We present spatially resolved scattered light images of the circumstellar disk around HK Tau B at 3.8 and 4.7 mu m taken with the Keck Telescope Laser Guide Star Adaptive Optics (AO) system, and 1.6-2.12 mu m images taken with the Very Large Telescope/NACO AO system. Combined with previously published optical Hubble Space Telescope data, we investigate the spatially resolved scattered light properties of this edge-on circumstellar disk and probe for the presence of large grains. The 0.6-3.8 mu m scattered light observations reveal strong, and in some cases, unusual, wavelength dependencies in the observed disk morphology. The separation between the two scattered light nebulae, which is directly proportional to the disk-mass-opacity product, decreases by 30% between 0.6 and 3.8 mu m. Over the same wavelength range, the FWHM of the disk nebulosity declines by a factor of two, while the flux ratio between the two nebulae increases by a factor of similar to 8. No other disk known to date shows a flux ratio that increases with wavelength. Both the FWHM and nebula flux ratio are affected by the scattering phase function and the observed behavior can most readily be explained by a phase function that becomes more forward throwing with wavelength. The multi-wavelength scattered light observations also confirm the asymmetric nature of the disk and show that the level of asymmetry is a function of wavelength. We use the MCFOST radiative transfer code to model the disk at four wavelengths, corresponding to the I, H, Ks, and L' bandpasses. A single power-law grain size distribution can recreate the observed disk properties simultaneously at all four wavelengths. Bayesian analysis of the dust parameters finds a 99% probability that the maximum grain size is 5.5 mu m or larger. We also find that the grain size distribution is steep, with a 99% probability of a power-law index of 4.2 or larger, suggesting that these large grains are a small fraction of the overall dust population. The best-fit dust asymmetry parameter for each individual wavelength shows an unusual behavior, increasing with wavelength from the optical through the near-infrared, peaking at similar to 0.8 between 2.2 and 3.8 mu m, then decreasing by a factor of two by similar to 12 mu m. Comparing the wavelength dependence of the asymmetry parameter for HK Tau B with those for the interstellar medium (ISM) and dark cloud dust models, we find considerable evolution from an ISM state and argue for the presence of grain growth within the disk. Further, comparing the wavelength dependence of the asymmetry parameter for GG Tau, HV Tau C, and HK Tau B, the three disks that have been spatially resolved in scattered light between 0.8 and 3.8 mu m, finds a diverse range of dust properties, indicating differing degrees of grain growth for disks at a similar age.

We present new high spatial resolution (<~ 0.1") 1-5 micron adaptive optics images, interferometric 1.3 mm continuum and 12CO 2-1 maps, and 350 micron, 2.8 and 3.3 mm fluxes measurements of the HV Tau system. Our adaptive optics images reveal an unusually slow orbital motion within the tight HV Tau AB pair that suggests a highly eccentric orbit and/or a large deprojected physical separation. Scattered light images of the HV Tau C edge-on protoplanetary disk suggest that the anisotropy of the dust scattering phase function is almost independent of wavelength from 0.8 to 5 micron, whereas the dust opacity decreases significantly over the same range. The images further reveal a marked lateral asymmetry in the disk that does not vary over a timescale of 2 years. We further detect a radial velocity gradient in the disk in our 12CO map that lies along the same position angle as the elongation of the continuum emission, which is consistent with Keplerian rotation around an 0.5-1 Msun central star, suggesting that it could be the most massive component in the triple system. We use a powerful radiative transfer model to compute synthetic disk observations and use a Bayesian inference method to extract constraints on the disk properties. Each individual image, as well as the spectral energy distribution, of HV Tau C can be well reproduced by our models with fully mixed dust provided grain growth has already produced larger-than-interstellar dust grains. However, no single model can satisfactorily simultaneously account for all observations. We suggest that future attempts to model this source include more complex dust properties and possibly vertical stratification. (Abridged)

Aims. The brown dwarf (BD) formation process has not yet been completely understood. To shed more light on the differences and similarities between star and BD formation processes, we study and compare the disk fraction among both kinds of objects over a large angular region in the Taurus cloud. In addition, we examine the spatial distribution of stars and BD relative to the underlying molecular gas. Methods. In this paper, we present new and updated photometry data from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope on 43 BDs in the Taurus cloud, and recalculate of the BD disk fraction in this region. We also useed recently available CO mm data to study the spatial distribution of stars and BDs relative to the cloud's molecular gas. Results. We find that the disk fraction among BDs in the Taurus cloud is 41 ± 12%, a value statistically consistent with the one among TTS (58 ± 9%). We find that BDs in transition from a state where they have a disk to a diskless state are rare, and we study one isolated example of a transitional disk with an inner radius of ≈0.1 AU (CFHT BD Tau 12, found via its relatively small mid-IR excess compared to most members of Taurus that have disks. We find that BDs are statistically found in regions of similar molecular gas surface density to those associated with stars. Furthermore, we find that the gas column density distribution is almost identical for stellar and substellar objects with and without disks.

We combine the results from several multiplicity surveys of pre-main-sequence stars located in four nearby star-forming regions with Spitzer data from three different Legacy Projects. This allows us to construct a sample of 349 targets, including 125 binaries, which we use to to investigate the effect of companions on the evolution of circumstellar disks. We find that the distribution of projected separations of systems with Spitzer excesses is significantly different (P∼ 2.4e-5, according to the K–S test for binaries with separations less than 400 AU) from that of systems lacking evidence for a disk. As expected, systems with projected separations less than 40 AU are half as likely to retain at least one disk than are systems with projected separations in the 40–400 AU range. These results represent the first statistically significant evidence for a correlation between binary separation and the presence of an inner disk (r∼ 1 AU). Several factors (e.g., the incompleteness of the census of close binaries, the use of unresolved disk indicators, and projection effects) have previously masked this correlation in smaller samples. We discuss the implications of our findings for circumstellar disk lifetimes and the formation of planets in multiple systems.

We present the Chandra High Energy Transmission Grating Spectrometer (HETGS) and Keck observations of HDE 245059, a young weak-lined T Tauri star (WTTS), member of the pre-main sequence group in the Lambda Orionis Cluster. Our high spatial resolution, near-infrared observations with Keck reveal that HDE 245059 a binary separated by 0.87". Based on this new information we have obtained an estimate of the masses of the binary components; 3M_{sun} and 2.5M_{sun} for the north and south components, respectively. We have estimated the age of the system to be ~2-3 Myr. We detect both components of the binary in the zeroth order Chandra image and in the grating spectra. Our fits to the spectrum of the binary have shown that the emission is dominated by a plasma between 8 and 15 MK, a soft component at 4 MK and a hard component at 50 MK are also detected. The value of the hydrogen column density was low, 8 x 10^{19} cm^{-2}, likely due to the clearing of the inner region of the Lambda Orionis cloud. The abundance pattern shows an inverse First Ionization Potential (FIP) effect for all elements from O to Fe, the only exception being Ca. A 3-T model was fitted to the individual zeroth order spectra using the abundances derived for the binary. We have also obtained several lines fluxes from the grating spectra. The fits to the triplets show no evidence of high densities. We conclude that the X-ray properties of the weak-lined T Tau binary HDE 245059 are similar to those generally observed in other weak-lined T Tau stars. Although its accretion history may have been affected by the clearing of the interstellar material around Lambda Ori, its coronal properties appears not to have been strongly modified.

We present Spitzer images of the Taurus Complex (TC). We take advantage of the sensitivity and the spatial resolution of the observations to characterize the diffuse infrared ( IR) emission across the cloud. This work highlights evidence of dust evolution within the translucent sections of the archetype reference for studies of quiescent molecular clouds. We combine the Spitzer 160 mu m and IRAS 100 mu m observations to produce a dust temperature map and a far-IR (FIR) dust opacity map at 5 ' resolution. The average dust temperature is about 14.5 K with a dispersion of +/- 1 K across the cloud. The FIR dust opacity is tightly correlated with the extinction derived from Two Micron All Sky Survey stellar colors and is a factor of 2 larger than the average value for the diffuse interstellar medium. This opacity increase and the attenuation of the radiation field both contribute to account for the lower emission temperature of the large grains. The structure of the TC significantly changes in the mid-IR (MIR) images that trace emission from polycyclic aromatic hydrocarbons (PAHs) and very small grains (VSGs). We focus our analysis of the MIR emission to a range of ecliptic latitudes away from the zodiacal bands and where the zodiacal light residuals are small. Within this cloud area, there are no 8 and 24 mu m counterparts to the brightest 160 mu m emission features. Conversely, the 8 and 24 mu m images reveal filamentary structure that is strikingly inconspicuous in the 160 mu m and extinction maps. The IR colors vary over subparsec distances across this filamentary structure. We compare the observed colors with model calculations quantifying the impact of the radiation field intensity and the abundance of stochastically heated particles on the dust spectral energy distribution. To match the range of observed colors, we have to invoke variations by a factor of a few of both the interstellar radiation field and the abundance of PAHs and VSGs. We conclude that within this filamentary structure a significant fraction of the dust mass cycles in and out the small-size end of the dust size distribution.

Circumstellar discs are expected to be the nursery of planets. Grain growth within such discs is the first step in the planet formation process. The Bok globule CB 26 harbours such a young disc. We present a detailed model of the edge-on circumstellar disc and its envelope in the Bok globule CB 26. The model is based on HST near-infrared maps in the I, J, H, and K bands, OVRO and SMA radio maps at 1.1mm, 1.3mm and 2.7mm, and the spectral energy distribution (SED) from 0.9 microns to 3mm. New photometric and spectroscopic data from the Spitzer Space Telescope and the Caltech Submilimeter Observatory have been obtained and are part of our analysis. Using the self-consistent radiative transfer code MC3D, the model we construct is able to discriminate parameter sets and dust properties of both its parts, namely envelope and disc. We find that the disc has an inner hole with a radius of 45 +/- 5 AU. Based on a dust model including silicate and graphite the maximum grain size needed to reproduce the spectral millimetre index is 2.5 microns. Features seen in the near-infrared images, dominated by scattered light, can be described as a result of a rotating envelope. Successful employment of ISM dust in both the disc and envelope hint that grain growth may not yet play a significant role for the appearance of this system. A larger inner hole gives rise to the assumption that CB 26 is a circumbinary disc.