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At least 271 records · Page 15
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DOE OSTI2011

POROSITIES OF PROTOPLANETARY DUST AGGLOMERATES FROM COLLISION EXPERIMENTS

The aggregation of dust through sticking collisions is the first step of planet formation. The basic physical properties of the evolving dust aggregates strongly depend on the porosity of the aggregates; e.g., mechanical strength, thermal conductivity, and the gas-grain coupling time. Also, the outcome of further collisions depends on the porosity of the colliding aggregates. In laboratory experiments we study the growth of large aggregates of {approx}3 mm to 3 cm through continuous impacts of small dust agglomerates of 100 {mu}m in size, consisting of {mu}m grains at different impact velocities. The experiments show that agglomerates grow by direct sticking as well as through gravitational reaccretion. The latter can be regarded as a suitable analog to the reaccretion of fragments by gas drag in protoplanetary disks. Experiments were carried out in the velocity range between 1.5 m s{sup -1} and 7 m s{sup -1}. With increasing impact velocities the volume filling factor of the resulting agglomerates increases from {phi} = 0.2 for 1.5 m s{sup -1} to {phi} = 0.32 for 7 m s{sup -1}. These values are independent of the target size. Extrapolation of the measured velocity dependence of the volume filling factor implies that higher collision velocities will not lead to more compact aggregates. Therefore, {phi} = 0.32 marks a degree of compaction suitable for describing structures forming at v > 6 m s{sup -1}. At small collision velocities below 1 m s{sup -1}, highly porous structures with {phi} Almost-Equal-To 0.10 will form. For intermediate collision velocities porosities vary. Depending on the disk model and resulting relative velocities, objects in protoplanetary disks up to decimeters in size might evolve from highly porous ({phi} Almost-Equal-To 0.10) to compact ({phi} = 0.32) with a more complex intermediate size range of varying porosity.

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DOE OSTI2011

MODELING MAGNETOROTATIONAL TURBULENCE IN PROTOPLANETARY DISKS WITH DEAD ZONES

Turbulence driven by magnetorotational instability (MRI) crucially affects the evolution of solid bodies in protoplanetary disks. On the other hand, small dust particles stabilize MRI by capturing ionized gas particles needed for the coupling of the gas and magnetic fields. To provide an empirical basis for modeling the coevolution of dust and MRI, we perform three-dimensional, ohmic-resistive MHD simulations of a vertically stratified shearing box with an MRI-inactive 'dead zone' of various sizes and with a net vertical magnetic flux of various strengths. We find that the vertical structure of turbulence is well characterized by the vertical magnetic flux and three critical heights derived from the linear analysis of MRI in a stratified disk. In particular, the turbulent structure depends on the resistivity profile only through the critical heights and is insensitive to the details of the resistivity profile. We discover scaling relations between the amplitudes of various turbulent quantities (velocity dispersion, density fluctuation, vertical diffusion coefficient, and outflow mass flux) and vertically integrated accretion stresses. We also obtain empirical formulae for the integrated accretion stresses as a function of the vertical magnetic flux and the critical heights. These empirical relations allow us to predict the vertical turbulent structure of a protoplanetary disk for a given strength of the magnetic flux and a given resistivity profile.

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DOE OSTI2011

TURBULENT LINEWIDTHS IN PROTOPLANETARY DISKS: PREDICTIONS FROM NUMERICAL SIMULATIONS

Submillimeter observations of protoplanetary disks now approach the acuity needed to measure the turbulent broadening of molecular lines. These measurements constrain disk angular momentum transport, and furnish evidence of the turbulent environment within which planetesimal formation takes place. We use local magnetohydrodynamic (MHD) simulations of the magnetorotational instability (MRI) to predict the distribution of turbulent velocities in low-mass protoplanetary disks, as a function of radius and height above the mid-plane. We model both ideal MHD disks and disks in which Ohmic dissipation results in a dead zone of suppressed turbulence near the mid-plane. Under ideal conditions, the disk mid-plane is characterized by a velocity distribution that peaks near v {approx_equal} 0.1c{sub s} (where c{sub s} is the local sound speed), while supersonic velocities are reached at z > 3H (where H is the vertical pressure scale height). Residual velocities of v Almost-Equal-To 10{sup -2} c{sub s} persist near the mid-plane in dead zones, while the surface layers remain active. Anisotropic variation of the linewidth with disk inclination is modest. We compare our MHD results to hydrodynamic simulations in which large-scale forcing is used to initiate similar turbulent velocities. We show that the qualitative trend of increasing v with height, seen in the MHD case, persists for forced turbulence and is likely a generic property of disk turbulence. Percentage level determinations of v at different heights within the disk, or spatially resolved observations that probe the inner disk containing the dead zone region, are therefore needed to test whether the MRI is responsible for protoplanetary disk turbulence.

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DOE OSTI2011

SOLAR MODELS WITH ACCRETION. I. APPLICATION TO THE SOLAR ABUNDANCE PROBLEM

We generate new standard solar models using newly analyzed nuclear fusion cross sections and present results for helioseismic quantities and solar neutrino fluxes. The status of the solar abundance problem is discussed. We investigate whether nonstandard solar models with accretion from the protoplanetary disk might alleviate this problem. We examine a broad range of models, analyzing metal-enriched and metal-depleted accretion and three scenarios for the timing of accretion. Only partial solutions are found. For metal-rich accreted material (Z{sub ac} {approx}> 0.018) there exist combinations of accreted mass and metallicity that bring the depth of the convective zone into agreement with the helioseismic value. For the surface helium abundance, the helioseismic value is reproduced if metal-poor or metal-free accretion is assumed (Z{sub ac} {approx}< 0.09). In both cases a few percent of the solar mass must be accreted. Precise values depend on when accretion takes place. We do not find a simultaneous solution to both problems but speculate that changing the hydrogen-to-helium mass ratio in the accreted material may lead to more satisfactory solutions. We also show that, with current data, solar neutrinos are already a very competitive source of information about the solar core and can help constraining possible accretion histories. Even without helioseismic constraints, solar neutrinos rule out the possibility that more than 0.02 M{sub Sun} from the protoplanetary disk were accreted after the Sun settled on the main sequence. Finally, we discuss how measurements of neutrinos from the CN cycle could shed light on the interaction between the early Sun and its protoplanetary disk.

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DOE OSTI2011

THERMAL SHADOWS AND COMPOSITIONAL STRUCTURE IN COMET NUCLEI

We use a fully three-dimensional thermal evolution model to examine the effects of a non-uniform surface albedo on the subsurface thermal structure of comets. Surface albedo markings cast 'thermal shadows' with strong lateral thermal gradients. Corresponding compositional gradients can be strong, especially if the crystallization of amorphous water ice is triggered in the hottest regions. We show that the spatial extent of the structure depends mainly on the obliquity, thermal conductivity, and heliocentric distance. In some circumstances, subsurface structure caused by the thermal shadows of surface features can be maintained for more than 10 Myr, the median transport time from the Kuiper Belt to the inner solar system. Non-uniform compositional structure can be an evolutionary product and does not necessarily imply that comets consist of building blocks accumulated in different regions of the protoplanetary disk.

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DOE OSTI2011

A FAST AND ACCURATE CALCULATION SCHEME FOR IONIZATION DEGREES IN PROTOPLANETARY AND CIRCUMPLANETARY DISKS WITH CHARGED DUST GRAINS

We develop a fast and accurate calculation method for ionization degrees in protoplanetary and circumplanetary disks including dust grains. We apply our method to calculate the ionization degree of circumplanetary disks. It is important to understand the structure and evolution of protoplanetary /circumplanetary disks since they are thought to be the sites of planet/satellite formation. The turbulence that causes gas accretion is supposed to be driven by magnetorotational instability (MRI) that occurs only when the ionization degree is high enough for magnetic field to be coupled to gas. We calculate the ionization degrees in circumplanetary disks to estimate the sizes of MRI-inactive regions. We properly include the effect of dust grains because they efficiently capture charged particles and make ionization degree lower. Inclusion of dust grains complicates the reaction equations and requires expensive computation. In order to accelerate the calculation of ionization reactions, we develop a semianalytic method based on the charge distribution model proposed previously. This method enables us to study the ionization state of disks for a wide range of model parameters. For a previous model of circum-Jovian disk, we find that an MRI-inactive region covers almost all regions even without dust grains. This suggests that the gas accretion rates in circumplanetary disks are much smaller than previously thought.

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DOE OSTI2011

MULTI-EPOCH OBSERVATIONS OF HD 69830: HIGH-RESOLUTION SPECTROSCOPY AND LIMITS TO VARIABILITY

The main-sequence solar-type star HD 69830 has an unusually large amount of dusty debris orbiting close to three planets found via the radial velocity technique. In order to explore the dynamical interaction between the dust and planets, we have performed multi-epoch photometry and spectroscopy of the system over several orbits of the outer dust. We find no evidence for changes in either the dust amount or its composition, with upper limits of 5%-7% (1{sigma} per spectral element) on the variability of the dust spectrum over 1 year, 3.3% (1{sigma}) on the broadband disk emission over 4 years, and 33% (1{sigma}) on the broadband disk emission over 24 years. Detailed modeling of the spectrum of the emitting dust indicates that the dust is located outside of the orbits of the three planets and has a composition similar to main-belt, C-type asteroids in our solar system. Additionally, we find no evidence for a wide variety of gas species associated with the dust. Our new higher signal-to-noise spectra do not confirm our previously claimed detection of H{sub 2}O ice leading to a firm conclusion that the debris can be associated with the break-up of one or more C-type asteroids formed in the dry, inner regions of the protoplanetary disk of the HD 69830 system. The modeling of the spectral energy distribution and high spatial resolution observations in the mid-infrared are consistent with a {approx}1 AU location for the emitting material.

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DOE OSTI2011

DISKS AROUND BROWN DWARFS IN THE EJECTION SCENARIO. I. DISK COLLISIONS IN TRIPLE SYSTEMS

We investigate the fate of disks around brown dwarfs in the ejection scenario and the implications on their observable properties. For that purpose, a parameter study of close triple approaches leading to escape is carried out where the ejected body is surrounded by a low-mass disk. We analyze the recircularized radial surface density profile of the post-encounter disk in dependence of the minimum two-body distances between the escaper and the perturbing bodies. Our results show that the general appearance of the disks is rather similar to disks after two-body encounters in as much as there is also an exponential drop in surface density for the outer disk regions as well as an enhancement of surface density for the innermost region relative to the initial disk profile. However, the disks after close triple approaches are mostly less massive, have generally flatter recircularized surface density disk profiles, and have radii that are similar or larger compared to disks after two-body encounters. From our results, we construct a simple scale-free model only depending on the minimum encounter distances of the two perturbers. Such a model is especially useful for statistical studies of disk collisions in triple systems that must cover a large range of encounter distances.

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DOE OSTI2011

CO ROVIBRATIONAL EMISSION AS A PROBE OF INNER DISK STRUCTURE

We present an analysis of CO emission lines from a sample of T Tauri, Herbig Ae/Be, and transitional disks with known inclinations in order to study the structure of inner disk molecular gas. We calculate CO inner radii by fitting line profiles with a simple parameterized model. We find that, for optically thick disks, CO inner radii are strongly correlated with the total system luminosity (stellar plus accretion) and consistent with the dust sublimation radius. Transitional disk inner radii show the same trend with luminosity, but are systematically larger. Using rotation diagram fits, we derive, for classical T Tauri disks, emitting areas consistent with a ring of width {approx}0.15 AU located at the CO inner radius; emitting areas for transitional disks are systematically smaller. We also measure lower rotational temperatures for transitional disks, and disks around Herbig Ae/Be stars, than for those around T Tauri stars. Finally, we find that rotational temperatures are similar to, or slightly lower than, the expected temperature of blackbody grains located at the CO inner radius, in contrast to expectations of thermal decoupling between gas and dust.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

FORMATION OF ORGANIC MOLECULES AND WATER IN WARM DISK ATMOSPHERES

Observations from Spitzer and ground-based infrared spectroscopy reveal significant diversity in the molecular emission from the inner few AU of T Tauri disks. We explore theoretically the possible origin of this diversity by expanding on our earlier thermal-chemical model of disk atmospheres. We consider how variations in grain settling, X-ray irradiation, accretion-related mechanical heating, and the oxygen-to-carbon ratio can affect the thermal and chemical properties of the atmosphere at 0.25-40 AU. We find that these model parameters can account for many properties of the detected molecular emission. The column density of the warm (200-2000 K) molecular atmosphere is sensitive to grain settling and the efficiency of accretion-related heating, which may account, at least in part, for the large range in molecular emission fluxes that have been observed. The dependence of the atmospheric properties on the model parameters may also help to explain trends that have been reported in the literature between molecular emission strength and mid-infrared color, stellar accretion rate, and disk mass. We discuss whether some of the differences between our model results and the observations (e.g., for water) indicate a role for vertical transport and freezeout in the disk midplane. We also discuss how planetesimal formation in the outer disk (beyond the snowline) may imprint a chemical signature on the inner few AU of the disk and speculate on possible observational tracers of this process.

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DOE OSTI2011

THE IONIZATION FRACTION IN THE DM Tau PROTOPLANETARY DISK

We present millimeter-wave observations of several molecular ions in the disk around the pre-main-sequence star DM Tau and use these to investigate the ionization fraction in different regions of the disk. New Submillimeter Array (SMA) observations of H{sub 2}D{sup +} J = 1{sub 1,0}-1{sub 1,1}, N{sub 2}H{sup +} J = 4-3, and CO J = 3-2 are presented. H{sub 2}D{sup +} and N{sub 2}H{sup +} are not detected and using the CO 3-2 disk size the observations result in an upper limit of <0.47 K km s{sup -1} for both lines, a factor of 2.5 below previous single-dish H{sub 2}D{sup +} observations. Assuming LTE, a disk midplane temperature of 10-20 K and estimates of the H{sub 2}D{sup +} o/p ratio, the observed limit corresponds to N{sub H{sub 2D{sup +}}}<2-21x10{sup 12} cm{sup -2}. We adopt a parametric model for the disk structure from the literature and use new IRAM 30 m telescope observations of the H{sup 13}CO{sup +} J = 3-2 line and previously published SMA observations of the N{sub 2}H{sup +} J = 3-2, HCO{sup +} J = 3-2, and DCO{sup +} J = 3-2 lines to constrain the ionization fraction, x{sub i} , in three temperature regions in the disk where theoretical considerations suggest different ions should dominate: (1) a warm, upper layer with T >20 K where CO is in the gas phase and HCO{sup +} is most abundant, where we estimate x{sub i} {approx_equal} 4 Multiplication-Sign 10{sup -10}; (2) a cooler molecular layer with T = 16-20 K where N{sub 2}H{sup +} and DCO{sup +} abundances are predicted to peak, with x{sub i} {approx_equal} 3 Multiplication-Sign 10{sup -11}; and (3) the cold, dense midplane with T <16 K where H{sup +}{sub 3} and its deuterated isotopologues are the main carriers of positive charge, with x{sub i} < 3 Multiplication-Sign 10{sup -10}. While there are considerable uncertainties, these estimates are consistent with a decreasing ionization fraction into the deeper, colder, and denser disk layers. Stronger constraints on the ionization fraction in the disk midplane will require not only substantially more sensitive observations of the H{sub 2}D{sup +} 1{sub 1,0}-1{sub 1,1} line, but also robust determinations of the o/p ratio, observations of D{sub 2}H{sup +}, and stronger constraints on where N{sub 2} is present in the gas phase.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

CARBON-RICH GIANT PLANETS: ATMOSPHERIC CHEMISTRY, THERMAL INVERSIONS, SPECTRA, AND FORMATION CONDITIONS

The recent inference of a carbon-rich atmosphere, with C/O {>=} 1, in the hot Jupiter WASP-12b motivates the exotic new class of carbon-rich planets (CRPs). We report a detailed study of the atmospheric chemistry and spectroscopic signatures of carbon-rich giant (CRG) planets, the possibility of thermal inversions in their atmospheres, the compositions of icy planetesimals required for their formation via core accretion, and the apportionment of ices, rock, and volatiles in their envelopes. Our results show that CRG atmospheres probe a unique region in composition space, especially at high temperature (T). For atmospheres with C/O {>=} 1, and T {approx}> 1400 K in the observable atmosphere, most of the oxygen is bound up in CO, while H{sub 2}O is depleted and CH{sub 4} is enhanced by up to two or three orders of magnitude each, compared to equilibrium compositions with solar abundances (C/O = 0.54). These differences in the spectroscopically dominant species for the different C/O ratios cause equally distinct observable signatures in the spectra. As such, highly irradiated transiting giant exoplanets form ideal candidates to estimate atmospheric C/O ratios and to search for CRPs. We also find that the C/O ratio strongly affects the abundances of TiO and VO, which have been suggested to cause thermal inversions in highly irradiated hot Jupiter atmospheres. A C/O = 1 yields TiO and VO abundances of {approx}100 times lower than those obtained with equilibrium chemistry assuming solar abundances, at P {approx} 1 bar. Such a depletion is adequate to rule out thermal inversions due to TiO/VO even in the most highly irradiated hot Jupiters, such as WASP-12b. We estimate the compositions of the protoplanetary disk, the planetesimals, and the envelope of WASP-12b, and the mass of ices dissolved in the envelope, based on the observed atmospheric abundances. Adopting stellar abundances (C/O = 0.44) for the primordial disk composition and low-temperature formation conditions (T {approx}< 30 K) for WASP-12b lead to a C/O ratio of 0.27 in accreted planetesimals, and, consequently, in the planet's envelope. In contrast, a C/O ratio of 1 in the envelope of WASP-12b requires a substantial depletion of oxygen in the disk, i.e., by a factor of {approx}0.41 for the same formation conditions. This scenario also satisfies the constraints on the C/H and O/H ratios reported for WASP-12b. If, alternatively, hotter conditions prevailed in a stellar composition disk such that only H{sub 2}O is condensed, the remaining gas can potentially have a C/O {approx} 1. However, a high C/O in WASP-12b caused predominantly by gas accretion would preclude superstellar C/H ratios which also fit the data.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CO AND H{sub 2} 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 Hubble Space Telescope Cosmic Origins Spectrograph, we measure column densities and rovibrational temperatures for CO and H{sub 2} 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{sub 10}(N({sup 12}CO)) = 17.5 {+-} 0.5 cm{sup -2} and T{sub rot}(CO) = 500{sup +500}{sub -200} K, although this rotational temperature may underestimate the local kinetic temperature of the CO-bearing gas. We also detect {sup 13}CO in absorption with an isotopic ratio of {approx}20. We do not observe H{sub 2} absorption against the continuum; however, hot H{sub 2} (v > 0) is detected in absorption against the Ly{alpha} emission line. We measure the column densities in eight individual rovibrational states, determining a total log{sub 10}(N(H{sub 2})) = 17.9{sup +0.6}{sub -0.3} cm{sup -2} with a thermal temperature of T(H{sub 2}) = 2500{sup +800}{sub -700} K. The high temperature of the molecules, the relatively small H{sub 2} column density, and the high inclination of the AA Tauri disk suggest that the absorbing gas resides in an inner disk atmosphere. If the H{sub 2} and CO are cospatial within a molecular layer {approx}0.6 AU thick, this region is characterized by {approx} 10{sup 5} cm{sup -3} with an observed (CO/H{sub 2}) ratio of {approx}0.4. We also find evidence for a departure from a purely thermal H{sub 2} distribution, suggesting that excitation by continuum photons and H{sub 2} formation may be altering the level populations in the molecular gas.

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DOE OSTI2012

PDS 144: THE FIRST CONFIRMED Herbig Ae-Herbig Ae WIDE BINARY

PDS 144 is a pair of Herbig Ae stars that are separated by 5.''35 on the sky. It has previously been shown to have an A2Ve Herbig Ae star viewed at 83 Degree-Sign inclination as its northern member and an A5Ve Herbig Ae star as its southern member. Direct imagery revealed a disk occulting PDS 144 N-the first edge-on disk observed around a Herbig Ae star. The lack of an obvious disk in direct imagery suggested PDS 144 S might be viewed face-on or not physically associated with PDS 144 N. Multi-epoch Hubble Space Telescope imagery of PDS 144 with a 5 year baseline demonstrates PDS 144 N and S are comoving and have a common proper motion with TYC 6782-878-1. TYC 6782-878-1 has previously been identified as a member of Upper Sco sub-association A at d = 145 {+-} 2 pc with an age of 5-10 Myr. Ground-based imagery reveals jets and a string of Herbig-Haro knots extending 13' (possibly further) which are aligned to within 7 Degree-Sign {+-} 6 Degree-Sign on the sky. By combining proper motion data and the absence of a dark mid-plane with radial velocity data, we measure the inclination of PDS 144 S to be i = 73 Degree-Sign {+-} 7 Degree-Sign . The radial velocity of the jets from PDS 144 N and S indicates they, and therefore their disks, are misaligned by 25 Degree-Sign {+-} 9 Degree-Sign . This degree of misalignment is similar to that seen in T Tauri wide binaries.

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DOE OSTI2012

MAGNETICALLY CONTROLLED ACCRETION FLOWS ONTO YOUNG STELLAR OBJECTS

Accretion from disks onto young stars is thought to follow magnetic field lines from the inner disk edge to the stellar surface. The accretion flow thus depends on the geometry of the magnetic field. This paper extends previous work by constructing a collection of orthogonal coordinate systems, including the corresponding differential operators, where one coordinate traces the magnetic field lines. This formalism allows for an (essentially) analytic description of the geometry and the conditions required for the flow to pass through sonic points. Using this approach, we revisit the problem of magnetically controlled accretion flow in a dipole geometry, and then generalize the treatment to consider magnetic fields with multiple components, including dipole, octupole, and split monopole contributions. This approach can be generalized further to consider more complex magnetic field configurations. Observations indicate that accreting young stars have substantial dipole and octupole components, and that accretion flow is transonic. If the effective equation of state for the fluid is too stiff, however, the flow cannot pass smoothly through the sonic points in steady state. For a multipole field of order l, we derive a general constraint on the polytropic index, n > l + 3/2, required for steady transonic flow to reach free-fall velocities. For octupole fields, inferred on surfaces of T Tauri stars, the index n > 9/2, so that the flow must be close to isothermal. The inclusion of octupole field components produces higher densities at the stellar surface and smaller areas for the hot spots, which occur at higher latitudes; the magnetic truncation radius is smaller (larger) for octupole components that are aligned (anti-aligned) with the stellar dipole. This contribution thus increases our understanding of magnetically controlled accretion for young stellar objects and can be applied to a variety of additional astrophysical problems.

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DOE OSTI2012

CHARGING AND COAGULATION OF DUST IN PROTOPLANETARY PLASMA ENVIRONMENTS

Combining a particle-particle, particle-cluster, and cluster-cluster agglomeration model with an aggregate charging model, the coagulation and charging of dust particles in plasma environments relevant for protoplanetary disks have been investigated, including the effect of electron depletion in high dust density environments. The results show that charged aggregates tend to grow by adding small particles and clusters to larger particles and clusters, and that cluster-cluster aggregation is significantly more effective than particle-cluster aggregation. Comparisons of the grain structure show that with increasing aggregate charge the compactness factor, {phi}{sub {sigma}}, decreases and has a narrower distribution, indicating a fluffier structure. Neutral aggregates are more compact, with larger {phi}{sub {sigma}}, and exhibit a larger variation in fluffiness. Overall, increased aggregate charge leads to larger, fluffier, and more massive aggregates.

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DOE OSTI2012

INDUCED TURBULENCE AND THE DENSITY STRUCTURE OF THE DUST LAYER IN A PROTOPLANETARY DISK

We study the turbulence induced in the dust layer of a protoplanetary disk based on the energetics of dust accretion due to gas drag. We estimate turbulence strength from the energy supplied by dust accretion, using the radial drift velocity of the dust particles in a laminar disk. Our estimate of the turbulence strength agrees with previous analytical and numerical research on the turbulence induced by Kelvin-Helmholtz and/or streaming instabilities for particles with stopping time less than the Keplerian time. For such small particles, the strongest turbulence is expected to occur when the dust-to-gas ratio of the disk is {approx}C{sup 1/2}{sub eff}(h{sub g} /r) {approx} 10{sup -2}, where C{sub eff} Almost-Equal-To 0.2 represents the energy supply efficiency to turbulence and h{sub g} /r {approx} 5 Multiplication-Sign 10{sup -2} is the aspect ratio of the gas disk. The maximum viscosity parameter is {alpha}{sub max} {approx} C{sub eff} T{sub s} (h{sub g} /r){sup 2} {approx} 10{sup -4} T{sub s} , where T{sub s} (< 1) is the non-dimensional stopping time of the dust particles. Modification to the dust-to-gas ratio from the standard value, 10{sup -2}, by any process, results in weaker turbulence and a thinner dust layer, and consequently may accelerate the growth process of the dust particles.

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DOE OSTI2012

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, provides 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 {mu}m. 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 Very Large Telescope CRIRES high-resolution (R {approx} 100,000) spectra of the CO fundamental rovibrational band at 4.7 {mu}m. 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 {approx}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 polycyclic aromatic hydrocarbon molecules, usually a gas tracer, within the hole makes the large CO hole even more unexpected.

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