Passive Disks Research Articles

Pinnatoxins and spirolides in Norwegian blue mussels and seawater

Fast-acting cyclic imines belonging to the pinnatoxin and pteriatoxin group of toxins were originally identified in shellfish of the genera Pinna and Pteria in Japan, after food poisoning events in China linked to consumption of Pinna spp. Recently, a range of new and known pinnatoxin analogs has been identified in shellfish, sediment, and seawater samples from Australia and New Zealand. Although the structurally closely-related spirolide toxins are better known, and have a worldwide distribution including Norway and other parts of Europe, the presence of pinnatoxins has not been reported in European waters or shellfish. Here we report results from a survey of Norwegian blue mussels for the presence of pinnatoxins and spirolides, by LC-MS/MS analysis of extracts obtained as part of Norway’s routine monitoring programme for regulated algal toxins during late autumn and early winter 2009. Spirolides and pinnatoxin G were widespread (pinnatoxin G (1), spirolide C (2), iso-spirolide C (3), 13-desmethylspirolide C (4), 13,19-didesmethylspirolide C (5), and 20-methylspirolide G (6) were detected in 69%, 13%, 60%, 22%, 33%, and 77%, respectively, of the shellfish samples) and, although levels were generally low, concentrations of up to 115 μg/kg of pinnatoxin G (1) and 226 μg/kg of 13-desmethylspirolide C (4) were detected. We also analyzed stored extracts from passive sampling disks deployed as part of a separate study in autumn 2007. All the stored extracts contained 20-methylspirolide G (which predominated at most locations), most contained pinnatoxin G (73%) and 13,19-didesmethylspirolide C (67%), but iso-spirolide C (36%) and 13-desmethylspirolide C (52%) were also detected in many of the samples. These results suggest that pinnatoxins may be much more widespread than previously suspected, and indicate that they or related compounds could be responsible for sporadic incidents of rapid-onset symptoms during mouse bioassays of shellfish in Europe and elsewhere. The toxicological significance of these levels of pinnatoxins and spirolides is at present unclear. However, although pinnatoxins appear to be less toxic than spirolides by intraperitoneal injection in the mouse bioassay, recently published preliminary toxicological data indicate that pinnatoxins may be as much as an order of magnitude more toxic than spirolides by oral ingestion via food.

Constraining the structure of the planet-forming region in the disk of the Herbig Be star HD 100546

Studying the physical conditions in circumstellar disks is a crucial step toward understanding planet formation. Of particular interest is the case of HD 100546, a Herbig Be star that presents a gap within the first 13 AU of its protoplanetary disk, that may originate in the dynamical interactions of a forming planet. We gathered a large amount of new interferometric data using the AMBER/VLTI instrument in the H- and K-bands to spatially resolve the warm inner disk and constrain its structure. Then, combining these measurements with photometric observations, we analyze the circumstellar environment of HD 100546 in the light of a passive disk model based on 3D Monte-Carlo radiative transfer. Finally, we use hydrodynamical simulations of gap formation by planets to predict the radial surface density profile of the disk and test the hypothesis of ongoing planet formation. The SED and the NIR interferometric data are adequately reproduced by our model. We show that the H- and K-band emissions are coming mostly from the inner edge of the internal dust disk, located near 0.24 AU from the star, i.e., at the dust sublimation radius in our model. We directly measure an inclination of $33^{\circ} \pm 11^{\circ}$ and a position angle of $140^{\circ} \pm 16^{\circ}$ for the inner disk. This is similar to the values found for the outer disk ($i \simeq 42^{\circ}$, $PA \simeq 145^{\circ}$), suggesting that both disks may be coplanar. We finally show that 1 to 8 Jupiter mass planets located at $\sim 8$ AU from the star would have enough time to create the gap and the required surface density jump of three orders of magnitude between the inner and outer disk. However, no information on the amount of matter left in the gap is available, which precludes us from setting precise limits on the planet mass, for now.

Characteristics of air pollution by polychlorinated dibenzo- p-dioxins and dibenzofurans in the typical industrial areas of Tangshan City, China

The ambient air in vicinity of different industrial sources for PCDD/PCDFs was sampled by TSP/PM 10 active samplers and passive PUF disk samplers in Tangshan City, a metropolis containing clusters of various industrial plants. The ΣTEQ concentrations of PCDD/PCDFs ranged from 44.2 to 394.1 fg I-TEQ/m 3 with an average of 169.9 fg I-TEQ/m 3. 2,3,4,7,8-PeCDF was the dominant contributor to (TEQ, contributing 41% (12% to 55%), while 1,2,3,4,6,7,8-HpCDF, OCDD and OCDF were the major congeners for the total concentrations. The ratios of Σ(PCDF/Σ(PCDD reached 2.54 on average, suggesting that de novo synthesis in thermal processes played an important role to the airborne pollution of PCDD/PCDFs. The similarities congener profiles indicated that TSP and PM 10 active sampling methods are comparable for the determination of the PCDD/PCDFs in ambient air, and the ratios of concentrations determined by the two methods suggested that the PCDD/PCDFs tended to stay in fine particles. It was found that 2,3,7,8-TCDF and OCDD were the dominating congeners in the passive PUF disks samples. Through principal components analysis, the coke industry was suggested to be a relatively high potential emission source for PCDD/PCDFs in the ambient air of Tangshan, which was possibly formed by de novo synthesis mechanism. In this study, the atmospheric impacts to the environment from different industrial sources could be ranked as follows (from high to low): coking, iron sintering, steel making, power generation and chlorinate alkali chemical production industries.

FORMING PLANETESIMALS BY GRAVITATIONAL INSTABILITY. II. HOW DUST SETTLES TO ITS MARGINALLY STABLE STATE

Dust at the midplane of a circumstellar disk can become gravitationally unstable and fragment into planetesimals if the local dust-to-gas density ratio mu is sufficiently high. We simulate how dust settles in passive disks and ask how high mu can become. We settle the dust using a 1D code and test for dynamical stability using a 3D shearing box code. This scheme allows us to explore the behavior of small particles having short but non-zero stopping times in gas: 0 < t_stop << the orbital period. The streaming instability is thereby filtered out. Dust settles until shearing instabilities in the edges of the dust layer threaten to overturn the entire layer. In this state of marginal stability, mu=2.9 for a disk whose bulk (height-integrated) metallicity is solar. For a disk whose bulk metallicity is 4x solar, mu reaches 26.4. These maximum values of mu, which depend on the background radial pressure gradient, are so large that gravitational instability of small particles is viable in disks whose bulk metallicities are just a few (<4) times solar. Earlier studies assumed that dust settles until the Richardson number Ri is spatially constant. Our simulations are free of this assumption but provide support for it within the dust layer's edges, with the proviso that Ri increases with bulk metallicity in the same way that we found in Paper I. Only modest enhancements in bulk metallicity are needed to spawn planetesimals directly from small particles.

THE RISE AND FALL OF PASSIVE DISK GALAXIES: MORPHOLOGICAL EVOLUTION ALONG THE RED SEQUENCE REVEALED BY COSMOS

The increasing abundance of passive “red-sequence” galaxies since z ∼ 1–2 is mirrored by a coincident rise in the number of galaxies with spheroidal morphologies. In this paper, however, we show in detail, that, the correspondence between galaxy morphology and color is not perfect, providing insight into the physical origin of this evolution. Using the COSMOS survey, we study a significant population of red-sequence galaxies with disk-like morphologies. These passive disks typically have Sa–Sb morphological types with large bulges, but they are not confined to dense environments. They represent nearly one-half of all red-sequence galaxies and dominate at lower masses (≲1010M☉) where they are increasingly disk-dominated. As a function of time, the abundance of passive disks with M* ≲ 1011M☉ increases, but not as fast as red-sequence spheroidals in the same mass range. At higher mass, the passive disk population has declined since z ∼ 1, likely because they transform into spheroidals. Based on these trends, we estimate that as much as 60% of galaxies transitioning onto the red sequence evolve through a passive disk phase. The origin of passive disks therefore has broad implications for our understanding of how star formation shuts down. Because passive disks tend to be more bulge-dominated than their star-forming counterparts, a simple fading of blue disks does not fully explain their origin. We explore the strengths and weaknesses of several more sophisticated explanations, including environmental effects, internal stabilization, and disk regrowth during gas-rich mergers. While previous work has sought to explain color and morphological transformations with a single process, these observations open the way to new insight by highlighting the fact that galaxy evolution may actually proceed through several separate stages.

Erratum to Thermal and hydrostatic structure of the protoplanetary nebula exposed to stellar radiation and stellar wind from the central star

A model for a nebula exposed to the radiation and stellar wind from the central star has been examined. T Tauri stars commonly have disks and stellar wind, though we have no theoretical model on nebular thermal structure under the wind. The aims of this paper are to propose a theoretical nebula model, and to perform mathematical analysis on the geometrical structure and temperature distribution of a passive nebula. Its geometrical surface is determined by the dynamical pressure of the stellar wind. The nebular surface is assumed to be a black body surface, heated by the incident stellar photon flux. We obtain temperature from the equation of energy balance between the stellar radiation upon this nebular surface and the black body radiation from it. The temperature distribution in passive disks is insensitive to the wind strength and is almost identical even if the wind strength changes by five orders of magnitude. The nebula temperature is not expressed by a simple power law function of the distance from the central star. This is an important difference between our results and those of previous works. Since the nebula surrounding a T Tauri star is influenced by stellar wind, our model may be more appropriate than any other simple single power law temperature for passive protoplanetary nebulae.

Galaxy Zoo: passive red spirals

We study the spectroscopic properties and environments of red spiral galaxies found by the Galaxy Zoo project. By carefully selecting face-on, disk dominated spirals we construct a sample of truly passive disks (not dust reddened, nor dominated by old stellar populations in a bulge). As such, our red spirals represent an interesting set of possible transition objects between normal blue spirals and red early types. We use SDSS data to investigate the physical processes which could have turned these objects red without disturbing their morphology. Red spirals prefer intermediate density regimes, however there are no obvious correlations between red spiral properties and environment - environment alone is not sufficient to determine if a spiral will become red. Red spirals are a small fraction of spirals at low masses, but are a significant fraction at large stellar masses - massive galaxies are red independent of morphology. We confirm that red spirals have older stellar popns and less recent star formation than the main spiral population. While the presence of spiral arms suggests that major star formation cannot have ceased long ago, we show that these are not recent post-starbursts, so star formation must have ceased gradually. Intriguingly, red spirals are ~4 times more likely than normal spirals to host optically identified Seyfert or LINER, with most of the difference coming from LINERs. We find a curiously large bar fraction in the red spirals suggesting that the cessation of star formation and bar instabilities are strongly correlated. We conclude by discussing the possible origins. We suggest they may represent the very oldest spiral galaxies which have already used up their reserves of gas - probably aided by strangulation, and perhaps bar instabilities moving material around in the disk.

The complex structure of the disk around HD 100546

Disclosing the structure of disks surrounding Herbig AeBe stars is important to expand our understanding of the formation and early evolution of stars and planets. We aim at revealing the sub-AU disk structure around the 10 Myr old Herbig Be star HD100546 and at investigating the origin of its near and mid-infrared excess. We used AMBER/VLTI observations to resolve the K-band emission and to constrain the location and composition of the hot dust in the innermost disk. Combining AMBER observations with photometric and MIDI/VLTI measurements from the litterature, we revisit the disk geometry using a passive disk model based on 3D radiative transfer. We propose a model that includes a tenuous inner disk made of micron-sized dust grains, a gap, and a massive optically thick outer disk, that successfully reproduces the interferometric data and the SED. We locate the bulk of the K-band emission at ~0.26 AU. Assuming that this emission originates from silicate, we show that micron-sized grains are required to enable the dust to survive at such a distance from the star. As a consequence, more than 40% of the K-band flux is related to scattering, showing that direct thermal emission is not sufficient to explain the near-infrared excess. In the massive outer disk, large grains in the mid-plane are responsible for the mm emission while a surface layer of small grains allows the mid and far infrared excesses to be reproduced. Such vertical structure may be an evidence for sedimentation. The observations are consistent with a model that includes a gap until ~13 AU and a total dust mass of ~0.008 lunar mass inside it. These values together with the derived scale height (~2.5 AU) and temperature (~220 K) at the inner edge of the outer disk (r=13 AU), are consistent with recent CO observations.

The flared inner disk of the Herbig Ae star AB Aurigae revealed by VLTI/MIDI in theN-band

Aims. We aim at using the long baselines of the VLT Interferometer and the mid-IR combiner MIDI (8-13 μm) to derive the morphology of the protoplanetary disk surrounding the Herbig Ae star AB Aurigae Methods: We present the first N-band analysis of AB Aur performed with a maximum angular resolution of 17 mas (2.5 AU at the Taurus-Auriga distance). We used the radiative transfer code MC3D and a silicate-dominated dust grain mixture to fit the spectral energy distribution (SED), together with the N-band dispersed visibilities (λ / δλ ~ 30) and to constrain the inner-disk spatial structure. Results: The silicate band is prominent in the ~ 300 mas FOV of the MIDI instrument, the emission reaches 70 to 90% of the total flux measured by ISO. The circumstellar emission (CSE) is resolved even at the shortest baselines. The spectrally dispersed visibilities show a steep drop between 8 and 9.5 μm, followed by a plateau between 10 and 13 μm. Our modelling shows that the observed SED and visibilities can be reproduced with a simple passive disk model. For such a weakly inclined disk (i ~ 30 deg), the mid-IR visibilities can directly determine the flaring index, while the scale height can be subsequently and unambiguously derived from the combination of the spectral and interferometric constraints. The modelling yields typical values for the scale height of about 8 AU at a radial distance of 100 AU and a flaring index in the range 1.25-1.30 for the explored range of model input parameters. Conclusions: The radial structure of the circumstellar inner disk around AB Aur is directly determined by MIDI. The radiative transfer modelling demonstrates the powerful synergy of interferometry and spectro-photometry to alleviate the degeneracy, which may hamper determining the disk morphology. Our analysis supports the classification of AB Aur among the flared disks of the first group in the Meeus classification. Based on observations collected at ESO (Paranal Observatory) with the VLT Interferometer - Prog ID: 074.C-552 & 076.C-252.

A convenient and cost-effective method for monitoring marine algal toxins with passive samplers

Passive sampling disks were developed based on the method of MacKenzie, L., Beuzenberg, V., Holland, P., McNabb, P., Selwood, A. [2004. Solid phase adsorption toxin tracking (SPATT): a new monitoring tool that simulates the biotoxin contamination of filter feeding bivalves. Toxicon 44, 901–918] and protocols were formulated for recovering toxins from the adsorbent resin via elution from small columns. The disks were used in field studies to monitor in situ toxin dynamics during mixed algal blooms at Flødevigen in Norway. Examples are given from time-integrated sampling using the disks followed by extraction and high performance liquid chromatography–mass spectrometry (HPLC–MS) analysis for azaspiracids, okadaic acid analogues, pectenotoxins, yessotoxins and spirolides. Profiles of accumulated toxins in the disks and toxin profiles in blue mussels ( Mytilus edulis) were compared with the relative abundance of toxin-producing algal species. Results obtained showed that passive sampling disks correlate with the toxin profiles in shellfish. The passive sampling disks were cheap to produce and convenient to use and, when combined with HPLC–MS or enzyme-linked immunosorbent assay (ELISA) analysis, provide detailed time-averaged information on the profile of lipophilic toxin analogues in the water. Passive sampling is therefore a useful tool for monitoring the exposure of shellfish to the toxigenic algae of concern in northern Europe.

Planet Shadows in Protoplanetary Disks. I. Temperature Perturbations

Planets embedded in optically thick passive accretion disks are expected to produce perturbations in the density and temperature structure of the disk. We calculate the magnitudes of these perturbations for a range of planet masses and distances. The model predicts the formation of a shadow at the position of the planet paired with a brightening just beyond the shadow. We improve on previous work on the subject by self-consistently calculating the temperature and density structures under the assumption of hydrostatic equilibrium and taking the full three-dimensional shape of the disk into account rather than assuming a plane-parallel disk. While the excursion in temperatures is less than in previous models, the spatial size of the perturbation is larger. We demonstrate that a self-consistent calculation of the density and temperature structure of the disk has a large effect on the disk model. In addition, the temperature structure in the disk is highly sensitive to the angle of incidence of stellar irradiation at the surface, so accurately calculating the shape of the disk surface is crucial for modeling the thermal structure of the disk.

Detection of an Inner Gaseous Component in a Herbig Be Star Accretion Disk: Near‐ and Mid‐Infrared Spectrointerferometry and Radiative Transfer modeling of MWC 147

Westudythegeometryandthephysicalconditionsintheinner(AU-scale)circumstellarregionaroundtheyoung HerbigBestarMWC147usinglong-baselinespectrointerferometryinthenear-infrared(NIR)K-band,VLTI/AMBER observations, and PTI archive data, as well as the mid-infrared (MIR) N-band, VLTI/MIDI observations. The emission from MWC 147 is clearly resolved and has a characteristic physical size of � 1.3 and � 9 AU at 2.2 and 11 � m, respectively (Gaussian diameter). The MIR emission reveals asymmetry consistent with a disk structure seen under intermediate inclination. The spectrally dispersed AMBER and MIDI interferograms both show a strong increase in the characteristic size toward longer wavelengths, much steeper than predicted by analytic disk models assuming power-law radial temperature distributions. We model the interferometric data and the spectral energy distribution of MWC 147 withtwo-dimensional, frequency-dependent radiationtransfer simulations. Thisanalysis showsthat models of spherical envelopes or passive irradiated Keplerian disks (with vertical or curved puffed-up inner rim) can easily fit the SED, but predict much lower visibilities than observed; the angular size predicted by such models is 2Y4 times larger than the size derived from the interferometric data, so these models can clearly be ruled out. Models of a Keplerian disk with optically thick gas emission from an active gaseous disk (inside the dust sublimation zone), however, yield a good fit of the SED and simultaneously reproduce the absolute level and the spectral dependence of the NIR and MIR visibilities. We conclude that the NIR continuum emission from MWC 147 is dominated by accretion luminosity emerging from an optically thick inner gaseous disk, while the MIR emission also contains contributions from the outer, irradiated dust disk. Subject headingg accretion, accretion disks — stars: formation — stars: individual (MWC 147) — stars: preYmain-sequence — techniques: interferometric

Thermal Waves in Irradiated Protoplanetary Disks

Protoplanetary disks are mainly heated by radiation from the central star. Since the incident stellar flux at any radius is sensitive to the disk structure near that location, an unstable feedback may be present. Previous investigations show that the disk would be stable to finite-amplitude temperature perturbations if the vertical height of optical surface is everywhere directly proportional to the gas scale height and if the intercepted fraction of stellar radiation is determined from the local grazing angle. We show that these assumptions may not be generally applicable. Instead, we calculate the quasi-static thermal evolution of irradiated disks by directly integrating the global optical depths to determine the optical surface and the total emitting area-filling factor of surface dust. We show that, in disks with modest mass accretion rates, thermal waves are spontaneously and continually excited in the outer disk, propagate inward through the planet-forming domains, and dissipated at small radii where viscous dissipation is dominant. This state is quasi-periodic over several thermal timescales and its pattern does not depend on the details of the opacity law. The viscous dissipation resulting from higher mass accretion stabilizes this instability such that an approximately steady state is realized throughout the disk. In passive protostellar disks, especially transitional disks, these waves induce significant episodic changes in SEDs, on the time scales of years to decades, because the midplane temperatures can vary by a factor of two between the exposed and shadowed regions. The transitory peaks and troughs in the potential vorticity distribution may also lead to baroclinic instability and excite turbulence in theplanet-forming regions.

TheSpitzerc2d Survey of Large, Nearby, Interstellar Clouds. IX. The Serpens YSO Population as Observed with IRAC and MIPS

We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy observations of the Serpens star-forming region. In particular we present a set of criteria for isolating bona fide young stellar objects, YSO's, from the extensive background contamination by extra-galactic objects. We then discuss the properties of the resulting high confidence set of YSO's. We find 235 such objects in the 0.85 deg^2 field that was covered with both IRAC and MIPS. An additional set of 51 lower confidence YSO's outside this area is identified from the MIPS data combined with 2MASS photometry. We describe two sets of results, color-color diagrams to compare our observed source properties with those of theoretical models for star/disk/envelope systems and our own modeling of the subset of our objects that appear to be star+disks. These objects exhibit a very wide range of disk properties, from many that can be fit with actively accreting disks to some with both passive disks and even possibly debris disks. We find that the luminosity function of YSO's in Serpens extends down to at least a few x .001 Lsun or lower for an assumed distance of 260 pc. The lower limit may be set by our inability to distinguish YSO's from extra-galactic sources more than by the lack of YSO's at very low luminosities. A spatial clustering analysis shows that the nominally less-evolved YSO's are more highly clustered than the later stages and that the background extra-galactic population can be fit by the same two-point correlation function as seen in other extra-galactic studies. We also present a table of matches between several previous infrared and X-ray studies of the Serpens YSO population and our Spitzer data set.

Thermal and hydrostatic structure of the protoplanetary nebula exposed to stellar radiation and stellar wind from the central star

Abstract A model for a nebula exposed to the radiation and stellar wind from the central star has been examined. T Tauri stars commonly have disks and stellar wind, though we have no theoretical model on nebular thermal structure under the wind. The aims of this paper are to propose a theoretical nebula model, and to perform mathematical analysis on the geometrical structure and temperature distribution of a passive nebula. Its geometrical surface is determined by the dynamical pressure of the stellar wind. The nebular surface is assumed to be a black body surface, heated by the incident stellar photon flux. We obtain temperature from the equation of energy balance between the stellar radiation upon this nebular surface and the black body radiation from it. The temperature distribution in passive disks is insensitive to the wind strength and is almost identical even if the wind strength changes by five orders of magnitude. The nebula temperature is not expressed by a simple power law function of the distance from the central star. This is an important difference between our results and those of previous works. Since the nebula surrounding a T Tauri star is influenced by stellar wind, our model may be more appropriate than any other simple single power law temperature for passive protoplanetary nebulae.

The inner gaseous accretion disk around a Herbig Be star revealed by near- and mid-infrared spectro-interferometry

AbstractHerbig Ae/Be stars are pre-main-sequence stars of intermediate mass, which are still accreting material from their environment, probably via a disk composed of gas and dust. Here we present a recent study of the geometry of the inner (AU-scale) circumstellar region around the Herbig Be star MWC 147 using long-baseline interferometry. By combining for the first time near- and mid-infrared spectro-interferometry on a Herbig star, our VLTI/AMBER and VLTI/MIDI data constrain not only the geometry of the brightness distribution, but also the radial temperature distribution in the disk. The emission from MWC 147 is clearly resolved and has a characteristic physical size of ∼1.3 AU and ∼9 AU at 2.2 μm and 11 μm respectively. This increase in apparent size towards longer wavelengths is much steeper than predicted by analytic disk models assuming power-law radial temperature distributions. For a detailed modeling of the interferometric data and the spectral energy distribution of MWC 147, we employ 2-D frequency-dependent radiation transfer simulations. This analysis shows that passive irradiated Keplerian dust disks can easily fit the SED, but predict much lower visibilities than observed, so these models can clearly be ruled out. Models of a Keplerian disk with emission from an optically thick inner gaseous accretion disk (inside the dust sublimation zone), however, yield a good fit of the SED and simultaneously reproduce the observed near- and mid-infrared visibilities. We conclude that the near-infrared continuum emission from MWC 147 is dominated by accretion luminosity emerging from an optically thick inner gaseous disk, while the mid-infrared emission also contains strong contributions from the passive irradiated dust disk.

Anomalous X-ray pulsars: persistent states with fallback disks

The anomalous X-ray pulsar 4U 0142+61 was recently detected in the mid infrared bands with the SPITZER Observatory (Wang, Chakrabarty and Kaplan: Nature 440, 772 (2006)). This observation is the first instance for a disk around an AXP. From a reanalysis of optical and infrared data, we show that the observations indicate that the disk is likely to be an active disk rather than a passive dust disk beyond the light cylinder, as proposed in the discovering paper. Furthermore, we show that the irradiated accretion disk model can also account for all the optical and infrared observations of the anomalous X-ray pulsars in the persistent state.

Probing Protoplanetary Disks with Silicate Emission: Where Is the Silicate Emission Zone?

Recent results indicate that the grain size and crystallinity inferred from observations of silicate features may be correlated with spectral type of the central star and/or disk geometry. In this paper, we show that grain size, as probed by the 10 um silicate feature peak-to-continuum and 11.3-to-9.8 um flux ratios, is inversely proportional to log L_star. These trends can be understood using a simple two-layer disk model for passive irradiated flaring disks, CGPLUS. We find that the radius, R_10, of the 10 um silicate emission zone in the disk goes as (L_star/L_sun)^0.56, with slight variations depending on disk geometry (flaring angle, inner disk radius). The observed correlations, combined with simulated emission spectra of olivine and pyroxene mixtures, imply a grain size dependence on luminosity. Combined with the fact that R_10 is smaller for less luminous stars, this implies that the apparent grain size of the emitting dust is larger for low-luminositysources. In contrast, our models suggest that the crystallinity is only marginally affected, because for increasing luminosity, the zone for thermal annealing (assumed to be at T>800 K) is enlarged by roughly the same factor as the silicate emission zone. The observed crystallinity is affected by disk geometry, however, with increased crystallinity in flat disks. The apparent crystallinity may also increase with grain growth due to a corresponding increase in contrast between crystalline and amorphous silicate emission bands.

Young, Low‐Mass Brown Dwarfs with Mid‐Infrared Excesses

We have combined new I, J, H, and Ks imaging of portions of the Chamaeleon II, Lupus I, and Ophiuchus star-forming clouds with 3.6 to 24 micron imaging from the Spitzer Legacy Program, From Molecular Clouds to Planet Forming Disks, to identify a sample of 19 young stars, brown dwarfs and sub-brown dwarfs showing mid-infrared excess emission. The resulting sample includes sources with luminosities of 0.5>log(L/Lsun)>-3.1. Six of the more luminous sources in our sample have been previously identified by other surveys for young stars and brown dwarfs. Five of the sources in our sample have nominal masses at or below the deuterium burning limit (~12 M_J). Over three decades in luminosity, our sources have an approximately constant ratio of excess to stellar luminosity. We compare our observed SEDs to theoretical models of a central source with a passive irradiated circumstellar disk and test the effects of disk inclination, disk flaring, and the size of the inner disk hole on the strength/shape of the excess. The observed SEDs of all but one of our sources are well fit by models of flared and/or flat disks.

A Keplerian Disk around the Herbig Ae Star HD 169142

We present Submillimeter Array observations of the Herbig Ae star HD 169142 in 1.3 mm continuum emission and 12CO J = 2-1 line emission at ~15 resolution that reveal a circumstellar disk. The continuum emission is centered on the star position and resolved, and it provides a mass estimate of ~0.02 M⊙ for the disk. The CO images show patterns in position and velocity that are well matched by a disk in Keplerian rotation with low inclination to the line of sight. We use radiative transfer calculations based on a flared, passive disk model to constrain the disk parameters by comparison to the spectral line emission. The derived disk radius is 235 AU, and the inclination is 13°. The model also necessitates modest depletion of the CO molecules, similar to that found in Keplerian disks around T Tauri stars.