−M A relation is theoretically expected for sub-Alfvénic MHD turbulence and confirmed by our synthetic observations of 12CO emission. It provides a new method for measuring the plane-of-the-sky magnetic fields in cold interstellar phases.

Time-resolved Rotational Velocities in the Upper Atmosphere of WASP-33 b*

Abstract While steady empirical progress has been made in understanding the structure and composition of hot-planet atmospheres, direct measurements of velocity signatures, including winds, rotation, and jets, have lagged behind. Quantifying atmospheric dynamics of hot planets is critical for a complete understanding of their atmospheres, and such measurements may even illuminate other planetary properties, such as magnetic field strengths. In this manuscript we present the first detection of the Balmer lines Hα and Hβ in the atmosphere of the ultra-hot Jupiter WASP-33 b. Using atmospheric models that include the effects of atmospheric dynamics, we show that the shape of the average Balmer line transmission spectrum is consistent with rotational velocities in the planet’s thermosphere of km s−1. We also measure a low-significance day- to nightside velocity shift of km s−1 in the transmission spectrum, which is naturally explained by a global wind across the planet’s terminator. In a separate analysis the time-resolved velocity centroids of individual transmission spectra show unambiguous evidence of rotation, with a best-fit velocity of km s−1, consistent with the value of v rot derived from the shape of the average Balmer line transmission spectrum. Our observations and analysis confirm the power of time-resolved transmission spectra with a high signal-to-noise ratio to measure the velocity structures in exoplanet atmospheres. The high rotational and wind velocities we measure highlight the need for more detailed 3D global climate simulations of the rarefied upper atmospheres of ultra-hot gas giants.

Thermal and Turbulent Properties of the Warm Neutral Medium in the Solar Neighborhood

Abstract The transition from the diffuse warm neutral medium (WNM) to the dense cold neutral medium (CNM) is what set the initial conditions to the formation of molecular clouds. The properties of the turbulent cascade in the WNM, essential to describe this radiative condensation process, have remained elusive in part due to the difficulty in mapping out the structure and kinematics of each H i thermal phase. Here we present an analysis of a 21 cm hyper-spectral data cube from the GHIGLS H i survey where the contribution of the WNM is extracted using ROHSA, a Gaussian decomposition tool that includes spatial regularization. The distance and volume of the WNM emission is estimated using 3D dust extinction map information. The thermal and turbulent contributions to the Doppler line width of the WNM were disentangled using two techniques, one based on the statistical properties of the column density and centroid velocity fields, and the other on the relative motions of CNM structures as a probe of turbulent motions. We found that the volume of WNM sampled here (5.2 × 105 pc3), located at the outer edge of the Local Bubble, shows thermal properties in accordance with expected values for heating and cooling processes typical of the solar neighborhood: P th/k B = (4.4 ± 2.6) × 103 K cm−3, n = 0.74 ± 0.41 cm−3, and T k = (6.0 ± 1.3) × 103 K. The WNM has the properties of sub/trans-sonic turbulence, with a turbulent Mach number at the largest scale probed here (l = 130 pc) of , a density contrast of , and velocity and density power spectra compatible with k −11/3. The low Mach number of the WNM provides dynamical conditions that allow the condensation mode of thermal instability to grow freely and form CNM structures, as predicted by theory.

Wave Equation of Micro Element Centroid Velocity in Thermodynamic System

As we all know, it is Maxwell’s electromagnetic wave equation and Hertz’s electromagnetic wave experiment that make our mobile communication wonderful today! Since the electromagnetic wave has been widely used after research in the field of electromagnetism, is there a corresponding wave in the field of thermodynamics? Starting from the relevant equations in non-equi- librium thermodynamics works, in some special cases, the wave equation of micro element centroid velocity of thermodynamic system is obtained by pure vector analysis. It is expected that the solution of the nonlinear equation can more accurately describe the causes and images of weather in theory, and then guide the artificial regulation of weather! Hope to find this wave and widely serve mankind! Or discuss it and get a more perfect result.

Paddling motion of a free-swimming jellyfish and Lagrangian coherent structure analysis

Swimming and prey capture by prolate and oblate jellyfishes are numerically examined in two-dimensions using multi-relaxation-time lattice Boltzmann (MRT-LB) and immersed boundary (IB) methods. The near-field fluid structure interaction (FSI) and predator-prey interaction mechanisms are revealed via the simulated Eulerian flow characteristics, finite-time Lyapunov exponent (FTLE) field, and Lagrangian coherent structures (LCS). We implement appropriate periodic body force (Fb) distribution at nodal points of the elastic bell in radial direction to model the paddled swimming as well as complex feeding behavior. For a paddling jellyfish the evolved starting and stopping vortices, as move close to each other in near-wake, create the necessary vortex induced thrust that facilitate the forward body motion. The forced bell contraction in power stroke assists quicker propulsive swimming, whereas passive bell expansion in resting phase facilitates the continued vortex induced forward movement for larger duration, via the refilled fluid momentum. For feeding the detailed prey interception and precise capture areas for various jellyfish models are identified hereby via the computed prey tracks, forward and backward time FTLE fields, and LCS. Swimming performances are analyzed based on interactive thrust and drag forces, input power (energy rate required for bell contraction), output power (thrust multiplied by centroid velocity), and cost of transport (COT). At low Reynolds number (Re) the COT becomes higher for an oblate jellyfish than that of a prolate one; while with increased Re the oblate species appears more economical. However, for enhanced paddling force (Fb) or reduced flapping frequency of bell, the COT for an oblate jellyfish steadily decreases. Hereby impacts of the varied force duration, flapping amplitude, flapping frequency, and bell-elasticity on the swimming are analyzed in greater detail. Notably, the propulsion efficiency increased for higher flapping frequency. The present numerical model efficiently unfolds prey capture mechanisms that are adopted by prolate and oblate medusae and quantifies their success rate (clearance rate, CR) in prey capture. Unlike in past studies, the FTLE fields and LCS that are computed here by tracking the transient motion of a large number of suspended Lagrangian prey particles exhibit the realistic predator-prey interaction and precise prey capture surface, which are difficult to measure or analyze empirically.

Study Turbulence and Probe Magnetic Fields Using the Gradient Technique: Application to H i-to-H2 Transition Regions

Abstract The atomic-to-molecular (H i-to-H2) transition in photodissociation regions (PDRs) has been investigated over the past several decades through analytic and numerical modeling. However, classical PDR models typically assume a uniform-density gas, ignoring the turbulent nature of the interstellar medium. Recently, Bialy et al. have presented a theoretical framework for studying the H i-to-H2 transition in a realistic turbulent medium with a nonhomogeneous density structure. Here we extend these turbulent-chemical models to explore the possibility of tracing the magnetic field direction in turbulent PDRs using the gradient technique. We utilize both subsonic and supersonic magnetohydrodynamic numerical simulations for chemical H i/H2 balance calculations. We confirm that the density fluctuations induced by turbulence can disperse the distribution of the H2 and H i fractions. We find that the energy spectrum of moment maps gets shallower when the sonic Mach number M S increases. We explore the ability in magnetic field tracing of gradients of higher-order velocity centroids and compare their performance with that of traditional velocity centroid gradients (VCGs) and with intensity gradients (IGs). We find that the velocity gradients of the second-order centroids (VC2Gs) are more accurate than VCGs and IGs in probing the magnetic field orientation.

Analysis of dynamic characteristics of bubble rise under a free surface

The rising process of a bubble occurs in several natural and industrial apparatuses. This process is computationally studied using the front tracking method for a moving interface whose surface properties are solved in terms of an immersed-boundary method. The results show that the free interface does not influence the bubble before the centroid velocity of the bubble reaches the terminal velocity, which reaches a stable value or fluctuates at it, with the distance h (between the centroid of the bubble and the free surface) reaching a certain value. When the Reynolds number increases, the time to reach terminal velocity will decrease, and the influence of the viscous factor on the terminal velocity is also weakened. The dramatic interaction between a bubble and free surface is beneficial to accelerate film draining out. It is also shown that the shape of the bubble gradually becomes an ellipse as the Weber number (We) decreases, and it is beneficial to reduce the resistance of the bubble. The free surface could accelerate the bubble breaking at high We values.

Anisotropy of Velocity Centroids and the Signature of Different MHD Modes in the Turbulent ISM

Abstract Magnetic turbulence is anisotropic because the directions of motion are constrained by the magnetic field. Such anisotropy can be observed in velocity centroids obtained from spectroscopic observations. We use magnetohydrodynamics (MHD) simulations to produce synthetic spectroscopic observations (position–position–velocity data) and study the anisotropy in the structure function of velocity centroid maps. We decomposed the velocity in the simulations into Alfvén, slow, and fast modes and studied how each of them contributes to the observed anisotropy. We found that when the angle between the line of sight and the mean magnetic field is large the Alfvén mode dominates the observed anisotropy, while for smaller angles the anisotropy is not large enough to be used to probe the magnetization of the medium, and it is dominated by the slow mode. Our results are in fair agreement with the theoretical predictions in Kandel et al.

Advancing the Velocity Gradient Technique: Using Gradient Amplitudes and Handling Thermal Broadening

Abstract The recent development of the velocity gradient technique allows observers to map magnetic field orientations and magnetization using the direction of velocity gradients. Aside from the directions, amplitudes of velocity gradients also contain valuable information about the underlying properties of magnetohydrodynamic (MHD) turbulence. In this paper, we explore what physical information is contained in the amplitudes of velocity gradients and discuss how this information can be used to diagnose properties of turbulence in both diffuse and self-gravitating interstellar media. We identify the relations between amplitudes of both intensity and velocity centroid gradients and the sonic Mach number M s , and they are consistent with the theory’s predictions. We test the robustness of the method and discuss how to utilize the amplitudes of gradients into self-gravitating media. To extend the velocity gradient technique, we also discuss the usage of amplitude method to position–position–velocity space as a possible way to retrieve the velocity channel maps before the contamination of thermal broadening. We discuss that the velocity gradient technique with these advancements could potentially give a significantly more accurate statistical insight into the properties of magnetized turbulence.

Gas Kinematics of the Massive Protocluster G286.21+0.17 Revealed by ALMA

Abstract We study the gas kinematics and dynamics of the massive protocluster G286.21+0.17 with the Atacama Large Millimeter/submillimeter Array using spectral lines of C18O(2–1), (3–2), (3–2), and (3–2). On the parsec clump scale, C18O emission appears highly filamentary around the systemic velocity, and are more closely associated with the dust continuum, and is strongly concentrated toward the protocluster center, where no or only weak detection is seen for and , possibly due to this region being at a relatively evolved evolutionary stage. Spectra of 76 continuum-defined dense cores, typically a few 1000 au in size, are analyzed to measure their centroid velocities and internal velocity dispersions. There are no statistically significant velocity offsets of the cores among the different dense gas tracers. Furthermore, the majority (71%) of the dense cores have subthermal velocity offsets with respect to their surrounding, lower-density C18O-emitting gas. Within the uncertainties, the dense cores in G286 show internal kinematics that are consistent with being in virial equilibrium. On clump scales, the core-to-core velocity dispersion is also similar to that required for virial equilibrium in the protocluster potential. However, the distribution in velocity of the cores is largely composed of two spatially distinct groups, which indicates that the dense molecular gas has not yet relaxed to virial equilibrium, perhaps due to there being recent/continuous infall into the system.

The H2O Spectrum of the Massive Protostar AFGL 2136 IRS 1 from 2 to 13 μm at High Resolution: Probing the Circumstellar Disk

Abstract We have observed the massive protostar AFGL 2136 IRS 1 in multiple wavelength windows in the near- to mid-infrared at high (∼3 km s−1) spectral resolution using VLT+CRIRES, SOFIA+EXES, and Gemini North+TEXES. There is an abundance of H2O absorption lines from the ν 1 and ν 3 vibrational bands at 2.7 μm, from the ν 2 vibrational band at 6.1 μm, and from pure rotational transitions near 10–13 μm. Analysis of state-specific column densities derived from the resolved absorption features reveals that an isothermal absorbing slab model is incapable of explaining the relative depths of different absorption features. In particular, the strongest absorption features are much weaker than expected, indicating optical depth effects resulting from the absorbing gas being well mixed with the warm dust that serves as the “background” continuum source at all observed wavelengths. The velocity at which the strongest H2O absorption occurs coincides with the velocity centroid along the minor axis of the compact disk in Keplerian rotation recently observed in H2O emission with ALMA. We postulate that the warm regions of this dust disk dominate the continuum emission at near- to mid-infrared wavelengths, and that H2O and several other molecules observed in absorption are probing this disk. Absorption line profiles are not symmetric, possibly indicating that the warm dust in the disk that produces the infrared continuum has a nonuniform distribution similar to the substructure observed in 1.3 mm continuum emission.