Circumstellar (CS) disks are ubiquitous around stars with
intermediate ages around a few million years. They are a natural
outcome of the star formation process, because of the need of
angular momentum conservation during the collapse of the initial
molecular core (Shu, F. H., & Adams, F. C., 1987, IAUS, 115, 417.). We consider normally that disks contain gas and dust.
As the star evolved, the disk changes: the gas is dissipated and a variety of processes (collisions...) leads to the growth of dust grains and the formation of planets. In order to characterise planets'formation,
it is interesting to understand the physics of the medium where they were born.
Herbig Ae (HAe) stars represent a particularly interesting
laboratory for studying disks evolution and planet formation since the planets
are still in the building process or eventually just formed. HAe stars are believed to be the more massive analogues of T Tauri
stars ( 2-10 M
). Although great progress had been made in modelling the disk struture with radiative transfer models able to reproduce the
SEDs (Chiang, E. I., & Goldreich, P. 1997, ApJ, 490, 368; Natta, A., Prusti, T., Neri, R., et al. 2001, A&A, 371, 186; Dullemond, C.P., & Dominik, C., & Natta, A. 2001, ApJ, 560, 957), the structure of the disks is not uniquely constrained.
Spatially resolved observations are also needed to constrain more the
geometry of the disks.
The observations were performed using the ESO mid-infrared instrument VISIR installed on the Very Large Telescope (Paranal, Chili), a mid-IR (around 10 and 20 m) camera and spectrometer.
With a 8 m telescope, the Full Width Half Maximum due to diffraction
gives a spatial resolution of 0.3" at 10
m. As a result, we are able to resolve disks with a typical
size of 100 AU around Herbig stars at a distance of 100 pc.
Around 10
m, the emission may be produced by non-equilibrium
reprocessing of ultraviolet radiation by very large organic molecules (like PAH: polycyclic aromatic hydrocarbons particle).
Such particles undergo transient heating : they do not reach thermal equilibrium with the radiation
field, but absorb individual photons, experiencing a rapid increase in temperature, and slowly cool,
re-radiating the absorbed energy at longer wavelengths. This radiation allows us to see much further since this grains could reach high temperature far away from the star.
Under the assumption of good seeing (0.5 arc second in the visible), the diffraction is the limitation at 10 m on a 8 m telescope. Unfortunatly, during our exposures, we observe a seeing of 0.8 arc second wich restricts our resolution. Indeed, the source is moving on the detector of about 5 pixels (pfov = 0.075"/pixel). In order to get the best
spatial resolution, we experiment a new imaging mode on the bright objects, the 'BURST' mode. The
principe is to take rapid images (exposure time = 50 ms) shifted and
added to correct from seeing effect and 'possible' bad positions of
the chopper in order to reach the best spatial resolution.
So, we present high angular imaging and spectroscopic observations of a sample of Herbig stars, extracted from the lists of Thé et al.(1994, A&AS, 104, 315) and Malfait et
al.(1998, A&AL, 332, 25).
We have spatially resolved most of them and an example, HD97048, is shown in Fig.1.
With both imagery and spectroscopy, the object is spatially resolved and is quite extended (around 1.5'') at 11.3 m (feature of PAH).
We can also resolve an asymmetry in the direction east/west due probably to a geometric effect of the flared disk.
We will discuss the comparison of the observed spatial distribution of the 11.3
m feature and the adjacent continuum emission with the predictions of a disk model that includes transiently heated small grains PAHs in addition to large grains in thermal equilibrium (Habart, E., Natta, A., Krugel, E., 2004, A&A, 427, 179).