[Abridged] We present the first direct comparison of the distribution of the gas, as traced by the [OI] 6300 AA emission, and the dust, as traced by the 10 micron emission, in the protoplanetary disk around three intermediate-mass stars: HD 101412, HD 135344 B and HD 179218. N-band visibilities were obtained with VLTI/MIDI. Simple geometrical models are used to compare the dust emission to high-resolution optical spectra in the 6300 AA [OI] line of the same targets. The disks around HD 101412 and HD 135344 B appear strongly flared in the gas, but self-shadowed in the dust beyond ~ 2 AU. In both systems, the 10 micron emission is rather compact (< 2 AU) while the [OI] brightness profile shows a double peaked structure. The inner peak is strongest and is consistent with the location of the dust, the outer peak is fainter and is located at 5-10 AU. Spatially extended PAH emission is found in both disks. The disk around HD 179218 is flared in the dust. The 10 micron emission emerges from a double ring-like structure with the first ring peaking at ~ 1 AU and the second at ~ 20 AU. No dust emission is detected between ~ 3 -- 15 AU. The oxygen emission seems also to come from a flared structure, however, the bulk of this emission is produced between ~ 1 -- 10 AU. This could indicate a lack of gas in the outer disk or could be due to chemical effects which reduce the abundance of OH -- the parent molecule of the observed [OI] emission -- further away from the star. The three systems, HD 179218, HD 135344 B and HD 101412, may form an evolutionary sequence: the disk initially flared becomes flat under the combined action of gas-dust decoupling, grain growth and dust settling.
We present sub-arcsecond resolution millimeter-wave images of a circumstellar disk in the Bok globule CB26. The presence of an edge-on disk is confirmed by the dust continuum morphology and the velocity field of 13CO emission, which displays a Keplerian rotation pattern about an axis perpendicular to the long axis of the dust emission. We deduce a mass ~0.3 Msun for the obscured central star. The disk is optically thick at mm wavelengths inside 120 AU, has a symmetric 20 degree warp beyond 120 AU, an outer radius of ~200 AU, and a mass of at least 0.1 Msun. We suggest that the CB26 system is i…
We present millimeter-wave continuum images of four wide (separations 210-800 AU) young stellar binary systems in the Taurus-Auriga star-forming region. For all four sources, the resolution of our observations is sufficient to determine the mm emission from each of the components. In all four systems, the primary star's disk has stronger millimeter emission than the secondary and in three of the four, the secondary is undetected; this is consistent with predictions of recent models of binary formation by fragmentation. The primaries' circumstellar disk masses inferred from these observations a…
Astronomical observations of flares from analogs of the early Sun have the potential to give critical insights into the high energy irradiation environment of protoplanetary disks. Solar-mass young stellar objects are significantly more X-ray luminous than the typical low-mass T Tauri star. They undergo frequent strong flaring on a several day time scale. Very powerful flares also occur, but on a longer time frame. The hard X-ray spectrum of these stars become even harder during flaring. The X-rays from these sun-like young stellar objects have the potential to ionize circumstellar material ou…
The origin of disks surrounding young stars has direct implications for our understanding of the formation of planetary systems. In the interstellar clouds from which star form, angular momentum is regulated by magnetic fields, preventing the spin up of contracting cores. When ~0.03 pc-sized dense cores decouple from the magnetic field and collapse dynamically, ~10^-3 km/s*pc of specific angular momentum is locked into the system. A viscous accretion disk is one of two possible mechanisms available for the necessary redistribution of angular momentum; the other one is the formation of a multip…
Circumstellar disks are expected to form early in the process that leads to the formation of a young star, during the collapse of the dense molecular cloud core. It is currently not well understood at what stage of the collapse the disk is formed or how it subsequently evolves. We aim to identify whether an embedded Keplerian protoplanetary disk resides in the L1489 IRS system. Given the amount of envelope material still present, such a disk would respresent a very young example of a protoplanetary disk. Using the Submillimeter Array (SMA) we have observed the HCO$^+$ $J=$ 3--2 line with a res…
We present high spectral resolution optical spectra of three young intermediate mass stars, in all of which we spectrally resolve the 6300 Angstrom [OI] emission line. Two of these have a double peaked line profile. We fit these data with a simple model of the [OI] emission caused by photo-dissociation of OH molecules in the upper layer of a circumstellar disk by stellar UV radiation and thus translate the Doppler broadened [OI] emission profile into an amount of emission as a function of distance from the central star. The resulting spectra are in agreement with the expected disk shapes as de…
Theoretical arguments suggest that protoplanetary disks around young stars should start small and grow with the addition of high angular momentum material to reach the radii of several hundred AUs that characterize the disks around optically visible T Tauri stars. Examples of much more compact disks, with radii much less than 100 AU, have been found around some very young stars, but in all cases tidal truncation from a near binary companion provides a ready explanation for the small disk size. We report here an example of a compact, possibly isolated disk around the class 0 object IRAS16293-24…
We present new sub-arcsecond-resolution near-infrared polarimetric imaging and millimetre interferometry data on the circumstellar disk system in the Bok globule CB26. The data imply the presence of a M > 0.01 M_sun edge-on disk of >400 AU in diameter, being in Keplerian rotation around a young ~0.35 M_sun star. The mm dust emission from the inner 200 AU is highly optically thick, but the outer parts are optically thin and made of small dust grains. Planetesimal growth in the inner disk could neither be comfirmed nor excluded. The outer optically thin part of the disk is strongly warped. We ar…
We characterize the crystalline silicate content and spatial distribution of small dust grains in a large sample of protoplanetary disks in the Taurus-Auriga young cluster, using Spitzer Space Telescope mid-infrared spectra. In turn we use the results to analyze the evolution of structure and composition of these 1-2 Myr-old disks around Solar- and later-type young stars, and test the standard models of dust processing which result in the conversion of originally amorphous dust into minerals. We find strong evidence of evolution of the dust crystalline mass fraction in parallel with that of th…
We show that grains have grown to ~mm size (sand sized) or larger in the terrestrial zone (within ~3 AU) of the protoplanetary disk surrounding the 3 Myr old binary star KH 15D. We also argue that the reflected light in the system reaches us by back scattering off the far side of the same ring whose near side causes the obscuration.
We study the properties of powerful X-ray flares from 161 pre-main sequence (PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula region. Relationships between flare properties, protoplanetary disks and accretion are examined in detail to test models of star-disk interactions at the inner edge of the accretion disks. Previous studies had found no differences in flaring between diskfree and accreting systems other than a small overall diminution of X-ray luminosity in accreting systems. The most important finding is that X-ray coronal extents in fast-rotating diskfree star…
We model the evolution of protoplanetary disks surrounding millisecond pulsars, using PSR 1257+12 as a test case. Initial conditions were chosen to correspond to initial angular momenta expected for supernova-fallback disks and disks formed from the tidal disruption of a companion star. Models were run under two models for the viscous evolution of disks: fully viscous and layered accretion disk models. Supernova-fallback disks result in a distribution of solids confined to within 1-2 AU and produce the requisite material to form the three known planets surrounding PSR 1257+12. Tidal disruption…
We present a new continuum 3D radiative transfer code, MCFOST, based on a Monte-Carlo method. MCFOST can be used to calculate (i) monochromatic images in scattered light and/or thermal emission, (ii) polarisation maps, (iii) interferometric visibilities, (iv) spectral energy distributions and (v) dust temperature distributions of protoplanetary disks. Several improvements to the standard Monte Carlo method are implemented in MCFOST to increase efficiency and reduce convergence time, including wavelength distribution adjustments, mean intensity calculations and an adaptive sampling of the radia…
Mid-infrared (MIR) images of the Herbig Ae star HD 142527 were obtained at 18.8 and 24.5 micron with the Subaru/COMICS. Bright extended arc-like emission (outer disk) is recognized at r=0.85" together with a strong central source (inner disk) and a gap around r=0.6" in the both images. Thermal emission of the eastern side is much brighter than that of the western side in the MIR. We estimate the dust size as a few micron from the observed color of the extended emission and the distance from the star. The dust temperature T and the optical depth tau of the MIR emitting dust are also derived fro…
The remanent magnetisation of meteorite material in the solar system indicates that magnetic fields of several Gauss are present in the protoplanetary disk. It is shown that such relativelystrong magnetic fields can be generated in dusty protoplanetary disks by relative shear motions of the charged dust and the neutral gas components. Self-consistent multi-fluid simulations show that for typical plasma parameters shear flows with collisional momentum transfer between the different components of the dusty plasma result in a very efficient generation of magnetic fields with strengths of about 0.…
The continuous spectra of the substars with surrounding protoplanetary disks were calculated. The results reveal that protoplanetary disc average temperature decreases to 3 K during the period of 5 Myr for substars with masses 0.01M_{Sun} and during the period of 160 Myr for substars with masses 0.08M_{Sun}. Estimations of protoplanetary discs flux maximum depending on the substar mass at the age of 1 Myr are: 4.6 kJy (for 0.01M_{Sun}) and 3.4 MJy (for 0.08M_{Sun}). Maximum of protoplanetary disc radiation before it reaches the temperature of the cosmic microwave background changes within the …
The recent identification of several groups of young stars within 100 parsecs of the Sun has generated widespread interest. Given their proximity and possible age differences, these systems are ideally suited for detailed studies of star and planet formation. Here we report on the first investigation of protoplanetary disks in one such group, the high-latitude cloud MBM 12 at a distance of $\sim$65 pc. We present mid-infrared observations of the eight candidate pre-main-sequence (PMS) members and the two main-sequence (MS) stars in the same line-of-sight which may or may not be associated with…
The discovery of now about 20 extrasolar planets orbiting solar-type stars with properties quite different from those in our Solar System has raised many questions about the formation and evolution of planets. The tidal interaction between the planet and the surrounding disk determines the orbital properties and the mass of the planet. We have performed numerical computations of a planet embedded in a protoplanetary disk and found that for typical values of the viscosity the planet may easily grow upto ten Jupiter masses. New results on the mass evolution and the migration of the planet are pr…
Radio images of maser spots in the infrared source GL2789, connected with the young stellar object V645 Cyg, have been obtained as a result of the radio interferometric observations of H2O maser at 22 GHz and methanol maser at 6.7 GHz, with the VLBI arrays VLBA and EVN. It was shown that the position of the masers coincide with the optical object within 0.2''. The maser spots are located along the line North-South, and their position and radial velocity can be described by a model of the Keplerian disk with a maximum radius of 40 AU for H2O maser and 800 AU for methanol maser. The H2O and meth…
Planetary systems, ours included, are formed in disks of dust and gas around young stars. Disks are an integral part of the star and planet formation process, and knowledge of the distribution and temperature of inner disk material is crucial for understanding terrestrial planet formation, giant planet migration, and accretion onto the central star. While the inner regions of protoplanetary disks in nearby star forming regions subtend only a few nano-radians, near-IR interferometry has recently enabled the spatial resolution of these terrestrial zones. Most observations have probed only dust, …
We report the detection of spatially-extended mid-infrared emission around the luminous embedded star IRc9 in OMC-1, as seen in 8.8, 11.7, and 18.3 micron images obtained with T-ReCS on Gemini South. The extended emission is asymmetric, and the morphology is reminiscent of warm dust disks around other young stars. The putative disk has a radius of roughly 1.5 arcsec (700 AU), and a likely dust mass of almost 10 Earth masses. The infrared spectral energy distribution of IRc9 indicates a total luminosity of about 100 Lsun, implying that it shall become an early A-type star when it reaches the ma…
Aims: We look for new regions for the search of substellar objects. Methods: Two circular areas, 45 arcmin-radius each, centred on the young massive star systems Alnilam and Mintaka in the Orion Belt, have been explored. The regions are very young (less than 10 Ma), have low extinction, and are neighbours to sigma Orionis (~3 Ma), a young open cluster very rich in brown dwarfs and planetary-mass objects. We have used Virtual Observatory tools, the astro-photometric Tycho-2, DENIS and 2MASS catalogues, 10 control fields at similar galactic latitudes, and X-ray, mid-infrared and spectroscopic da…
Observations of interstellar gas-phase and solid-state species in the 2.4-200 micron range obtained with the spectrometers on board the Infrared Space Observatory are reviewed. Lines and bands due to ices, polycyclic aromatic hydrocarbons, silicates and gas-phase atoms and molecules (in particular H2, CO, H2O, OH and CO2) are summarized and their diagnostic capabilities illustrated. The results are discussed in the context of the physical and chemical evolution of star-forming regions, including photon-dominated regions, shocks, protostellar envelopes and disks around young stars.
We study protoplanetary disc evolution assuming that angular momentum transport is driven by gravitational instability at large radii, and magnetohydrodynamic (MHD) turbulence in the hot inner regions. At radii of the order of 1 AU such discs develop a magnetically layered structure, with accretion occurring in an ionized surface layer overlying quiescent gas that is too cool to sustain MHD turbulence. We show that layered discs are subject to a limit cycle instability, in which accretion onto the protostar occurs in bursts with an accretion rate of 10^{-5} solar masses / yr, separated by quie…
We report the codiscovery of the spatially-resolved dust disk of the Vega-like star HR 4796A. Images of the thermal dust emission at $λ= 18 μ$m show an elongated structure approximately 200 AU in diameter surrounding the central A0V star. The position angle of the disk, $30^{\circ} \pm 10^{\circ}$, is consistent to the position angle of the M companion star, $225^{\circ}$, suggesting that the disk-binary system is being seen nearly along its orbital plane. The surface brightness distribution of the disk is consistent with the presence of an inner disk hole of approximately 50 AU radius, as was…
During a program of spectroscopic monitoring of V838 Mon, we serendipitously discovered that a neighboring 16th-mag star is of type B. We then carried out a spectroscopic survey of other stars in the vicinity, revealing two more B-type stars, all within 45" of V838 Mon. We have determined the distance to this sparse, young cluster, based on spectral classification and photometric main-sequence fitting of the three B stars. The distance is found to be 6.2+-1.2 kpc, in excellent agreement with the geometric distance to V838 Mon of 5.9 kpc obtained from Hubble Space Telescope polarimetry of the l…
The formation and evolution of young low mass stars are characterized by important processes of mass loss and accretion ocurring in the innermost regions of their placentary circumstellar disks. Because of the large obscuration of these disks at optical and infrared wavelengths in the early protostellar stages (Class 0 Sources), they were previously detected only at radio wavelengths using interferometric techniques. We have detected with the Infrared Space Observatory (ISO) the mid-infrared emission associated with the Class 0 protostar VLA1 in the HH1-2 region located in the Orion nebula. Th…
We study magnetic field steepening due to ambipolar diffusion (Brandenburg & Zweibel 1994) in protoplanetary disk environments and draw the following conclusions. Current sheets are generated in magnetically active regions of the disk where the ionization fraction is high enough for the magnetorotational instability to operate. In late stages of solar nebula evolution, the surface density is expected to have lowered and dust grains to have gravitationally settled to the midplane. If the local dust-to-gas mass ratio near the midplane is increased above cosmic abundances by factors > 10^3, curre…
We report on a limited search for pure-rotational molecular hydrogen emission associated with young, pre-main-sequence stars. We looked for H_2 v=0 J = 3->1 and J = 4->2 emission in the mid-infrared using the Texas Echelon-Cross-Echelle Spectrograph (TEXES) at NASA's 3m Infrared Telescope Facility. The high spectral and spatial resolution of our observations lead to more stringent limits on narrow line emission close to the source than previously achieved. One star, AB Aur, shows a possible (2sigma) H_2 detection, but further observations are required to make a confident statement. Our non-det…
The process of turbulent radial mixing in protoplanetary disks has strong relevance to the analysis of the spatial distribution of crystalline dust species in disks around young stars and to studies of the composition of meteorites and comets in our own solar system. A debate has gone on in the recent literature on the ratio of the effective viscosity coefficient $ν$ (responsible for accretion) to the turbulent diffusion coefficient $D$ (responsible for mixing). Numerical magneto-hydrodynamic simulations have yielded values between $ν/D\simeq 10$ (Carballido, Stone & Pringle, 2005) and $ν/D\si…
Circumstellar disks provide the material reservoir for the growth of young stars and for planet formation. We combine a high-level radiative transfer program with a thermal-chemical model of a typical T Tauri star disk to investigate the diagnostic potential of the far-infrared lines of water for probing disk structure. We discuss the observability of pure rotational H2O lines with the Herschel Space Observatory, specifically the residual gas where water is mainly frozen out. We find that measuring both the line profile of the ground 110-101 ortho-H2O transition and the ratio of this line to t…
The TW Hya system is perhaps the closest analog to the early solar nebula. We have used the Very Large Array to image TW Hya at wavelengths of 7mm and 3.6 cm with resolutions 0.1 arcseconds (about 5 AU) and 1.0 arcseconds (about 50 AU), respectively. The 7mm emission is extended and appears dominated by a dusty disk of radius larger than 50 AU surrounding the star. The 3.6 cm emission is unresolved and likely arises from an ionized wind or gyrosynchrotron activity. The dust spectrum and spatially resolved 7mm images of the TW Hya disk are fitted by a simple model with temperature and surface d…
We study the efficiency of the noble gases sequestration by the ion H3+ in the form of XH3+ complexes (with X = argon, krypton or xenon) in gas phase conditions similar to those encountered during the cooling of protoplanetary disks, at the epoch of icy planetesimals formation. We show that XH3+ complexes form very stable structures in the gas phase and that their binding energies are much higher than those involved in the structures of X-H2O hydrates or pure X-X condensates. This implies that, in presence of H3+ ions, argon, krypton or xenon are likely to remain sequestrated in the form of XH…
We use a high order accuracy spectral code to carry out two-dimensional time-dependent numerical simulations of vortices in accretion disks. In particular, we examine the stability and the life time of vortices in circumstellar disks around young stellar objects. The results show that cyclonic vortices dissipate quickly, while anticyclonic vortices can survive in the flow for many orbits. When more than one vortex is present, the anticyclonic vortices interact through vorticity waves and merge together to form larger vortices. The decay time of anticyclonic vortices is of the order of 30-60 or…
Hundreds of circumstellar disks in the Orion nebula are being rapidly destroyed by the intense ultraviolet radiation produced by nearby bright stars. These young, million-year-old disks may not survive long enough to form planetary systems. Nevertheless, the first stage of planet formation -- the growth of dust grains into larger particles -- may have begun in these systems. Observational evidence for these large particles in Orion's disks is presented. A model of grain evolution in externally irradiated protoplanetary disks is developed and predicts rapid particle size evolution and sharp out…
We present models for the HI 21 cm emission from circumstellar disks and use them to convert observed upper limits on the HI 21 cm flux to limits on the total disk gas mass. The upper limits we use come from earlier Australia Telescope Compact Array observations of the debris disk around beta Pictoris as well as fresh Giant Meterwave Radio Telescope observations of HD135344, LkCa 15 and HD163296. Our observations and models span a range of disk types, from young proto-planetary disks to old debris disks. The models self-consistently calculate the gas chemistry (H/H2 balance) and the thermal st…
The temperature, density and kinematics of the gas and dust surrounding the luminous young stellar object GL~2591 are investigated on scales as small as 100 AU, probed by 4.7 micron absorption spectroscopy, to over 60,000 AU, probed by single-dish submillimeter spectroscopy. These two scales are connected by interferometric 86-226 GHz images of size 60,000 AU and resolution 2000 AU in continuum and molecular lines. The data are used to constrain the physical structure of the envelope and investigate the influence of the young star on its immediate surroundings.
