1 Introduction
In the early 1980s, optical polarimetric observations [13] revealed the existence of elongated and flattened circumstellar dust material around some Pre-Main-Sequence (PMS) stars such as the low-mass TTauri stars, similar to the Sun when it was about
IR excess around TTauri stars was integrated as the signature of a dust disk orbiting the central stars, the material being the residue of the molecular cloud which formed the central star. With total masses (
Proto-planetary disks orbiting around young stars of a few million years old are now routinely observed by large single-aperture telescopes and interferometers from the radio up to the optical domain. Large optical telescopes provide images of the disk surface where the dust grains scatter the impinging stellar light. When the dust disk is observed close to edge-on, these images reveal that the disks are flaring, as expected from the theory of disks in hydrostatic equilibrium [23]. In the meantime, spectro-imaging of the CO gas provided by millimetre/submillimetre (mm/submm) arrays have shown that protoplanetary disks are in Keplerian rotation around their central star and that disks can be as large as 500–800 AU, in radius. At the distance of the closest star forming regions (
Fig. 1 is a sketch showing a ‘classical’ accretion disk orbiting a TTauri star of a few million years and located at the Taurus distance (150 pc). From the observations, one can then naturally distinguish three parts in disks. The very inner disk, located close to its inner radius, is extending up to radius of

Sketch of a disk of a few million years old orbiting a TTauri star located at 150 pc from the Earth. This figure also displays the area sampled in disks by large astronomical facilities depending on their wavelengths, sensitivity and angular resolution. ALMA, the submm array, currently in construction in Chili, will have enough sensitivity and resolution to provide the first images of the inner disk with a linear resolution of a few AUs. Multi-wavelengths approaches are necessary to sample the whole disk. Bottom: the three lines, from left to right, correspond to the very inner disk, the inner disk, the outer disk, respectively. Note that the star is not at the right scale. Masquer
Sketch of a disk of a few million years old orbiting a TTauri star located at 150 pc from the Earth. This figure also displays the area sampled in disks by large astronomical facilities depending on their wavelengths, sensitivity and angular ... Lire la suite
Fig. 1. Représentation schématique d’un « demi-disque » d’environ
Fig. 1. Représentation schématique d’un « demi-disque » d’environ

From [14]. Observation of the gas disk orbiting the TTauri star DM Tau of
From [14]. Observation of the gas disk orbiting the TTauri star DM Tau of
Fig. 2. Observation du disque de gaz entourant l’étoile TTauri DM Tau (0.5 M⊙,
Fig. 2. Observation du disque de gaz entourant l’étoile TTauri DM Tau (0.5 M⊙,
More generally, these last years have revealed the potential of interferometric techniques as powerful tools to resolve and model such disks. This is true both for small hot inner disks and large colder outer disks. It appears that only aperture synthesis (interferometry) can nowadays technically achieve the angular resolution needed.
2 Disk properties: from large to smaller scales
The next section presents some major highlights resulting from the technical improvements of large telescopes and interferometers.
2.1 Gas disk observed with millimeter/submillimeter arrays
Current mm/submm interferometers provide images of the dust and gas outer disks at an angular resolution of
CO maps not only reveal that disks are in Keplerian rotation around TTauri [18] and Herbig Ae stars [20], they also allow us to retrieve the physical conditions in outer disks. Comparing resolved CO maps to a disk model by performing a
The radial profiles of the temperature deduced from12CO images appear to be consistent with models of stellar heating in flared disks, the outer disk being colder than the inner one. Moreover, since the
It is also important to mention that CO in the gas phase has been observed in all TTauri disks at a temperature which appears to be below the CO freeze out point (17 K). This suggests that the vertical turbulence should play an important role by providing chemical mixing between the vertical molecular layers.
Searching for molecules other than CO remains sensitivity limited to the most abundant molecules found in molecular clouds. So far, there are only a few attempts to survey a large set of molecules in protoplanetary disks [10,17,27,11]. In addition to
2.2 MID interferometry: mineralogy of the inner disk
Continuum observations of the dust, both at mm and NIR wavelengths, imply that the dust in such objects has evolved compared to that found in the interstellar medium. In particular, the particle size has started to increase from sub-micron size up to a few centimeters [25]. One key problem in planetary formation is the understanding of the dust evolution in term of grain size, vertical and radial distribution, composition and chemical nature from the inner disk to the outer disk. For example, in the interstellar medium (ISM), the silicate grains are amorphous while in our Solar system, comets and meteorites present crystalline silicates. The new generation of optical, Near-IR and MID-IR interferometers begin to provide direct insights on the inner disks where planetary formation is thought to occur. The observations discussed below present a qualitative but major improvement in the knowledge of the dust properties in inner disks.
Van Boekel et al. [26] recently observed three disks orbiting young stars of

From [26]. MIDI spectra of the silicate bands around 10
From [26]. MIDI spectra of the silicate bands around 10
Fig. 3. Spectres MIDI autour de 10
Fig. 3. Spectres MIDI autour de 10
So far, these recent data bring the first observational constraints on the properties of dust grains at the scale of a few AU in disks. These kinds of observations will likely provide in the next years the first detailed analysis of the dust mineralogy in inner disks, leading to a better understanding of the dust evolution in the earliest phases of the planetary formation.
More generally, recent progress in observational astrophysics begins to provide the sensitivity and the angular resolution needed to study the regions of proto-planetary disks where planetary formation should occur.
3 Dissipation of dust and gas disks
We have seen in the previous section that the dust found around disks a few million years old has strongly evolved compared to that found in the ISM and in star forming regions. The IR excess is characteristic of the amount of dust found in inner disks; the observation of this excess is often used as a tracer of the inner disk evolution while the submm/mm flux can be reasonably considered as a tracer of the dust in the outer disk. Tracing the evolution of the gas (the bulk of the disk mass) is difficult because it is essentially based on the observation of the low J CO rotational lines. These CO lines also strongly emit in molecular clouds where most of these stars are, at least partly, embedded. As a consequence, the high opacity of the CO lines in the clouds does not allow astronomers to detect CO disks, except if the disks are located at the periphery. It is then difficult to get significant statistics on the gas component evolution. This problem will likely disappear in a few years as soon as the submm array ALMA provides enough sensitivity to systematically survey the rarer isotopes of CO.
3.1 The lifetime of disks
Observations of numerous TTauri stars located in star forming regions of various ages have been made by several groups. Fig. 4 is a compilation by [15] of the IR excess (traced by the H–K excess sampling the very inner disk) versus the age of the stars. There are many uncertainties associated to this kind of analysis, the most important being (1) the errors on the estimate of the IR excess and (2) the derivation of the stellar age (which is taken as the median apparent age of the individual objects belonging to the same cluster). However, there is a clear trend which can be seen on this plot: at age around 1 million years, 80–90% of the TTauri stars are surrounded by an innermost dust disk while the fraction of inner disks drops to 30–40% at 2–3 million years and beyond 5 million years, almost all disks have disappeared.

After [15]. Observations of the IR excess in disks, traced by the H–K excess, the difference of magnitudes H (1.6
After [15]. Observations of the IR excess in disks, traced by the H–K excess, the difference of magnitudes H (1.6
Fig. 4. Observation de l’excès IR dans les disques, caractérisé par H–K, la différence de magnitude entre la bande H (
Fig. 4. Observation de l’excès IR dans les disques, caractérisé par H–K, la différence de magnitude entre la bande H (
Comparison of the IR excess with the observations of the dust observed in the mm/submm domain [3] and CO data [12] suggest that the dissipation of the inner and outer disks are roughly simultaneous. However, since the statistics in the mm/submm domain are poor, it is difficult to derive quantitative numbers, yet.
Nevertheless, taking into account the various types of surveys and their results, it seems reasonable to conclude that the dissipation is globally a fast process since more than half of the inner disks disappear after 2–3 Myr.
3.2 Dissipation of disks
There are many physical reasons which should explain the apparent dispersal of disks:
- • the formation of a planetary system;
- • the formation of a binary or a multiple stellar system;
- • the tidal truncation by stellar encounters;
- • the photo-evaporation of the disk.
Large optical/NIR surveys of star forming clusters have shown that many stars form in binary and/or multiple systems (e.g., [19]). The formation of a binary or a multiple stellar system strongly affects the original distribution of material which is tidally disrupted. For example, observations of the TTauri binary system GG Tau [9] have shown that the gas and the dust are located inside a circumbinary disk orbiting the binary while very small individual disks exist around each stellar component, inside the Roche lobe. Hence, even if the innermost disks can exist, statistics provided by studies such as those presented above, may be partially biased by the presence in the samples of unresolved multiple stellar systems which are not yet known.
In some clusters, such as
The photo-evaporation of disks is invoked as a possible phenomenon responsible for the disruption of inner disks [2]. UV photons coming from the young stars reach the disk surface where they can create a ionization front, heating the material behind. The ionized gas becomes gravitationally unbound and flows in a wind away from the disk surface. This phenomenon is observed for the proplyds orbiting TTauri stars located in the Orion Trapezium region [4], close to the OB association. The strong ionizing UV flux from the OB stars photo-evaporates the outer disks in their surroundings. Alexander and collaborators have shown that in some cases photo-evaporation should clear the inner disk very efficiently, creating a central hole, and disrupt the whole disk on a time-scale of about
Moreover, whatever the frequencies of the various physical processes described in this section are, all of them may affect planet formation since they have a direct impact on the dust and gas distribution close to the stars with the same time-scales.
4 Towards planetary formation: the LkCa15 case
The extension of the IRAM array baselines performed in summer 2006 provides an angular resolution of

From [21]. IRAM interferometer observations of the dust disks surrounding MWC480 and LkCa15 at 1.3 and 2.8 mm. From left to right: 2.8-mm images, 1.3-mm images, residual images after subtracting the best model to the data, circularly averaged visibility versus baselines length (filled squares) and best model (black curve). Top: MWC480. Bottom: LkCa15. MWC480 and LkCa15 are young stars of a few million years and masses of 2 and 1 M⊙, respectively. Contrary to previous observations at low angular resolution, these observations performed with a linear resolution of
From [21]. IRAM interferometer observations of the dust disks surrounding MWC480 and LkCa15 at 1.3 and 2.8 mm. From left to right: 2.8-mm images, 1.3-mm images, residual images after subtracting the best model to the data, circularly averaged visibility versus ... Lire la suite
Fig. 5. Observations IRAM de l’émission thermique des disques de poussières entourant MWC480 et LkCa15 d’après [21]. De gauche à droite : images obtenues à 2,8 mm, à 1,3 mm, image des résidus après soustraction du meilleur modèle aux données, visibilités moyennées en fonction de la ligne de base (carrés pleins) et meilleur modèle (courbe noire). Haut : pour MWC480. Bas : pour LkCa15.
The LkCa15 disk has been observed at mm wavelengths with lower angular resolutions (ranging from to 200 to 70 AU) by several interferometers. None of these observations were suggesting such a wide cavity. This example illustrates how an increase in linear resolution (and sensitivity) can change the physics of an astrophysical object, as soon as the spatial scales needed to study the physical processes are reached.
5 Summary and open questions
In the last fifteen years, resolved observations, from the mm to the optical domain, of the circumstellar disks orbiting TTauri stars have opened the process leading to the formation of planetary systems to direct investigations. Today, we are far from understanding the whole process but the characterization of the physical and chemical properties of these proto-planetary disks represents a major step since planets are built from the disk material. Our current knowledge allows us to tell that:
- • many disks can have large outer radii, within the range 100–800 AU;
- • disks are usually in Keplerian rotation;
- • optical images reveal that disks are flaring;
- • vertical temperature gradients in proto-planetary disks appear to be compatible with heating by the central star, in agreement with disk models;
- • both mm and optical data show that grains in disks are more evolved than in the parent molecular clouds, with sizes up to a few centimeters;
- • NIR surveys suggest that inner disk typical lifetimes are 2–3 million years, beyond 5 million years, most proto-planetary disks may have disappeared.
We have seen through several examples that multi-wavelength approaches are required to get a global understanding of the physical properties of these disks. Optical to MID-IR interferometry has started to unveil the physics of the inner disk. The inner disk will be imaged by the next generation of instruments. With its capability of reaching AU-scale angular resolution at wavelengths where dust has a moderate opacity, ALMA is expected to be the premium instrument to probe these regions and allow astronomers to constrain the models of planet formation. ALMA will also have the potential of providing much higher angular resolution images of the emission of molecular lines allowing detailed modelling of the gas content and of the disk kinematics.
The forthcoming generation of astronomical facilities will likely change our views on planetary formation. In particular, they will bring new information on the following fundamental questions:
- • what is the impact of the parent cloud and its vicinity on the disk properties and its evolution?
- • what is the feedback of the young star on the gas and dust disk and its evolution (photo-evaporation - photo-chemistry at the disk surface) and how does it affect planetary formation?
- • what are the time-scales for grain growth and for vertical settlement of the dust on the disk mid-plane?
- • more generally, what is the mass distribution (gas + dust) radially and vertically in proto-planetary disks and how does it evolve with time?
- • what are the indirect/direct evidences of the formation of a planet in a proto-planetary disk?
Finally, the comparison of these proto-planetary disks which are expected to be representative of the first phases of a planetary system with our own Solar system remains fundamental, as a guideline for each other domain. In the coming years, the understanding of the dynamical and physical/chemical evolution of the dust and gas content in disks will naturally provide a better estimate of the chronology of the planetary formation and a better understanding of the formation of our own Solar System.
Acknowledgements
Anne Dutrey thanks her collaborators: S. Guilloteau, M. Simon, E. Dartois and V. Piétu for a fruitful long-term collaboration. Drs L. Hillenbrand and R. Van Boekel are acknowledged for providing figures.
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