15 pages and 5 figures, invited review talk, to appear in the ASP conference proceedings of the "Frank N. Bash Symposium 2007: New Horizons in Astronomy", editors: A. Frebel, J. Maund, J. Shen, M. Siegel
Circumstellar disks are an integral part of the star formation process and the sites where planets are formed. Understanding the physical processes that drive their evolution, as disks evolve from optically thick to optically thin, is crucial for our understanding of planet formation. Disks evolve through various processes including accretion onto the star, dust settling and coagulation, dynamical interactions with forming planets, and photo-evaporation. However, the relative importance and timescales of these processes are still poorly understood. In this review, I summarize current models of the different processes that control the evolution of primordial circumstellar disks around low-mass stars. I also discuss recent observational developments on circumstellar disk evolution with a focus on new Spitzer results on transition objects.
Few circumstellar disks have been directly observed. Here we use sensitive differential polarimetric techniques to overcome atmospheric speckle noise in order to image the circumstellar material around HD 169142. The detected envelope or disk is considerably smaller than expectations based on the measured strength of the far-IR excess from this system.
I briefly review recent developments in the study of circumstellar debris disks, particularly at infrared and sub-millimeter wavelengths, and discuss possible avenues of research for the near future.
We present results of a study aimed at deriving fundamental properties of circumstellar debris disks from observed infrared to submillimeter spectral energy distributions. This investigation is motivated by increasing telescope/detector sensitivity, in particular the expected availability of the Space Infrared Telescope Facility (SIRTF) followed by the Stratospheric Observatory for Infrared Astronomy (SOFIA), which will enable detailed studies with large source samples of late stage circumstellar disk and planetary system evolution. We base our study on an analytic model of the disk density di…
This article is a tutorial on deriving physical properties from the spectral energy distributions of circumstellar disks. It includes a new derivation of disk mass from millimeter-wave flux density using recent disk model parameters. The energetics for most disks are dominated by radiative heating and cooling, and almost any spectral energy distribution may be understood from physically plausible models in the literature.
We investigate the possibility to detect giant planets that are still embedded in young circumstellar disks. Based on models with different stellar, planetary, and disk masses, and different radial positions of the planet we analyze the resulting submillimeter appearance of these systems. We find that the influence of the planet on the spectral energy distribution could not be distinguished from that of other disk parameters. However, dust reemission images of the disks show that the hot region in the proximity of a young planet, along with the gap, could indeed be detected and mapped with the…
A question central to understanding the origin of our solar system is: how do planets form in circumstellar disks around young stars? Because of the complex nature of the physical processes involved, multi-wavelength observations of large samples will be required in order to obtain a complete answer to this question. Surveys undertaken with ISO have helped to solve pieces of this puzzle in addition to uncovering new mysteries. We review a variety of studies aimed at understanding; i) the physical structure and composition of circumstellar disks commonly found surrounding young stellar objects;…
We present results of a coronographic imaging search for circumstellar dust disks with the Adaptive Optics Near Infrared System (ADONIS) at the ESO 3.6m telescope in La Silla (Chile). 22 candidate stars, known to be orbited by a planet or to show infrared excess radiation, were examined for circumstellar material. In the PSF-subtracted images no clear disk was found. We further determine the detection sensitivities and outline how remaining atmospheric fluctuations still can hamper adaptive optics observations.
We present a model of the circumstellar environment of the so-called ``Butterfly Star'' in Taurus (IRAS 04302+2247). The appearance of this young stellar object is dominated by a large circumstellar disk seen edge-on and the light scattering lobes above the disk. The model is based on multi-wavelength continuum observations: Millimeter maps and high-resolution near-infrared images obtained with HST/NICMOS. It was found that the disk and envelope parameters are comparable with those of the circumstellar environment of other young stellar objects. A main result is that the dust properties must b…
We describe new, deep MIPS photometry and new high signal-to-noise optical spectroscopy of the 2.5 Myr-old IC 348 Nebula. To probe the properties of the IC 348 disk population, we combine these data with previous optical/infrared photometry and spectroscopy to identify stars with gas accretion, to examine their mid-IR colors,and to model their spectral energy distributions. IC 348 contains many sources in different evolutionary states, including protostars and stars surrounded by primordial disks, two kinds of transitional disks, and debris disks. Most disks surrounding eary/intermediate spect…
We argue that the interaction of stellar wind with the surface of a circumstellar accretion (or protoplanetary) disk can result in the acceleration of relativistic electrons in an external layer of the disk, and produce synchrotron radiation. Conservative estimates give a total synchrotron luminosity $L_s\sim 10^{-5}\lsun$ for a central star with $\dot M=10^{-6}\msun$ yr$^{-1}$, comparable with the value observed around the TW object in the W3(OH) region.
There is strong evidence that the planets in the solar system evolved from a disk-shaped solar nebula ~4.56 Gyr ago. By studying young stars in various evolutionary stages, one aims at tracing back the early history of the solar system, in particular the timescales for disk dissipation and for the formation of planetary systems. We used the VLT & ISAAC, and ESA's Infrared Space Observatory & ISOCAM to study the circumstellar environment of young low-mass stars.
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…
Detailed evidence on the system AX J0049.4-7323 is presented here to show how the passage of the neutron star in the binary system disrupts the circumstellar disk of the mass donor Be star. A similar effect is noted in three other Be/X-ray binary systems. Together the observational data should provide valuable tools for modelling these complex interactions.
We investigate the microlensing effects on a source star surrounded by a circumstellar disk, as a function of wavelength. The microlensing light curve of the system encodes the geometry and surface brightness profile of the disk. In the mid- and far-infrared, the emission of the system is dominated by the thermal emission from the cold dusty disk. For a system located at the Galactic center, we find typical magnifications to be of order 10-20% or higher, depending on the disk surface brightness profile, and the event lasts over one year. At around 20 microns, where the emission for the star an…
This review covers the properties of disks around pre-main--sequence stars. It is at this time in the evolution that planets form, and it is important to understand the properties of these disks to understand planet formation. I discuss disk shapes, masses and temperatures, the properties of the host stars, disk lifetimes and dissipation processes. Finally, evidence of grain growth during pre-main-sequence evolution is briefly summarized.
Keck adaptive optics imaging with a physical resolution of 0.4 AU resolves the inner (15-80 AU) disk of AU Mic (GJ 803), the nearest known scattered light disk to Earth. The inner disk is asymmetric and possesses a sharp change in structure at 35 AU. The disk also shows spatially localized enhancements and deficits at 25-40 AU separations. The overall morphology points to the influence of unseen larger bodies and resembles structures expected from recent planet formation. AU Mic is coeval with the archetypical debris disk system beta Pic, and the similarities between their two disks point to s…
We discuss how we obtain the spatial distribution of ice on the surface of the circumstellar disk around young stars. Ice in the disks plays a very important role in various issues, for instance, on the disk structure, on the planet formation, on the isotopic anomaly in meteorites, and on the origin of the sea on the Earth. Therefore, the spatially resolved observation of the condensation/sublimation front of ice, so-called ``snow line'' is strongly required. Here, we propose a new method for obtaining the spatially resolved ``snow line'' on the circumstellar disks by observing 3 \micron H$_2$…
Context: the observation of young stars with circumstellar disks suggests that the disks are dissipated, starting from the inner region, by the radiation of the central star and eventually by the formation of rocky planetesimals, over a time scale of several million years. It was also shown that strong UV radiation emitted by nearby massive stars can heat a circumstellar disk up to some thousand degrees, inducing the photoevaporation of the gas. This process strongly reduces the dissipation time scale. Aims: we study whether there exists a correlation between the spatial distribution of stars …
Over the past 10 years abundant evidence has emerged that many (if not all) stars are born with circumstellar disks. Understanding the evolution of post-accretion disks can provide strong constraints on theories of planet formation and evolution. In this review, we focus on developments in understanding: a) the evolution of the gas and dust content of circumstellar disks based on observational surveys, highlighting new results from the Spitzer Space Telescope; b) the physical properties of specific systems as a means to interpret the survey results; c) theoretical models used to explain the ob…
We present the first high angular resolution 1.4mm and 2.7mm continuum maps of the T Tauri binary system HK Tau obtained with the Plateau de Bure Interferometer. The contributions of both components are well disentangled at 1.4mm and the star previously known to host an edge-on circumstellar disk, HK Tau B, is elongated along the disk's major axis. The optically bright primary dominates the thermal emission from the system at both wavelengths, confirming that it also has its own circumstellar disk. Its non-detection in scattered light images indicates that the two disks in this binary system a…
We present a comparative study of the circumstellar disks in Be/X-ray binaries and isolated Be stars based upon the H-alpha emission line. From this comparison it follows that the overall structure of the disks in the Be/X-ray binaries is similar to the disks of other Be stars, i.e. they are axisymmetric and rotationally supported. The factors for the line broadening (rotation and temperature) in the disks of the Be stars and the Be/X-ray binaries seem to be identical. However, we do detect some intriguing differences between the envelopes. On average, the circumstellar disks of the Be/X-ray b…
We report here the first resolved images of the circumstellar dust disk around the Pre-Main-Sequence star HD 100546. These near-infrared images were obtained in J and short K bands with the adaptive optics system ADONIS at the ESO observatory. A bright disk extending 2 arcsec (200 AU) far from the star and viewed with an inclination with respect to the line of sight around 50 degrees is revealed by the observations. Using a simplified model of light scattering, we find that the dust disk density peaks at a distance of 40 AU from the star, and the FWHM of this dense ring is typically 20 AU. Thi…
We explore test particle orbits in the orbital plane of eccentric stellar binary systems, searching for ``invariant loops'': closed curves that change shape periodically as a function of binary orbital phase as the test particles in them move under the stars' gravity. Stable invariant loops play the same role in this periodically-varying potential as stable periodic orbits do in stationary potentials; in particular, when dissipation is weak, gas will most likely follow the non-intersecting loops, while nearby particle orbits librate around them. We use this method to set bounds on the sizes of…
We report HST/NICMOS coronagraphic images of the HD 15115 circumstellar disk at 1.1\micron. We find a similar morphology to that seen in the visible and at H band--an edge-on disk that is asymmetric in surface brightness. Several aspects of the 1.1\micron data are different, highlighting the need for multi-wavelength images of each circumstellar disk. We find a flattening to the western surface brightness profile at 1.1\micron interior to 2\arcsec (90 AU) and a warp in the western half of the disk. We measure the surface brightness profiles of the two disk lobes and create a measure of the dus…
Circumstellar disks play an important role in many stages of the evolution of stars. However, it is only possible to directly image circumstellar disks for a few of the nearest stars. For massive stars, the situation is even more difficult, as they are on average further away than the more numerous low mass stars. Here, we discuss spectropolarimetry which is a technique that can reveal the presence of disks on spatial scales of order stellar radii. It is therefore extremely useful in studying disks around young stars, while the method has prospects for quantitative modelling. In this review we…
We investigate the analogy between circumstellar disks and the Taylor-Couette flow. Using the Reynolds similarity principle, the analogy results in a number of parameter-free predictions about stability of the disks, and their turbulent transport properties, provided the disk structure is available. We discuss how the latter can be deduced from interferometric observations of circumstellar material. We use the resulting disk structure to compute the molecular transport coefficients, including the effect of ionization by the central object. The resulting control parameter indicates that the dis…
We present a numerical model describing a circularly symmetric gaseous disk around the Be star chi Ophiuchi. The model is constrained by long-baseline interferometric observations that are sensitive to the H-alpha Balmer line emission from the disk. For the first time our interferometric observations spatially resolve the inner region of the circumstellar disk around chi Ophiuchi and we use these results to place a constraint on the physical extent of the H-alpha-emitting region. We demonstrate how this in turn results in very specific constraints on the parameters that describe the variation …
We present the results of a 2.7 mm continuum interferometric survey of 24 young stellar objects in 11 fields. The target objects range from deeply embedded Class 0 sources to optical T Tauri sources. This is the first sub-arcsecond survey of the 2.7 mm dust continuum emission from young, embedded stellar systems. The images show a diversity of structure and complexity. The optically visible T Tauri stars (DG Tauri, HL Tauri, GG Tauri,and GM Aurigae) have continuum emission dominated by compact, less than 1", circumstellar disks. The more embedded near-infrared sources (SVS13 and L1551 IRS5) ha…
We present synthetic spectra for circumstellar disks that are heated by radiation from a central brown dwarf. Under the assumption of vertical hydrostatic equilibrium, our models yield scaleheights for brown dwarf disks in excess of three times those derived for classical T Tauri (CTTS) disks. If the near-IR excess emission observed from brown dwarfs is indeed due to circumstellar disks, then the large scaleheights we find could have a significant impact on the optical and near-IR detectability of such systems. Our radiation transfer calculations show that such highly flared disks around brown…
We investigate the stellar environment of the Beta Pictoris-like star HD 141569 with optical spectroscopy and near infrared imaging. The B9.5Ve primary, and an M2 and an M4 type star both located in projection less than nine arseconds away have the same radial velocity and proper motion and therefore almost certainly form a triple star system. From their x-ray flux, lithium absorption and location on pre-main-sequence evolutionary tracks, the companions appear to be 5 Myr old. HD 141569 A is now one of the few stars with a circumstellar disk that has a well-determined age. The circumstellar di…
Molecular lines hold valuable information on the physical and chemical composition of disks around young stars, the likely progenitors of planetary systems. This invited contribution discusses techniques to calculate the molecular emission (and absorption) line spectrum based on models for the physical and chemical structure of protoplanetary disks. Four examples of recent research illutrate these techniques in practice: matching resolved molecular-line emission from the disk around LkCa15 with theoertical models for the chemistry; evaluating the two-dimensional transfer of ultraviolet radiati…
We show that in many methanol maser sources the masers are located in lines, with a velocity gradient along them which suggests that the masers are situated in edge-on circumstellar, or protoplanetary, disks. We present VLBI observations of the methanol maser source G309.92+0.48, in the 12.2 GHz transition, which confirm previous observations that the masers in this source lie along a line. We show that such sources are not only linear in space but, in many cases, also have a linear velocity gradient. We then model these and other data in both the 6.7 GHz and the 12.2 GHz transition from a num…
There is increasing evidence that low mass stars with circumstellar disks can be born close to massive stars, in some cases within tenths of a pc. If the disks have lifetimes greater than those of the more massive stars, they are exposed to the radiation fields and gas flows from the late evolutionary phases and supernovae of the massive stars. The fast flows from supernovae are likely to give some stripping of mass from the disks, but do not typically lead to complete disruption of the disks. In the slow wind from a red supergiant star, there is the possibility of gravitational accretion of w…
Interpretation of molecular line observations in tenuous circumstellar disks around young G-type stars in terms of a disk mass is difficult without a model that describes the chemical structure of these disks. This paper now discusses the chemistry in tenuous disks around young solar-type stars based on disk models that take into account the presence of a stellar chromosphere. The example of the disk around a 70 Myr old solar-type star shows that the dissociating radiation from the chromosphere is stronger than the interstellar ultraviolet radiation field up to a distance of ~400 AU from the s…
Using interferometric data from BIMA observations, combined with detailed modeling in Fourier space of the physical structures predicted by models, we constrain the circumstellar envelope parameters for four Class 0 young stellar objects, as well as their embedded circumstellar disks. The envelopes of these objects are still undergoing collapse, and theoretical collapse models can be compared to the observations. Since it has been suggested in a previous study that both the Larson-Penston and Shu similarity solutions underestimate the age of the system, we adopt Tassis & Mouschovias' model of …
We present 3-5 micron VLT-ISAAC spectroscopy searching for evidence of methanol ices in edge-on disks of young embedded stars. Examples include the disks of L1489 IRS in Taurus and CRBR 2422.8-3423 in Ophiuchus, the last of which has the highest column density of solid CO known toward a YSO. We find no unambiguous evidence for abundant methanol in the observed disks, but give strict upper limits. Several additional low-mass sources in the Serpens and Chameleon molecular clouds exhibit abundant solid methanol, although it is not clear if the ice is associated with a disk or with the envelope. T…
We report the first resolved scattered light images of the circumstellar dust disk around the old Pre-Main Sequence star HD 141569 A. The disk seen in HST/NICMOS2 images shapes a bright annulus inclined at 37.5 degrees +/- 4.5 degrees from edge-on. This ring peaks at 325+/-10 AU from the star with a characteristic width of ~150 AU. At 1.6 microns, the dust grains scatter a total flux density of at least 4.5 +/- 0.5 mJy. Our disk model using the spatial distribution implied by the images does not explain the 10 microns excess and requires an additional grain population closer to the star. Some …
We present high resolution 0.45" x 0.32" observations from the BIMA array toward the Herbig Ae system MWC 480 in the lambda = 1.4 mm dust continuum. We resolve a circumstellar disk of radius ~170 AU and constrain the disk parameters by comparing the observations to flat disk models. These results show that the typical fit parameters of the disk, such as the mass, Md ~ 0.04-0.18 Mo, and the surface density power law index, p=0.5 or 1, are comparable to those of the lower mass T Tauri stars. The dust in the MWC 480 disk can be modeled as processed dust material (beta ~ 0.8), similar to the Herbi…
We present the first ground-based near-infrared image of the circumstellar disk around the post-Herbig Ae/Be star HD141569A initially detected with the HST. Observations were carried out in the near-IR (2.2 $μ$m) at the Palomar 200-inch telescope using the adaptive optics system PALAO. The main large scale asymmetric features of the disk are detected on our ground-based data. In addition, we measured that the surface brightness of the disk is slightly different than that derived by HST observations (at 1.1 $μ$m and 1.6 $μ$m). We interpret this possible color-effect in terms of dust properties …
Our efficient numerical approach has been applied to modeling the asymmetric circumstellar dust disk around Beta Pictoris as observed with the HST/STIS. We present a new model on the origin of the warping of the Beta Pic disk. We suggest that the observed warp is formed by the gravitational influence of a planet with a mass of about 10 masses of Earth, at a distance of 70 AU, and a small inclination ($\sim 2.5^\circ$) of the planetary orbit to the main dust disk. Results of our modeling are compared with the STIS observations.
In circumstellar disks or shells it is often assumed that gas and dust temperatures are equal where the latter is determined by radiative equilibrium. This paper deals with the question whether this assumption is applicable for tenous circumstellar disks around young A stars. In this paper the thin hydrostatic equilibrium models described by Kamp & Bertoldi (2000) are combined with a detailed heating/cooling balance for the gas. The most important heating and cooling processes are heating through infrared pumping, heating due to the drift velocity of dust grains, and fine structure and molecul…
We present mid-infrared nulling interferometric and direct imaging observations of the Herbig Ae star HD 100546 obtained with the Magellan I (Baade) 6.5 m telescope. The observations show resolved circumstellar emission at 10.3, 11.7, 12.5, 18.0, and 24.5 microns. Through the nulling observations (10.3, 11.7 and 12.5 microns), we detect a circumstellar disk, with an inclination of 45 +- 15 degrees with respect to a face-on disk, a semimajor axis position angle of 150 +- 10 degrees (E of N), and a spatial extent of about 25 AU. The direct images (18.0 and 24.5 microns) show evidence for cooler …
We investigate the X-ray and near-infrared emission properties of a sample of pre-main sequence (PMS) stellar systems in the Orion Nebula Cluster (ONC) that display evidence for circumstellar disks (``proplyds'') and optical jets in Hubble Space Telescope (HST) imaging. Approximately 70% of ~140 proplyds were detected as X-ray sources in the Chandra Orion Ultradeep Program observation of the ONC, including ~25% of proplyds that do not display central stars in HST imaging. Many proplyds display near-infrared excesses, suggesting disk accretion is ongoing onto the central, PMS stars. Among circu…
Aims. We attempt to understand the presence of gas phase CO below its freezing temperature in circumstellar disks. We study two promising mechanisms to explain this phenomenon: turbulent mixing and photodesorption. Methods. We compute the chemical evolution of circumstellar disks including grain surface reactions with and without turbulent mixing and CO photodesorption. Results. We show that photodesorption significantly enhances the gas phase CO abundance, by extracting CO from the grains when the visual extinction remains below about 5 magnitudes. However the resulting dependence of column d…
Coronagraphic imaging with the Near Infrared Camera and Multi Object Spectrometer on the Hubble Space Telescope reveals a large, ~400 AU (4'') radius, circumstellar disk around the Herbig Ae/Be star HD 141569. A reflected light image at 1.1 micron shows the disk oriented at a position angle of 356 +/- 5 deg and inclined to our line of sight by 51 +/- 3 deg; the intrinsic scattering function of the dust in the disk makes the side inclined toward us, the eastern side, brighter. The disk flux density peaks 185 AU (1.''85) from the star and falls off to both larger and smaller radii. A region of d…
Near-infrared imaging with the Hubble Space Telescope recently revealed a circumstellar dust disk around the A star HD 32297. Dust scattered light is detected as far as 400 AU radius and the linear morphology is consistent with a disk ~10 degrees away from an edge-on orientation. Here we present the first optical images that show the dust scattered light morphology from 560 to 1680 AU radius. The position angle of the putative disk midplane diverges by 31 degrees and the color of dust scattering is most likely blue. We associate HD 32297 with a wall of interstellar gas and the enigmatic region…
Grain alignment theory suggests that grains should be aligned in circumstellar regions and the observational data available supports this conclusion. We discuss the alignment of grains via (1) magnetic relaxation, (2) mechanical processes, and (3) radiative torques. We show that ferromagnetic relaxation is likely to be more important than superparamagnetic relaxation if the dust in circumstellar regions is similar to species recently captured in Earth atmosphere. Outflows and stellar winds provide grain streaming along magnetic field lines and therefore mechanical alignment competes with the f…
We highlight two recent studies of circumstellar disks in the Taurus and Orion star forming regions. Using the JCMT and CSO, we measure disk fluxes in Taurus over a wide range of submillimeter wavelengths and determine the frequency dependence of the dust opacity. We find clear evidence for a systematic change in its behavior with time, most readily explained by grain growth. Using the SMA, we observed the protoplanetary disks (proplyds) in the Orion Trapezium cluster. The combination of high resolution, high frequency, and high sensitivity that this instrument provides made it possible to res…
Observations of the rho Ophiuchi star forming region with VLT ANTU and ISAAC under 0.35" seeing conditions reveal two bipolar reflection nebulosities intersected by central dust lanes. The sources (OphE-MM3 and CRBR 2422.8-3423) can be identified as spatially resolved circumstellar disks viewed close to edge-on, similar to edge-on disk sources discovered previously in the Taurus and Orion star forming regions. Millimeter continuum fluxes yield disk masses of the order of 0.01 Mo, i.e. about the mass deemed necessary for the minimum solar nebula. Follow-up spectroscopic observations with SUBARU…
We present new multiwavelength submillimeter continuum measurements of the circumstellar dust around 48 young stars in the $ρ$ Ophiuchus dark clouds. Supplemented with previous 1.3 mm observations of an additional 99 objects from the literature, the statistical distributions of disk masses and submillimeter colors are calculated and compared to those in the Taurus-Auriga region. These basic submillimeter properties of young stellar objects in both environments are shown to be essentially identical. As with their Taurus counterparts, the $ρ$ Oph circumstellar dust properties are shown to evolve…
A search for previously undetected optically thin disks around stars in the nearby, young, TW Hydrae Association was conducted around sixteen stars with sensitive 12 and 18 micron photometry. The survey could detect Zodiacal-like dust, with temperature 200-300 K, at levels of L(IR)/L(*)=7e-3. Possible mid-infrared excess emission from TWA 17 was detected at the 2-sigma level, but none of the other stars showed evidence for circumstellar dust. The rapid disappearance of large amounts of dust around the K and M-type stars in this sample may mean that any planet formation in the terrestrial plane…
The first results from a near-contemporaneous optical and infrared spectroscopic observing program designed to probe the detailed density structure of classical Be circumstellar disks are presented. We report the discovery of asymmetrical infrared emission lines of He I, O I, Fe II, and the Brackett, Paschen, and Pfund series lines of H I which exhibit an opposite V/R orientation (V $>$ R) to that observed for the optical Balmer H$α$ line (V $<$ R) in the classical Be star $ζ$ Tau. We interpret these data as evidence that the density wave which characterizes $ζ$ Tau's disk has a significantly …
We present 4.5, 8, and 16um photometry from the Spitzer Space Telescope for 204 stars in the Upper Scorpius OB Association. The data are used to investigate the frequency and properties of circumstellar disks around stars with masses between ~ 0.1 and 20 Msun at an age of ~ 5 Myr. We identify 35 stars that have emission at 8um or 16um in excess of the stellar photosphere. The lower mass stars (~ 0.1-1.2 Msun) appear surrounded by primordial optically thick disks based on the excess emission characteristics. Stars more massive than ~ 1.8 Msun have lower fractional excess luminosities suggesting…
A remarkable correlation between the H-alpha emission line and the radio behaviour of LSI+61 303 (V615 Cas, GT 0236+610) over its 4 yr modulation is discovered. The radio outburst peak is shifted by a quarter of the 4 yr modulation period (about 400 days) with respect to the equivalent width of the H-alpha emission line variability. The onset of the LSI+61 303 radio outbursts varies in phase with the changes of the H-alpha emission line, at least during the increase of H-alpha equivalent width. This is the first clear correlation between the emission associated to the compact object and the Be…
The dust disk around Beta Pic must be produced by collision or by evaporation of orbiting Kuiper belt-like objects. Here we present the Orbiting-Evaporating-Bodies scenario in which the disk is a gigantic multi-cometary tail supplied by slowly evaporating bodies like Chiron. If dust is produced by evaporation, a planet with an eccentric orbit can explain the observed asymmetry of the disk, because the periastron distribution of the parent bodies are then expected to be non-axisymmetric. We investigate the consequence for the Kuiper belt-like objects of the formation and the migration of an out…
The sigma Orionis cluster is a young association evolving under the disruptive influence of its massive O-star namesake. We are analysing this cluster as part of a program to characterise the influence of O-stars on the early stages of stellar evolution. At an age of approximately 4 Myr, this cluster is at a crucial stage in terms of disk evolution and therefore it is a key case to better constrain disk dissipation timescales. We have obtained RI photometry and optical spectroscopy of the sigma Ori cluster; we have analysed the Li I and Na I features to establish cluster membership. We have th…
We present a radiative transfer model which computes the spectral energy distribution of a passive, irradiated, circumstellar disk, assuming the grains are in radiative equilibrium. Dependence on radial density profile, grain temperature estimation, and optical depth profiles on the output SED are discussed. The bist fit model for HR4796A has a minimum and maximum spherical grain size of 2.2 and 1000 μ$m respectively, a size distribution slightly steeper than the "classical" -3.5 MRN power law, grains composed of silicates, trolite, ice, and organics and a peak radial density of 1.0 x 10^-17 g…
We propose that the two armed spiral features seen in visible Hubble Space Telescope images of scattered light in HD100546's circumstellar disk are caused by the illumination of a warped outer disk. A tilt of 6-15 degrees from the symmetry plane can cause the observed surface brightness variations providing the disk is very twisted (highly warped) at radii greater than 200 AU where the spiral features are seen. Dust lanes are due in part to shadowing in the equatorial plane from the inner disk within a radius of 100 AU. HD100546's outer disk, if viewed edge-on, would appear similar to that of …
In this paper we present results on the size distribution of circumstellar disks in the Trapezium cluster as measured from HST/WFPC2 data. Direct diameter measurements of a sample of 135 bright proplyds and 14 silhouettes disks suggest that there is a single population of disks well characterized by a power-law distribution with an exponent of -1.9 +- 0.3 between disk diameters 100-400 AU. For the stellar mass sampled (from late G to late M stars) we find no obvious correlation between disk diameter and stellar mass. We also find that there is no obvious correlation between disk diameter and t…
